AQA AS Level Biology Unit 2 - Exchange and transport

The rather large topic of Exchange and transport from Unit 2 Biology AQA AS

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Exchange between organisms and their environments

For survival organisms must transfer materials between themselves and their environment. Once absorbed, materials must be rapidly distributed to the cells that  require them and the waste products returned to the exchange surface for removal. This requires a transport system.

The size and metabolic rate of an organism will affect the amount og each material that needs to be exchanged. In turn this will influence the type of exchange surface and transport sustem that has evolved to meet the requirements of each organism.

Examples of things which need to be interchanged between an organism and its environment include:

  • respiratory gases (oxygen and carbon dioxide)
  • nutrients (glucose, fattyacids, amino acids, vitamins minerals)
  • excretory products(urea and carbon dioxide)
  • heat

This exchange can be passive (no energy required - diffusion or osmosis) or active (energy is required - active transport)

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Surface area to volume ratio

Exchange taked place at the surface of an organism, but the materials absorbed are used by the cells that mostly make up its volume. For exchange to be effective, the surface area of the organism must be large compared with its volume.

Small organisms have a surface area that is large enough, compared with their volume, to allow efficient exchange across their body surface. However, as organisms become larger their volume increase at a faster rate than their surface area. Because of this simple diffusion of materials across the surface can only meet the needs of relatively inactive organisms. Even if the surface area could supply enough material, it would still take too long for it to reach the middle of the organism if diffusion alone was the method of transport. To overcome this problem, organisms have evolved one or more of the followinf features:

  • a flattened shape so that no cell is ever far from the surface (flatworm)
  • specialised exchange surfaces with large area to increase the surface area to volume ratio (lungsin mammals, gills in fish)
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Features of a specialised exchange surface

To allow effective transfer of materials across them by diffusion or active transport, exchange surfaces show the following characteristics:

  • a large surface area to volume ratio to increase the rate of exchange
  • very thing so short diffusion distance
  • partially permeable membrane
  • movement of the environmental medium
  • movement of the internal medium

Diffusion is proportional to surface area x difference in concentration divided bt length of diffusion path

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Gas exchange in single celled organisms

Single-celled organisms are small and therefore have a large surface area to volume ratio. Oxygen is absorbed by diffusion across their body surface, which is covered only by a cell-surface membrance. In the same way, carbon dioxide from respiration diffused out across their body surface. Where a living cell is surrounded by a cell wall, this is completely permeable and so there is no barrier to the diffusion of gases

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Gas exchange in insects

Most insects are terrestrial(live on land) The problem for them is water evaporates easily from the surface of their bodies and they can become dehydrated.They therefore need to conserve water. However, efficient gas exchange requires a thin, permeable surface with a large area. These features conflict with the need to conserve water. Overall, as a terrestrial organism, the insect has to balance the opposing needs of exchanging respiratory gases with reducing water loss.

To reduce water loss, terrestrial organisms usually exhibit two features:

  • Wateproof coverings - over their body surfaces. in the cases of insects this covering is a rigid outer skeleton that is covered with a waterproof covering.
  • Small surface area to volume ration - to minimise the area over which water is lost.

These features mean that insects cannot use their body surface to diffuse respiratory gases in the way single-celled organisms can. Instead they have developed an internal network of tubes called tacheae. The tracheae are supported by strengthened rings to prevent them from collapsing. The tracheae divide into smalled tubes called tracheoles. The tracheoles extend throughout all the body tissues of the insect. In this way atomospheric air, with the oxygen it contains, is brought directly to the respiring tissue

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Gas exchange in insects (2)

Respiratory gases move in and out of the tracheal system in two ways:

  • Along a diffusion gradient
  • Ventilation
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Gas exchange in fish

Fish have a water-proof, gas-tight outer converin. Being large they have a small surface are to volume ratio. Their body surface is therefore not adequate to supply and remove their respiratory gases and so they have a specialised internal gas exchange surface: the gills

Structure of gills:

