The individual may feel relaxed but does not realise that his sense of judgement, sense of timing, and muscular coordination have been seriously impaired.


Unlike ethanol, intake of methanol in very small quantities can cause death.


Methanol is oxidised to methanal in the liver.


Methanal reacts rapidly with the components of cells.


It causes the protoplasm to get coagulated, in much the same way an egg is coagulated by cooking.


Methanol also affects the optic nerve, causing blindness.


Ethanol is an important industrial solvent.


To prevent the misuse of ethanol produced for industrial use, it is made unfit for drinking by adding poisonous substances like methanol to it.


Dyes are also added to colour the alcohol blue so that it can be identified easily.


This is called denatured alcohol.


More to Know!


Alcohol as a fuel.


Sugarcane plants are one of the most efficient convertors of sunlight into chemical energy.


Sugarcane juice can be used to prepare molasses which is fermented to give alcohol (ethanol).


Some countries now use alcohol as an additive in petrol since it is a cleaner fuel which gives rise to only carbon dioxide and water on burning in sufficient air (oxygen).


Properties of Ethanoic Acid


Ethanoic acid is commonly called acetic acid and belongs to a group of acids called carboxylic


acids.


Compare the pH of dilute acetic acid called vinegar and is used widely as a preservative and dilute hydrochloric acid using in pickles.


The melting point of pure ethanoic acid both litmus paper and universal is 290 K and hence it often freezes during winter indicator.


This gave rise to its name glacial.


Are both acids indicated by the acetic acid.


Take 1 mL ethanol (absolute alcohol) and 1 mL glacial acetic acid along with a few drops of concentrated sulphuric acid in a test tube.


Warm in a water-bath for at least five minutes as shown.


Pour into a beaker containing 20-50 mL of water and smell the resulting mixture.


Reactions of ethanoic acid:


Esterification reaction:


Esters are most commonly  formed by reaction of an acid and an alcohol.


Ethanoic acid reacts with absolute ethanol in the presence of an acid catalyst to give an ester.


Esters are sweet-smelling substances.


These are used in making formation of ester noperfumes and as flavouring agents.


On treating with sodium hydroxide, which is an alkali, the ester is converted back to alcohol and sodium salt of carboxylic acid.


This reaction is known as saponification because it is used in the preparation of soap.


Reaction with a base: Like mineral acids, ethanoic acid reacts with a base such as sodium hydroxide to give a salt (sodium ethanoate or commonly called sodium acetate) and water:


NaOH + CH3COOH → CH3COONa + H2O


How does ethanoic acid react with carbonates and hydrogencarbonates?


Let us perform an activity to find out.


Set up the apparatus as shown in train/chapter 2.


Take a spatula full of sodium carbonate in a test tube and add 2 mL of dilute ethanoic acid.


What do you observe?


Pass the gas produced through freshly prepared lime-water.


Can the gas produced by the reaction between ethanoic acid and sodium carbonate be identified by this test?


Repeat this Activity with sodium hydrogencarbonate instead of sodium carbonate.


Reaction with carbonates and hydrogencarbonates:


Ethanoic acid reacts with carbonates and hydrogencarbonates to give rise to a salt, carbon dioxide and water.


The salt produced is commonly called sodium acetate.


2CH3COOH + Na2CO3 → 2CH 3COONa + H2O + CO2


CH 3COOH + NaHCO 3 → CH3COONa + H2O + CO2


QUESTIONS


How would you distinguish experimentally between an alcohol and  a carboxylic acid?


What are oxidising agents?


SOAPS AND DETERGENTS


Take about 10 mL of water each in two test tubes.


Add a drop of oil (cooking oil) to both the test tubes and label them as A and B.


To test tube B, add a few drops of soap solution.


Now shake both the test tubes vigourously for the same period of time.


Can you see the oil and water layers separately in both the test tubes immediately after you   stop shaking them?


Formation of micelles


Leave the test tubes undisturbed for some time  and observe.


Does the oil layer separate out?


In which test tube does this happen first?


This activity demonstrates the effect of soap in cleaning.


Most dirt is oily in nature and as you know, oil does not dissolve in water.


The molecules of soap are sodium or potassium salts of long-chain carboxylic acids.


The ionic-end of soap dissolves in water while the carbon chain dissolves in oil.


The soap molecules, thus form structures called micelles where one end of the molecules is towards the oil droplet while the ionic-end faces outside.


This forms an emulsion in water.


The soap micelle thus helps in dissolving the dirt in water and we can wash our clothes clean.


Can you draw the structure of the micelle that would be formed if you dissolve soap in a hydrocarbon?


Micelles


Soaps are molecules in which the two ends have differing properties, one is hydrophilic, that is, it dissolves in water, while the other end is hydrophobic, that is, it dissolves in  hydrocarbons.


When soap is at the surface of water, the hydrophobic ‘tail’ of soap will not be soluble in water and the soap will align along the surface of water with the ionic end in water and the hydrocarbon ‘tail’ protruding out of water.


More to Know!


These molecules have a unique orientation that keeps the hydrocarbon portion out of the water.


This is achieved by forming clusters of molecules in which the hydrophobic tails are in the interior of the cluster and the ionic ends are on the surface of the cluster.


This formation is called a micelle.


Soap in the form of a micelle is able to clean, since the oily dirt will be collected in the centre of the micelle.


The micelles stay in solution as a colloid and will not come together to precipitate because of ion-ion repulsion.


Thus, the dirt suspended in the micelles is also easily rinsed away.


The soap micelles are large enough to scatter light.


Hence a soap solution appears cloudy.


Effect of soap in cleaning.


Take about 10 mL of distilled water (or rain water) and 10 mL of hard water (from a tubewell or hand-pump) in separate test tubes.


Add a couple of drops of soap solution to both.


Shake the test tubes vigorously for an equal period of time and observe the amount of foam formed.


In which test tube do you get more foam?


In which test tube do you observe a white curdy precipitate?


Note for the teacher : If hard water is not available in your locality,


prepare some hard water by dissolving hydrogencarbonates/ sulphates/chlorides of calcium or magnesium in water.


Take two test tubes with about 10 mL of hard water in each.


Add five drops of soap solution to one and five drops of detergent solution to the other.


Shake both test tubes for the same period.


Do both test tubes have the same amount of foam?


In which test tube is a curdy solid formed?


Have you ever observed while bathing that foam is formed with difficulty and an insoluble substance (scum) remains after washing with water?


This is caused by the reaction of soap with the calcium and magnesium salts, which cause the hardness of water.


Hence you need to use a larger amount of soap.


This problem is overcome by using another class of compounds called detergents as cleansing agents.


Detergents are generally ammonium or sulphonate salts of long chain carboxylic acids.


The charged ends of these compounds do not form insoluble precipitates with the calcium and magnesium ions in hard water.


Thus, they remain effective in hard water.


Detergents are usually used to make shampoos and products for cleaning clothes.


Would you be able to check if water is hard by using a detergent?


People use a variety of methods to wash clothes.


Usually after adding the soap, they ‘beat’ the clothes on a stone, or beat it with a paddle,   scrub with a brush or the mixture is agitated in a washing machine.


Why is agitation necessary to get clean clothes?


What you have learnt?


Carbon is a versatile element that forms the basis for all living organisms and many of the things we use.


This large variety of compounds is formed by carbon because of its tetravalency and the property of catenation that it exhibits.


Covalent bonds are formed by the sharing of electrons between two atoms so that both can achieve a completely filled outermost shell.


Carbon forms covalent bonds with itself and other elements such as hydrogen, oxygen, sulphur, nitrogen and chlorine.


Carbon also forms compounds containing double and triple bonds between carbon atoms.


These carbon chains may be in the form of straight chains, branched chains or rings.


The ability of carbon to form chains gives rise to a homologous series of compounds in which the same functional group is attached to carbon chains of different lengths.


The functional groups such as alcohols, aldehydes, ketones and carboxylic acids bestow characteristic properties to the carbon compounds that contain them.


Carbon and its compounds are some of our major sources of fuels.


Ethanol and ethanoic acid are carbon compounds of importance in our daily lives.


The action of soaps and detergents is based on the presence of both hydrophobic and hydrophilic groups in the molecule and this helps to emulsify the oily dirt and hence its removal.


Periodic Classification of Elements


In Class 12 we have learnt that matter around us is present in the form of elements, compounds and mixtures and the elements contain atoms of only one type.


Do you know how many elements are known till date?


At present, 118 elements are known to us.


All these have different properties.


Out of these 118, only 98 are naturally occurring.


As different elements were being discovered, scientists gathered more and more information about the properties of these elements.


They found it difficult to organise all that was known about the elements.


They started looking for some pattern in their properties, on the basis of which they could study such a large number of elements with ease.


MAKING ORDER OUT OF CHAOS  – EARLY ATTEMPTS AT THE CLASSIFICATION OF ELEMENTS


We have been learning how various things or living beings can be classified on the basis of their properties.


Even in other situations, we come across instances of organisation based on some properties.


For example, in a shop, soaps are kept together at one place while biscuits are kept together elsewhere.


Even among soaps, bathing soaps are stacked separately from washing soaps.


Similarly, scientists made several attempts to classify elements according to their properties and obtain an orderly arrangement out of chaos.


The earliest attempt to classify the elements resulted in grouping the then known elements as metals and non-metals.


Imagine you and your friends have  found pieces of an old map to reach a treasure.


Would it be easy or later further classifications were tried out as our knowledge chaotic to find the way to the of elements and their properties increased.


Chemistry as elements were  known but there was no clue as to how to classify and study about them.


In the year 1817, Johann Wolfgang Döbereiner, a German chemist, tried to arrange the elements with similar properties into groups.


