BL2131 Notes
Foundations in Biology II
Topics 1 to 7

This is a compilation of topics taught in BL2131.
Made by LQ 

“Double the initial number of the module code, double the pain!” ~Einstein or someone idk

Links of All LQ Notes: LQ Notes Links Document

Topic
Page
Topic 1: Transport in Plants
2
Topic 2: Transport in Hoomanz
22
Topic 3: Respiration
40
Topic 4: Heredity
48
Topic 5: Reproduction in Plants
56
Topic 6: Reproduction in Hoomanz
66
Topic 7: Homeostasis and Thermoregulation
85
Sources
95



Author’s Note
Please use at your own discretion, information in this set of notes may not be correct (im not good at bio okay), do confirm with your Lovely Biology Teachers if you have any doubt, and if you can, drop a comment in the notes if you find a mistake. 

Information marked with [confirm] need further clarification, if you have knowledge on the content, pleeeeeaaaassse help.

Everything that says “FYI” is purely for additional reading, and is not tested (at this level).

Thanks for reading and good luck! :D

Note: Topic 1 - Transport in Plants is NOT tested for EOY Exams.

Topic 1: Transport in Plants

Recap: If you need a Recap for Y1 Biology BL1131, consider referring to BL1131 Notes Topic 7: Nutrition in Plants before continuing.


1.1 Parts of a Leaf
Consider the transverse section of a Privet (ligustrum) plant below, in Figure 1.1.



Figure 1.1: A Transverse Section of a Ligustrum Leaf

This is a typical dicotyledonous (or dicot, for short) leaf. We can label this transverse section as such:



Figure 1.2: Labelled Transverse Section of Ligustrum Leaf

However, as you might notice, the stoma and vascular bundles are not labelled.


Figure 1.3: Labelled Stoma of Ligustrum Leaf

Figure 1.3 above shows the stoma of the Ligustrum Leaf.

For clarity’s sake, here’s a clearer image of what the stoma looks like, from an Alfalfa (Medicago Sativa) plant:

Figure 1.4: Labelled Stoma of Medicago Sativa Leaf



Figure 1.5: Labelled Vascular Bundle of Ligustrum Leaf

Note that the xylem is always above the phloem (when the leaf is positioned naturally, i.e. stomata on the bottom).

1.2 Vascular Tissues in the Stem 


Figure 1.6: Longitudinal Section of Cucurbita Stem

Figure 1.6 above shows the Longitudinal Section of Vascular Tissues of a Squash (Cucurbita) Plant Stem. The xylem is always positioned closer to the centre of the stem than the phloem, as in Figure 1.7 below:

Figure 1.7: Cross Section of a Plant Stem

1.3 Vascular Tissues in the Roots



Figure 1.8: Cross Section of a Ranunculus Plant Root

Figure 1.8 above shows the xylem and phloem of a Buttercup (Ranunculus) Plant Root. As you can see, we can observe that the phloem and xylem are not packed into vascular bundles like they are in the stem or leaves.

The xylem is also shaped in the shape of a star (or X, or cross, or whatever you call it).
The xylem and phloem alternate with each other.


1.4 Xylem Vessels
Xylems are the water-carrying tubes of a plant.

1.4.1 Function of Xylem

The xylem has 2 main functions:
Providing mechanical support for the plant; and
Conducting water and dissolved mineral salts from the roots to the stem and leaves.

The xylem is a long, continuous, hollow tube, similar to a straw, that is made up of dead cells fused together.

It contains no cross-walls or protoplasm. But what are cross-walls? Refer to Figure 1.9 below.


Figure 1.9: Formation of Xylem

1.4.2 Lignin

Lignin is a hard and rigid substance that is found on the walls of the xylem, which strengthens it and provides mechanical support for the plant. Deposits of lignin are formed on the walls of the xylem through a process called lignification.

Figure 1.10 below shows three lignification patterns that we need to know, where the red parts denote lignin deposits:


Figure 1.10: Lignification Patterns

The benefits of lignification are that the plant gets mechanical support, ensuring that the vessels do not collapse and hence helps in the transport of water and mineral salts.

1.5 Processes Involved in Transport of Water 
There are three main processes involved in the transport of water - Root Pressure, Capillary Action, and finally Transpiration Pull.

1.5.1 Root Pressure

Root Pressure is the pressure resulting from the constant entry of water (by osmosis) through the roots. Figure 1.11 below is an illustration of this:



Figure 1.11: Root Pressure

As illustrated, the water molecules “push” the entire stream of water upwards as they try to enter by osmosis, as shown by the red arrows.

Illustrated by Figure 1.12 below is the effect of root pressure.

Figure 1.12: Effect of Root Pressure

1.5.2 Capillary Action

 Capillary Action is dependent on the forces of cohesion and adhesion. It is defined as the tendency of water molecules to move up inside very narrow tubes called capillaries.

Cohesion is the attraction of water molecules to other water molecules held together.  When one molecule goes up, the others are pulled up by cohesion. Refer to Figure 1.13 below.


Figure 1.13: Cohesion of Water

Adhesion is the attraction of water molecules to other molecules (like the glass test tube, or the xylem walls). Refer to Figure 1.14 below.



Figure 1.14: Adhesion of Water
When we consider both forces, the resultant force is called Capillary Action. 

Figure 1.15: Effect of Capillary Action

1.5.3 Transpiration Pull

Transpiration pull is the suction force caused by transpiration which results in water moving up the xylem in a stream of water called the transpiration stream.

Recall: 

Transpiration is the loss of water from the aerial parts of a plant, especially through the stomata of leaves.


The effect of Transpiration Pull is demonstrated in Figure 1.16 below, where the orange arrows denote the water molecules diffusing out of the stoma, and the green arrows denote Transpiration Pull.

Figure 1.16: Diffusion and Transpiration Pull


1.6 Phloem Vessels
The phloem is the food-carrying tube in a plant.


Figure 1.17: Longitudinal Section of Phloem

The phloem consists of Companion Cells, Sieve Plates and Sieve Tube Cells (also known as Sieve Tube Elements). 

The phloem conducts manufactured food from the green parts of the plant (i.e. parts with chlorophyll; mostly the leaves) to the rest of the plant in the form of Sucrose and Amino Acids. This process is called Translocation of Food.

Translocation occurs from Sugar Sources (for example, leaves) to Sugar Sinks (for example, roots, fruits, etc.).

1.6.1 Aphid Feeding Experiment

Aphids have a mouthpart called a stylet, which acts like a mini syringe, pulling the phloem sap out of the phloem. The flow of the phloem sap can then be tracked by using aphids at different points between a source and a sink. 

Research found that the concentration of sugar near sugar sinks is lower, and the pressure of the phloem sap is higher near sugar sources.

FYI: This supports a hypothesis known as the Pressure Flow Hypothesis, which is not tested.

1.6.2 Ringing Experiment

Ringing, or Girdling, is the removal of the layer of bark containing the phloems.

Figure 1.18: Girdling Experiment

After time, the manufactured food from the leaves can no longer be transported down to the roots, which also shows the path which food travels in.

The food accumulates just above the ring that was removed, creating a swollen region. This also lowers the water potential of the cells in that region, allowing water to enter and swell the area further.

1.6.3 Radioactive Carbon Experiment

Carbon is most commonly found as Carbon-12,   612C. However, there is a radioactive isotope of carbon, Carbon-14,   614C, written as:14C. Carbon-14 can then form radioactive carbon dioxide, written as:14CO2.

We supply the plant with radioactive carbon dioxide using a container as in Figure 1.19 below:



Figure 1.19: Supplying Radioactive Carbon Dioxide to Leaf

This special radioactive carbon dioxide is then used in photosynthesis, creating radioactive glucose, and then radioactive sucrose as it is transported. 

This radioactive sucrose in the stem can then be captured by an X-ray, which allows the places where radioactive sucrose are transported to to be captured, thereby pinpointing the translocation flow.

1.6.4 Sieve Tube Cells

Sieve Tube cells have very little protoplasm, which reduces the resistance to the flow of substances within the phloem.

Simply, it means there is barely anything inside the cells to block the way of food being transported.


Figure 1.20: Sieve Tube Cell and Companion Cell

As we can see from Figure 1.20 above, the Sieve Tube Cell is mostly empty, while the Companion Cell is filled with organelles.

1.6.5 Sieve Plates

Sieve Plates have pores, which are the tiny holes running through them.



 Figure 1.21: Sieve Plate

These pores within the sieve plates allow for rapid flow of manufactured food substances.

The sieve plates also act as a barrier to prevent the loss of sap when the phloem is cut or damaged, often by an insect or herbivorous animal. (FYI: This is done by the formation of a substance known as Callose.)

1.6.6 Companion Cells

Since Sieve Tube Cells have most of their organelles removed, they naturally don’t have mitochondria to release energy. Hence, another cell nearby, called the companion cell, supplies it with energy to keep it alive and for the loading of sucrose into the sieve tubes (see Figure 1.22).



Figure 1.22: Companion Cell

The companion cell is considerably tinier than the sieve tubes, as shown in Figure 1.22 above.

These cells have numerous mitochondria to provide energy for the loading of sucrose into the sieve tubes, i.e., “pushing” the sucrose through the sieve plate. Companion cells also provide nutrients to the sieve tubes to keep them alive.


1.7 Cortex and Pith (FYI)
Well, it’s supposed to be FYI only, but somehow it got tested in a worksheet so :/

In the stem, there is also the cortex and the pith, which are both used for storage of starch, mainly. Refer to Figure 1.23 below:


Figure 1.23: Cross-Section of Sunflower Stem

The root also has a cortex, which is used for a similar purpose. Refer to Figure 1.24 below:


Figure 1.24: Cross Section of Dicot Root


1.8 Boundary Layer
Around the leaves of a plant, there exists a Boundary Layer, which is a layer of unstirred (i.e. calm) air. In this region, the wind is weaker and both the humidity and the temperature are higher.

A thinner boundary layer would result in a more abrupt change from the unstirred air to the bulk air (i.e. surrounding, normal air), which would also then increase the diffusion gradient of  water.

Hence, a thinner boundary layer would cause water to diffuse out of the stomata, as a result of transpiration, faster. 

Furthermore, a thinner boundary layer also causes carbon dioxide to diffuse into the stomata faster and increase transfer of heat.

Figure 1.25: Boundary Layer of a Leaf

1.9 Factors Affecting Rate of Transpiration
There are 4 main factors affecting rate of transpiration:
Humidity
Wind
Temperature
Light

When the atmosphere is more humid, the concentration gradient of water is less steep, so diffusion and hence transpiration occurs at a slower rate.

When the wind is stronger, the water vapour accumulating on the outside of the stomata (in the boundary layer) is blown away, hence increasing the concentration gradient of water, so diffusion and hence transpiration occurs at a higher rate.