  • located within th ebody of the fish behind the head
  • made up of gill filaments
  • the gill filaments are stacked up in a pile
  • at right angles to the filaments are the gill lamellae which increase surface area
  • water is taken in through the mouth and forced over the gills and out through an opening on each side of the body
  • the flow of water over the gill lamellae and the flow of blood within them are in opposite directions - countercurrent flow
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The countercurrent exchange principle

The essential feature of the countercurrent exchange system us that the blood and the water that flow over the gill lamellae do so in opposite directions. This arrangement means that:

  • Blood that is already well loaded with oxygen meets water which has its maximum concentration of oxygen and diffusion of oxygen from the water to the blood takes place
  • Blood with little or no oxygen will meet with water which has had most but not all oxygen removed and again diffusion of oxygen from the water to teh blood takes place

There is a fairly constant rate of diffusion across the entire length of the gill lamellae and about 80% of the oxygen available in the water is absorbed into the blood of the fish. If the flow of blood had been in the same direction as the flow of water the diffusion gradient would only be maintained across part of the length of the gill lamellae and only 50 % of the available oxygen would be absorbed in to the blood

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Gas exchange in the leaf of a plant

Like animal cells all plant cells take in oxygen and produce carbon dioxide during respiration. When it comes to fas exchange however plants show one important difference from animals. some plant cells carry out photosynthesis.

During photosynthesis, plant cells take in co2 and produce o2. at times the gases produced in one process can be used for other. this reduces the need for gas exchange with the external air. Overall this means that the volumes and types of gases that are being exchange by a plant leaf change. This depends on the balance between the rates of photosynthesis and respiration.

  • When photosynthese is taking place, although some co2 comes from respiration of cells, most of it has to be obtained from the external air. in the same way some oxygen from photosyntesis is used in respiration but most of it diffuses out of the plant
  • When photosynthesis is not occurring e.e in the dark, o2 diffuses into the leaf because it is constantly being used by cells during respiration. in the same way, carbon dioxide produced druring respiration diffuses out 
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Stucture of a plant leaf and gas exchange

In some ways gas exchange in plants is not unlike that of insects:

  • No living cell is far from the external air and therefore a source of o2 and co2
  • diffusion takes place in the gas phase (air) which makes it more rapid than if it were in water
  • Overall therefore there is a short fast diffusion pathway. In addition a plant leaf has a very large surface area compared with the volume of living tissue. For these reasons, no specialised transport system is needed for gases, which simply move in and through the plant by diffusion.

Most gaseous exchange occurs in leaves which show the following adaptations for rapid diffusion:

  • a thin, flat shape that provides a large surface area
  • many small pores called stomate mostly in the lower epidermis
  • numerous interconnecting air-spaces that occur throughout the mesophyll
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  • Stomata are minute pores which occur mainly but not exclusively on the leaves, especially the underside.
  • Each stoma (singular) is surrounded by a pair of special cells (guard cells).
  • These cells can open and closed the stomatal pore.
  • In this way they can control the rate of gaseous exchange.
  •  This is important because terrestrial organisms lose water by evaporation.
  • Plants have to balance the conflicting needs of gas exchange and control of water loss.
  • They do this by completely or partly closing stomata at times when water loss would be excessive.
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For a detailed diagram of a closed stoma go to the image was too large in size to put onto a card

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Circulatory system in a mammal

Diffusion is adequate for transport over short distances. The efficient supply of materials over large distances requires a mass transport system.

Why large organims need a transport system:

All organisms need to exchange materiasl between themselves and their environment. In small organisms this exchange takes place over the surface of the body. However with increasing size, the surface area to volume ratio decrease to a point where the needs of the organism cannot be met by the body surface alone. A specialist exchange surface is therefore needed to absorb nutrients and respiratory gases, and to remove excretory produces. These exchange surfaces are loacted in specific regions of the organism. A transport system is required to take materials from cells to the exchange surfaces and from the exchange surfaces to the cells. Materials have to be transported between exchange surfaces and the environment. They also need to be transported between different parts of the organism. As tissues and organs of which they are made have become more specialised and dependent upon one another. This makes the transport sustem all the more essential

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Circulatory system of a mammal (2)

Whether of not there is a specialised transport medium and whether or not it is circulate by a pump depends on two factors:

  • the surface area to volume ratio
  • how active the organism is

The lower the surface are to volume ratio the more active the organism, the greater is the need for a specialised transport system with a pump.