He identified some groups having three elements each.


So he called these groups ‘triads’.


Döbereiner showed that when the three elements in a triad were written in the order of increasing atomic masses.


The atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.


For example, take the triad consisting of lithium (Li), sodium (Na) and potassium (K) with the respective atomic masses 6.9, 23.0 and 39.0.


What is the average of the atomic masses of Li and K?


How does this compare with the atomic mass of Na?


Given below (Table 5.1) are some groups of three elements.


These elements are arranged downwards in order of increasing atomic masses.


Can you find out which of these groups form Döbereiner triads?


Newlands’ Law of Octaves


The attempts of Döbereiner encouraged other chemists to correlate the properties of elements with their atomic masses.


In 1866, John Newlands, an English scientist, arranged the then known elements in the order of increasing atomic masses.


He started with the element having the lowest atomic mass (hydrogen) and ended at thorium which was the 56th element.


He found that every eighth element had properties similar to that of the first.


He compared this to the octaves found in music.


Therefore, he called it the ‘Law of Octaves’.


It is known as ‘Newlands’ Law of Octaves’.


In Newlands’ Octaves, the properties of lithium and sodium were found to be the same.


Sodium is the eighth element after lithium.


Similarly, beryllium and magnesium resemble each other.


A part of the original form of Newlands’ Octaves is given.


Do You Know?


Are you familiar with musical notes?


In the Indian system of music, there are seven musical notes in a scale – sa, re, ga, ma, pa,


da, ni.


In the west, they use the notations – do, re, mi, fa, so, la, ti.


The notes in a scale are


separated by whole and half-step frequency intervals of tones and semitones.


A musician


uses these notes for composing the music of a song.


Naturally, there must be some repetition


of notes.


Every eighth note is similar to the first one and it is the first note of the next scale.


It was found that the Law of Octaves was applicable only upto calcium, as after calcium every eighth element did not possess properties similar to that of the first.


It was assumed by Newlands that only 56 elements existed in nature and no more elements would be discovered in the future.


But, later on, several new elements were discovered, whose properties did not fit into the Law of Octaves.


In order to fit elements into his Table, Newlands adjusted two elements in the same slot, but also put some unlike elements under the same note.


Note that cobalt and nickel are in the same slot and these are placed in the same column as fluorine, chlorine and bromine which have very different properties than these elements.


Iron, which resembles cobalt and nickel in properties, has been placed far away from these elements.


Thus, Newlands’ Law of Octaves worked well with lighter elements only.


QUESTIONS


Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves?


What were the limitations of Döbereiner’s classification?


What were the limitations of Newlands’ Law of Octaves?


MAKING ORDER OUT OF CHAOS – MENDELÉEV ’S PERIODIC TABLE


Even after the rejection of Newlands’ Law of Octaves, many scientists continued to search for a pattern that correlated the properties of elements with their atomic masses.


The main credit for classifying elements goes to Dmitri Ivanovich Mendeléev, a Russian chemist.


He was the most important contributor to the early development of a Periodic Table of elements wherein the elements were arranged on the basis of their fundamental property, the atomic mass, and also on the similarity of chemical properties.


Dmitri lvanovich Mendeléev (1834-1907)


Dmitri lvanovich Mendeléev was born in Tobolsk in Western Siberia, Russia on 8 February 1834.


After his early education, Mendeléev could join a university only due to the efforts of his mother.


Dedicating his investigations to his mother he wrote, “She instructed with example, corrected with love and travelled with me to places spending her last resources and strength.


She knew that with the aid of science without violence, with love but firmness, all superstitions, untruth and errors can be removed.”


The arrangement of elements he proposed is called Mendeléev’s Periodic Table.


The Periodic Table proved to be the unifying principle in chemistry.


It was the motivation for the discovery of some new elements.


When Mendeléev started his work, 63 elements were known.


He examined the relationship between the atomic masses of the elements and their physical and chemical properties.


Among chemical properties, Mendeléev concentrated on the compounds formed by elements with oxygen and hydrogen.


He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements.


The formulae of the hydrides and oxides formed by an element were treated as one of the basic properties of an element for its classification.


He then took 63 cards and on each card he wrote down the properties of one element.


He sorted out the elements with similar properties and pinned the cards together on a wall.


He observed that most of the elements got a place in a Periodic Table and were arranged in the order of their increasing atomic masses.


It was also observed that there occurs a periodic recurrence of elements with similar physical and chemical properties.


On this basis, Mendeléev formulated a Periodic Law, which states that ‘the properties of elements are the periodic function of their atomic masses’.


Mendeléev’s Periodic Table contains vertical columns called ‘groups’ and horizontal rows called ‘periods’.


Mendeléev’s Periodic Table was published in a German journal in 1872.


In the formula for oxides and hydrides at the top of the columns, the letter ‘R’ is used to represent any of the elements in the group.


Note the way formulae are written.


For example, the hydride of carbon, CH4, is written as RH4 and the oxide CO2, as RO2.


Achievements of Mendeléev’s Periodic Table


While developing the Periodic Table, there were a few instances where Mendeléev had to place an element with a slightly greater atomic mass before an element with a slightly lower atomic mass.


The sequence was inverted so that elements with similar properties could be grouped together.


For example, cobalt (atomic mass 58.9) appeared before nickel (atomic mass 58.7).


Further, Mendeléev left some gaps in his Periodic Table.


Instead of looking upon these gaps as defects, Mendeléev boldly predicted the existence of some elements that had not been discovered at that time.


Mendeléev named them by prefixing a Sanskrit numeral, Eka (one) to the name of preceding element in the same group.


For instance, scandium, gallium and germanium, discovered later, have properties similar to Eka–boron, Eka–aluminium and Eka–silicon, respectively.


The properties of Eka–Aluminium predicted by Mendeléev and those of the element, gallium which was discovered later and replaced Eka- aluminium.


This provided convincing evidence for both the correctness and usefulness of Mendeléev’s Periodic Table.


Further, it was the extraordinary success of Mendeléev’s prediction that led chemists not only to accept his Periodic Table but also recognise him, as the originator of the concept on which it is based.


Noble gases like helium (He), neon (Ne) and argon (Ar) have been mentioned in many a context before this.


These gases were discovered very late because they are very inert and present in extremely low concentrations in our atmosphere.


One of the strengths of Mendeléev’s Periodic Table was that, when these gases were discovered, they could be placed in a new group without disturbing the existing order.


Limitations of Mendeléev’s Classification


On the other hand, just like halogens, hydrogen also exists as diatomic molecules and it combines with metals and non-metals to form covalent compounds.


Looking at its resemblance to alkali metals and the halogen family, try to assign hydrogen a correct position in Mendeléev’s Periodic Table.


To which group and period should hydrogen be assigned?


Certainly, no fixed position can be given to hydrogen in the Periodic Table.


This was the first limitation of Mendeléev’s Periodic Table.


He could not assign a correct position to hydrogen in his Table.


Isotopes were discovered long after Mendeléev had proposed his periodic classification of elements.


Let us recall that isotopes of an element have similar chemical properties, but different atomic masses.


Consider the isotopes of chlorine, Cl-35 and Cl-37.


Would you place them in different slots because their atomic masses are different?


Or would you place them in the same position because their chemical properties are the same?


Thus, isotopes of all elements posed a challenge to Mendeleev’s Periodic Law.


Another problem was that the atomic masses do not increase in a regular manner in going from one element to the next.


So it was not possible to predict how many elements could be discovered between two elements especially when we consider the heavier elements.


Use Mendeléev’s Periodic Table to predict the formulae for the oxides of the following elements:


K,     C,     AI,     Si,   Ba.


Besides gallium, which other elements have since been discovered that were left by Mendeléev in his periodic Table?


What were the criteria used by Mendeléev in creating his Periodic Table?


Why do you think the noble gases are placed in a separate group?


MAKING ORDER OUT OF CHAOS – THE MODERN PERIODIC TABLE


In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass as described below.


Accordingly, Mendeléev’s Periodic Law was modified and atomic number was adopted as the basis of Modern Periodic Table and the Modern


Periodic Law can be stated as follows:


‘Properties of elements are a periodic function of their atomic number.’


Let us recall that the atomic number gives us the number of protons in the nucleus of an atom and this number increases by one in going from one element to the next.


Elements, when arranged in order of increasing atomic number Z, lead us to the classification known as the Modern Periodic Table.


Prediction of properties of elements could be made with more precision when elements were arranged on the basis of increasing atomic number.


How were the positions of cobalt and nickel resolved in the Modern Periodic Table?


How were the positions of isotopes of various elements decided in the Modern Periodic Table?


Is it possible to have an element with atomic number 1.5 placed between hydrogen and helium?


Where do you think should hydrogen be placed in the Modern Periodic Table?


As we can see, the Modern Periodic Table takes care of three limitations of Mendléev’s Periodic Table.


The anomalous position of hydrogen can be discussed after we see what are the bases on which the position of an element in the Modern Periodic Table depends.


Position of Elements in the Modern Periodic Table


The Modern Periodic Table has 18 vertical columns known as ‘groups’ and 7 horizontal rows known as ‘periods’.


Let us see what decides the placing of an element in a certain group and period.


Look at the group 1 of the Modern Periodic Table, and name the elements present in it.


Write down the electronic configuration of the first three elements of group 1.


What similarity do you find in their electronic configurations?


How many valence electrons are present in these three elements?


You will find that all these elements contain the same number of valence electrons.


Similarly, you will find that the elements present in any one group have the same number of valence electrons.


For example, elements fluorine (F) and chlorine (Cl), belong to group 17, how many electrons do fluorine and chlorine have in their outermost shells?


