Transport in Plants Transport in Plants Carbon Fixation by Roots The main source of carbon for plants comes from the atmosphere as carbon dioxide and fixation of this carbon dioxide by leaves and other green parts of the shoot system in the presence of light as an energy source - the process of photosynthesis . Some of this fixed carbon is converted into fuels for respiration, such as glucose, some is stored as compact materials like starch, and some is used in building blocks, such as proteins, nucelotides and phospholipids, to build the plant body. Fixation is an odd word, and one that stems all the way back to alchemy. It makes perfect sense, however, when you think of gases as mobile, with their molecules constantly moving around by diffusion due to their thermal kinetic energy. Fixation means to render less mobile, and was historically depicted by clipping the 'wings' of the gaseous or volatile substance (by converting it into a solid or non-volatile liquid, for example). However, it is a little known fact that roots can also fix carbon. Roots do not photosynthesise, however, and they fix carbon in the absence of light with the use of enzymes.

Ordinarily roots only fix enough carbon for their own secretory activities, with the bulk of carbon fixation occurring in the shoot system. Root-fixed carbon is used, for example, to produce root secretions which leak into the soil around the roots (the rhizosphere). These secretions probably have various functions, but appear to encourage plant-friendly microbes, signaling to mycorhizal fungi, for example, advertising the root and encouraging the fungus to form a symbiosis with it. These secretions are also thought to be important in helping to 'mobilise, plant nutrients - many nutrients are locked onto soil particles in an insoluble form and so can not be absorbed by the roots until they have been dissolved or mobilised. In some herbaceous plants, it has been shown that roots can fix enough surplus carbon to contribute to shoot carbon, that is the roots export carbon that they fix and which is in excess of their own needs, into the shoot system. To what extent the roots contribute to the stem carbon of woody plants is not known.

The Role of Root Pressure As well as contributing a small amount to the pressure gradient which drives xylem sap part-way up a tree, or perhaps to the top of some short plants, or even small trees, in certain conditions, root pressure possibly has several important functions. In very humid climates where evapotranspiration is too low to drive xylem flow, root pressure ensures that some xylem travels up the plant to deliver mineral nutrients from the roots to the shoot system. This root pressure may cause droplets of water to exude from vessels at the edges and tips of leaves (guttation) and some plants have special pores (hydathodes) at the ends of the veins to allow this water out (after the minerals are extracted from it by the cells). This flow due to root pressure also occurs in many plants at night, contributing to early morning due. It is also important in very early spring, supplying growing buds with minerals before the leaves have opened enough for evapotranspiration to take over as the main driving force. Root pressure may also help unblock cavitated vessels.

Sugar in the Xylem In maples and birches, cold-tolerant trees, xylem sap is driven up the stem in winter and carries sucrose with it to fuel the developing flowers which open early before the leaves. The sucrose is loaded into the xylem from ray parenchyma and other storage cells in the xylem. This sap ascent can not be utilising the transpiration stream as no leaves are available in winter to drive it. It occurs on warmer days that follow cold nights and is thought to involve a night-freeze, warm-day cycle of pressure changes in the trunk. At night the xylem sap freezes and this is thought to trap and compress gases in the xylem as the sap freezes. The daytime heat melts the ice in the xylem, expanding the trapped gases and generating a positive pressure to drive xylem sap flow up the trunk. At this point the sugary xylem sap can be collected for use in making syrup and wines. In the evenings, the colling temperatures dissolve more gases in the xylem, seeded from adjoining tissues such as parenchyma, ready to form compressed gases when the xylem freezes at night.

A good article, on an external web site, describing this process in maple can be found here: maple syrup . Foliar Absorption It should be remembered that plants can also absorb nutrients through their leaves. Gases, liquids and solid mineral dusts falling on the leaf can all lead to increased nutrient absorption. The nutrients are absorbed through the plasmodesmata and also to some extent through the cuticle. Apparently, a European forest may absorb up to 30% of its nitrogen through the canopy in this way (Thomas, 2000). Bibliography Schulz, 1992. Living sieve cells of conifers as visualized by confocal, laser-scanning fluorescence microscopy. Protoplasma 166: 153-164. Scheirer, 1978. Cell wall chemistry and fine structure in leptoids of Dendroligotrichum (Bryophyta): The end wall. Amer. J. Bot. 65: 1027-1031. Esau, 1976. Anatomy of Seed plants, second edition. Thomas, P. 2000.

Trees: their natural history. Cambridge University Press. Other sources to be added... Article last updated: 27/2/15 We've all sat under enormous old trees, whether it's been for a picnic, or for shade, or just to have a quiet place to sit and think. Many of us vaguely remember our school science classes and can recall the basics of photosynthesis and the life-cycle of a tree. However, there's actually a lot more going on under that bark than you might realize. Take another look at how a tree waters, feeds and grows and all the intricate mechanisms it has developed to do it and you may find it quite fascinating! At the least, it'll certainly give you something to consider the next time you lean up against a 100 year old oak tree to consider your finances . Oak trees, like the one illustrated above (a 3D Pov-Ray model) may take-up more than 400 litres of water each day! This water moves up the stem in the outermost rings of xylem (wood).