When the temperature is higher, the rate of evaporation increases, so transpiration occurs at a higher rate.

When there is more light, the stomata will open and become wider, increasing the rate of transpiration. 


1.10 Temporary Wilt
A normal plant is held up by the turgor pressure in the leaf mesophyll cells, which is why leaves don’t crumple up usually.

Figure 1.26: Turgor Pressure due to Accumulation of Water

However, when the rate of transpiration exceeds the rate of absorption of water by the roots (i.e. too much water is lost), the mesophyll cells lose water and lose their turgor pressure. Hence, the cells and the leaf become flaccid. 

This is called wilting.



Figure 1.27: Wilting of a Plant

Refer to Figure 1.27 above. This shows a plant (Dendropanax Morbiferus) wilting from picture A to B to C. As we see, in C, the leaves have become flaccid and are drooping down.


1.10.1 Benefits of Wilting

When a leaf folds up, the surface area exposed to sunlight is reduced. Excessive loss of water also causes the guard cells to become flaccid, which causes the stomata to close. These two factors reduce the transpiration rate (i.e. reduces water loss). The plant is also cooled.


1.10.2 Disadvantages of Wilting

The rate of photosynthesis is reduced, since less carbon dioxide can enter the plant when the stomata is closed. Furthermore, the surface area of leaves exposed to sunlight decreases, which also reduces the rate of photosynthesis.

Topic 2: Transport in Humans
2.1 Why a Transport System?
We know that cells do not need a transport system, as nutrients can directly diffuse in, and waste can directly diffuse out, so why do we need one?
    uwucellular
Well, in a Unicellular Environment (uni - one; unicellular - single celled), the movement of substances can occur by simple diffusion, because no part of the cell is too far from the external environment.

However, in a Complex Multicellular Organism, a transport system is needed to transport substances from one part of the body to another because many cells are located deep in the body, far from the external environment, too far for substances to be transported by diffusion.

For example, for mammals, a transport system consisting of blood, blood vessels, and a heart was developed.

2.2 Composition of Blood
Here is the composition of blood:

Figure 2.1 - Composition of Blood


2.3 Plasma
Plasma is a yellow substance which consists of 90% water and 10% proteins, mineral salts, food substances, waste products and hormones.

Its function is to transport materials throughout the body.


2.4 Red Blood Cells
Red Blood Cells, also known as erythrocytes, have a biconcave shape, as shown in Figure 2.2 below.


Figure 2.2: Biconcave shape of Red Blood Cell


This biconcave shape increases Surface Area to Volume Ratio to increase rate of uptake of oxygen.

Red Blood Cells are filled with a red pigment Haemoglobin (or Hemoglobin) for binding oxygen. Haemoglobin is broken down in the spleen and liver.

Figure 2.3: Haemoglobin

Red Blood Cells do not have a nucleus, so as to carry more Haemoglobin to carry more oxygen.

Red Blood Cells are elastic to enable cells to squeeze through blood capillaries.

Red Blood Cells are synthesised in the bone marrow and are broken down in the spleen and liver.


2.5 White Blood Cells
White Blood cells, also known as leucocytes, have an irregular shape and contain a nucleus, as shown in Figure 2.4 below:

Figure 2.4: A White Blood Cell

White Blood Cells are colourless as they do not contain haemoglobin.

White Blood Cells are also mobile as they are able to move, change their shape and squeeze through walls of thin blood capillaries.

Leucocytes can be further subdivided into two types - Phagocytes and Lymphocytes, which have different functions.


2.5.1 Lymphocytes

Lymphocytes produce antibodies that protect the body from diseases. 

When a pathogen (i.e. harmful foreign particle) enters the bloodstream, lymphocytes are stimulated to produce antibodies.

Antibodies are specialised Y-shaped proteins, as shown in Figure 2.5 below:

Figure 2.5: An Antibody

These antibodies bind to antigens on bacteria and viruses like a lock and key (similar to enzyme lock-and-key hypothesis). This causes the bacteria and viruses to clump together in a process known as agglutination, as shown in Figure 2.6 below.


Figure 2.6: Agglutination

The clumped pathogens are then ingested by Phagocytes by Phagocytosis, as covered in Section 2.5.2 below.


2.5.2 Phagocytes

Phagocytes surround and engulf foreign particles like bacteria by phagocytosis.

Phagocytes can destroy foreign bodies like Bacteria, Viruses, Toxins, Allergens, and other Pathologically Altered Cells of Organisms.

Figure 2.7: The process of Phagocytosis

2.6 Platelets
Platelets, also known as thrombocytes, are membrane-bound fragments of cytoplasm. Their main function is blood clotting.

When platelets are damaged, they release an enzyme known as thrombokinase, and this begins the process of Blood Clotting (Refer to Section 2.7 below).


2.7 Blood Clotting
The main purpose for Blood Clotting is to prevent excessive blood loss and prevent pathogens from entering the blood through the wound.

Blood Clotting, also known as Coagulation of Blood,  is a 3-step process, known as a Cascade Reaction, as one reaction sets off the next.

It starts with Damaged Tissue Platelets releasing Thrombokinase, which is an enzyme. 

Thrombokinase, along with the Ca2+ ion, catalyses the reaction converting the inactive enzyme Prothrombin to its active form, Thrombin.

Thrombin then catalyses the reaction converting soluble Fibrinogen into insoluble Fibrin.

Fibrin is like a net that blocks off the wound, while Prothrombin and Fibrinogen are Clotting Factors, along with White Blood Cells and Red Blood Cells. These substances help to block the wound.


Figure 2.8: Clotting of Blood

Figure 2.9 below shows a summary of the Coagulation Cascade.


Figure 2.9: Coagulation Cascade


2.8 Functions of Blood
Blood has 3 key functions:
To act as a transport medium carrying many substances from one part of the body to another;
To protect the body against disease-carrying organisms (known as pathogens); and
To coagulate (i.e. the clotting of blood) at wounds by sealing the wound.


2.8.1 Substances Transported by Blood

Table 2.10 below shows some substances transported by blood:

Substance Transported
Source
Destination

Digested Food Products

Small Intestine
Liver, then to All Other Parts of Body
Excretory Products
(i.e. waste)
Nitrogenous Waste
All Parts of Body
Kidney
Carbon Dioxide
All Parts of Body
Lungs
Hormones
Glands
Target Cells & Organs
Heat
Respiring Body Tissue
All Parts of Body
Oxygen
Lungs
All Parts of Body


Table 2.10: Substances Transported by Blood

2.8.2 Gaseous Exchange

Figure 2.11 below shows the process of Gaseous Exchange in the air-sac (also known as alveolus).



Figure 2.11: Gaseous Exchange in the Alveolus


2.8.3 Transport of Oxygen

As the blood passes through lungs, O2 diffuses from the air sacs in the lungs into the blood. The O2 then combines with Haemoglobin in the red blood cells to form Oxyhaemoglobin.

Taking a unit of Haemoglobin as Hb, the formula for Oxyhaemoglobin is then Hb4O2.

This process is easily reversible. As the blood leaves the lungs and passes through a tissue that lacks oxygen, the oxyhaemoglobin breaks apart into Oxygen and Haemoglobin, and the Oxygen is then supplied to the tissue by diffusion.


While 98.5% of Oxygen is transported in the form of Oxyhaemoglobin, Oxygen can also dissolve in the plasma of blood, which is how the remaining 1.5% of Oxygen is transported.


2.8.4 Transport of Carbon Dioxide

Carbon Dioxide can be transported in 3 main ways in the blood.

Firstly, it can be dissolved in and transported in the plasma directly. 

This accounts for about 7% of CO2 transported.
 
Secondly, it can be transported as the Hydrogen Carbonate Ion in the plasma.

CO2 first diffuses into the Red Blood Cell, where it reacts with H2O to form Carbonic 
Acid, H2CO3. This reaction is catalysed by an enzyme present in Red Blood Cells, 
Carbonic Anhydrase.

Then, Carbonic Acid dissociates to form Hydrogen Ion, H+, and Hydrogen Carbonate 
Ion (also known as Bicarbonate Ion), HCO3-.

HCO3- can then be transported in the plasma.

The chemical equations are:
H2O+CO2H2CO3H++HCO3-

This accounts for 70% of CO2 transported.

Lastly, it can be transported by Haemoglobin.

	Taking a unit of Haemoglobin as Hb, the reaction is CO2+HbHbCO2.
	This form of Haemoglobin, HbCO2, is known as Carbaminohaemoglobin.

	This accounts for 23% of CO2 transported.


2.9 Steps to Wound Healing (FYI)
The first step is Haemostasis (or Hemostasis), which involves coagulation of blood (i.e. clotting of blood).
The second step is Inflammation, where pathogens are attacked and destroyed.
The third step is Proliferation, where the epidermis (outer skin tissues) is repaired.
The final step is Remodelling, where collagen, a protein, is rearranged within the dermal layer.



2.10 The Human Heart


Figure 2.12: Four Main Chambers of the Heart

Figure 2.12 above shows the four main chambers of the heart.

Figure 2.13: Major Blood Vessels of the Heart

Figure 2.13 above shows the major blood vessels related to the heart.


Figure 2.14: Four Valves of the Heart

Figure 2.14 above shows the four main valves of the heart.

With reference from Figure 2.11, Figure 2.12 and Figure 2.13 above, 

Deoxygenated Blood first enters the Right Atrium, through the Superior and Inferior Vena Cava, then enters the Right Ventricle through the Tricuspid Valve (also known as the Right Atrioventricular Valve). 

Blood then enters the Pulmonary Artery through the Pulmonary Valve (also known as the Right Semi-Lunar Valve).

The deoxygenated blood travels through the Pulmonary Artery into the lungs, where gaseous exchange occurs, turning the blood into Oxygenated Blood.

The Oxygenated Blood returns to the heart and enters the Left Atrium through the Pulmonary Veins, then enters the Left Ventricle through the Bicuspid Valve (also known as the Left Atrioventricular Valve, or the Mitral Valve).

Blood then enters the Aorta through the Aortic Valve (also known as the Left Semi-Lunar Valve). The blood is then distributed all around the body through the Aorta.


Figure 2.14 shows some miscellaneous parts (FYI), in the heart.


Figure 2.14: Miscellaneous Parts of the Heart


2.11 Blood Groups
Some people have Red Blood Cells with antigens present. There are two types of antigens that can be present, Antigen A and Antigen B. 

One person’s blood can have either one of these antigens, neither, or both. These types of blood are known as blood groups.

These are the four possible “types” of blood cells:

Figure 2.15: Antigens on Red Blood Cells

We then name the blood group based on what antigens are present. For example, if the blood cells have Antigen A but not Antigen B, the blood group is known as Blood Group A.