Hence, we can say that groups in the Periodic Table signify an identical outer- shell electronic configuration.


On the other hand, the number of shells increases as we go down the group.


There is an anomaly when it comes to the position of hydrogen because it can be placed either in group 1 or group 17 in the first period.


Can you say why?


If you look at the long form of the Periodic Table, you will find that the elements Li, Be, B, C, N, O, F, and Ne are present in the second period.


Write down their electronic configuration.


Do these elements also contain the same number of valence electrons?


Do they contain the same number of shells?


You will find that these elements do not have the same number of valence electrons, but they contain the same number of shells.


You also observe that the number of valence shell electrons increases by one unit, as the atomic number increases by one unit on moving from left to right in a period.


Or we can say that atoms of different elements with the same number of occupied shells are placed in the same period.


Na, Mg, Al, Si, P, S, Cl and Ar belong to the third period of the Modern Periodic Table, since the electrons in the atoms of these elements are filled in K, L and M shells.


Write the electronic configuration of these elements and confirm the above statement.


Each period marks a new electronic shell getting filled.


How many elements are there in the first, second, third and fourth periods?


We can explain the number of elements in these periods based on how electrons are filled into various shells.


You will study the details of this in higher classes.


Recall that the maximum number of electrons that can be accommodated in a shell depends on the formula 2n2 where ‘n’ is the number of the given shell from the nucleus.


For example,


K Shell – 2 × (1)2 = 2, hence the first period has 2 elements.


L Shell – 2 × (2)2 = 8, hence the second period has 8 elements.


M Shell – 2 × (3)2 = 18, but the outermost shell can have only 8 electrons, so the third period also has only 8 elements.


The position of an element in the Periodic Table tells us about its chemical reactivity.


As you have learnt, the valence electrons determine the kind and number of bonds formed by an element.


Can you now say why Mendeléev’s choice of formulae of compounds as the basis for deciding the position of an element in his Table was a good one?


How would this lead to elements with similar chemical properties being placed in the same group?


Trends in the Modern Periodic Table


Valency : As you know, the valency of an element is determined by the number of valence electrons present in the outermost shell of its atom.


How do you calculate the valency of an element from its electronic configuration?


What is the valency of magnesium with atomic number 12 and sulphur with atomic number 16?


Similarly find out the valencies of the first twenty elements.


How does the valency vary in a period on going from left to right?


How does the valency vary in going down a group?


Atomic size: The term atomic size refers to the radius of an atom.


The atomic size may be visualised as the distance between the centre of the nucleus and the outermost shell of an isolated atom.


The atomic radius of hydrogen atom is 37 pm (picometre, 1 pm = 10–12m).


Let us study the variation of atomic size in a group and in a period.


Arrange them in decreasing order of their atomic radii.


Are the elements now arranged in the pattern of a period in the Periodic Table?


Which elements have the largest and the smallest atoms?


How does the atomic radius change as you go from left to right in a period?


You will see that the atomic radius decreases in moving from left to right along a period.


This is due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.


Study the variation in the atomic radii of first group elements given below and arrange them in an increasing order.


Name the elements which have the smallest and the largest atoms.


How does the atomic size vary as you go down a group?


You will see that the atomic size increases down the group.


This is because new shells are being added as we go down the group.


This increases the distance between the outermost electrons and the nucleus


so that the atomic size increases in spite of the increase in nuclear charge.


Metallic and Non-metallic Properties


Examine elements of the third period and classify them as metals and non-metals.


On which side of the Periodic Table do you find the metals?


On which side of the Periodic Table do you find the non-metals?


As we can see, the metals like Na and Mg are towards the left-hand side of the Periodic Table while the non-metals like sulphur and chlorine are found on the right-hand side.


In the middle, we have silicon, which is classified as a semi-metal or metalloid because it exhibits some properties of both metals and non-metals.


In the Modern Periodic Table, a zig-zag line separates metals from non-metals.


The borderline elements – boron, silicon, germanium, arsenic, antimony, tellurium and polonium – are intermediate in properties and are called metalloids or semi-metals.


As you have seen in train/chapter 3, metals tend to lose electrons while forming bonds, that is, they are electropositive in nature.


How do you think the tendency to lose electrons will change in a group?


How will this tendency change in a period?


As the effective nuclear charge acting on the valence shell electrons increases across a period, the tendency to lose electrons will decrease.


Down the group, the effective nuclear charge experienced by valence electrons is decreasing because the outermost electrons are farther away from the nucleus.


Therefore, these can be lost easily.


Hence metallic character decreases across a period and increases down a group.


Non-metals, on the other hand, are electronegative.


They tend to form bonds by gaining electrons.


Let us learn about the variation of this property.


How would the tendency to gain electrons change as you go from left to right across a period?


How would the tendency to gain electrons change as you go down a group?


As the trends in the electronegativity show, non-metals are found on the right-hand side of the Periodic Table towards the top.


These trends also help us to predict the nature of oxides formed by the elements because it is known to you that the oxides of metals are basic and that of non-metals are acidic in general.


QUESTIONS


How could the Modern Periodic Table remove various anomalies of Mendeléev’s Periodic Table?


Name two elements you would expect to show chemical reactions similar to magnesium.


What is the basis for your choice?


Name three elements that have a single electron in their outermost shells.


Name two elements that have two electrons in their outermost shells.


Name three elements with filled outermost shells.


Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas.


Is there any similarity in the atoms of these elements?


Helium is an unreactive gas and neon is a gas of extremely low reactivity.


What, if anything, do their atoms have in common?


In the Modern Periodic Table, which are the metals among the first ten elements?


By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic?


What you have learnt


Elements are classified on the basis of similarities in their properties.


Döbereiner grouped the elements into triads and Newlands gave the Law of Octaves.


Mendeléev arranged the elements in increasing order of their atomic masses an according to their chemical properties.


Mendeléev even predicted the existence of some yet to be discovered elements on the basis of gaps in his Periodic Table.


Anomalies in arrangement of elements based on increasing atomic mass could be removed when the elements were arranged in order of increasing atomic number, a fundamental property of the element discovered by Moseley.


Elements in the Modern Periodic Table are arranged in 18 vertical columns called groups and 7 horizontal rows called periods.


Elements thus arranged show periodicity of properties including atomic size, valency or combining capacity and metallic and non-metallic character.


EXERCISE


Which of the following statements is not a correct statement about the trends when going from left to right across the periods of periodic Table.


The elements become less metallic in nature.


The number of valence electrons increases.


The atoms lose their electrons more easily.


The oxides become more acidic.


Element X forms a chloride with the formula XCl2, which is a solid with a high melting point.


Which element has two shells, both of which are completely filled with electrons?


Which element has the electronic configuration 2, 8, 2?


Which element has a total of three shells, with four electrons in its valence shell?


Which element has a total of two shells, with three electrons in its valence shell?


Which element has twice as many electrons in its second shell as in its first shell?


What property do all elements in the same column of the Periodic Table as boron have in common?


What property do all elements in the same column of the Periodic Table as fluorine have in common?


An atom has electronic configuration 2, 8, 7.


What is the atomic number of this element?


To which of the following elements would it be chemically similar?


Life Processes


How do we tell the difference between what is alive and what is not alive?


If we see a dog running, or a cow chewing cud, or a man shouting loudly on the street, we know that these are living beings.


What if the dog or the cow or the man were asleep?


We would still think that they were alive, but how did we know that?


We see them breathing, and we know that they are alive.


What about plants?


How do we know that they are alive?


We see them green, some of us will say.


But what about plants that have leaves of colours other than green?


They grow over time, so we know that they are alive, some will say.


In other words, we tend to think of some sort of movement, either growth-related or not, as common evidence for being alive.


But a plant that is not visibly growing is still alive, and some animals can breathe without visible movement.


So using visible movement as the defining characteristic of life is not enough.


Movements over very small scales will be invisible to the naked eye movements of molecules, for example.


Is this invisible molecular movement necessary for life?


If we ask this question to professional biologists, they will say yes.


In fact, viruses do not show any molecular movement in them (until they infect some cell), and that is partly why there is a controversy about whether they are truly alive or not.


Why are molecular movements needed for life?


We have seen in earlier classes that living organisms are well-organised structures.


They can have tissues, tissues have cells, cells have smaller components in them, and so on.


Because of the effects of the environment, this organised, ordered nature of living structures is very likely to keep breaking down over time.


If order breaks down, the organism will no longer be alive.


So living creatures must keep repairing and maintaining their structures.


Since all these structures are made up of molecules, they must move molecules around all the time.


What are the maintenance processes in living organisms?


Let us explore.


WHAT ARE LIFE PROCESSES?


The maintenance functions of living organisms must go on even when they are not doing anything particular.


Even when we are just sitting in class, even if we are just asleep, this maintenance job has to go on.


The processes which together perform this maintenance job are life processes.


Since these maintenance processes are needed to prevent damage and break-down, energy is needed for them.


This energy comes from outside the body of the individual organism.


So there must be a process to transfer a source of energy from outside the body of the organism, which we call food, to the inside, a process we commonly call nutrition.


If the body size of the organisms is to grow, additional raw material will also be needed from outside.


Depending on the complexity of these carbon sources, differented.


Since life on earth depends on carbon based molecules, most of these food sources are also carbon-based.


Organisms can then use different kinds of nutritional processes.


The outside sources of energy could be quite varied, since the environment is not under the control of the individual organism.


These sources of energy, therefore, need to be broken down or built up in the body, and must be finally converted to a uniform source of energy that can be used for the various molecular movements needed for maintaining living structures, as well as to the kind of molecules the body needs to grow.


For this, a series of chemical reactions in the body are necessary.


Oxidising-reducing reactions are some of the most common chemical means to break-down molecules.