Some of this water becomes (temporarily) incorporated into the cells have a vast surface area of leaves to catch enough sunlight for photosynthesis and these leaves need carbon dioxide which is reacted with water to form the organic building blocks of the plant's cells, using the energy harnessed by sunlight. These building blocks include amino acids and sugars . The much needed carbon dioxide comes from the atmosphere and leaves have closable pores called stomata (singular stoma) to take up this carbon dioxide by diffusion. Leaves are generally born on stalks which have a hinge at their base (called the pulvinus) and this allows the leaves to vibrate in the wind, stirring the air to help the leaves obtain a fresh supply of carbon dioxide (diffusion in still air is slow and a zone of stagnant carbon dioxide depleted air would surround each leaf). A general equation for photosynthesis is given below: Left: a section through a leaf blade. Both the upper and lower surfaces of the leaf are bounded by a layer of epithelium (E) (covering tissue) which is coated by a cuticle (C) of its own secretion.

The cuticle is thickest on the upper surface of the leaf and contains waxy materials to make it water proof in order to reduce water loss by evaporation. Stomata occur mostly among the lower epidermis and each stoma (S) is surrounded by a pair of sausage-shaped guard cells (red). (palisade mesophyll, PM) whilst the lower leaf contains more loosely packed isodiametric cells (spongy mesophyll, SM). traversing the middle of the leaf blade are vessels which branch from the central midrib. Each vessel or vascular bundle is enclosed in a cylinder of cells called the bundle sheath (BS) and contains two principle types of vascular or water-conducting tissue: xylem (X) and phloem (P). Note the many spaces between the cells, especially the spongy mesophyll - these are a series of interconnected air spaces. The airs spaces connect to the outside air through the stomata via an air space beneath each stoma, called the sub-stomatal air space (visible in this photomicrograph). Air carrying carbon dioxide diffuses into the leaf through the stomata and then diffuses throughout the air-space chambers to reach the mesophyll.

The mesophyll (especially the palisade mesophyll) are the site of photosynthesis and contain chloroplasts which contain the green light-harvesting pigment chlorophyll . In plants, epidermal cells do not contain chloroplasts (with the exception of guard cells) and so do not photosynthesise. Left: a varnish cast of the undersurface of a leaf showing the stomata and their accompanying guard cells. Click here to see more about leaf morphology and vasculature. Stomata will close to conserve water at night-time when photosynthesis can no longer continue, and they will close if the plant is losing too much water. Stomata will open and close according to a circadian rhythm as well as in direct response to light and dark. Stomata may also close in response to wounding - plants can lose a lot of water through open wounds and some plants, e.g. tomato plants, react rapidly to damage by transmitting electrical signals throughout their leaves which trigger the stomata to close. Despite these measures, plants lose a lot of water through their stomata and this water needs to be replaced. Xylem vessels carry water from the roots, up the stem and to the leaves.

Plants exploit this situation, making the most of a 'bad thing' and they utilise the so-called transpiration stream of sap ascending in the xylem to carry mineral nutrients, obtained by the roots from the soil, with it. Transpiration from the leaves also helps to keep them cool (important as they are purposefully exposed to the sunlight!). The xylem sap is carried into each leaf, along the central vein in the midrib and into the leaf's vascular network . Water then traverses the leaf from cell-to-cell and evaporates into the air spaces. How does water travel from cell-to-cell? In plant tissues like leaf mesophyll, neighbouring cells are connected by pores called plasmodesmata . These are tiny pores that traverse the cell walls and cell membranes of neighbouring cells. They are membrane-lined and the cytoplasms of neighbouring cells are continuous as the cytoplasm fills the plasmodesmatal channels. Thus, the cytoplasms of plant cells form a connected continuous system called the symplast. Water can travel through the cytoplasm from cell-to-cell through the symplast. Water can, however, also move through the extracellular spaces and cell walls that together form the apolast.

The cell walls are principally made of cellulose microfibrils which form a porous mesh. Water can move through the apoplast by capillarity (and probably moves faster than through the symplast). Capillarity Water will move upwards inside a very narrow glass tube (capillary tube) for a short distance. Water does this because it is sticky. Water molecules are electrically charged and dipolar (one end or pole of the molecule is negatively charged, the other pole positively charged). Opposite charges attract and water molecules bond or stick to each other (by hydrogen bonds in a process called cohesion - like sticking to like) and they are also electrically attracted to other materials, like the glass wall of a capillary tube, or the cellulose microfibrils of the apoplast (in a process called adhesion - two unlike materials sticking together). This attraction pulls water along inside narrow spaces (this is how a sponge can passively soak-up water). The force that drives this movement of water is the capillary force of cohesion-adhesion. This force drives the movement of water through plant apoplasts.