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Features of transport systems

Any large organism encounters the same problems in transporting materials within itself. Not surprisingly the transport systems of many organisms have many common features:

  • A suitable medium in which to carry materials e,g blood. This is normally a liquid bases on water because water readily dissolves substances and can be moved around easily
  • A form of mass transport in which the transport medium is moves around in bulk over large distances
  • A closed system of tubular vesselshat contains the transport medium and forms a branching network to distribute it toall parts of the organism
  • A mechanism for moving the transport medium within vessels. This requires a pressure difference between one part of the system and another, This is achieved in two main ways: (a) Animals use muscular contraction either of the body muscles or of a specialised pumping organ e.g. heart. or (b) plants do not possess muscles and so often rely on passive natural physical processes such as evaporation of water
  • a mechanism to maintain the mass flow movement in one direction e.g valves
  • A means of controlling the flow of the transport medium to suit the changing needs of different parts of the organism
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Transport systems in mammals

  • Mammals have a closed blood system in which blood is confined to vessels.
  • A musclular pump called the heart circulates the blood around the body,
  • Mammals have a double circulatory system.
  • This refers to the fact that blood passes twice through the heart for each complete circuit of thebody.
  • This is because when blood is passed through the lungs its pressure is reduces. If it were to pass immediately to the rest of the body its low pressure would make circulation very slow. Blood is therefore returned to the heart to boost its pressure before being circulated to the rest of the tissues.
  • Substances are then delivered to the rest of the body quickly, which is necessary as mammals have a high body temperature and hence a high rate of metabolism.
  • The vessels that make up the circulatory sustem of a mammal are divided into three types: arteries, veins and capillaries.

Although a transport system is used to move substances longer distances, the final part of the journey into cells is by diffusion. The final exchange from blood vessels into cells is rapid because it take place over a large surface area, across a short diffusion distance and there is a steep diffusion gradient.

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mammalian circulatory system


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Structure of blood vessels

There are different types of blood vessels:

  • Arteries - carry blood away from the heart and into arterioles
  • Arterioles - are smaller arteries that control blood flow from arteries to capillaries
  • Capillaries  - are tiny vessels that link arterioles to veins
  • Veins - carry blood from capillaries back to the heart

Arteries, arterioles and veins all have the same basic layered structures. For the outside inwards, these layers are:

  • tough outer layer - that resists pressure changes from both within and outside
  • Muscle layer - that can contract and so control the flow of blood
  • Elastic layer - that helps to maintain blood pressure by stretching and springing back
  • Thin inner lining (endothelium) - that is smooth to prevent friction and thin to allow diffusion
  • Lumen - that is not actually a layer but the central cavity of the blood vessel through which the blood flows.
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Artery and Vein structures


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Artery structure related to function

The function of arteries is to transport blood rapidly under high pressure from the heart to the tissue. Their structure is adapted to this funstion as follows:

  • The muscle layes is thick compared to veins - this means smaller arteries can be constricted and dilated in order to control the volume of blood passing through them
  • The elastic layer is relatively thick compared to veins - because it is important that blood pressure in arteries is kept high if blood is to reach the extremeties of the body. The elastic wall is stretched at each beat of the heart (systole). It then springs back when the heart relaxes (diastole) in the same way as an elastic band. This stretching and recoil action helps to maintain high pressure and smooth pressure surges created b the beating of the heart.
  • The overall thickness of the wall is large - this also resists the vessel bursting under pressure
  • There are no valves - (except in the arteries leaving the heart) because blood is under constant high pressure and therefore does not tend to flow backwards
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Arteriole structure related to function

Arterioles carry blood under low pressure than arteries, from arteries to capillaries. They also control the flow of blood between the two. Their structure is related to these functions as follows:

  • The muscle layer is relatively thicker than arteries - the contraction of this muscle layer allows constriction of the lumen of the arteriole. This restricts the flow of blood and so controls its movement into the capillaries that supply the tissue with blood.
  • The elastic layer is relatively thinner than in arteries - because bloos pressure is lower.
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Vein structure related to function