For this, many organisms use oxygen sourced from outside the body.


The process of acquiring oxygen from outside the body, and to use it in the process of break-down of food sources for cellular needs, is what we call respiration.


In the case of a single-celled organism, no specific organs for taking in food, exchange of gases or removal of wastes may be needed because the entire surface of the organism is in contact with the environment.


But what happens when the body size of the organism increases and the body design becomes more complex?


In multi-cellular organisms, all the cells may not be in direct contact with the surrounding environment.


Thus, simple diffusion will not meet the requirements of all the cells.


We have seen previously how, in multi-cellular organisms, various body parts have specialised in the functions they perform.


We are familiar with the idea of these specialised tissues, and with their organisation in the body of the organism.


It is therefore not surprising that the uptake of food and of oxygen will also be the function of specialised tissues.


However, this poses a problem, since the food and oxygen are now taken up at one place in the body of the organisms, while all parts of the body need them.


This situation creates a need for a transportation system for carrying food and oxygen from one place to another in the body.


When chemical reactions use the carbon source and the oxygen for energy generation, they create by-products that are not only useless for the cells of the body, but could even be harmful.


These waste by- products are therefore needed to be removed from the body and discarded outside by a process called excretion.


Again, if the basic rules for body design in multi-cellular organisms are followed, a specialised tissue for excretion will be developed, which means that the transportation system will need to transport waste away from cells to this excretory tissue.


Let us consider these various processes, so essential to maintain life, one by one.


Why is diffusion insufficient to meet the oxygen requirements of multi-cellular organisms like humans?


What criteria do we use to decide whether something is alive?


What are outside raw materials used for by an organism?


What processes would you consider essential for maintaining life?


NUTRITION


When we walk or ride a bicycle, we are using up energy.


Even when we are not doing any apparent activity, energy is needed to maintain a state of order in our body.


We also need materials from outside in order to grow, develop, synthesise protein and other substances needed in the body.


This source of energy and materials is the food we eat.


How do living things get their food?


The general requirement for energy and materials is common in all organisms, but it is fulfilled in different ways.


Some organisms use simple food material obtained from inorganic sources in the form of carbon dioxide and water.


These organisms, the autotrophs, include green plants and some bacteria.


Other organisms utilise complex substances.


These complex substances have to be broken down into simpler ones before they can be used for the upkeep and growth of the body.


To achieve this, organisms use bio-catalysts called enzymes.


Thus, the heterotrophs survival depends directly or indirectly on autotrophs.


Heterotrophic organisms include animals and fungi.


Autotrophic Nutrition


Carbon and energy requirements of the autotrophic organism are fulfilled by photosynthesis.


It is the process by which autotrophs take in substances from the outside and convert them into stored forms of  energy.


This material is taken in the form of carbon dioxide and water which is converted into carbohydrates in the presence of sunlight and chlorophyll.


Carbohydrates are utilised for providing energy to the plant.


We will study how this takes place in the next section.


The carbohydrates which are not used immediately are stored in the form of starch, which serves as the internal energy reserve to be used as and when required by the plant.


A somewhat similar situation is seen in us where some of the energy derived from the food we eat is stored in our body in the form of glycogen.


Let us now see what actually happens during the process of photosynthesis.


The following events occur during this process –


Absorption of light energy by chlorophyll.


Conversion of light energy to chemical energy and splitting of water molecules into hydrogen and oxygen.


Reduction of carbon dioxide to carbohydrates.


These steps need not take place one after the other immediately.


For example, desert plants take up carbon dioxide at night and prepare an intermediate which is acted upon by the energy absorbed by the chlorophyll during the day.


Let us see how each of the components of the above reaction are necessary for photosynthesis.


If you carefully observe a cross-section of a leaf under the microscope you will notice that some cells contain green dots.


These green dots are cell organelles called chloroplasts which contain chlorophyll.


Let us do an activity which demonstrates that chlorophyll is essential for photosynthesis.


Take a potted plant with variegated leaves – for example, money plant or crotons.


Keep the plant in a dark room for three days so that all the starch gets used up.


Now keep the plant in sunlight for about six hours.


Pluck a leaf from the plant.


Mark the green areas in it and trace them on a sheet of paper.


Dip the leaf in boiling water for a few minutes.


After this, immerse it in a beaker containing alcohol.


Carefully place the above beaker in a water-bath and heat till the alcohol begins to boil.


What happens to the colour of the leaf?


What is the colour of the solution?


Now dip the leaf in a dilute solution of iodine for a few minutes.


Take out the leaf and rinse off the iodine solution.


Observe the colour of the leaf and compare this with the tracing of the leaf done in the beginning.


What can you conclude about the presence of starch in various areas of the leaf?


Now, let us study how the plant obtains carbon dioxide.


In Class 12, we had talked about stomata which are tiny pores present on the surface of the leaves.


Massive amounts of gaseous exchange takes place in the leaves through these pores for the purpose of photosynthesis.


But it is important to note here that exchange of gases occurs across the surface of stems, roots and leaves as well.


Since large amounts of water can also be lost through these stomata, the plant closes these pores when it does not need carbon dioxide for photosynthesis.


The opening and closing of the pore is a function of the guard cells.


The guard cells swell when water flows into them, causing the stomatal pore to open.


Similarly the pore closes if the guard cells shrink.


Take two healthy potted plants which are nearly the same size.


Keep them in a dark room for three days.


Now place each plant on separate glass plates.


Place a watch-glass containing potassium hydroxide by the side of one of the plants.


The potassium hydroxide is used to absorb carbon dioxide.


Cover both plants with separate bell-jars as shown.


Use vaseline to seal the bottom of the jars to the glass plates so hydroxide without potassium hydroxide       that the set-up is air-tight.


Keep the plants in sunlight for about two hours.


Pluck a leaf from each plant and check for the presence of starch as in the above activity.


Do both the leaves show the presence of the same amount of starch?


What can you conclude from this activity?


Based on the two activities performed above, can we design an experiment to demonstrate that sunlight is essential for photosynthesis?


So far, we have talked about how autotrophs meet their energy requirements.


But they also need other raw materials for building their body.


Water used in photosynthesis is taken up from the soil by the roots in terrestrial plants.


Other materials like nitrogen, phosphorus, iron and magnesium are taken up from the soil.


Nitrogen is an essential element used in the synthesis of proteins and other compounds.


This is taken up in the form of inorganic nitrates or nitrites.


Or it is taken up as organic compounds which have been prepared by bacteria from atmospheric nitrogen.


Heterotrophic Nutrition


Each organism is adapted to its environment.


The form of nutrition differs depending on the type and availability of food material as well as how it is obtained by the organism.


For example, whether the food source is stationary (such as grass) or mobile (such as a deer), would allow for differences in how the food is accessed and what is the nutritive apparatus used by a cow and a lion.


There is a range of strategies by which the food is taken in and used by the organism.


Some organisms break-down the food material outside the body and then absorb it.


Examples are fungi like bread moulds, yeast and mushrooms.


Others take in whole material and break it down inside their bodies.


What can be taken in and broken down depends on the body design and functioning.


Some other organisms derive nutrition from plants or animals without killing them.


This parasitic nutritive strategy is used by a wide variety of organisms like cuscuta (amar-bel), ticks, lice, leeches and tape-worms.


How do Organisms obtain their Nutrition?


Since the food and the way it is obtained differ, the digestive system is different in various organisms.


In single-celled organisms, the food may be taken in by the entire surface.


But as the complexity of the organism increases, different parts become specialised to perform different functions.


For example, Amoeba takes in food using temporary finger-like extensions of the cell surface which fuse over the food particle forming a food-vacuole.


Inside the food-vacuole, complex substances are broken down into simpler ones which then diffuse into the cytoplasm.


The remaining undigested material is moved to the surface of the cell and thrown out.


In Paramoecium, which is also a unicellular organism, the cell has a definite shape and food is taken in at a specific spot.


Food is moved to this spot by the movement of cilia which cover the entire surface of the cell.


Nutrition in Amoeba


Nutrition in Human Beings


The alimentary canal is basically a long tube extending from the mouth to the anus.


We can see that the tube has different parts.


Various regions are specialised to perform different functions.


What happens to the food once it enters our body?


We shall discuss this process here.


Take 1 mL starch solution (1%) in two test tubes (A and B).


Add 1 mL saliva to test tube A and leave both test tubes undisturbed for 20-30 minutes.


Now add a few drops of dilute iodine solution to the test tubes.


In which test tube do you observe a colour change?


What does this indicate about the presence or absence of starch in the two test tubes?


What does this tell us about the action of saliva on starch?


We eat various types of food which has to pass through the same digestive tract.


Naturally the food has to be processed to generate particles which are small and of the same texture.


This is achieved by crushing the food with our teeth.


Since the lining of the canal is soft, the food is also wetted to make its passage smooth.


When we eat something we like, our mouth ‘waters’.


This is actually not only water, but a fluid called saliva secreted by the salivary glands.


Another aspect of the food we ingest is its complex nature.


If it is to be absorbed from the alimentary canal, it has to be broken into smaller molecules.


This is done with the help of biological catalysts called enzymes.


The saliva contains an enzyme called salivary amylase that breaks down starch which is a complex molecule to give sugar.


The food is mixed thoroughly with saliva and moved around the mouth while chewing by the muscular tongue.


It is necessary to move the food in a regulated manner along the digestive tube so that it can be processed properly in each part.


The lining of canal has muscles that contract rhythmically in order to push the food forward.


These peristaltic movements occur all along the gut.


From the mouth, the food is taken to the stomach through the food-pipe or oesophagus.


The stomach is a large organ which expands when food enters it.