Figure 2.16: Blood Groups

We call the blood group with neither antigen Blood Group O.


2.11.1 Antibodies and Blood Groups

If a person’s Red Blood Cell does not contain Antigen A, then their blood plasma will contain the Antibody known as Anti-A.

Similarly, if a person’s Red Blood Cell does not contain Antigen B, then their blood plasma will contain the Antibody known as Anti-B.



Table 2.17: Antibodies and Antigens Present in Blood


2.11.2 Blood Donors and Recipients

Sometimes, patients require blood donations, but not all blood groups are compatible with others. Blood Groups with Antigen A cannot be compatible with those with Anti-A Antibody. Similarly, Blood Groups with Antigen B cannot be compatible with those with Anti-B Antibody.

This is because Agglutination will occur in the blood. Refer to Section 2.11.3 below.

For example, a person with Blood Group A, which contains the Antibody Anti-B, cannot receive blood from a person with Blood Group B, since it contains Antigen B.

On the other hand, for example, a person with Blood Group A can receive blood from a person with Blood Group O.


Table 2.18 below summarises which Blood Groups are compatible with which Blood Groups.



Blood
Recipient
Blood Donor
A
B
AB
O
A
✔
✖
✖
✔
B
✖
✔
✖
✔
AB
✔
✔
✔
✔
O
✖
✖
✖
✔


Table 2.18: Blood Recipient and Donor


2.11.3 Agglutination in the Blood

When both Antigen A and Antibody Anti-A, or both Antigen B and Antibody Anti-B are present, agglutination (Refer to Section 2.5.1) will occur, as the antigens and antibodies bind to each other.



Figure 2.19: Agglutination of Red Blood Cells

This causes the Red Blood Cells to stop being able to function properly. Furthermore, agglutinated red blood cells can clog blood vessels which can prevent the circulation of blood in the body.

This can have fatal consequences.


2.12 Blood Vessels
A vein is a blood vessel carrying blood towards the heart. An artery is a blood vessel carrying blood away from the heart.  A capillary is a microscopic blood vessel that links arteries to veins.

2.12.1 Arteries

Arteries are typically oxygen-rich, except for the Pulmonary Arteries. 

It has thick muscular walls to prevent it from rupturing when there is an influx of blood at high pressure. Its muscles contract and relax to cause constriction and dilation of the artery.

It has a small lumen relative to its diameter. It has elastic fibres which can stretch and spring back, which helps to maintain the pulse.


2.12.2 Veins

Veins are typically deoxygenated, except for the Pulmonary Veins. It is thinner than an artery, and has a large lumen relative to its diameter.

Only veins have valves within them, as shown from Figure 2.20 below. This prevents backflow of blood as the blood pressure is lower in the veins.


Figure 2.20: Valves in the Veins
2.12.3 Capillaries

It is only one-cell thick endothelium to make it partially permeable and to reduce diffusion distance of substances. It has the largest lumen relative to its diameter. Figure 2.21 below shows how capillaries are connected to arteries and veins.


Figure 2.21 - Capillaries

The network of capillaries increases surface area to volume ratio and total cross-sectional area, to allow for efficient movement of substances between blood and cells.

2.13 The Cardiac Cycle
A systole refers to when the atrium/ventricle is contracting.
A diastole refers to when the atrium/ventricle is relaxed.

2.13.1 Phases of the Cardiac Cycle

The cardiac cycle is made up of three steps:

Atrioventricular Diastole (~0.4s)
Atrial Systole & Ventricular Diastole (~0.1s)
Ventricular Systole & Atrial Diastole (~0.3s)




Figure 2.22 - Blood Pressure against Time graph

For Sections 2.13.2 to 2.13.4, refer to Figure 2.22 above.

2.13.2 Atrial Systole

During Atrial Systole, the atria (plural of atrium) contracts, forcing the blood inside the atrium to enter the ventricles. During this time, the AV Valves are open, and the Semi-lunar Valves are closed.


2.13.3 Ventricular Systole

During Ventricular Systole, the atrium relaxes and the ventricles contract. This causes the blood pressure of the ventricles increases to be greater than that of the atria and the aorta/pulmonary arteries, which forces the AV Valves to close, and the Semi-lunar Valves to open. 

As the AV Valves close, backflow of blood is prevented and a “lub” sound is produced.

The wall of the aorta stretches to accommodate the high pressure of blood exiting from the left ventricle, pushing blood into systemic circulation.


2.13.4 Atrioventricular Diastole

The ventricles begin to relax as the Semi-lunar Valves close to prevent backflow of blood. This produces a “dub” sound. The pressure in the ventricles drops to be lower than that of the atria, causing the AV Valves to open.

At this time, there is a slight increase in aortic pressure as the walls of the aorta spring back to maintain the driving force to push blood into systemic circulation, causing what is known as the dicrotic notch on the blood pressure-time graph (Refer to Figure 2.22 above).

During Atrioventricular Diastole, blood passively flows into the heart from the veins into the atria, and then through the AV Valves into the ventricles.


2.14 Coronary Heart Disease
Coronary Arteries lie on the surface of the heart and supply the muscles of the heart with oxygenated blood, since the walls of the heart are too thick for the muscles to directly obtain blood from the chambers.

Figure 2.23 - Coronary Arteries

Excess cholesterol can build up along the sides of the inner arterial walls, causing blockages and disrupting blood flow. This causes the lumen of arteries to become narrower and is known as atherosclerosis.

A blood clot can then form in the Coronary Arteries, a process known as thrombosis. This reduces blood flow to the heart muscles, which receive less oxygen and nutrients. This causes sections of the heart muscles to weaken and become damaged, possibly leading to a heart attack.
Topic 3: Respiration
“BREATHE AIR” ~ Drake
~Andrew Tate

3.1 Why is Respiration Important?
Living things respire because they require energy to sustain life. Energy is required for processes like growth, cell division (reproduction), movement, and excretion.

Living organisms first break down food into simpler food molecules through digestion, then use these molecules to release energy through respiration.


3.2 Aerobic Respiration

Aerobic Respiration takes place as the following reaction:

C6H12O6+6 O26 CO2+6 H2O + Energy
			     (glucose)

It is the breakdown of food substances in the presence of oxygen. It results in the release of a large amount of energy in the form of ATP and it gives out carbon dioxide and water as waste products. Energy is also released as heat.

It occurs in the mitochondria of all cells.

FYI: Adenosine Triphosphate (ATP)

This is the structure of Adenosine Triphosphate.

Figure 3.1 - Adenosine Triphosphate

The final phosphate is said to have an Energy-Rich bond and when broken, releases a lot of energy. The reaction to break this bond is:

ATP+H2OADP+Pi+Energy

Where ADP is Adenosine Diphosphate:

Figure 3.2 - Adenosine Diphosphate

…and Pi is an inorganic phosphate (usually in the form of H2PO4- or HPO42-)[confirm]




3.3 Anaerobic Respiration
Anaerobic Respiration takes place in the following reaction in humans:

C6H12O62 C3H6O3+ Energy
				     (glucose)         (lactic acid)

Anaerobic Respiration is the breakdown of food substances in the absence of oxygen. It releases significantly less energy than aerobic respiration. (FYI: around 19 times less!)

Less energy is released as heat in anaerobic respiration than in aerobic respiration.


3.3.1 Anaerobic Respiration in Yeasts

In yeast, anaerobic respiration takes place in a slightly different reaction:

C6H12O62 CO2+2 C2H5OH+ Energy
			           (glucose)		         (ethanol)

This reaction cannot happen in humans as ethanol is toxic to humans, but not to yeasts.

3.4 Photosynthesis vs Respiration
Photosynthesis is like the “reverse reaction” of Respiration.

Respiration
Photosynthesis
Energy is used
Energy is released
Oxygen is used
Oxygen is released
Carbon Dioxide is released
Carbon Dioxide is used
Water is released
Water is used
Catabolic process 
(“breaking down”)
Anabolic process
(“building up”)
Occurs all the time, independent of chlorophyll and light
Occurs only in cells containing chlorophyll and under presence of light
Results in loss of dry mass
Results in gain of dry mass


Table 3.3 - Respiration vs Photosynthesis

Table 3.3 above illustrates why photosynthesis and respiration are “opposites”.


3.5 Respiration During Exercise
During exercise, the muscles contract vigorously to enable movement. Both the rate of respiration and heart rate increase to enable more oxygen to reach the muscles.

If there is not enough oxygen to produce the energy required, an oxygen debt results as anaerobic respiration takes place to provide the energy required. This can cause lactic acid to accumulate in the muscle cells, which can lead to fatigue and muscle pain.

In the liver, some lactic acid is oxidised by enzymes present to release some energy, converting the rest of the lactic acid back into glucose.

When all the lactic acid has been converted back to glucose, the oxygen debt is said to be repaid as glucose is transported back to the muscles.

3.6 The Human Respiratory System

Figure 3.4: The Human Respiratory System


3.6.1 Nasal Cavity

The fringe of hair and mucous layer on the walls of the nasal cavity trap dust and foreign particles. As the air passes through the air passages, it is warmed and moistened.

3.6.2 Trachea

The trachea, also known as the windpipe, is supported by C-shaped rings of cartilage that ensures that the trachea is always open.

3.6.3 Walls of Trachea and Bronchi

The walls of trachea have gland cells, which secrete mucus that trap dust, and ciliated cells, which have cilia that sweep the trapped dust particles and bacteria up the bronchi and trachea back into the pharynx.

3.6.4 Lungs

In the lungs, the bronchial tubes divide repeatedly to form bronchioles, which end in an alveoli.

3.6.5 Alveoli

Alveoli are the site of gas exchange. The numerous alveoli in the lungs increase Surface Area to Volume Ratio for higher rate of gaseous exchange. 

The alveoli are also well-supplied with blood capillaries, which contain a constant flow of blood to enable the efficient exchange of gases.

The alveolar surface is coated with a thin film of water, and the wall of an alveolus is one-cell thick.

3.6.6 Ribs

Ribs support the chest wall, and have two main sets of muscles - internal intercostal muscles, and external intercostal muscles, as seen in Figure 3.5 below.


Figure 3.5: Intercostal Muscles in the Ribs

FYI: Humans have 12 pairs of ribs! Yay!

3.6.7 Diaphragm 

The diaphragm is a dome-shaped muscle and elastic tissue that can contract and relax to change the volume of the thoracic cavity (chest cavity).

Figure 3.6: Contraction and Relaxation of Diaphragm


3.7 Breathing in Humans
Breathing consists of two parts - Inspiration (Breathing in) and Expiration (Breathing out). 

Tip: Remember that the diaphragm and the external intercostal muscles perform the same action (contract or relax) at the same time, while the internal intercostal muscles perform the opposite.