Veins transport blood slowly, under low pressure, from the tissues to the heart. Their structure is related to this function as follows:

  • The muscle layer is relatively thin - compared to arteries because veins carry blood away from tissues and thereore their constriction and dilation cannot control the flow of blood to the tissues
  • The elastic layer is relatively thin - compared to arteries because the low pressure of blood within the veins will not cause them to burst and pressure is too low to create a recoil action.
  • The overall thickness of the wall is small - because there is no need for a thick wall as the pressure within the veins is too low to create any risk of bursting. It also allows them to be flattened easily, aiding the flow of blood within them.
  • There are valves throughout - to ensure that blood does not flow backwards, which it might otherwise do because the pressure is so low. When body muscle contract, veins are compressed pressurising the blood within them. The valves ensure that this pressure directs the blood in one direction only: towards the heart
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Capillary structure related to function

The function of capillaries is to exchange metabolic materials such as oxygen, carbon dioxide and glucose between the blood and the cells of the body. The flow of blood in capillaries is much slower. This allows more time for the exchange of materials. The structure of capillaries is related to their function as follows:

  • Their walls consist only of the lining layer - making them extremely thin, so the distanceover which diffusion takes place is short. This allows for rapid diffusion of materials between the blood and the cells.
  • They are numerous and highly branched - thus providing a large are for diffusion
  • They have a narrow diameter - and so permeate tissues, which means that no cell is far from a capillary
  • Their lumen is so narrow - that red blood cells are squeezed flat against the side of the capillary. This brings them even closer to the cells to which they supply oxygen. This again reduces the diffusion distance
  • There are spaces between the lining(endothelial) cells - that allow whilte blood cells to escape in order to deal with infections within tissues

Although capillaries are small they cannot serve every single cell directly. Therefore the final journey of metabolic materials is made in a liquid solution that bathes the tissues. This is called tissue fluid.

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Tissue fluid and its formation

  • Tissue fluid is a watery liquid that contains glucose, amino acids, fattty acids, salts and oxygen.
  •  Tissue fluid supplies all of these substances to the tissues.
  • In return, it receives carbon dioxide and other waste materials from the tissues.
  • Tissue fluid is therefore the means by which materials are exchanged between blood and cells and, as such, it bathes all the cells and is, in effect, where they live.
  • Tissue fluid is formed from blood plasma, and the composition of blood plasma is controlled by various homeostatic systems.
  • As a result tissue fluid provides a mostly constant environment for the cells it surrounds.
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Formation of tissue fluid

Blood pumped by the heart passes along arteries, then the narrower arterioles and finally, the even narrower capillaries. This creates a pressure, called hydrostatic pressure, at the arterial end of the capillaries. This hydrostatic pressure forces tissue fluid out of the blood plasma. The outward pressure is, however, opposed by two other forces:

  • hydrostatic pressure of the tissue fluid outside the capillaries, which prevents outward movement of liquid
  • the lower water potential of the blood due to the plasma proteins, that pulls water back into the blood within the capillaries

However, the combined effect of all these forces is to create an overall pressure that pushes tissue fluid out of the capillaries. This pressure is only enough to force small molecules out of the capillaries, leaving all cells and proteins in the blood. The type of filtration under pressure is called ultrafiltration.

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Return of tissue fluid to the circulatory system

Once tissue fluid has exchange metabolic materials with the cells it bathes, it must be returned to the circulatory system. Most tissue fluid returns to the blood plasma directly via the capillaries. This return occurs as follows:

  • The loss of the tissue fluid from the capillaries reduces the hydrostatic pressure inside them
  • As a result, by the time the blood has reached the venous end of the capillary network its hydrostatic pressure is less than that of the tissue fluid outside it
  • Therfore the tissue fluid id forced back into the capillaries by the higher hydrostatic pressure outside them
  • In additions, the osmotic forces resulting form the proteins in the blood plasma pull water back into the capillaries
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Return of tissue fluid to the circulatory system (

The tissue fluid has lost much of its oxygen and nutrients by diffusion into the cells that it bathed, but it has gained carbon dioxide and waste materials in return

Not all the tissue fluid can return to the capillaries; the remainder is carried back via the lymphatic system. This is a system of vessels that begin in the tissues, Initially the resemble capillaries, but they gradually merge into larger vessels that form a network around the body. Tehse larger vessels drain their contents back into the bloodstream via two ducts that join veins close to the heart.