Capillarity is of key importance in water transport along the outside of moss stems in ectohydric Water moves from the xylem across the leaf to the air spaces by the apoplast and symplast and then evaporates through the stomata (transpiration). Click here and here for more information on the structure of xylem and wood. It is the transpiration of water from leaves which is the main driving force for the movement of water in xylem. Loss of water from the leaves creates a negative suction pressure that draws water up the stem. Peak flow rates in xylem are about 1 mm/s, though maximum velocities as high as 0.8 m/s have been reported. Flow in the xylem stops at night and then velocities rise in the morning, peaking around midday. Wider vessels have larger maximum velocities but are more prone to cavitation (the formation of air bubbles that block xylem vessels) in cold weather. For this reason, the evergreen cold-tolerant conifers have narrower vessels and deciduous trees produce narrower vessels in Autumn and wider vessels in Spring and Summer - this annual cycle in vessel size creates the annual growth ring seen in trees.

Cavitation occurs when the water column breaks - the water is being pulled up to a great height and may break under its won weight, especially if cold and 'brittle' and fracture of the water column is also more likely in wider vessels. The fact that columns of water can be lifted up against gravity at all is due to cohesion: water molecules stick to one another and so they move up the xylem as a continuous column. Water will also adhere to the inside of the vessel wall. This physical explanation of xylem sap ascent is called the cohesion-tension theory, as the water is pulled up the tree (and so is in tension). The movement of sap in the xylem is called the transpiration stream. shoot on a tree or the stem or branch of an herbaceous plant) the vascular tissue is arranged in bundles. The stem is covered in a layer of cells forming an epidermis. Beneath this there are often several layers of thick-walled cells called collenchyma which strengthen the stem. Each bundle contains xylem on the inside and another vascular tissue, the phloem on the outside. Xylem vessels and phloem fibres also strengthen the stem.

Plant Roots Before considering the phloem transport system, let's look at water uptake in roots. The arrangement of the xylem and phloem in a typical young (non-woody) dicotyledonous root is shown on the left and below. Compare this pattern with that found in the stem. The xylem is central, so water must enter the root and travel across the cortex and cross a cylinder of cells called the endodermis in order to reach it. One piece of experimental evidence for suction forces driving xylem sap ascent in the stem comes from stem-girth measurements. These show that at dawn the trunk contracts slightly, reaching its narrowest at midday and then increases again to its night-time value. This corresponds to the stomata opening at dawn, as the tree begins photosynthesis, creating a suction which draws the xylem vessels in, narrowing them slightly and this narrowing is greatest when transpiration is greatest at around midday. More direct methods are available, such as heat-pulse measurements. An electrical wire is placed just beneath the bark at one point of the stem and a thermocouple placed about 4 cm further up.

A brief electric current heats the wire and the xylem near it and then the time measured for the heat pulse to travel up the stem can be measured, demonstrating the ascent of sap in the wood. The detailed structure of a sector of such a root is shown below: In woody stems, a ring of secondary xylem or wood is added each year. The outer rings of wood are the newest and only these contain functional vessels. Older vessels become blocked and non-functional. The oldest parts form the heartwood. Also within the wood are radial plates of parenchyma cells called rays, which can transport materials across the stem, for example depositing waste compounds into the heartwood giving it a different colour. The bark has two main layers - the inner fibrous layer of phloem and the outer layer of cork cells. Some of the root epidermal cells, situated near the growing root tips, have projections called root hairs (unicellular hairs) which increase the surface area of the root for water absorption, though other epidermal cells may also absorb water. The diagram below shows two possible routes of this water absorption.

The bottom arrow (blue) shows water moving through plasmodesmata in the symplast and is the symplastic pathway. Note that to reach the symplast the water first had to cross the cell membrane of the root hair cell. (See plant cell structure). The top arrow (red) shows water being absorbed through the apoplast (apoplastic pathway) traveling through the sponge-like meshes of the cell walls by capillarity, until it reaches a cylinder of cells called the endodermis. (The apoplast pathway is probably faster than the symplast pathway). These cells have a hoop of water-proof material in their cell walls (suberin) forming the Casparian strip. At this point the apolastic pathway is blocked and the water is forced to detour along the dashed part of the arrow, which leads into the symplast. At this point the water was forced to cross the cell membrane of the endodermis cell. This is the key point - whichever route the water takes to reach the xylem, it has to cross at least one cell-surface membrane. These cell-surface membranes are able to control water entry and so the root can control water and mineral uptake at these points.

Protein pumps in these membranes actively pump mineral salts, obtained from the soil water, into their cytoplasm, using cellular energy in the form of ATP (adenosine trisphosphate). This loading with salts creates an osmotic gradient (the water potential inside the cell is lowered - see below for an explanation of water potential) which then drives water into the cell, across the membrane (as water moves from a region of low solute gradient, or high water potential, to a region of high solute concentration, or low water potential). Thus, by using energy to accumulate the dilute and much needed mineral slats from the soil, the cells are essentially pumping water into the root and into the xylem! The xylem will then carry this water and these minerals (the minerals are essential nutrients for the plant) in the transpiration stream. This pumping creates a positive pressure in the root xylem, helping to push water up the stem - the so-called root pressure. This root pressure may force water up by a metre or so and may be sufficient tod rive water transport in small plants, however, in tall trees transpiration is by far the main driving force for xylem sap movement.