The muscular walls of the stomach help in mixing the food.


Human alimentary canal thoroughly with more digestive juices.


These digestion functions are taken care of by the gastric glands present in the wall of the stomach.


These release hydrochloric acid, a protein digesting enzyme called pepsin, and mucus.


The hydrochloric acid creates an acidic medium which facilitates the action of the enzyme pepsin.


What other function do you think is served by the acid?


The mucus protects the inner lining of the stomach from the action of the acid under normal conditions.


We have often heard adults complaining about ‘acidity’.


Can this be related to what has been discussed above?


The exit of food from the stomach is regulated by a sphincter muscle which releases it in small amounts into the small intestine.


From the stomach, the food now enters the small intestine.


This is the longest part of the alimentary canal which is fitted into a compact space because of extensive coiling.


The length of the small intestine differs in various animals depending on the food they eat.


Herbivores eating grass need a longer small intestine to allow the cellulose to be digested.


Meat is easier to digest, hence carnivores like tigers have a shorter small intestine.


The small intestine is the site of the complete digestion of carbohydrates, proteins and fats.


It receives the secretions of the liver and pancreas for this purpose.


The food coming from the stomach is acidic and has to be made alkaline for the pancreatic enzymes to act.


Bile juice from the liver accomplishes this in addition to acting on fats.


Fats are present in the intestine in the form of large globules which makes it difficult for enzymes to act on them.


Bile salts break them down into smaller globules increasing the efficiency of enzyme action.


This is similar to the emulsifying action of soaps on dirt that we have learnt about in train/chapter 4.


The pancreas secretes pancreatic juice which contains enzymes like trypsin for digesting proteins and lipase for breaking down emulsified fats.


The walls of the small intestine contain glands which secrete intestinal juice.


The enzymes present in it finally convert the proteins to amino acids, complex carbohydrates into glucose and fats into fatty acids and glycerol.


The digested food is taken up by the walls of the intestine.


The inner lining of the small intestine has numerous finger-like projections called villi which increase the surface area for absorption.


The villi are richly supplied with blood vessels which take the absorbed food to each and every cell of the body, where it is utilised for obtaining energy, building up new tissues and the repair of old tissues.


The unabsorbed food is sent into the large intestine where more villi absorb water from this material.


The rest of the material is removed from the body via the anus.


The exit of this waste material is regulated by the anal sphincter.


More to Know!


Dental caries


Dental caries or tooth decay causes gradual softening of enamel and dentine.


It begins when bacteria acting on sugars produce acids that softens or demineralises the enamel.


Masses of bacterial cells together with food particles stick to the teeth to form dental plaque.


Saliva cannot reach the tooth surface to neutralise the acid as plaque covers the teeth.


Brushing the teeth after eating removes the plaque before the bacteria produce acids.


If untreated, microorganisms may invade the pulp, causing inflammation and infection.


What are the differences between autotrophic nutrition and heterotrophic nutrition?


Where do plants get each of the raw materials required for photosynthesis?


What is the role of the acid in our stomach?


What is the function of digestive enzymes?


How is the small intestine designed to absorb digested food?


RESPIRATION


Take some freshly prepared lime water in a test tube.


Blow air through this lime water.


Note how long it takes for the lime Rubber tube


Tube water to turn milky.


Use a syringe or pichkari to pass air through some fresh lime water


Test tube containing lime water taken in another test tube.


Note how long it takes for this lime water to turn milky.


What does this tell us about the amount of carbon dioxide in the air.


Air being passed into lime water with a pichkari/ syringe.


Air being exhaled into lime water that we breathe out?


Take some fruit juice or sugar solution and add some yeast to this.


Take this mixture in a test tube fitted with a one-holed cork.


Fit the cork with a bent glass tube.


Dip the free end of the glass tube into a test tube containing freshly prepared lime water.


What change is observed in the lime water and how long does it take for this change to occur?


What does this tell us about the products of fermentation?


We have discussed nutrition in organisms in the last section.


The food material taken in during the process of nutrition is used in cells to provide energy for various life processes.


Diverse organisms do this in different ways – some use oxygen to break-down glucose completely into carbon dioxide and water, some use other pathways that do not involve oxygen.


In all cases, the first step is the break-down of glucose, a six-carbon molecule, into a three-carbon molecule called pyruvate.


This process takes place in the cytoplasm.


Further, the pyruvate may be converted into ethanol and carbon dioxide.


This process takes place in yeast during fermentation.


Since this process takes place in the absence of air (oxygen), it is called anaerobic respiration.


Break-down of pyruvate using oxygen takes place in the mitochondria.


This process breaks up the three-carbon pyruvate molecule to give three molecules of carbon dioxide.


The other product is water.


Since this process takes place in the presence of air (oxygen), it is called aerobic respiration.


The release of energy in this aerobic process is a lot greater than in the anaerobic process.


Sometimes, when there is a lack of oxygen in our muscle cells, another pathway for the break-down of pyruvate is taken.


Here the pyruvate is converted into lactic acid which is also a three-carbon molecule.


This build-up of lactic acid in our muscles during sudden activity causes cramps.


The energy released during cellular respiration is immediately used to synthesise a molecule called ATP which is used to fuel all other activities in the cell.


In these processes, ATP is broken down giving rise to a fixed amount of energy which can drive the endothermic reactions taking place in the cell.


ATP is the energy currency for most cellular processes.


The energy released during the process of respiration is used to make an ATP molecule from ADP and inorganic phosphate.


More to Know!


The process of respiration is used to make an ATP molecule from ADP and inorganic


Endothermic processes in the cell then use this ATP to drive the reactions.


When the terminal phosphate linkage in ATP is broken using water, the energy equivalent to 30.5 kJ/mol is released.


Think of how a battery can provide energy for many different kinds of uses.


It can be used to obtain mechanical energy, light energy, electrical energy and so on.


Similarly,  ATP can be used in the cells for the contraction of muscles, protein synthesis, conduction of nervous impulses and many other activities.


Since the aerobic respiration pathway depends on oxygen, aerobic organisms need to ensure that there is sufficient intake of oxygen.


We have seen that plants exchange gases through stomata, and the large inter-cellular spaces ensure that all cells are in contact with air.


Carbon dioxide and oxygen are exchanged by diffusion here.


They can go into cells, or away from them and out into the air.


The direction of diffusion depends upon the environmental conditions and the requirements of the plant.


At night, when there is no photosynthesis occurring, CO2 elimination is the major exchange activity going on.


During the day, CO2 generated during respiration is used up for photosynthesis, hence there is no CO2 release.


Instead, oxygen release is the major event at this time.


Animals have evolved different organs for the uptake of oxygen from the environment and for getting rid of the carbon dioxide produced.


Terrestrial animals can breathe the oxygen in the atmosphere, but animals that live in water need to use the oxygen dissolved in water.


Observe fish in an aquarium.


They open and close their mouths and the gill-slits (or the operculum which covers the gill-slits) behind their eyes also open and close.


Are the timings of the opening and closing of the mouth and gill-slits coordinated in some manner?


Count the number of times the fish opens and closes its mouth in a minute.


Compare this to the number of times you breathe in and out in a minute.


Since the amount of dissolved oxygen is fairly low compared to the amount of oxygen in the air, the rate of breathing in aquatic organisms is much faster than that seen in terrestrial organisms.


Fishes take in water through their mouths and force it past the gills where the dissolved oxygen is taken up by blood.


Terrestrial organisms use the oxygen in the atmosphere for respiration.


This oxygen is absorbed by different organs in different animals.


All these organs have a structure that increases the surface area which is in contact with the oxygen-rich atmosphere.


Since the exchange of oxygen and carbon dioxide has to take place across this surface, this surface is very fine and delicate.


In order to protect this surface, it is usually placed within the body, so there have to be passages that will take air to this area.


In addition, there is a mechanism for moving the air in and out of this area where the oxygen is absorbed.


In human beings, air is taken into the body through the nostrils.


The air passing through the nostrils is filtered by fine hairs that line the passage.


The passage is also lined with mucus which helps in this process.


From here, the air passes through the throat and into the lungs.


Rings of cartilage are present in the throat.


Within the lungs, the passage divides into smaller and smaller tubes which finally terminate in balloon-like structures which are called alveoli.


The alveoli provide a surface where the exchange of gases can take place.


The walls of the alveoli contain an extensive network of blood-vessels.


As we have seen in earlier years, when we breathe in, we lift our ribs and flatten our diaphragm, and the chest cavity becomes


Because of this, air is sucked into the lungs and fills the expanded alveoli.


The blood brings carbon dioxide from the rest of the body for release into the alveoli, and the oxygen in the alveolar air is taken up by blood in the alveolar blood vessels to be transported to all the cells in the body.


During the breathing cycle, when air is taken in and let out, the lungs always contain a residual volume of air so that there is sufficient time for oxygen to be absorbed and for the carbon dioxide to be released.


When the body size of animals is large, the diffusion pressure alone cannot take care of oxygen delivery to all parts of the body.


Instead, respiratory pigments take up oxygen from the air in the lungs and carry it to tissues which are deficient in oxygen before releasing it.


In human beings, the respiratory pigment is haemoglobin which has a very high affinity for oxygen.


This pigment is present in the red blood corpuscles.


Carbon dioxide is more soluble in water than oxygen is and hence is mostly transported in the dissolved form in our blood.


Do You Know?


If the alveolar surface were spread out, it would cover about 80 m2.


How much do you think the surface area of your body is?


Consider how efficient exchange of gases becomes because of the large surface available for the exchange to take place.


If diffusion were to move oxygen in our body, it is estimated that it would take 3 years for a molecule of oxygen to get to our toes from our lungs.