The contents of the lymphatic system (lymph) are not moved by the pumping heart. Instead they are moved by:

  • hydrostatic pressure - of the tissue fluid that has left the capillaries
  • contraction of body muscles - valves in the lymph vessels ensure that the fluid inside them moves away from the tissues in the direction of the heart
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Basic model of capillary structure


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Movement of water through roots

The vast majority of plants are errestrial organisms. As a result they need to conserve water and so they are covered by a waterproof layer. Therefore they cannot absorb water over the general body surface. Instead, they have a special exchange surface in the soil: the root hairs.

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Uptake of water by root hairs

Root haris are the exchange surfaces in plants that are responsible for the absorption of water and mineral ions. Plants constantly lose water by the process of transpiration. This loss can amount to up to 700dm3  per day in a  large tree. All of this water must be replaced by water that is absorbed through the root hairs.

Each rot hair is a long, thin extension of a root epidermal cell. These root hairs remain functional for a few weeks before dying back, to be replaced by others nearer the growing tip. They are efficient surfaces for the exchange of water and mineral ions because:

  • they provide a large surface area as they are very long extensions and occur in thousands on each of the branches of a root
  • they have a thin surface layer (the cell-surface membrane and cellulose wall) across which materials can move easily
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Uptake by root hairs (2)

Root hairs arise from epidermal cells a little way behind the tips of younf roots. These hairs gro into the spaces around soil particles. In damp conditions they are surrounded by a soil solution which contains small quantities of mineral ions. The soil solution is, however, mostly water and therefore has a very high water potential - only slightly less than than zero. In contrast, the root hairs and other cells of the root, have sugars, amino acids and other cells of the roots, ahve sugars, amino acids and mineral ions dissolved in them. These cells therefore have a much lower water potenitial. As a result, water moves by osmosis from the soil solution into the root-hair cells down this water potential gradient.

After being absorbed into he root-hair cell, water continues its journey across the root in two ways:

  • The apolplastic pathway (the apoplast)
  • The symplastic pathway (the symplast)
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The apoplastic pathway

  • As water is drawn into endodermal cells, it pulls more water along behind it, due to conhesive propeties of the water molecules.
  •  This creates a tension that draws water along the cell walls of these cells of the root cortex.
  • The mesh-like structure of the cellulose cell walls of these cells has many water-filled spaces and so there is little or not resistance to this pull of water along the cell walls.     
  •                 (
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The symplastic pathway

This takes place across the cytoplasm of the cells of the cortex as a result of osmosis. The water passes through the cell walls along tiny openings called plasmodesmata. Each plasmodesma (singlular) is filled with a thin strand of cytoplasm. Therefore there is a continuous column of cytoplasm extending from the root-hair cell to the xylem at the centre of the root. Water moves along this column as follows:

  • Water entering by osmosis increase the water potential of the root hair cell
  • The root hair cell now has a higher water potential than the first cell in the cortex
  • Water therefore moves from the root hair cell to the first cell in the cortex by osmosis, down the water potential gradient
  • The first cell now has a higher water potential than its neigbour to the inside of the stem
  • Water therefore moves into this neighbouring cell by osmosis along the water potential gradient
  • This second cell now has a higher water potential that its neighbour to the inside, and so water moves from the second cell to the third cell by osmosis along the water potential gradient
  • At the same time, this loss of water from the first cortical cell loweds its water potential causeing more water to enter it by osmosis from the root hair cell.
  • In this way, a water potential gradient is set up across all the cells of the cortex which carries water along the cytoplasm from the root-hari cell to the endodermis
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Passage of water into the xylem