Water potential (given the Greek letter psi as its symbol) is simply the potential energy possessed by a unit volume (a set volume, e.g. 1 metre cubed) of water. It is the sum total of the various kinds of potential energy the water can possess. The most familiar is the gravitational potential energy. Take a ball and raise it in the air and you have increased its gravitational potential energy. Energy is the ability to cause change and this gravitational energy is potential energy because it has the potential to cause change, but will not do so until the ball is released. When released, the ball falls to the ground as it loses gravitational potential energy which is turned into kinetic energy (the energy of movement). Water potential is a useful concept for describing water movement in plants. It is often said that water will always move from a region of high water potential to a region of low water potential (down a water potential gradient) - like the falling ball. However, this is not strictly true, water will move down a water potential gradient of its accord if allowed to do so.

However, it is possible to move water from a region of low water potential to a region of high water potential, by supplying energy (as you did when raising the ball) - the water will then gain potential energy. In xylem water does move from high to low water potential. The plant expends none of its own energy to move the water, it simply opens the stomata and transpiration does the rest and water moves from high water potential in the roots to low water potential in the air above the canopy. (transpiration creates a negative pressure potential at the top of the tree, sucking the water up the stem). However, in phloem water can move from low to high water potential because the plant uses cellular energy (in the form of ATP) to actively pump water up or down the stem (it creates positive pressure that pushes the water along in the phloem). In both xylem and phloem it is the pressure which drives water movement. The phloem contains a system of vessels for transporting photoassimilates (the products of photosynthesis like amino acids and sugars) around the plant. Sugar is transported mainly as sucrose and nitrogen as amino acids (the building blocks of proteins).

The reason why sucrose is transported rather than glucose is probably because sucrose is harder for bacteria to metabolise and so transporting sucrose reduces the risk of infection. (Similarly insects transport the sugar trehalose and seaweeds mannitol. Mammals, however, transport glucose in their blood stream since this can be rapidly utilised by nerve cells and having such large brains necessitates a ready fuel supply. This, combined with their constantly warm temperatures, makes mammals very prone to infection and so they have evolved highly sophisticated immune systems. Still, however, major wounds to large animals in nature often result in death, not from the wound, but from the ensuing infection). Phloem is a tissue comprising several different cell types, including parenchyma, tough sclerenchyma fibres, and the phloem vessels, called sieve tubes, which are made of sieve-tube members or elements (sieve tube cells). Phloem carries sugars (and other photoassimilates) from sources where they are released (e.g. photosynthesising leaves where they are made, or storage organs like bulbs germinating in Spring) to sinks where they used (e.g.

growing fruit, shaded leaves, storage organs like roots which store the sugars as starch). This process is called translocation (a name sometimes also used to describe movement of materials in xylem). It is now known that it is not just the sugars and other solutes that move through the phloem, but the water moves with it - the whole phloem sap moves in bulk flow (mass flow or mass transport). In photosynthesising leaves, the sugars can be transported toward the phloem sieve tubes in the veins by either the symplast or apoplast pathways, as shown below: Note that the apoplastic pathway involves the sugars crossing the cell membranes of specialised parenchyma cells called companion cells (there is typically one companion cell per sieve element). This cell-surface membrane can regulate the transport of the sugar and also drives it by actively pumping sugar from the neighbouring parenchyma cells into the phloem sieve tubes (using membrane protein pumps that require cellular energy in the form of ATP). This pumping of sugars into the companion cell/sieve tube element creates an osmotic gradient, or put another way, lowers the solute potential of the companion cell and hence lowers its water potential.

(Solute potential is highest for pure water which is given the value zero, so other solutions will have a lower or more negative solute potential). Water then follows the sugar as it moves from high to low water potentials and enters the sieve tubes. As the phloem travels along the sieve tubes in the source, passing from element to element, it is given a push as more sugars are loaded in, creating a pulsatile pressure (positive pressure potential) that pushes the phloem along. Unloading of the sugars occurs at the sink. Here companion cells actively pump the sugars out from the phloem, causing water to follow. This unloading adds to the pressure gradient in the sieve tubes - the phloem is pushed along by sugar loading at the source and pulled along by sugar unloading at the sink. This movement from source to sink can occur in any direction within the plant, but one of the major routes will be from the leaves, down the stem, to the roots. Roots can not and do not photosynthesise and need sugars to meet their own energy needs, they also store excess sugars as starch, safely underground and away from browsing herbivores! Above: The structure of a phloem sieve tube and its companion cells.

Note the peripheral layer of cytoplasm against the walls of the sieve tube members (sieve tube elements, sieve elements or sieve tube cells) bounded by a degraded tonoplast (the membrane around the original cell vacuole) which loosely separates the cytoplasm from the lumen. During differentiation, condensed and confined to the margins of the tube and may be condensed into P-protein bodies. Smooth endoplasmic reticulum (SER) gradually degenerates and may be absent altogether in mature sieve tube elements. Adjacent sieve tube members are separated by porous sieve plates with pore diameters ranging from about 1 micrometre to 14 micrometres. responsible for providing most of the energy needed to pump materials into and out of the phloem tubes to neighbouring parenchyma cells. Large single plasmodesmata and groups of plasmodesmata, called sieve areas, connect the cytoplasm of the companion cell to its sieve tube element. 'Sieve area' is a general term for an area of pores connecting a sieve tube cell to another cell.