3.7.1 Inspiration

During inspiration, the diaphragm contracts and flattens. Around the same time, the external intercostal muscles contract and the internal intercostal muscles relax, moving the rib cage upwards and outwards.

The thoracic volume increases, decreasing pressure in the lungs to below that of the surrounding air, so air is drawn into the lungs.


3.7.2 Expiration

During expiration, the diaphragm relaxes and arches up. Around the same time, the external intercostal muscles relax and the internal intercostal muscles contract, moving the rib cage downwards and inwards.

The thoracic volume decreases, compressing the lungs and increasing pressure in the lungs, forcing the air out of the lungs.


3.8 Chemicals Present in Tobacco Smoke
We all know that smoking can have many adverse effects on your health. This is because tobacco smoke contains chemicals such as Nicotine, Carbon Monoxide and Tar, as well as certain other Irritants.


3.8.1 Nicotine

Nicotine is highly addictive and makes blood clot more easily, increasing risk of heart disease.


Figure 3.7: Nicotine

3.8.2 Carbon Monoxide

Carbon Monoxide decreases the ability of red blood cells to transport oxygen, as it binds strongly with haemoglobin to form carboxyhaemoglobin. This bond is typically too strong to be broken so the haemoglobin is just stuck in this form and can no longer bind with oxygen.

It also increases the rate at which fats are deposited on the inner arterial walls and increases the risk of atherosclerosis (Refer to Section 2.14).

3.8.3 Tar

Tar is a carcinogenic (cancer-causing) chemical that paralyses the cilia lining in air passages and reduces the efficiency of gaseous exchange.

3.8.4 Other Irritants

These irritants paralyse the cilia lining in air passages and increase the risk of Chronic Bronchitis and Emphysema (Refer to Section 3.9 below).


3.9 Diseases Caused by Tobacco Smoke
We focus on three conditions caused by tobacco smoke - Chronic Bronchitis, Emphysema and Lung Cancer.


3.9.1 Chronic Bronchitis

Chronic Bronchitis refers to infection of the bronchi. 

In patients with Chronic Bronchitis, the epithelium lining the airways is inflamed. The cilia lining the airways are paralysed and breathing difficulties are often developed.

There is also excessive mucus secretion, leading to persistent coughing as a natural response to try to clear the mucus. 

3.9.2 Emphysema

Emphysema refers to damage to the alveoli of the lungs, in which the alveoli could be collapsed, destroyed, narrowed, overinflated or stretched.

In patients with Emphysema, the partition walls of alveoli eventually break down due to violent coughing, resulting in the reduction of surface area of alveoli available for gaseous exchange. 

The lungs also lose their elasticity as the elastic fibres of the lungs are damaged, and become constantly inflated with air.

Symptoms of emphysema include severe coughs, breathing difficulties and wheezing.

3.9.3 Lung Cancer

In heavy smokers, lung cancer can develop due to uncontrolled division of cells producing outgrowths or lumps of tissues.
Topic 4: Heredity
Actually fun biology?!!

4.1 Basic Definitions
Chromosomes are formed from chromatin, which is found in the nucleus of a cell.
Chromosomes come in homologous pairs, where one is from the father, and one is from the mother.
Humans have 23 pairs of chromosomes.
Each chromosome can split into two identical sister chromatids during cell division.

Figure 4.1 - Chromosome

A gene is a section of DNA at a specific location on a chromosome which contains certain information that determines a trait. It can be duplicated and controls the production of one protein. It is inherited from parent to child.

A Mendelian trait is a trait determined by one gene.

The ploidy level of cells describes the number of chromosomes present. 
In diploid cells, we say its ploidy level is 2n, and in haploid cells, we say its ploidy level is n.

A somatic cell is any non-sex cell. In humans, somatic cells are always diploid.

A gamete is a sex cell. In humans, gametes are always haploid.

4.2 Sexual vs Asexual Reproduction
Sexual reproduction is the process involving the fusion of nuclei to form a zygote and the production of genetically dissimilar offspring from two parents.

Asexual reproduction is the process resulting in the production of genetically identical offspring from one parent.

In general, sexual reproduction takes more time than asexual reproduction.

4.3 Mitosis
Cells can undergo Mitosis to produce two identical daughter cells. Each daughter cell has the same number of chromosomes (same ploidy level) as the parent cell.

FYI: The Mitotic Phase consists of 5 stages - Prophase, Metaphase, Anaphase, Telophase (which are referred to collectively as Mitosis) and Cytokinesis.

Figure 4.2 - Mitosis

Mitosis is considered as Asexual Reproduction.

Mostly, only diploid cells reproduce by Mitosis.

4.3.1 Functions of Mitosis

Mitosis allows for growth, increasing the number of cells within the organism.
Mitosis also allows for renewal and repair, where the cells and tissues lost in the processes of wear and tear are regenerated.

4.4 Meiosis
Cells can also undergo cell division through Meiosis. The cells that reproduce through meiosis are typically sex cells, and it causes the haploid gametes produced to have half the number of chromosomes from its parent cells.

In humans, a sex cell from the father will undergo meiosis to form 4 sperm cells. 

On the other hand, a sex cell from the mother will undergo meiosis to form 4 cells, one of which becomes an egg cell and the other three are non-functional cells termed polar bodies.

4.4.1 Why half the chromosome number?

The cells generated must be haploid in order to ensure that the number of chromosomes over successive generations through sexual reproduction remains constant.

4.4.2 Genetic Variation

Since meiosis occurs with sexual reproduction, there is crossing over of genes between the paternal cell and the maternal cell, which causes genetic variation in the daughter cells.

4.5 Homozygous, Heterozygous, whaaaaat???
Since each chromosome comes in a homologous pair, where one chromosome comes from the Mother, and one comes from the Father, for every gene on one chromosome, there is essentially another copy of this gene on the other chromosome in the pair. 

This means, there are two alleles (that can be different!) for every gene that controls a trait!

Normally, a gene has two alleles, or “versions” - the dominant one, and the recessive one. We denote by “T” the dominant allele and by “t” the recessive allele.

The word homozygous refers to having two copies of the same allele (TT or tt), while heterozygous means having one of each (Tt, order doesn’t matter).



Figure 4.3 - The Three Configurations of Alleles

As seen in Figure 4.3 above, if we have a dominant allele present, the dominant allele always dominates and will take priority over the recessive allele. Hence in Homozygous Dominant and Heterozygous cases, we say that the dominant allele is expressed.

Similarly in Homozygous Recessive environments, we say the recessive allele is expressed.

4.6 Punnett Squares
Punnet Squares and Genetic Diagrams are tools to determine and explain genotypes (the genetic makeup of an organism) and phenotypes (the observable traits of an organism) arising from genetic crosses.

When the dominant and recessive alleles are expressed, we can expect slightly different phenotypes. 

For example, for the allele controlling height of a pea plant, where the dominant allele is the “Tall” allele, if the dominant allele is expressed, we expect the plant to be tall. On the other hand if the recessive allele is expressed, we expect it to be dwarf. See the example below.

Worked Example

Consider a pure-bred tall and pure-bred dwarf plant. Given that the allele for dwarfism is recessive, explain why all the offspring produced are tall, by means of a Punnett Square.

Solution

Let the alleles representing dwarf and tall be t and T respectively. Then we construct a Punnett Square:

Figure 4.4 - Punnett Square

Notice that each of the Tt’s are constructed by taking one allele from each parent. That is, we take the topmost row and leftmost column of each cell.


Figure 4.5 - Construction of Punnett Square

From this, we see that the genotypes of all the offspring are Tt, which means that the phenotypes of the offspring will all be tall.




4.7 Genetic Diagrams
A Genetic Diagram is like a more comprehensive version of a Punnett Square, that shows the intermediary steps of how genes work.

Using the same example from before, we propose a Genetic Diagram.

Worked Example

Consider a pure-bred tall and pure-bred dwarf plant. Given that the allele for dwarfism is recessive, explain why all the offspring produced are tall, by means of a Genetic Diagram.

Solution

Pure-bred simply means homozygous. Let the alleles representing dwarf and tall be t and T respectively. Then we construct a Genetic Diagram:



Figure 4.6 - Genetic Diagram




F1 Generation (First Filial Generation) means the plants produced by cross-breeding of the parents. Do remember that the fertilisation arrows must point from different parents, and should not repeat.

Of course, the colours are not needed, but do take note the shapes (rectangle vs circle). Circles are only used for gametes.

Worked Example

An allele controls whether a person has black or red hair. The allele for black hair is dominant. It is given Male A is heterozygous, and Female B has red hair. What is the probability a child from A and B has red hair?

Solution

If B has red hair, it means the recessive allele has been expressed, so she must be homozygous recessive. Now we can construct a Genetic Diagram.



Figure 4.7 - Genetic Diagram

The probability that a child of A and B has red hair is then 2(2+2)100%=50%.



4.8 Co-dominance and Incomplete Dominance
Sometimes, the allele isn’t just split into dominant and recessive. There are actually 3 cases. The dominant-recessive case in the previous examples are known as Complete Dominance.

Co-dominance refers to how two alleles can be expressed together and are equally expressed in heterozygous environments, while Incomplete Dominance refers to how two alleles’ effects can be “mixed” together in heterozygous environments. 

For example, consider two flowers and denote by CR the allele for red flowers, and CW the allele for white flowers. The three cases of dominance can be shown below:


Figure 4.8 - Complete, Incomplete and Co-Dominance

4.9 Multiple Alleles
We have only explored genes with exactly 2 alleles. What if there are more? A gene is said to have multiple alleles if it has more than 2 alleles (“versions”).

4.9.1 ABO Blood Group

If you need a recap on Blood Groups, you may refer to Section 2.11 - Blood Groups.

The ABO Blood Group System is one we need to be familiar with for BL2131. We denote by IA the presence of antigen A, by IB the presence of antigen B and IO the presence of neither.

IA and IB are co-dominant to each other, while IO is recessive to both IA and IB.

Blood Group
Possible Alleles
A
IAIO (heterozygous) or IAIA (homozygous dominant)
B
IBIO (heterozygous) or IBIB (homozygous dominant)
AB
IAIB (co-dominant)
O
IOIO (homozygous recessive)


Table 4.9 - Blood Groups and their Alleles

From this, we can construct genetic diagrams to find the possible blood types of children, if we know the parents’ alleles. For example, 2 parents with Blood Type O will only produce children of Blood Type O.

Topic 5: Reproduction in Plants
Boo. No, I’m not trying to scare you. Just booing this chapter. It’s halloween now :3

5.1 Parts of the Flower
Here are some parts of a typical flower that we need to know.


Figure 5.1: Longitudinal Section of Flower

5.1.1 Collective Nouns

Here are the collective words of the flower parts:

Singular
Collective
Petal
Corolla
Sepal
Calyx
Stamen
Androecium
Carpel
Gynoecium

		               Table 5.2

Also, a cluster of flowers is also referred to as an inflorescence.