Aren’t you glad that we have haemoglobin?


What advantage over an aquatic organism does a terrestrial organism have with regard to obtaining oxygen for respiration?


What are the different ways in which glucose is oxidised to provide energy in various organisms?


How is oxygen and carbon dioxide transported in human beings?


How are the lungs designed in human beings to maximise the area for exchange of gases?


TRANSPORTATION


Transportation in Human Beings


Visit a health centre in your locality and find out what is the normal range of haemoglobin content in human beings.


Is it the same for children and adults?


Is there any difference in the haemoglobin levels for men and women?


Visit a veterinary clinic in your locality.


Find out what is the normal range of haemoglobin content in an animal like the buffalo or cow.


Is this content different in calves, male and female animals?


Compare the difference seen in male and female human beings and animals.


How would the difference, if any, be explained?


We have seen in previous sections that blood transports food, oxygen and waste materials in our bodies.


In Class 9, we learnt about blood being a fluid connective tissue.


Blood consists of a fluid medium called plasma in which the cells are suspended.


Plasma transports food, carbon dioxide and nitrogenous wastes in dissolved form.


Oxygen is carried by no the red blood cells.


Many other substances like salts, are also transported by the blood.


We thus need a pumping organ to push blood around the body, a network of tubes to reach all the tissues and a system in place to ensure that this network can be repaired if damaged.


Our pump — the heart


The heart is a muscular organ which is as big as our fist.


Because both oxygen and carbon dioxide have to be transported by the blood, the heart has different chambers to prevent the oxygen-rich blood from mixing with the blood containing carbon dioxide.


The carbon dioxide-rich blood has to reach the lungs for the carbon dioxide to be removed, and the oxygenated blood from the lungs has to be brought back to the heart.


This oxygen-rich blood is then pumped to the rest of the body.


We can follow this process step by step.


Oxygen-rich blood from the lungs comes to the thin-walled upper Schematic sectional chamber of the heart on the left, the left atrium.


The left atrium relaxes view of the human heart   when it is collecting this blood.


It then contracts, while the next chamber, the left ventricle, expands, so that the blood is transferred to it.


When the muscular left ventricle contracts in its turn, the blood is pumped out to the body.


De-oxygenated blood comes from the body to the upper chamber on the right, the right atrium, as it expands.


As the right atrium contracts, the corresponding lower chamber, the right ventricle, dilates.


This transfers blood to the right ventricle, which in turn pumps it to the lungs for oxygenation.


Since ventricles have to pump blood into various organs, they have thicker muscular walls than the atria do.


Valves ensure that blood does not flow backwards when the atria or ventricles contract.


Oxygen enters the blood in the lungs.


The separation of the right side and the left side of the heart is useful to keep oxygenated and de- oxygenated blood from mixing.


Such separation allows a highly efficient supply of oxygen to the body.


This is useful in animals that have high energy needs, such as birds and mammals, which constantly use energy to maintain their body temperature.


In animals that do not use energy for this purpose, the body temperature depends on the temperature in the environment.


Such animals, like amphibians or many reptiles have three-chambered hearts, and tolerate some mixing of the oxygenated and de-oxygenated blood.


Schematic representation of transport and exchange streams.


Fishes, on the other hand, have only two chambers to their hearts, and the blood is pumped to the gills, is oxygenated there, and passes directly to the rest of the body.


Thus, blood goes only once of oxygen and carbon dioxide through the heart in the fish during one cycle of passage through the body.


On the other hand, it goes through the heart twice during each cycle in other vertebrates.


This is known as double circulation.


Blood pressure


The force that blood exerts against the wall of a vessel is called blood pressure.


This pressure is much greater in arteries than in veins.


The pressure of blood inside the artery during ventricular systole (contraction) is called systolic pressure and pressure.


in artery during ventricular diastole (relaxation) is called diastolic pressure.


The normal systolic pressure is about 120 mm of Hg and diastolic pressure is 80 mm of Hg.


More to Know!


Blood pressure is measured with an instrument called sphygmomanometer.


High blood pressure is also called hypertension and is caused by the constriction of arterioles, which results in increased resistance to blood flow.


It can lead to the rupture of an artery and internal bleeding.


The tubes – blood vessels


Arteries are the vessels which carry blood away from the heart to various organs of the body.


Since the blood emerges from the heart under high pressure, the arteries have thick, elastic walls.


Veins collect the blood from different organs and bring it back to the heart.


They do not need thick walls because the blood is no longer under pressure, instead they have valves that ensure that the blood flows only in one direction.


On reaching an organ or tissue, the artery divides into smaller and smaller vessels to bring the blood in contact with all the individual cells.


The smallest vessels have walls which are one-cell thick and are called capillaries.


Exchange of material between the blood and surrounding cells takes place across this thin wall.


The capillaries then join together to form veins that convey the blood away from the organ or tissue.


Maintenance by platelets


What happens if this system of tubes develops a leak?


Think about situations when we are injured and start bleeding.


Naturally the loss of blood from the system has to be minimised.


In addition, leakage would lead to a loss of pressure which would reduce the efficiency of the pumping system.


To avoid this, the blood has platelet cells which circulate around the body and plug these leaks by helping to clot the blood at these points of injury.


Lymph


There is another type of fluid also involved in transportation.


This is called lymph or tissue fluid.


Through the pores present in the walls of capillaries some amount of plasma, proteins and blood cells escape into intercellular spaces in the tissues to form the tissue fluid or lymph.


It is similar to the plasma of blood but colourless and contains less protein.


Lymph drains into lymphatic capillaries from the intercellular spaces, which join to form large lymph vessels that finally open into larger veins.


Lymph carries digested and absorbed fat from intestine and drains excess fluid from extra cellular space back into the blood.


Transportation in Plants


We have discussed earlier how plants take in simple compounds such as CO2 and photosynthesise energy stored in their chlorophyll-containing organs, namely leaves.


The other kinds of raw materials needed for building plant bodies will also have to be taken up separately.


For plants, the soil is the nearest and richest source of raw materials like nitrogen, phosphorus and other minerals.


The absorption of these substances therefore occurs through the part in contact with the soil, namely roots.


If the distances between soil-contacting organs and chlorophyll- containing organs are small, energy and raw materials can easily diffuse to all parts of the plant body.


But if these distances become large because of changes in plant body design, diffusion processes will not be sufficient to provide raw material in leaves and energy in roots.


A proper system of transportation is therefore essential in such situations.


Energy needs differ between different body designs.


Plants do not move, and plant bodies have a large proportion of dead cells in many tissues.


As a result, plants have low energy needs, and can use relatively slow transport systems.


The distances over which transport systems have to operate, however, can be very large in plants such as very tall trees.


Plant transport systems will move energy stores from leaves and raw materials from roots.


These two pathways are constructed as independently organised conducting tubes.


One, the xylem moves water and minerals obtained from the soil.


The other, phloem transports products of photosynthesis from the leaves where they are synthesized to other parts of the plant.


We have studied the structure of these tissues in detail in Class 9.


Transport of water


In xylem tissue, vessels and tracheids of the roots, stems and leaves are interconnected to form a continuous system of water-conducting channels reaching all parts of the plant.


At the roots, cells in contact with the soil actively take up ions.


This creates a difference in the concentration of these ions between the root and the soil.


Water, therefore, moves into the root from the soil to eliminate this difference.


This means that there is steady movement of water into root xylem, creating a column of water that is steadily pushed upwards.


However, this pressure by itself is unlikely to be enough to move water over the heights that we commonly see in plants.


Plants use another strategy to move water in the xylem upwards to the highest points of the plant body.


Take two small pots of approximately the same size and having the same amount of soil.


One should have a plant in it.


Place a stick of the same height as the plant in the other pot.


Cover the soil in both pots with a plastic sheet so that moisture cannot escape by evaporation.


Cover both sets, one with the plant and the other with the stick, with plastic sheets and place in bright sunlight for half an hour.


Do you observe any difference in the two cases?


Provided that the plant has an adequate supply of water, the water which is lost through the stomata is replaced by water from the xylem vessels in the leaf.


In fact, evaporation of water molecules from the cells of a leaf creates a suction which pulls water from the xylem cells of roots.


The loss of water in the form of vapour from the aerial parts of the plant is known as transpiration.


Thus, transpiration helps in the absorption and upward movement of water and minerals dissolved in it from roots to the leaves.


It also helps in temperature regulation.


The effect of root pressure in transport of water is more important at night.


During the day when the stomata are open, the transpiration pull becomes the major driving force in the movement of water in the xylem.


Movement of water during transpiration in a tree


Transport of food and other substances.


So far we have discussed the transport of water and minerals in plants.


Now let us consider how the products of metabolic processes, particularly photosynthesis, are moved from leaves, where they are formed, to other parts of the plant.


This transport of soluble products of photosynthesis is called translocation and it occurs in the part of the vascular tissue known as phloem.


Besides the products of photosynthesis, the phloem transports amino acids and other substances.


These substances are especially delivered to the storage organs of roots, fruits and seeds and to growing organs.


The translocation of food and other substances takes place in the sieve tubes with the help of adjacent companion cells both in upward and downward directions.


Unlike transport in xylem which can be largely explained by simple physical forces, the translocation in phloem is achieved by utilizing energy.


Material like sucrose is transferred into phloem tissue using energy from ATP.


This increases the osmotic pressure of the tissue causing water to move into it.


This pressure moves the material in the phloem to tissues which have less pressure.


This allows the phloem to move material according to the plant’s needs.


For example, in the spring, sugar stored in root or stem tissue would be transported to the buds which need energy to grow.


What are the components of the transport system in human beings?


What are the functions of these components?


Why is it necessary to separate oxygenated and deoxygenated blood in mammals and birds?