When these pores are especially large and confined to a distinct piece of cell wall, as in the end-walls of the sieve tube cells, they are called sieve plates. When a sieve tube is cut or wounded, the P-protein spreads out (unwinds) rapidly to fill the lumen and block the pores in the sieve plates, so blocking the vessel and preventing further loss of phloem sap. Gymnosperms, including conifers like the pine tree, do not have sieve tubes. Instead they have sieve cells that are connected together by sieve areas with smaller pores (the pores are generally less than 0.8 micrometres in diameter), and so do not form an open tube. Sieve cells are much narrower (less than about 5 micrometres in diameter, compared to sieve tube cells which are up to about 50 micrometres in diameter) and several times longer than sieve tube cells. Specialised parenchyma cells function much like companion cells, but are called albuminous cells in gymnosperms. (In development, a sieve tube cell and its companion cell are produced from the same parent cell by cell division, whereas sieve cells and albuminous cells do not have a common parent cell).

Sieve cells in gymnosperms retain their SER (smooth endoplasmic reticulum) as a network that terminates on either side of the sieve are pores, and do not have P-protein. When a gymnosperm sieve cell is wounded, the SER swells up and expands to block the pores. This SER network may also have some role in transport and it is possible that translocation in gymnosperm phloem operates by a different mechanism to that described for angiosperm sieve tubes. Mosses and algae also have phloem-like tissues. In mosses elongated leptoid cells are connected by fields of plasmodesmata (with pore diameters of 0.12 to 0.15 micrometres in Dendroligotricum) and so resemble sieve cells. In seaweeds , trumpet hyphae are made up of sieve cells connected by sieve areas or sieve plates, with pore diameters ranging from 0.04-0.09 micrometres in Laminaria, to 6.0 micrometres in the giant kelp Macrocystis.

In gymnosperms the pore diameter is typically less than 0.8 micrometres, and 6.5 micrometres in Fagus ( beech ). The general pattern seems to be that as plants moved onto land and grew larger, so did their pore sizes, which reduces resistance to sap movement and increases the maximum rate of translocation in a vessel of a given diameter. Gymnosperms may form very large trees and are at odds with this trend and more will be said about this later. which expand to fill all the space right up to the rigid cell walls ('boxes') that contain them. This will happen if the cells are exposed to a sugar solution of low concentration - water will move into the cells until the cells are full. Right - cells which have lost water by osmosis, causing their protoplasts to shrink away from the cell walls, a phenomenon called plasmolysis (the cells are said to be plasmolysed). This happened because the tissue was soaked in a sugar solution of high concentration, causing water to diffuse out from the cells by osmosis.

As water makes up some 70% of the volume of cells, the protoplasts have shrunk and come away from their surrounding cell walls. This phenomenon is important in plants. In soft green or fleshy plant parts, when the plant has sufficient water, the cells are full and the protoplasts push against the cell walls, keeping them rigid (rather like tyres filled with water at high pressure) and giving the plant parts support - the cells and plant parts are said to be turgid (swollen and rigid). This is important for keeping leaves and green stems upright to intercept sunlight - the tissues (especially parenchyma) functions as a pressurised cellular solid. A common example of a pressurised cellular solid is polystyrene - tiny balls of foam filled with air which when packed together become rigid - bending the polystyrene slightly squeezes the air inside the balls which resist bending. If a plant is dehydrated (due to water loss by evaporation) however, the cells plasmolyse and lose pressure and the cell walls lose rigidity and the cells become flaccid and the plant wilts.

This is also protective - as leaves droop so they become less-exposed to the drying sun and/or wind. Of course, the plant cannot survive in this desiccated state indefinitely, as water is vital to cell chemistry, but they can recover quickly if the plant receives water. Animal cells, such as mammalian red blood cells, similarly shrink if they lose water by osmosis to a concentrated solution. However, if immersed in distilled water, which is fairly pure, then water enters the cells by osmosis, causing them to swell and burst - mammalian cells will explode within seconds on contact with distilled water. Single-celled organisms that resemble animal cells, like amoeba , can survive because they have contractile vacuoles, which swell up with water, as excess water enters the cell, and then contract to expel the water. Comment on this article! Osmosis and Plasmolysis Not all scientists define water potential in the same way. We have adopted the definition in terms of potential energy per unit volume. More or less equivalently, some define it as potential energy per mole (one mole being about 6.022 x 10^23 particles, or molecules of water in this case).

Other definitions refer to water potential as the free energy of water per unit volume or per mole. Free energy is the energy available to do work, and so is essentially equivalent to potential energy in most cases. However, free energy is sometimes defined as internal energy only. A system, such as a mass of water, has both internal energy (due to the movements of its molecules relative to each other) and external energy (due to the movement of the body of water as a whole). In plant biology, we need to include both external and internal energy - external energy is important when we consider movement of water up a tree - as it ascends, the water as a whole gains gravitational potential energy, which has the potential to do work should the water be released and allowed to fall to the ground. Internal energy is important as this drives diffusion. Diffusion is the net or overall movement of molecules from a region of higher concentration to a region of lower concentration. (By concentration we mean the number of molecules in a given volume, such as the number of molecules per cubic meter). This movement occurs because molecules are in constant thermal motion (they move about and the hotter they are, the more they move) and this movement is random.