5.2 Sepal
The sepal (collective: calyx) is a modified leaf that protects the flower in the bud stage. Some flowers have another layer outside the calyx called the epicalyx.

5.3 Petal
The petal (collective: corolla) is a modified leaf, and in insect-pollinated flowers, it is brightly coloured to attract insects for pollination, and provides a landing platform for insects. 

Sometimes, the petals have nectar guides to guide the insect towards the nectar to increase the chance of pollination.

5.4 Stamen
The stamen is the male reproductive parts of the flower. (A way to remember this is “stamen”)
The stamen is made up by an anther and a filament, and all the stamens together make up the androecium (plural: androecia) of the flower.

The filament keeps the anther upright in a position suitable to disperse pollen grains, and the anther produces pollen grains.

The longitudinal section of the anther reveals two anther lobes, with two pollen sacs each.

In the centre, there is a vascular bundle comprising a xylem and phloem.


Figure 5.3 - Stamen and its Longitudinal Section

5.4.1 Pollen Grain

Pollen grains contain a haploid set of chromosomes. 
Each pollen grain has two nuclei - the generative nucleus and the tube nucleus (also known as the vegetative nucleus).

As seen in Figure 5.4 below, insect-pollinated flowers have pollen grains with rough surfaces to stick to the insect’s body. They are also larger and less numerous. On the other hand, wind-pollinated flowers have pollen grains that are smaller, smoother, and more numerous.


Figure 5.4 - Pollen Grains under Microscope

5.5 Carpel
The carpel is the female reproductive parts of the flower.
The carpel is made up of at least one stigma, a style and an ovary, and all the carpels (usually just one) make up the pistil or gynoecium.


The stigma is where the pollen grains land. When the stigma matures, it secretes a sugary fluid which stimulates the growth of any pollen grains that land on it.

The style connects the stigma to the ovary, holding the stigma upright in a position to trap pollen grains.

The ovary contains one or more ovules. Each ovule contains an ovum and the definitive nucleus, and is connected to the placenta by a stalk called the funicle. The ovum has a haploid set of chromosomes.

Figure 5.5 - Carpel of Flower


5.6 Pollination
Pollination is the transfer of pollen grains from an anther to a stigma. If the anther and stigma are from flowers of the same plant, it is self-pollination. Otherwise, it is cross-pollination.

In self-pollinated plants, the flowers are bisexual (with both male and female reproductive parts in the same flower) with anther and stigmas maturing at the same time. The stigmas are typically situated directly below the anthers.

On the other hand, in cross-pollinated plants, the flowers can be dioecious (with two types of flowers, either male or female reproductive parts) or bisexual/monoecious. The anther and stigmas mature at different times, and the stigma is situated some distance away from the anthers.

We focus on insect-pollinated and wind-pollinated plants.

In insect-pollinated plants, typically, the insect brushes its back against the anther of one flower while trying to collect nectar, causing pollen grains to be stuck onto it. When the insect visits the next flower, it brushes its back against the flower’s stigma, completing pollination.

In wind-pollinated plants, the reproductive parts are often long and pendulous, sticking out of the flower. This allows the pollen to be carried away by the wind more easily, and allows the large, feathery stigma to capture the pollen more easily.


5.6.1 Advantages and Disadvantages of Self vs Cross Pollination

Here are some advantages and disadvantages of self-pollination and cross-pollination. Advantages are marked in Green while Disadvantages are marked in Red. (sorry Ethan Tan)

Point
Self-Pollination
Cross-Pollination
Beneficial Qualities
All Beneficial Qualities are inherited
Can be inherited from both parents
Number of Parent
Only one
Multiple
Reliance on External Factors
Not reliant on external factors
Requires Wind/Insects
Likeliness
More likely to take place
Less likely to take place
Speed
Faster
Slower
Wastage of Pollen and Energy
Less wastage
More wastage
Genetic Variation
Less Genetic Variation
More Genetic Variation
Harmful Recessive Alleles
More likely to be expressed in offspring
Less likely to be expressed in offspring
Disease
More susceptible to disease
Less susceptible to disease
Seeds
Seeds survive for a shorter period before germination
Seeds can survive longer before germination


Table 5.6 - Advantages and Disadvantages of Self and Cross Pollination



5.7 Clitoria Ternatea (Butterfly Pea)
Figure 5.7 below is the petal of the Butterfly Pea flower (Clitoria Ternatea), which is a local insect-pollinated flower.

Figure 5.7 - Clitoria Ternatea Flowers



Figure 5.8 - Clitoria Petals

When the insect lands on the standard petal, it follows the nectar guides and forces the wing petals open. 

The insect’s back then forces the keel petals upwards, revealing the reproductive parts of the flower inside. This allows pollen grains to stick to the insect’s back, allowing for pollination.


5.8 Ischaemum Muticum (Murainagrass)
Ischaemum Muticum is another local species of plant we should be familiar with. It is a wind-pollinated plant, as can be seen from the pendulous anthers and the feathery stigma in Figure 5.9 below.



Figure 5.9 - Ischaemum Muticum

5.9 Fertilisation
Recall that fertilisation is the fusion of a male gamete with a female gamete to form a zygote.

5.9.1 Inside the Ovule

Figure 5.10 - Ovule
Figure 5.10 above shows the cells present inside the ovule. All these cells are formed through meiosis and are all haploid.

5.9.2 The Pollen Tube

When a pollen grain lands on a mature stigma, the sugary fluid stimulates the pollen grain to germinate, as a pollen tube grows out of it. The pollen tube secretes enzymes that digest the tissues of the stigma and style, allowing the pollen tube to grow towards the ovary, or specifically the micropyle, which is the opening of the ovule.

At this point, the tube nucleus is the one “leading” the pollen tube, as the generative nucleus follows behind.


Figure 5.11 - Growth of Pollen Tube


Eventually, the pollen tube reaches the ovary and enters an ovule through the micropyle, as shown in Figure 5.12 below. At this point, the tube nucleus has finished its job and disintegrates.

Figure 5.12: Pollen Tube entering Micropyle

5.9.3 Double Fertilisation

The generative nucleus undergoes mitosis to form two haploid sperm cells.

One of the sperm cells fuses with the ovum to form a diploid zygote. Fertilisation has occurred.

The other sperm cell fuses with the two haploid polar nuclei cells, and forms a triploid (3n) endosperm nucleus, which supplies nutrients. This is Double Fertilisation.

In summary, we say that most flowering plants (also known as angiosperms) undergo Double Fertilisation. Double Fertilisation saves energy such that plants do not waste energy in forming endosperms for ovules that would not get fertilised.

Figure 5.13: Double Fertilisation in Angiosperms


5.9.4 Post-Fertilisation Changes

Table 5.14 below summarises the post-fertilisation changes.

Before
After
Ovule
Seed
Zygote
Embryo
Endosperm Nucleus
Endosperm
Ovule Stalk
Seed Stalk
Ovary
Fruit
Ovary Wall
Fruit Wall (Pericarp)
Stigma, Style and Sepal
Wither or modified to help in fruit dispersal
Stamens, Petals
Wither and fall off


		               Table 5.14: Post-Fertilisation Changes

Topic 6: Reproduction in Humans
Teaching repro to 14-year-olds… hmm…

6.0 Notation
In the rest of these notes, let GnRH, FSH and LH denote Gonadotropin-Releasing Hormone, Follicle Stimulating Hormone and Luteinising Hormone respectively.


Symbol
Meaning

Blue Box


Organ

Orange Box




Hormone

Red Arrow


A Secretes B

Orange Arrow


A Stimulates B

Green Arrow


A Develops Into B

Circle


Cell


Table 6.0: Notation for Chapter 6

6.1 Puberty
Puberty refers to the period of rapid development during which a person gains the ability to reproduce. During puberty, the sex organs mature, and sex hormones are released.


6.1.1 Puberty in Boys vs Girls

Puberty is a time of physical, emotional and social changes for teenagers. Specifically, Table 6.1 below highlights some of the changes that occur.

Question
Boys
Girls
When does Puberty occur?
Around 14 Years Old
Around 10-12 Years Old
What visible changes occur?
Larynx (Voice Box) Enlarges

Voice Deepens

Penis and Testes enlarge

Growth of Beard, Facial Hair, Pubic Hair, Armpit Hair, etc.
Menstruation Begins

Growth of Pubic Hair

Breasts and Uterus Enlarge

Hips Broaden
What other changes occur?
Sex organs mature and start producing sperms.
Sex organs mature and ova (eggs) are produced, beginning of ovarian cycle
What causes these changes?
Increased levels of testosterone produced in the testes.
Increased levels of oestrogen and progesterone produced in the ovaries.


Table 6.1: Effects of Puberty

6.1.2 What Triggers Puberty?

The term “Gonads” refers to the Testes in Males, and Ovaries in Females.



Figure 6.2: How is Puberty Triggered?


Common Misconceptions: 
✖  FSH, LH, Progesterone and Oestrogen are only found in Females
✖  Testosterone is only found in Males
✔  All 5 hormones are present (in varying levels) in both Males and Females.

6.1.3 Gametogenesis

Puberty also causes the gonads to start producing gametes through a process called Gametogenesis. It starts with the division of a Germ Cell (parent cell) into two cells by Mitosis.

These cells then further divide through Meiosis.

Spermatogenesis (Production of Sperm) occurs in males, while Oogenesis (Production of Ova/Eggs) occurs in females. Refer to Figure 6.3 below.


Figure 6.3: Gametogenesis

6.2 The Male Reproductive System
Figure 6.4 below shows the Male Reproductive System.

Figure 6.4: Male Reproductive System

6.2.1 Testis

The testis (plural: testes) produces sperms and sex hormones like testosterone.

6.2.2 Scrotum

The scrotum is a sac of skin located outside the abdominal cavity that holds and supports the testes. It helps to maintain a lower temperature for the development of sperms.

The optimal temperature for sperm development is 33 to 34 °C.

6.2.3 Epididymis

The epididymis is a coiled duct next to the testis. It stores inactive sperms temporarily, before they are transported into the vas deferens.

6.2.4 Vas Deferens

The vas deferens (ductus deferens, sperm duct) conducts sperms from the testis to the penis.

6.2.5 Seminal Vesicle, Cowper’s Gland and Prostate Gland

These three organs release fluids that contain nutrients and enzymes needed to activate sperm cells, creating a fluid called semen.

The Seminal Vesicle also temporarily stores sperms before they are released into the urethra.

6.2.6 Urethra

Urethra conducts semen from the vas deferens or urine from the urinary bladder to the penis.

Note: The Urethra is unable to do these two things at the same time, just one at a time.