What are the components of the transport system in highly organized plants?


How are water and minerals transported in plants?


How is food transported in plants?


EXCRETION


We have already discussed how organisms get rid of gaseous wastes generated during photosynthesis or respiration.


Other metabolic activities generate nitrogenous materials which need to be removed.


The biological process involved in the removal of these harmful metabolic wastes from the body is called excretion.


Different organisms use varied strategies to do this.


Many unicellular organisms remove these wastes by simple diffusion from the body surface into the surrounding water.


As we have seen in other processes, complex multi-cellular organisms use specialised organs to perform the same function.


Excretion in Human Beings


The excretory system of human beings includes a pair of kidneys, a pair of ureters, a urinary bladder and a urethra.


Kidneys are located in the abdomen, one on either side of the backbone.


Urine produced in the kidneys passes through the ureters into the urinary bladder where it is stored until it is released through the urethra.


How is urine produced?


The purpose of making urine is to filter out waste products from the blood.


Excretory system in human beings


Just as CO2 is removed from the blood in the lungs, nitrogenous waste such as urea or uric acid are removed from blood in the kidneys.


It is then no surprise that the basic filtration unit in the kidneys, like in the lungs, is a cluster of very thin-walled blood capillaries.


Each capillary cluster in the kidney is associated with the cup-shaped end of a tube that collects the filtered urine.


Each kidney has large numbers of these filtration units called nephrons packed close together.


Some substances in the initial filtrate, such as glucose, amino acids, salts and a major


amount of water, are selectively re-absorbed as the urine flows along the tube.


The amount of water re- absorbed depends on how much excess water there is in the body, and on how much of dissolved waste there is to be excreted.


The urine forming in each kidney eventually enters a long tube, the ureter, which connects the kidneys with the urinary bladder.


Urine is stored in the urinary bladder until the pressure of the expanded bladder leads to the urge to pass it out through the urethra.


The bladder is muscular, so it is under nervous control, as we have discussed elsewhere.


As a result, we can usually control the urge to urinate.


Structure of a nephron


Kidneys are vital organs for survival.


Several factors like infections, injury or restricted blood flow to kidneys reduce the activity of kidneys.


This leads to accumulation of poisonous wastes in the body, which can even lead to death.


In case of kidney failure, an artificial kidney can be used.


An artificial kidney is a device to remove nitrogenous waste products from the blood through dialysis.


Artificial kidneys contain a number of tubes with a semi-permeable lining, suspended in a tank filled with dialysing fluid.


More to Know!


This fluid has the same osmotic pressure as blood, except that it is devoid of nitrogenous wastes.


The patient’s blood is passed through these tubes.


During this passage, the waste products from the blood pass into dialysing fluid by diffusion.


The purified blood is pumped back into the patient.


This is similar to the function of the kidney, but it is different since there is no re- absorption involved.


Normally, in a healthy adult, the initial filtrate in the kidneys is about 180 L daily.


However, the volume actually excreted is only a litre or two a day, because the remaining filtrate is re- absorbed in the kidney tubules.


Excretion in Plants


Plants use completely different strategies for excretion than those of animals.


Oxygen itself can be thought of as a waste product generated during photosynthesis! We have discussed earlier how plants deal with oxygen as well as CO2.


They can get rid of excess water by transpiration.


For other wastes, plants use the fact that many of their tissues consist of dead cells, and that they can even lose some parts such as leaves.


Many plant waste products are stored in cellular vacuoles.


Waste products may be stored in leaves that fall off.


Other waste products are stored as resins and gums, especially in old xylem.


Plants also excrete some waste substances into the soil around them.


Describe the structure and functioning of nephrons.


What are the methods used by plants to get rid of excretory products?


How is the amount of urine produced regulated?


What you have learnt?


Movement of various types can be taken as an indication of life.


The maintenance of life requires processes like nutrition, respiration, transport of materials within the body and excretion of waste products.


Autotrophic nutrition involves the intake of simple inorganic materials from the environment and using an external energy source like the Sun to synthesise complex high-energy organic material.


Heterotrophic nutrition involves the intake of complex material prepared by other organisms.


In human beings, the food eaten is broken down by various steps along the alimentary canal and the digested food is absorbed in the small intestine to be sent to all cells in the body.


During the process of respiration, complex organic compounds such as glucose are broken down to provide energy in the form of ATP.


ATP is used to provide energy for other reactions in the cell.


Respiration may be aerobic or anaerobic.


Aerobic respiration makes more energy available to the organism.


In human beings, the transport of materials such as oxygen, carbon dioxide, food and excretory products is a function of the circulatory system.


The circulatory system consists of the heart, blood and blood vessels.


In highly differentiated plants, transport of water, minerals, food and other materials is a function of the vascular tissue which consists of xylem and phloem.


In human beings, excretory products in the form of soluble nitrogen compounds are removed by the nephrons in the kidneys.


Plants use a variety of techniques to get rid of waste material.


For example, waste material may be stored in the cell-vacuoles or as gum and resin, removed in the falling leaves, or excreted into the surrounding soil.


Control and Coordination


In the previous chapter, we looked at life processes involved in the maintenance functions in living organisms.


There, we had started with a notion we all have, that if we see something moving, it is alive.


Some of these movements are in fact the result of growth, as in plants.


A seed germinates and grows, and we can see that the seedling moves over the course of a few days, it pushes soil aside and comes out.


But if its growth were to be stopped, these movements would not happen.


Some movements, as in many animals and some plants, are not connected with growth.


A cat running, children playing on swings, buffaloes chewing cud – these are not movements caused by growth.


Why do we associate such visible movements with life?


A possible answer is that we think of movement as a response to a change in the environment of the organism.


The cat may be running because it has seen a mouse.


Not only that, we also think of movement as an attempt by living organisms to use changes in their environment to their advantage.


Plants grow out into the sunshine.


Children try to get pleasure and fun out of swinging.


Buffaloes chew cud to help break up tough food so as to be able to digest it better.


When bright light is focussed on our eyes or when we touch a hot object, we detect the change and respond to it with movement in order to protect ourselves.


If we think a bit more about this, it becomes apparent that all this movement, in response to the environment, is carefully controlled.


Each kind of a change in the environment evokes an appropriate movement in response.


When we want to talk to our friends in class, we whisper, rather than shouting loudly.


Clearly, the movement to be made depends on the event that is triggering it.


Therefore, such controlled movement must be connected to the recognition of various events in the environment, followed by only the correct movement in response.


In other words, living organisms must use systems providing control and coordination.


In keeping with the general principles of body organization in multicellular organisms, specialised tissues are used to provide these control and coordination activities.


1 ANIMALS – NERVOUS SYSTEM


In animals, such control and coordination are provided by nervous and muscular tissues, which we have studied in Class 9.


Touching a hot object is an urgent and dangerous situation for us.


We need to detect it, and respond to it.


How do we detect that we are touching a hot object?


All information from our environment is detected by the specialised tips of some nerve cells.


These receptors are usually located in our sense organs, such as the inner ear, the nose, the tongue, and so on.


So gustatory receptors will detect taste while olfactory receptors will detect smell.


This information, acquired at the end of the dendritic tip of a nerve cell sets off a chemical reaction that creates an electrical impulse.


This impulse travels from the dendrite to the cell body, and then along the axon to its end.


At the end of the axon, the electrical impulse sets off the release of some chemicals.


These chemicals cross the gap, or synapse, and start a similar electrical impulse in a dendrite of the next neuron.


This is a general scheme of how nervous impulses travel in the body.


A similar synapse finally allows delivery of such impulses from neurons to other cells, such as muscles cells or gland.


It is thus no surprise that nervous tissue is made up of an organized network of nerve cells or neurons, and is specialised for conducting information via electrical impulses from one part of the body to another.


identify the parts of a neuron where information is acquired through which information travels as an electrical impulse, and where this impulse must be converted into a chemical signal for onward transmission.


Put some sugar in your mouth.


How does it taste?


Block your nose by pressing it between your thumb and index finger.


Now eat sugar again.


Is there any difference in its taste?


While eating lunch, block your nose in the same way and notice if you can fully appreciate the taste of the food you are eating.


Is there a difference in how sugar and food taste if your nose is blocked?


If so, why might this be happening?


Read and talk about possible explanations for these kinds of differences.


Do you come across a similar situation when you have a cold?


What happens in Reflex Actions?


‘Reflex’ is a word we use very commonly when we talk about some sudden action in response to something in the environment.


We say ‘I jumped out of the way of the bus reflexly’, or ‘I pulled my hand back from the flame reflexly’, or ‘I was so hungry my mouth started watering reflexly’.


What exactly do we mean?


A common idea in all such examples is that we do something without thinking about it, or without feeling in control of our reactions.


Yet these are situations where we are responding with some action to changes in our environment.


How is control and coordination achieved in such situations?


Let us consider this further.


Take one of our examples.


Touching a flame is an urgent and dangerous situation for us, or in fact, for any animal!


How would we respond to this?


One seemingly simple way is to think consciously about the pain and the possibility of getting burnt,    and therefore move our hand.


An important question then is, how long will it take us to think all this?


The answer depends on how we think.


If nerve impulses are sent around the way we have talked about earlier, then thinking is also likely to involve the creation of such impulses.


Thinking is a complex activity, so it is bound to involve a complicated interaction of many nerve impulses from many neurons.


If this is the case, it is no surprise that the thinking tissue in our body consists of dense networks of intricately arranged neurons.


It sits in the forward end of the skull, and receives signals from all over the body which it thinks about before responding to them.


Obviously, in order to receive these signals, this thinking part of the brain in the skull must be connected to nerves coming from various parts of the body.