If a membrane separates the regions of higher and lower concentration, then diffusion will only take place if the membrane is permeable to the substance diffusing - for example if it is porous and the pores are large enough to allow the molecules through. The cell-surface membrane works in this way - it is a selectively-permeable membrane, allowing only certain substances to cross it. Water is one such substance. However, because cell membranes are made principally of lipids (fats/oils) in the form of phospholipids, water does not cross easily (oil and water do not mix - water does not dissolve well in fat and vice versa) and will only slowly leak across a phospholipid membrane. However, cells need water and so cell membranes have protein pores, called aquaporins, that are the right size to allow water molecules to cross easily. This is an example of facilitated diffusion - the protein pores help or facilitate the diffusion of water across the membrane. Diffusion of water across cell membranes is so important that it is given its own special name - osmosis. In biology we rarely deal with 'pure' water, since biological fluids are all solution.

A solution is a mixture of materials, in which certain substances, called solutes, are dissolved in a solvent (which is the main component of the mixture). Most often in biology the solvent of interest is water and this will contain a variety of dissolved solutes such as salts (like sodium chloride or common table salt, NaCl, and slats of potassium, calcium and iron and other metals) sugars and amino acids. When salt, NaCl, dissolves in water, it splits up into ions of sodium, Na+ and chloride, Cl- (an ion is an atom or molecule that gains one or more units of net electric charge, either positive or negative). In such a solution it is inconvenient to think of the concentration of water as this is hard to measure, and we usually think in terms of concentration of solute. Since ions, atoms or molecules of solvent occupy space that would otherwise be occupied by water molecules, the higher the total solute concentration (of all solutes) the lower the concentration of water. Thus water will diffuse from a region of lower solute concentration (higher water concentration) to a region of higher solute concentration (lower water concentration). Osmosis across plant cell membranes can be easily demonstrated using plant tissues.

A classic experiment involves taking strips of onion epidermis (such as the translucent tissue-paper like layer on the inside of the fleshy leaves of the onion bulb which is one-cell thick and so easily observed under the light microscope) and immersing them for twenty minutes or so in different concentrations of solution, typically a solution of sucrose sugar. Some results from such an experiment are shown below: Other related pages on Cronodon: WHAT IS PLANT NUTRITION? The Orchid House WHAT IS PLANT NUTRITION? Plants use inorganic minerals for nutrition, whether grown in the field or in a container. Complex interactions involving weathering of rock minerals, decaying organic matter, animals, and microbes take place to form inorganic minerals in soil. Roots absorb mineral nutrients as ions in soil water. Many factors influence nutrient uptake for plants. Ions can be readily available to roots or could be "tied up" by other elements or the soil itself. Soil too high in pH (alkaline) or too low (acid) makes minerals unavailable to plants.

FERTILITY OR NUTRITION The term "fertility" refers to the inherent capacity of a soil to supply nutrients to plants in adequate amounts and in suitable proportions. The term "nutrition" refers to the interrelated steps by which a living organism assimilates food and uses it for growth and replacement of tissue. Previously, plant growth was thought of in terms of soil fertility or how much fertilizer should be added to increase soil levels of mineral elements. Most fertilizers were formulated to account for deficiencies of mineral elements in the soil. The use of soilless mixes and increased research in nutrient cultures and hydroponics as well as advances in plant tissue analysis have led to a broader understanding of plant nutrition. Plant nutrition is a term that takes into account the interrelationships of mineral elements in the soil or soilless solution as well as their role in plant growth. This interrelationship involves a complex balance of mineral elements essential and beneficial for optimum plant growth. ESSENTIAL VERSUS BENEFICIAL The term essential mineral element (or mineral nutrient) was proposed by Arnon and Stout (1939). They concluded three criteria must be met for an element to be considered essential. These criteria are: 1.

A plant must be unable to complete its life cycle in the absence of the mineral element. 2. The function of the element must not be replaceable by another mineral element. 3. The element must be directly involved in plant metabolism. These criteria are important guidelines for plant nutrition but exclude beneficial mineral elements. Beneficial elements are those that can compensate for toxic effects of other elements or may replace mineral nutrients in some other less specific function such as the maintenance of osmotic pressure. The omission of beneficial nutrients in commercial production could mean that plants are not being grown to their optimum genetic potential but are merely produced at a subsistence level. This discussion of plant nutrition includes both the essential and beneficial mineral elements. WHAT ARE THE MINERAL ELEMENTS? There are actually 20 mineral elements necessary or beneficial for plant growth. Carbon (C), hydrogen (H), and oxygen (O) are supplied by air and water. The six macronutrients, nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) are required by plants in large amounts. The rest of the elements are required in trace amounts (micronutrients).