6.2.7 Penis

The penis is the erectile organ that becomes hard as it is filled with blood during sexual intercourse, such that when it enters the vagina, it is able to deposit sperm-containing semen.

6.2.8 Spermatic Cord

The spermatic cord is a cord-like structure containing blood vessels which supplies blood to the testis. 


Figure 6.5: Spermatic Cord

6.2.9 Vasectomy

Vasectomy is a procedure by which the vas deferens is either tied up (temporary) or cut (permanent). It is a contraceptive method (that is, it prevents pregnancy) that prevents sperms from reaching the urethra, preventing fertilisation.

6.3 The Female Reproductive System
Figure 6.6 below shows the female reproductive system.


Figure 6.6: Female Reproductive System

6.3.1 Ovary

The ovary produces the ovum (egg, plural: ova) and female sex hormones such as oestrogen and progesterone.

6.3.2 Fimbria

The fimbria is the opening of the ovary, and has cilia that sweep the eggs into the oviduct.

6.3.3 Oviduct

The oviduct, also known as the fallopian tube, is a narrow muscular tube that conducts mature eggs from the ovary to the uterus, with the cilia present on its surface.

By right, the oviduct should be the site of fertilisation.

6.3.4 Uterus

The uterus (or womb) is a muscular chamber where the fetus develops during pregnancy. It is often described as having the shape of an inverted pear.

6.3.5 Endometrium

The endometrium (or uterine lining) is the outer muscular layer of the uterus, and is the site of implantation.

6.3.6 Cervix

The cervix is a circular ring of muscles that closes off the lower end of the uterus, which allows menstrual blood to flow out into the vagina during menstruation. See 6.8 Menstrual Cycle.

6.3.7 Vagina

The vagina is the birth canal. It is also the receptacle for semen, where semen is deposited. Furthermore, it is where menstrual blood flows out.

6.3.8 Tubal Ligation

Tubal Ligation is similar to a vasectomy, but it is for females instead of males. 

It is a contraceptive method (that is, it prevents pregnancy) where the oviduct is tied (temporary) or cut (permanent).

6.4 Sperm
Sperms (spermatozoa) are produced by the testes. They contain 3 main parts - the head, middle piece, and tail. Refer to Figure 6.7 below.

Figure 6.7: Sperm

Sperms are produced throughout the entire lifetime of a male after puberty. Sperms are quite small, around 55 to 65 µm (micrometer: 1 µm = 10-6 m) in length, and 3.5 µm in width.

The acrosome contains an enzyme called acrozyme, which is able to digest the jelly coat and cell membrane of a mature egg to allow the sperm to penetrate into the egg, in order for fertilisation to occur.

The nucleus contains one haploid set of chromosomes.

The tail is also called a flagellum, and allows the sperm to move. We say the sperm is motile.

Common Misconceptions:
✖  Motility is the same as Mobility
✔ Motility refers to something being able to move on its own, while mobility refers to something moving either on its own or by external assistance (e.g. by wind).

6.5 Ovum
The ovum (egg, oocyte) is produced by the ovary. Refer to Figure 6.8 below.


Figure 6.8: Ovum

The ova are present in the ovaries of a female at birth. Eggs are released into the oviducts from puberty to when a female reaches 45-55 years of age. At that age, we say that menopause has occurred.

The nucleus of the ovum contains one haploid set of chromosomes.

The ovum is unable to move on its own, and hence we say its non-motile. However, it is able to move as the cilia brushes it along the fallopian tube. Hence, despite being non-motile, it is mobile. (Recall the difference between mobility and motility!)

Factor
Sperm
Ovum
Motility
Motile
Non-motile
Size
Small
Large
Shape
Long, has a head, middle piece and tail
Circular
Number
~300 million are released at once
1 is released at a time


Table 6.9: Differences between Sperm and Ovum

6.6 Fertilisation
Recall that fertilisation is the fusion of a male gamete with a female gamete to form a zygote. In humans, it is the fusion of a sperm with an egg to form a zygote.

sz6.6.1 The Path Taken by the Sperm

Figure 6.10 below shows the path taken by the sperm, from its production in the testes to the site of fertilisation, the oviduct.


<
Figure 6.10: Path Taken by Sperm

6.6.2 Sperm Penetrating the Egg

The sperms first swim towards the egg. When some sperms eventually reach the egg, they scatter the follicle cells, possibly using enzymes they secrete. One sperm secretes acrozyme to digest the zona pellucida, passing through the cell membrane and entering the cytoplasm of the egg.

The sperm and egg nuclei fuse to form a zygote. Fertilisation has occurred. This triggers the egg to form a protective fertilisation envelope around it, preventing any other sperm from entering, since acrozyme is ineffective against this protection envelope.
6.6.3 Post-fertilisation

After fertilisation, the zygote undergoes mitosis, eventually forming a blastocyst, which is a ball of cells. Around day 7, the blastocyst reaches the uterus and implants itself onto the thickened endometrium. 

This blastocyst will eventually develop into an embryo, and into a fetus. 

6.7 Ovarian Cycle
Since birth, a female’s ovaries contain many immature eggs known as follicles. These follicles are diploid, and form a matured, haploid ovum by oogenesis.

A mature egg is released into the oviduct in regular intervals due to the ovarian cycle, which consists of 3 parts - Follicular Phase, Ovulation Phase and Luteal Phase.

6.7.1 Follicular Phase

The hypothalamus secretes GnRH, which stimulates the Pituitary Gland to secrete FSH and LH. In the follicular phase, FSH stimulates a primary follicle to develop into a Graafian follicle, while secreting oestrogen throughout. The oestrogen is used for the repair and growth of the endometrium in the uterus.


Figure 6.11: Follicular Phase

As the Graafian follicle develops, oogenesis occurs as a mature haploid ovum develops. At the same time, high levels of oestrogen causes the pituitary glands to secrete LH.

6.7.2 Ovulation Phase

High levels of LH eventually causes the Graafian follicle to rupture, releasing the haploid ovum into the oviduct through the fimbria. This occurs around Day 14 of the 28-day Ovarian/Menstrual Cycle. We extend Figure 6.11 as such:



Figure 6.12: Ovulation Phase

6.7.3 Luteal Phase

As the ovum is released in the ovulation phase, something else called the corpus luteum forms alongside it. The corpus luteum produces oestrogen and progesterone. 

The oestrogen causes the endometrium to grow thicker, while the progesterone maintains the thickness of the endometrium, so that implantation of embryo can occur if fertilisation is successful.

The high levels of progesterone and oestrogen causes negative feedback, decreasing the amount of FSH and LH secreted. Specifically, high levels of oestrogen inhibit FSH production, and high levels of progesterone inhibit FSH and LH production[confirm].

This negative feedback ensures that only one ovum is released at a time by inhibiting the development of more primary follicles. Refer to Figure 6.13 below.

Figure 6.13: Luteal Phase

Finally, the corpus luteum breaks down if no successful fertilisation occurs, causing a sharp decline in progesterone and oestrogen levels. As progesterone is what maintains the thickness of the endometrium, this causes the endometrium to begin breaking down as well, which leads to menstruation.

However, if fertilisation is successful, the corpus luteum DOES NOT break down. 

Instead, it continues to produce oestrogen and progesterone so that the endometrium grows thicker and maintains the endometrium thickness until the placenta develops, when the placenta takes over the production of these hormones.

Figure 6.14 below is a summary of the ovarian cycle.


Figure 6.14: Summary of Ovarian Cycle


6.8 Menstrual Cycle
Instead of focusing on what happens in the ovary like the ovarian cycle, the menstrual cycle focuses on the changes in the uterus over the same duration.

The menstrual cycle consists of 4 stages over 28 days - Menstrual Phase, Proliferative Phase, Ovulation and Secretory Phase.

6.8.1 Menstrual Phase (Day 1 - Day 5)

In the menstrual phase, menstruation occurs as the shedding of endometrium, degenerated egg and blood from the endometrium takes place. They are removed through the vagina.

Around the same time, the pituitary gland begins to secrete FSH into the bloodstream to restart the ovarian cycle.

6.8.2 Proliferative Phase (Day 6 - Day 13)

During this phase, high levels of FSH causes the primary follicles to secrete oestrogen, and the high levels of oestrogen, allows for repair and growth of endometrium. The high levels of oestrogen also stimulates the pituitary gland to secrete LH.

6.8.3 Ovulation (Day 14)

On Day 14, high levels of LH trigger ovulation and the ovary releases a mature haploid ovum. This ovum remains viable for up to 3 days (but typically 24 hours). For comparison, sperm remains viable for up to 3 days (but typically 48 hours) in the female reproductive tract.

Hence, the optimum time for sexual intercourse such that the chance of fertilisation is maximum is around Day 13 to 16 of the ovarian cycle.

6.8.4 Secretory Phase (Day 15 - Day 28)

During the secretory phase, the endometrium of the uterus is thickened by the high levels of oestrogen and is maintained by the high levels of progesterone, to prepare for implantation if fertilisation occurs.

6.9 Contraceptive Pills
Contraceptive Pills typically contain progesterone and oestrogen. High levels of oestrogen and progesterone cause negative feedback to be sent to the hypothalamus, which suppresses the FSH and LH production from the pituitary glands. 

Hence, no primary follicles are able to mature and develop into Graafian follicles and no ovulation occurs. Thus fertilisation will not occur and no conception takes place.

6.10 Placenta
The placenta is a temporary organ that consists of both maternal tissue and embryonic tissue. It is temporary as it will be shed off after giving birth.

Figure 6.15: Placenta in Uterus
Figure 6.15 above shows the placenta in the uterus, while Figure 6.16 below shows the placenta in more detail.

Figure 6.16: Placenta

Let’s zoom in further. Refer to Figure 6.17 below.


Figure 6.17: Zoomed in Placenta

Notice that in the mother’s blood, we mark the arteries red (oxygen-rich) by convention, and the veins blue (oxygen-poor). On the other hand, in the fetal portion, we mark the arteries in blue and the veins in red. This means, the veins carry oxygen-rich blood instead of the arteries and vice versa.

6.10.1 Functions of Placenta

The placenta secretes oestrogen and progesterone, to suppress the pituitary gland from producing FSH and LH so that no ovulation would occur, and to maintain the thickness of the endometrium.

The placenta is the site of exchange of substances between the mother and the fetus.

The placenta allows antibodies to diffuse from the mother’s blood into the fetus’ blood to protect the fetus against disease.

6.10.2 Diffusion Through the Placenta

The substances transported from Mother to Fetus are hormones, oxygen and nutrients.
The substances transported from Fetus to Mother are hormones, waste products like carbon dioxide and urea.

For example, oxygen from the maternal blood pools diffuses into the chorionic villi and into the fetal venule, which then connects to the umbilical vein, which carries the oxygen towards the fetus. 