Similarly, if this part of the brain is to instruct muscles to move, nerves must carry this signal back to different parts of the body.


If all of this is to be done when we touch a hot object, it may take enough time for us to get burnt!


How does the design of the body solve this problem?


Rather than having to think about the sensation of heat, if the nerves that detect heat were to be connected to the nerves that move muscles in a simpler way, the process of detecting the signal or the input and responding to it by an output action might be completed quickly.


Such a connection is commonly called a reflex arc.


Where should such reflex arc connections be made between the input nerve and the output nerve?


The best place, of course, would be at the point where they first meet each other.


Nerves from all over the body meet in a bundle in the spinal cord on their way to the brain.


Reflex arcs are formed in this spinal cord itself, although the information input also goes on to reach the brain.


Of course, reflex arcs have evolved in animals because the thinking process of the brain is not fast enough.


In fact many animals have very little or none of the complex neuron network needed for thinking.


So it is quite likely that reflex arcs have evolved as efficient ways of functioning in the absence of true thought processes.


However, even after complex neuron networks have come into existence, reflex arcs continue to be more efficient for quick responses.


Can you now trace the sequence of events which occur when a bright light is focussed on your eyes?


Human Brain Is reflex action the only function of the spinal cord?


Obviously not, since we know that we are thinking beings.


Spinal cord is made up of nerves which supply information to think about.


Thinking involves more complex mechanisms and neural connections.


These are concentrated in the brain, which is the main coordinating centre of the body.


The brain and spinal cord constitute the central nervous system.


They receive information from all parts of the body and integrate it.


We also think about our actions.


Writing, talking, moving a chair, clapping at the end of a programme are examples of voluntary actions which are based on deciding what to do next.


So, the brain also has to send messages to muscles.


This is the second way in which the nervous system communicates with the muscles.


The communication between the central nervous system and the other parts of the body is facilitated by the peripheral nervous system consisting of cranial nerves arising from the brain and spinal nerves rising from the spinal cord.


The brain thus allows us to think and take actions based on that thinking.


As you will expect, this is accomplished through a complex design, with different parts of the brain responsible for integrating different inputs and outputs.


The brain has three such major parts or regions, namely the fore-brain, mid-brain and hind-brain.


The fore-brain is the main thinking part of the brain.


It has regions which receive sensory impulses from various receptors.


Separate areas of the fore-brain are specialised for hearing, smell, sight and so on.


There are separate areas of association where this sensory information is interpreted by putting it together with information from other receptors as well as with information that is already stored in the brain.


Based on all this, a decision is made about how to respond and the information is passed on to the motor areas which control the movement of voluntary muscles, for example, our leg muscles.


However, certain sensations are distinct from seeing or hearing.


For example, how do we know that we have eaten enough?


The sensation of feeling full is because of a centre associated with hunger, which is in a separate part of the fore-brain.


Human brain


Study the labelled diagram of the human brain.


We have seen that the different parts have specific functions.


Can we find out the function of each part?


Let us look at the other use of the word ‘reflex’ that we have talked about in the introduction.


Our mouth waters when we see food we like without our meaning to.


Our hearts beat without our thinking about it.


In fact, we cannot control these actions easily by thinking about them even if we wanted to.


Do we have to think about or remember to breathe or digest food?


So, in between the simple reflex actions like change in the size of the pupil, and the thought out actions such as moving a chair, there is another set of muscle movements over which we do not have any thinking control.


Many of these involuntary actions are controlled by the mid-brain and hind-brain.


All these involuntary actions including blood pressure, salivation and vomiting are controlled by the     medulla in the hind-brain.


Think about activities like walking in a straight line, riding a bicycle, picking up a pencil.


These are possible due to a part of the hind-brain called the cerebellum.


It is responsible for precision of voluntary actions and maintaining the posture and balance of the body.


Imagine what would happen if each of these events failed to take place if we were not thinking about it.


How are these Tissues protected?


A delicate organ like the brain, which is so important for a variety of activities, needs to be carefully protected.


For this, the body is designed so that the brain sits inside a bony box.


Inside the box, the brain is contained in a fluid-filled balloon which provides further shock absorption.


If you run your hand down the middle of your back, you will feel a hard, bumpy structure.


This is the vertebral column or backbone which protects the spinal cord.


How does the Nervous Tissue cause Action?


So far, we have been talking about nervous tissue, and how it collects information, sends it around the body, processes information, makes decisions based on information, and conveys decisions to muscles for action.


In other words, when the action or movement is to be performed, muscle tissue will do the final job.


How do animal muscles move?


When a nerve impulse reaches the muscle, the muscle fibre must move.


How does a muscle cell move?


The simplest notion of movement at the cellular level is that muscle cells will move by changing their shape so that they shorten.


So the next question is, how do muscle cells change their shape?


The answer must lie in the chemistry of cellular components.


Muscle cells have special proteins that change both their shape and their arrangement in the cell in response to nervous electrical impulses.


When this happens, new arrangements of these proteins give the muscle cells a shorter form.


Remember when we talked about muscle tissue in Class 9, there were different kinds of muscles, such as voluntary muscles and involuntary muscles.


Based on what we have discussed so far, what do you think the differences between these would be?


What is the difference between a reflex action and walking?


What happens at the synapse between two neurons?


Which part of the brain maintains posture and equilibrium of the body?


How do we detect the smell of an agarbatti (incense stick)?


What is the role of the brain in reflex action?


COORDINATION IN PLANTS


Animals have a nervous system for controlling and coordinating the activities of the body.


But plants have neither a nervous system nor muscles.


So, how do they respond to stimuli?


When we touch the leaves of a chhui-mui (the ‘sensitive’ or ‘touch-me-not’ plant of the Mimosa family), they begin to fold up and droop.


When a seed germinates, the root goes down, the stem comes up into the air.


What happens?


Firstly, the leaves of the sensitive plant move very quickly in response to touch.


There is no growth involved in this movement.


On the other hand, the directional movement of a seedling is caused by growth.


If it is prevented from growing, it will not show any movement.


So plants show two different types of movement – one dependent on growth and the other independent of growth.


Immediate Response to Stimulus


Let us think about the first kind of movement, such as that of the sensitive plant.


Since no growth is involved, the plant must actually move its leaves in response to touch.


But there is no nervous tissue, nor any muscle tissue.


How does the plant detect the touch, and how do the leaves move in response?


If we think about where exactly the plant is touched, and what part of the plant actually moves, it is apparent that movement happens at a point different from the point of touch.


So, information that a touch has occurred must be communicated.


The plants also use electrical-chemical means to convey this information from cell to cell, but unlike in animals, there is no specialised tissue in plants for the conduction of information.


Finally, again as in animals, some cells must change shape in order for movement to happen.


Instead of the specialised proteins found in animal muscle cells, plant cells change shape by changing the amount of water in them, resulting in swelling or shrinking, and therefore in changing shapes.


Movement Due to Growth


Some plants like the pea plant climb up other plants or fences by means of tendrils.


These tendrils are sensitive to touch.


When they come in contact with any support, the part of the tendril in contact with the object does not grow as rapidly as the part of the tendril away from the object.


This causes the tendril to circle around the object and thus cling to it.


More commonly, plants respond to stimuli slowly by growing in a particular direction.


Because this growth is directional, it appears as if the plant is moving.


Let us understand this type of movement with the help of an example.


Fill a conical flask with water.


Cover the neck of the flask with a wire mesh.


Keep two or three freshly germinated bean seeds on the wire mesh.


Take a cardboard box which is open from one side.


Keep the flask in the box in such a manner that the open side of the box faces light coming from a window .


After two or three days, you will notice that the shoots bend towards light and roots away from light.


Now turn the flask so that the shoots are away from light and the roots towards light.


Leave it.


Response of the plant to the direction of light undisturbed in this condition for a few days.


Have the old parts of the shoot and root changed direction?


Are there differences in the direction of the new growth?


What can we conclude from this activity?


Environmental triggers such as light, or gravity will change the directions that plant parts grow in.


These directional, or tropic, movements can be either towards the stimulus, or away from it.


So, in two different kinds of phototropic movement, shoots respond by bending towards light while roots respond by bending away from it.


How does this help the plant?


Plants show tropism in response to other stimuli as well.


The roots of a plant always grow downwards while the shoots usually grow upwards and away from the earth.


This upward and downward growth of shoots and roots, respectively, in response to the pull of earth or gravity is, obviously, geotropism.


If ‘hydro’ means water and ‘chemo’ refers to chemicals, what would ‘hydrotropism’ and ‘chemotropism’ mean?


Can we think of examples of these kinds of directional growth movements?


One example of chemotropism is the growth of pollen tubes towards ovules, about which we will learn more when we examine the reproductive processes of living organisms.


Let us now once again think about how information is communicated in the bodies of multicellular organisms.


The movement of the sensitive plant in response to touch is very quick.


The movement of sunflowers in response to day or night, on the other hand, is quite slow.


Growth-related movement of plants will be even slower.


Even in animal bodies, there are carefully controlled directions to growth.


Our arms and fingers grow in certain directions, not haphazardly.


So controlled movements can be either slow or fast.


If fast responses to stimuli are to be made, information transfer must happen very quickly.


For this, the medium of transmission must be able to move rapidly.


Electrical impulses are an excellent means for this.


But there are limitations to the use of electrical impulses.


Firstly, they will reach only those cells that are connected by nervous tissue, not each and every cell in the animal body.


Secondly, once an electrical impulse is generated in a cell and transmitted, the cell will take some time to reset its mechanisms


before it can generate and transmit a new impulse.


In other words, cells cannot continually create and transmit electrical impulses.


It is thus no wonder that most multicellular organisms use another means of