Essential trace elements include boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), sodium (Na), zinc (Zn), molybdenum (Mo), and nickel (Ni). Beneficial mineral elements include silicon (Si) and cobalt (Co). The beneficial elements have not been deemed essential for all plants but may be essential for some. The distinction between beneficial and essential is often difficult in the case of some trace elements. Cobalt for instance is essential for nitrogen fixation in legumes. It may also inhibit ethylene formation (Samimy, 1978) and extend the life of cut roses (Venkatarayappa et al., 1980). Silicon, deposited in cell walls, has been found to improve heat and drought tolerance and increase resistance to insects and fungal infections. Silicon, acting as a beneficial element, can help compensate for toxic levels of manganese, iron, phosphorus and aluminum as well as zinc deficiency. A more holistic approach to plant nutrition would not be limited to nutrients essential to survival but would include mineral elements at levels beneficial for optimum growth.

With developments in analytical chemistry and the ability to eliminate contaminants in nutrient cultures, the list of essential elements may well increase in the future. THE MINERAL ELEMENTS IN PLANT PRODUCTION The use of soil for greenhouse production before the 1960s was common. Today a few growers still use soil in their mixes. The bulk of production is in soilless mixes. Soilless mixes must provide support, aeration, nutrient and moisture retention just as soils do, but the addition of fertilizers or nutrients are different. Many soilless mixes have calcium, magnesium, phosphorus, sulfur, nitrogen, potassium and some micronutrients incorporated as a pre-plant fertilizer. Nitrogen and potassium still must be applied to the crop during production. Difficulty in blending a homogenous mix using pre-plant fertilizers may often result in uneven crops and possible toxic or deficient levels of nutrients. Soilless mixes that require addition of micro and macronutrients applied as liquid throughout the growth of the crop, may actually give the grower more control of his crop. To achieve optimum production, the grower can adjust nutrient levels to compensate for other environmental factors during the growing season. The absorption of mineral ions is dependent on a number of factors in addition to weather conditions.

These include the cation exchange capacity or CEC and the pH or relative amount of hydrogen (H+) or hydroxyl ions (OH-) of the growing medium, and the total alkalinity of the irrigation water. CEC OR CATION EXCHANGE CAPACITY The Cation Exchange Capacity refers to the ability of the growing medium to hold exchangeable mineral elements within its structure. These cations include ammonium nitrogen, potassium, calcium, magnesium, iron, manganese, zinc and copper. Peat moss and mixes containing bark, sawdust and other organic materials all have some level of cation exchange capacity. pH: WHAT DOES IT MEAN? The term pH refers to the alkalinity or acidity of a growing media water solution. This solution consists of mineral elements dissolved in ionic form in water. The reaction of this solution whether it is acid, neutral or alkaline will have a marked effect on the availability of mineral elements to plant roots. When there is a greater amount of hydrogen H+ ions the solution will be acid (<7.0). If there is more hydroxyl OH- ions the solution will be alkaline (>7.0). A balance of hydrogen to hydroxyl ions yields a pH neutral soil (=7.0).

The range for most crops is 5.5 to 6.2 or slightly acidic. This creates the greatest average level for availability for all essential plant nutrients. Extreme fluctuations of higher or lower pH can cause deficiency or toxicity of nutrients. THE ELEMENTS OF COMPLETE PLANT NUTRITION The following is a brief guideline of the role of essential and beneficial mineral nutrients that are crucial for growth. Eliminate any one of these elements, and plants will display abnormalities of growth, deficiency symptoms, or may not reproduce normally. Macronutrients Nitrogen is a major component of proteins, hormones, chlorophyll, vitamins and enzymes essential for plant life. Nitrogen metabolism is a major factor in stem and leaf growth (vegetative growth). Too much can delay flowering and fruiting. Deficiencies can reduce yields, cause yellowing of the leaves and stunt growth. Phosphorus is necessary for seed germination, photosynthesis, protein formation and almost all aspects of growth and metabolism in plants. It is essential for flower and fruit formation. Low pH (<4) results in phosphate being chemically locked up in organic soils. Deficiency symptoms are purple stems and leaves; maturity and growth are retarded. Yields of fruit and flowers are poor.

Premature drop of fruits and flowers may often occur. Phosphorus must be applied close to the plant's roots in order for the plant to utilize it. Large applications of phosphorus without adequate levels of zinc can cause a zinc deficiency. Potassium is necessary for formation of sugars, starches, carbohydrates, protein synthesis and cell division in roots and other parts of the plant. It helps to adjust water balance, improves stem rigidity and cold hardiness, enhances flavor and color on fruit and vegetable crops, increases the oil content of fruits and is important for leafy crops. Deficiencies result in low yields, mottled, spotted or curled leaves, scorched or burned look to leaves.. Sulfur is a structural component of amino acids, proteins, vitamins and enzymes and is essential to produce chlorophyll. It imparts flavor to many vegetables. Deficiencies show as light green leaves. Sulfur is readily lost by leaching from soils and should be applied with a nutrient formula. Some water supplies may contain Sulfur. Magnesium is a critical structural component of the chlorophyll molecule and is necessary for functioning of plant enzymes to produce carbohydrates, sugars and fats. It is used for fruit and nut formation and essential for germination of seeds.