The villi in the placenta described in this process are similar to the alveoli found within the lungs or the villi found within the intestines. These structures have an increased surface area to allow for faster diffusion.

6.10.3 Adaptations of Placenta

Table 6.18 below shows the adaptations of the placenta.

Feature
Function
Maternal blood directly bathes the entire surface of the chorionic villi
Constant diffusion is ensured by maintaining a high concentration of oxygen and nutrients.
There is continuous blood flow in fetal capillaries and maternal blood spaces
To maintain a constant diffusion gradient.
Chorionic villi have highly folded structure to increase Surface Area to Volume Ratio
Increased rate of diffusion.
Maternal blood not confined in blood vessels but found in maternal blood spaces
Facilitates efficient diffusion.


Table 6.18: Adaptations of Placenta



6.10.4 Why don’t the Maternal and Fetal Blood Mix?

If the maternal and fetal blood mixes directly, the maternal blood pressure would be too high and could cause damage to the organs and blood vessels of the fetus.

Furthermore, the mother and fetus could have different blood types, so if the maternal and fetal blood mix directly, agglutination could occur.

6.10.5 Umbilical Cord

The umbilical cord usually contains two umbilical arteries and one umbilical vein, and is connected to the fetal capillaries in the placenta.

Exchange of substances between the Mother and Fetus also must pass through the umbilical cord.


Application Question

(Biology Matters Chapter 18, Let’s Review, Q4(c) Modified)

Figure 6.19 below is the cross section of an umbilical cord. Name the structures P and Q and justify your answer with explanations.


Figure 6.19: Cross Section of Umbilical Cord




Solution:[verify]

P and Q are the umbilical arteries and the umbilical vein respectively.

The umbilical arteries have a smaller lumen relative to their diameters, carrying blood which flows at a higher blood pressure from the baby to the mother as compared to the umbilical vein, which have a larger lumen relative to its diameter, carrying blood which flows at a lower blood pressure from the mother to the baby.

Hence, the umbilical arteries have thicker walls than the umbilical veins to prevent them from rupturing when an influx of high pressure blood occurs.




6.10.6 Amniotic Sac and Fluid

The amniotic sac is a bag of fluid, called the amniotic fluid, in a female’s uterus where the unborn baby develops and grows. This fluid is sterile and must not be exposed to the outside world.

Most of the amniotic fluid comes from the mother’s blood by diffusion. 

After the first 3 months, the fetus also begins to excrete urine into the sac to fill the amniotic cavity. The fetus then swallows the fluid and the cycle repeats.

The amnion is the outer membrane of the amniotic sac.

6.10.7 Functions of Amniotic Fluid

The amniotic fluid has many functions, namely:
Supports and cushions the fetus
Protects the fetus from physical injury by absorbing shock
Allows the fetus to move freely during growth
Assists in regulating fetal body temperature
Lubricates the vagina during birth

6.11 Human Chorionic Gonadotropin (FYI)
hCG, short for Human Chorionic Gonadotropin, is a hormone that is secreted by cells in the placenta when successful fertilisation occurs, and hCG levels remain high during most of the pregnancy.

hCG maintains the corpus luteum and prevents it from breaking down, which keeps progesterone and oestrogen levels consistently high. 

High levels of progesterone ensures that the thickness of endometrium is maintained and ensures that menstruation does not occur, and at the same time inhibiting FSH and LH production.

High levels of oestrogen also inhibits FSH production.

The inhibition of FSH and LH prevents more primary follicles to develop into Graafian follicles and prevents further ovulation.

Figure 6.19 below highlights how well the female reproductive system is designed, by showing how all of the hormones interact with one another in many ways.


Figure 6.20: Interaction of Hormones in the Female Reproductive System

Topic 7: Homeostasis and Thermoregulation
is that a PC2131 reference (ew phys)

7.1 Maintaining Thermal Balance
There are 4 main processes that are involved in heat loss or heat gain: Evaporation, Radiation, Conduction, Convection.

Thermal Balance is important as exercise requires energy, releasing heat as a byproduct in the body. If thermal balance isn’t maintained, this could lead to heat injury like heatstroke.

Furthermore, because Singapore is humid, the rate of evaporation is lowered, which leads to less heat loss. Humidity is a very crucial factor that must be considered carefully in answering questions about Thermal Balance.

7.1.1 Evaporation

Heat is lost as latent heat of vaporisation. More accurately, heat is conducted from our skin and used to evaporate water in sweat (or other liquids) present on the surface of our skin.

7.1.2 Radiation

Objects exchange radiation with each other, and hotter objects tend to lose heat to cooler objects.

7.1.3 Conduction

Conduction is the direct transfer of heat when objects of different temperatures come into contact. Again, hotter objects tend to lose heat to cooler objects.

7.1.4 Convection

Convection can be described in terms of convection currents, but more simply it happens when a stream of air or fluid is cooler than body surface temperature.

7.2 Heat in the Body
When respiration occurs, heat is produced as a byproduct.

This heat must then be distributed from the active muscles to everywhere around the body, to ensure that not too much heat is collected at one location. But how?

The active muscles transfer heat to the blood in surrounding blood vessels by conduction, causing the muscles to lose heat, and the blood to gain heat. As the blood then flows into the blood vessels that surround the other body organs, it loses heat by conduction again to the body organs, evenly distributing the heat.


7.2.1 Core Temperature and Skin Temperature

Refer to Figure 7.1 below.

Figure 7.1: Core vs Skin Temperature

As we can see, the core temperature (that is, the temperature around your face and chest region) should always be maintained at around 37 °C. 

One of the reasons for this is that many processes such as digestion require enzymes, and many enzymes have an optimal temperature of around 37 °C. 

If the temperature gets too high, the enzymes are denatured and no longer work, and if the temperature gets too low, the enzymes are not as effective.

We can also see that the core temperature is usually higher than the skin temperature. This is why oral thermometers (which measure core temperature) usually have a higher reading than thermometers which measure skin temperature.

7.2.2 Hair in Mammals

Many mammals (including us!) have layers of fur or hair, or blubber with adipose tissue (body fat). These adaptations trap air, which is a poor conductor of heat. This reduces heat loss by conduction and convection.

7.3 Homeostasis
Homeostasis refers to maintaining a relatively stable internal environment regardless of changes in the external environment, which requires energy, obtained from respiration.

At the organism level, it gives the organism independence from reliance on the external environment, and enables it to travel between highly variable environments.

At the cellular level, it helps the cell maintain optimal pH, temperature and water potential for enzymatic reactions like digestion and cellular respiration, so it is more efficient.

7.4 What is Thermoregulation?
Thermoregulation is a process where animals maintain an internal temperature within a tolerable range as the external environment changes.

Thermoregulation is achieved by striking a balance between the heat gain and heat loss mechanisms in the body.

The hypothalamus is the control centre of thermoregulation.


7.4.1 How does the Hypothalamus Work?

The hypothalamus receives signals from receptors, such as thermoreceptors or pressure receptors present in our skin. It compares these signals with the set-point (“optimum value”).

It then decides on the appropriate response, sending signals to effectors (the organs that carry out the instructions to restore balance). The effectors then produce a response to change the controlled condition. This is called a self-regulatory corrective mechanism.


7.4.2 Negative and Positive Feedback

Negative Feedback refers to when the response reduces the stimulus. (e.g. when it is too hot, the body performs actions to reduce body temperature) 

Positive Feedback refers to when the response amplifies the stimulus. (e.g. apples ripen, producing ethylene, stimulating more apples to ripen, causing more ethylene, and so on…)

Figure 7.2 below summarises this idea.

Figure 7.2: Homeostasis

7.4.3 Examples of Positive Feedback

As mentioned earlier, ripening of apples is an example of positive feedback, since the ethylene gas produced by ripe apples stimulates more apples to ripen, which, in turn, produces more ethylene, as shown in Figure 7.3 below.



Figure 7.3: Apples Ripening


Another example is childbirth, where a positive feedback loop occurs as the baby’s head pushes against cervix, which causes signals to be transmitted to the brain, causing the brain to stimulate the pituitary gland to secrete oxytocin, which leads to uterine contractions, and pushes the baby’s head against the cervix more.

The feedback loop is shown in Figure 7.4 below.



Figure 7.4: Childbirth

The last example provided in the notes is milk production, shown in Figure 7.5 below.



Figure 7.5: Milk Production

7.5 Skin Structure
Figure 7.6 below shows a human’s skin structure. Human skin consists of 3 main sections: the epidermis, the dermis, and the subcutaneous layer.

Figure 7.6: Skin Structure

7.5.1 Cornified Layer

The cornified layer is the outermost layer of the epidermis, consisting of dead dry cells with keratin deposits. Dead skins are constantly shed from the surface.

FYI: Keratin helps to form hair, nails, and epidermis! How useful! :D

7.5.2 Granular Layer

The granular layer is the middle layer of the epidermis, consisting of living cells that move up constantly when they become dry and dead to replace the cells that are shedded from the cornified layer.

7.5.3 Malpighian Layer

The malpighian layer is the innermost layer of the epidermis, made of actively dividing cells that contain melanin, the pigment that contributes to the colour of human skin, hair and eyes.

Dark-skinned people have more melanin in their skin than light-skinned people.

Melanin is produced by cells called melanocytes.


7.5.4 Blood Vessels in the Dermis

The dermis is found below the malpighian layer, and has numerous blood capillaries supplying blood to the skin.

Figure 7.7: Vasoconstriction and Vasodilation

The arterioles in the dermis can constrict in a process called Vasoconstriction. When the arterioles constrict, the shunt vessels dilate, and the blood flow through the capillaries are reduced, while the blood flow through the shunt vessel is increased.

The shunt vessel is a way for the blood to bypass capillaries in certain areas.

On the other hand, the arterios can also dilate, called Vasodilation. When the arterioles dilate, the shunt vessels constrict, and the blood flow through the capillaries are increased, while the blood flow through the shunt vessel is decreased.

Since the capillaries are closer to the surface of the skin, if more blood flows through the capillaries, more heat will be lost by conduction from the blood to the surroundings. On the other hand, when less blood flows through the capillaries, less heat is lost by conduction.

This is one of the thermoregulatory mechanisms of the human body.

7.5.5 Hairs and Hair Erector Muscles

Hairs are embedded within the dermis. The malpighian layer sinks into parts of the dermis to form Hair Follicles.

Hair Papilla are found at the base of hair follicles and consist of blood capillaries and nerves.

Hair Erector Muscles, also known as arrector pili, contract and relax, causing hairs to stand up and lie flat respectively. When the hair erector muscles contract, goosebumps also form.

7.5.6 Sebaceous Gland

Sebaceous Glands are found in the dermis and open into each hair follicle. These glands secrete sebum, which is an oily substance that lubricates the hair, keeps the skin soft and prevents microbial growth.

7.5.7 Sweat Gland

Sweat Glands are richly surrounded by blood capillaries, secreting sweat which flows through the sweat duct and out the sweat pore to the skin surface. Sweat is another thermoregulatory mechanism of the human body.

7.5.8 Subcutaneous Layer

Subcutaneous fat is found below the dermis. The subcutaneous layer consists of adipose, which are cells which store fats.

7.5.9 Metabolic Wastes

We say a substance is excreted if the substance is a metabolic waste. Hence, we say that the sweat pores excrete sweat, but we don’t say that the sebaceous glands excrete sebum, because sebum isn’t considered as a metabolic waste.

More specifically, metabolic wastes are substances like carbon dioxide and urea, where urea (shown in Figure 7.8 below) is a nitrogenous organic compound that is commonly found in urine and sweat.

Figure 7.8: Urea


7.6 Corrective Mechanisms for Thermoregulation
Table 7.8 below gives some examples of corrective mechanisms for thermoregulation. Notice that the effects on a hot day are typically the opposite to those on a cold day, and all of these effects are considered negative feedback.


Corrective Mechanism
On a hot day
On a cold day
Arterioles and Shunt Vessels
Vasodilation to increase heat lost by conduction.
Vasoconstriction to decrease heat lost by conduction.


Sweat Gland
More active sweat glands to increase heat lost through evaporation by removing latent heat of vaporisation.
Less active sweat glands to decrease heat lost through evaporation by removing latent heat of vaporisation.


Hair Erector Muscles


Hair erector muscles relax to allow hairs to lie flat.
Hair erector muscles contract to make hairs stand up, trapping an insulating layer of warm air over the skin, reducing heat lost by conduction and convection.

Metabolic Rate
Rate of metabolic activities like respiration decreases so that less heat is produced.
Rate of metabolic activities increases so that more heat is produced.


Skeletal Muscles


-
Reflex contraction of skeletal muscles generates heat, increasing body temperature, through shivering.


Table 7.9 - Corrective Mechanisms for Thermoregulation

7.7 Skin Shenanigans: Alcohol and Showers
Here are two commonly asked questions.

Application Question 1: LQ the Alcoholic

(BL2131 Revision Worksheet 8 Homeostasis and Thermoregulation Q8 Modified)

LQ is climbing a mountain in -10 °C weather. He then has the urge to drink alcohol. Explain why drinking alcohol would make LQ feel warm initially. A while later, he died due to hypothermia. Explain how the alcohol contributed to LQ losing more heat.


Application Question 2: LQ the Showering Scientist

(BL2131 2023 EOY Revision Package Homeostasis and Thermoregulation Q30 (c) Modified)

LQ has come back to life! Maybe reincarnation is real. Either way, he becomes a scientist with core body temperature 38.3 °C. One day, he is showering while monitoring his core body temperature. Under 35 °C water, his temperature decreased to 37.3 °C after 10 minutes. The next day, with the same starting temperature and duration, under 15 °C water, his temperature only decreased to 37.6 °C. Account for this difference, with explanation.




Solutions can be found on the next page.

Solution to Question 1

When LQ drinks alcohol, the alcohol enters his bloodstream, which causes his skin arterioles to vasodilate. This diverts blood flow to and increases blood flow in the skin capillaries, which are closer to the thermoreceptors. Hence, more warm blood passes near the thermoreceptors, causing LQ to feel warm initially.

Over time, since the alcohol causes LQ’s skin arterioles to vasodilate, more blood is diverted to and flows in the skin capillaries, which is closer the cooler external environment. Hence more heat is lost to the environment by conduction, (convection, radiation and evaporation) and LQ’s body loses too much heat (which causes him to die due to hypothermia).


Solution to Question 2

When LQ showers at 15 °C, his thermoreceptors detect his skin temperature decreasing rapidly below the set-point, signalling to the hypothalamus to start thermoregulatory mechanisms to reduce heat loss. On the other hand, when LQ showers at 35 °C, his skin temperature remains above the set-point, so no thermoregulatory mechanisms occur.

In the 15 °C shower, the hypothalamus signals to the skin arterioles to vasoconstrict and the shunt vessels to vasodilate, diverting blood from and decreasing blood flow in the blood capillaries, which decreases heat lost by conduction. On the other hand, in the 35 °C shower, no vasoconstriction of the skin arterioles occur, so more blood flows to the blood capillaries and more heat is lost by conduction.

Furthermore, in the 15 °C shower, the hypothalamus also signals to the muscles to begin shivering, which consists involuntary vigorous contraction and relaxation of muscles, which requires energy from respiration. Hence metabolic rate increases, and heat is produced as a byproduct of respiration. On the other hand, shivering does not occur in the 35 °C shower.

Hence, LQ loses more heat in the 35 °C shower over 10 minutes than in the 15 °C shower.





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https://www.iitianacademy.com/igcse-biology-0610-16-3-sexual-reproduction-in-plants-igcse-style-questions-paper-3/, IITian Academy, Retrieved 1 Oct 2023 (Figure 5.4 - Pollen Grains)

https://www.123rf.com/photo_124796333_structure-of-stamen-and-carpel-flower-part-diagram.html, fancytapis, 123RF, Retrieved 1 Oct 2023 (Figure 5.5 - Carpel of Flower)

https://www.arogyayogaschool.com/blog/amazing-health-benefits-of-aprajita-clitoria-ternatea/, Arogya Yoga School (2022), Retrieved 30 Sep 2023 (Figure 5.7 - Clitoria Ternatea Flowers)

https://slideplayer.com/slide/13805880/, Smith (2018), Slides 46-52, Retrieved 30 Sep 2023 (Figure 5.8 - Clitoria Ternatea Petals)

https://www.natureloveyou.sg/Ischaemum%20muticum/Main.html, Kwan (2023), Retrieved 1 Oct 2023 (Figure 5.9 - Ischaemum Muticum)

https://www.quora.com/How-much-celled-structure-is-a-flower-ovule, Gupta (2018), Retrieved 1 Oct 2023 (Figure 5.10 - Ovule)

https://byjus.com/question-answer/the-growth-of-pollen-tube-through-the-style-of-a-flower-is-an-example-of-4/, byjus.com (2022), Retrieved 1 Oct 2023 (Figure 5.11 and Figure 5.12 - Growth of Pollen Tube)

https://www.vecteezy.com/vector-art/24066084-male-reproductive-system-icon-with-description-isolated-on-white-background-anatomy-of-the-internal-organs-of-man-illustration-vector, Vizir (2023), Retrieved 2 Oct 2023 (Figure 6.4 - Male Reproductive System)

https://www.news-medical.net/health/What-Is-Testicular-Torsion.aspx, Aliouche (2023), Retrieved 2 Oct 2023 (Figure 6.5 - Spermatic Cord)

https://www.istockphoto.com/vector/female-reproductive-system-vector-flat-illustration-gm467459878-60598796, PrettyVectors, istockphoto.com (2015), Retrieved 2 Oct 2023 (Figure 6.6 - Female Reproductive System)

https://www.lifecell.in/blog/health-check/sperm-morphology-basics-and-overview, Arulraj (2023), Retrieved 2 Oct 2023 (Figure 6.7 - Sperm)

https://quizlet.com/382869074/ovum-diagram/, mcenteeke, quizlet.com, Retrieved 2 Oct 2023 (Figure 6.8 - Ovum) 

https://ib.bioninja.com.au/higher-level/topic-11-animal-physiology/114-sexual-reproduction/placenta.html, BioNinja, Retrieved 3 Oct 2023 (Figure 6.16 and Figure 6.17 - Placenta)

Arpi, L.M. (2018). Histology of Umbilical Cord in Mammals. Retrieved 17 Oct 2023, from https://www.semanticscholar.org/paper/Histology-of-Umbilical-Cord-in-Mammals-Arpi/ff601c12b2614204ee8a5db212411c2f58191c17 (Figure 6.19 - Cross Section of Umbilical Cord)

https://www.cosinuss.com/en/measured-data/vital-signs/body-temperature/, Kreuzer (2021), Retrieved 3 Oct 2023 (Figure 7.1 - Core vs Skin Temperature)

https://www.freepik.com/premium-vector/red-apple-vector-healthy-sweet-fruit_34862686.htm, Retrieved 4 Oct 2023 (Figure 7.3 - Apple.)

https://www.toppr.com/ask/question/explain-the-terms-vasodilation-and-vasoconstrictionhow-do-these-processes-contribute-to-the-temperature-regulation-of/, toppr.com (2022), Retrieved 4 Oct 2023 (Figure 7.7 - Vasodilation and Vasoconstriction)

https://oneclass.com/homework-help/chemistry/7028085-nh2-2co-lewis-structure.en.html, oneclass.com (2020), Retrieved 17 Oct 2023 (Figure 7.8 - Urea)


Informative Sources
BL2131 Foundations in Biology II, Topic 1-7 Booklets (2023)

Biology Matters for GCE ‘O’ Level 2nd Edition (2013), Lam, Peng Kwan; Lam, Eric Y. K., Marshall Cavendish Education

Blood Groups and Red Cell Antigens (https://www.ncbi.nlm.nih.gov/books/NBK2263/) (2005), Dean, L., National Center for Biotechnology Information (US)

BL1131 Semester 1 & 2 Notes, Foundations in Biology I Notes (2022)

https://gpnmag.com/article/the-boundary-layer-and-its-importance/, Runkle (2016), Greenhouse Product News, Retrieved 6 Feb 2023

https://inside.upmc.com/collagen-benefits/, Brunner, University of Pittsburgh Medical Center (2019),  Retrieved 15 Feb 2023

https://www.whitman.edu/academics/majors-and-minors/biology/virtual-pig/circulatory-system/chordae-tendinae, Whitman College (2022), Retrieved 18 Feb 2023

https://educationalgames.nobelprize.org/educational/medicine/landsteiner/readmore.html, Landsteiner (2001), Retrieved 7 Mar 2023

https://saylordotorg.github.io/text_the-basics-of-general-organic-and-biological-chemistry/s23-01-atp-the-universal-energy-curre.html, Retrieved 16 Apr 2023

https://www.nhsinform.scot/illnesses-and-conditions/lungs-and-airways/bronchitis, NHS  Inform (2023), Retrieved 22 Apr 2023

https://www.hopkinsmedicine.org/health/conditions-and-diseases/pulmonary-emphysema, Johns Hopkins Medicine (2019), Retrieved 22 Apr 2023

Wildings et al. (2010), Retrieved 2 Oct 2023, from https://www.researchgate.net/publication/47756549_Intracytoplasmic_injection_of_morphologically_selected_spermatozoa_IMSI_improves_outcome_after_assisted_reproduction_by_deselecting_physiologically_poor_quality_spermatozoa


Way too many sources.

Good luck for Bio <3