Deficient plants appear chlorotic, show yellowing between veins of older leaves; leaves may droop. Magnesium is leached by watering and must be supplied when feeding. It can be applied as a foliar spray to correct deficiencies. Calcium activates enzymes, is a structural component of cell walls, influences water movement in cells and is necessary for cell growth and division. Some plants must have calcium to take up nitrogen and other minerals. Calcium is easily leached. Calcium, once deposited in plant tissue, is immobile (non-translocatable) so there must be a constant supply for growth. Deficiency causes stunting of new growth in stems, flowers and roots. Symptoms range from distorted new growth to black spots on leaves and fruit. Yellow leaf margins may also appear. Micronutrients Iron is necessary for many enzyme functions and as a catalyst for the synthesis of chlorophyll. It is essential for the young growing parts of plants. Deficiencies are pale leaf color of young leaves followed by yellowing of leaves and large veins. Iron is lost by leaching and is held in the lower portions of the soil structure. Under conditions of high pH (alkaline) iron is rendered unavailable to plants.

When soils are alkaline, iron may be abundant but unavailable. Applications of an acid nutrient formula containing iron chelates, held in soluble form, should correct the problem. Manganese is involved in enzyme activity for photosynthesis, respiration, and nitrogen metabolism. Deficiency in young leaves may show a network of green veins on a light green background similar to an iron deficiency. In the advanced stages the light green parts become white, and leaves are shed. Brownish, black, or grayish spots may appear next to the veins. In neutral or alkaline soils plants often show deficiency symptoms. In highly acid soils, manganese may be available to the extent that it results in toxicity. Boron is necessary for cell wall formation, membrane integrity, calcium uptake and may aid in the translocation of sugars. Boron affects at least 16 functions in plants. These functions include flowering, pollen germination, fruiting, cell division, water relationships and the movement of hormones. Boron must be available throughout the life of the plant. It is not translocated and is easily leached from soils. Deficiencies kill terminal buds leaving a rosette effect on the plant. Leaves are thick, curled and brittle.

Fruits, tubers and roots are discolored, cracked and flecked with brown spots. Zinc is a component of enzymes or a functional cofactor of a large number of enzymes including auxins (plant growth hormones). It is essential to carbohydrate metabolism, protein synthesis and internodal elongation (stem growth). Deficient plants have mottled leaves with irregular chlorotic areas. Zinc deficiency leads to iron deficiency causing similar symptoms. Deficiency occurs on eroded soils and is least available at a pH range of 5.5 - 7.0. Lowering the pH can render zinc more available to the point of toxicity. Copper is concentrated in roots of plants and plays a part in nitrogen metabolism. It is a component of several enzymes and may be part of the enzyme systems that use carbohydrates and proteins. Deficiencies cause die back of the shoot tips, and terminal leaves develop brown spots. Copper is bound tightly in organic matter and may be deficient in highly organic soils. It is not readily lost from soil but may often be unavailable. Too much copper can cause toxicity. Molybdenum is a structural component of the enzyme that reduces nitrates to ammonia.

Without it, the synthesis of proteins is blocked and plant growth ceases. Root nodule (nitrogen fixing) bacteria also require it. Seeds may not form completely, and nitrogen deficiency may occur if plants are lacking molybdenum. Deficiency signs are pale green leaves with rolled or cupped margins. Chlorine is involved in osmosis (movement of water or solutes in cells), the ionic balance necessary for plants to take up mineral elements and in photosynthesis. Deficiency symptoms include wilting, stubby roots, chlorosis (yellowing) and bronzing. Odors in some plants may be decreased. Chloride, the ionic form of chlorine used by plants, is usually found in soluble forms and is lost by leaching. Some plants may show signs of toxicity if levels are too high. Nickel has just recently won the status as an essential trace element for plants according to the Agricultural Research Service Plant, Soil and Nutrition Laboratory in Ithaca, NY. It is required for the enzyme urease to break down urea to liberate the nitrogen into a usable form for plants. Nickel is required for iron absorption. Seeds need nickel in order to germinate.

Plants grown without additional nickel will gradually reach a deficient level at about the time they mature and begin reproductive growth. If nickel is deficient plants may fail to produce viable seeds. Sodium is involved in osmotic (water movement) and ionic balance in plants. Cobalt is required for nitrogen fixation in legumes and in root nodules of nonlegumes. The demand for cobalt is much higher for nitrogen fixation than for ammonium nutrition. Deficient levels could result in nitrogen deficiency symptoms. Silicon is found as a component of cell walls. Plants with supplies of soluble silicon produce stronger, tougher cell walls making them a mechanical barrier to piercing and sucking insects. This significantly enhances plant heat and drought tolerance. Foliar sprays of silicon have also shown benefits reducing populations of aphids on field crops. Tests have also found that silicon can be deposited by the plants at the site of infection by fungus to combat the penetration of the cell walls by the attacking fungus. Improved leaf erectness, stem strength and prevention or depression of iron and manganese toxicity have all been noted as effects from silicon. Silicon has not been determined essential for all plants but may be beneficial for many. Written by Dorothy Morgan.

Staff Horticulturist employed by Dyna-Gro Corporation. Dorothy holds a B. S. Degree in Horticulture from Delaware Valley College of Science and Agriculture and Penn State University. Her experience has included managing commercial greenhouses, nurseries, hydroponics, and teaching vocational agriculture - Reproduced with permission of the author.

Question: What word refers to liquid absorption, as by plants through their roots?
Answer: OSMOSIS