  • When water reaches the endodermis by the apolplastic pathway, the waterproof band that makes up the Casparian ***** in endodermal cells prevents it from progressing further along the cell wall. At this point, water iss forces into the living protoplast of the cell, where it joins water that has arrives there by the symplastic pathway
  • Active transport of salt is the most likely mechanism by which water now gets into the xylem. Endodermal cells actively transport salts into the xylem. This proces requires energy and therefore can only occur within living tissue. It takes place along carrier proteins in the cell surface membrane. If water is to enter the xylem, it must first enter the cytoplasm of the endodermal cells. This explains why the water from the apoplastic pathway is forced into the cytoplasm of the endodermal cells by the Casparian *****
  • The active transport of mineral ions into the xylem by the endodermal cells creastes a lower water potential in the xylem. Water now moves into the xylem, by osmosis, along a water potential gradient. This water potential gradient is the result of the active transport of salts into the xylem from the endodermal cells. This creates a force that helps to move water up the plant. This force is called root pressure. While its contribution to water movement up a large tree is minimal compared to the transpiration pull, root pressure can make a significant contribution to water movement in small, herbaceous plants.
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Evidence for root pressure existence

Evidence for the existence of root pressure due to the active pumping of salts into the xylem includes the following:

  • The pressure increases with a rise in temperature and decreases at lower temperatures
  • Metabolic inhibitors, such as cyanide, prevent most energ release by respiration and also cause root pressure to cease
  • A decrease in the availability of oxygen or respiratory substrates causes a reduction in root pressure.
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Movement of water up stems

The main force that pulls water up the stem of a plant is the evaporation of water from leaves - a process called transpiration. It is therefore logical to begin from the oiubt where water molecule evaporate from the leaves, through the tiny openings called stomata, on the surface of the leaf.

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Movement of water out through stomata

  • The humidity of the atmosphere is usually less that that of the air spaces next to the stomata.
  •  Provided the stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air.
  • Water lost from the air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells.
  • By changing the size of the stomatal pores, plants can control their rate of transpiration.
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movement of water across the cells of a leaf

Water is lost from mesophyll  cells by evaporation from their surfaces. to the air spaces of the leaf. This is replaced by water reaching the mesophyll cells from the xylemby either the apoplastic or symplastics pathways. In the case of the symplastic pathway, the movement occurs because:

  • mesophyll cells lose waater to the air spaces,
  • these cells now have a lower water potential and so enters by osmosis into neighboring cells
  • the loss of water from these neighbouring cells lowers their water potential
  • they in turn take in wter from their neighbours by osmosis

In this way, a water potential gradient is established that pulls the water from the xylem, across the leaf mesophyll and finally out into the atmosphere

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Movement of water up the stem in the xylem

The two main factors that are responsible for the movement of water up the xylem, from the roots to the leaves, are cohesion-tension and root pressure. Cohesion-tension theory  operates as follows:

  • Water evaporates from the leaves as a result of transpiration
  • Water molecules form hydrogen bonds between one another and hence stick togehter. This is known as cohesion.
  • Water forms a continuous unbroken pathway across the mesophyll cells and down the xylem
  • As water evaporates from the mesophyll cells in the lead into he air spaces beneath the stomata, more molecules of water are drawn up behind it as a result of cohesion.
  • Water is hence pulled up the xylem as a result of transpiration. This is called the transpiration pull
  • Transpiration pull puts the xylem under tension i.e there is a negative pressure within the xylem, hence the name cohesion-tension theory
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Evidence that supports cohesion theory

Such is the force of transpiration pull that it can easily raise water up the 100m or mnore of the tallest trees. There are several pieces of evidence to support the cohension-tension theory: These include:

  • Change in diameter of tree trunks according to the rate of transpiration. During the day, when transpiration is at its greatest there is more tension(more negative pressure) in the xylem. This causes the trunk to shrink in diameter. At night when transpiration is at its lowest, there is less tension in the xylem and so the diameter of the trunk increases
  • If a xylem vessel is broken and air enters it, the tree can no longer draw up water. This is because the continuous column of water is broken and so the water molecules can no longer stick together
  • When xylem vessel is broken, water does not leak outm as would be the case under pressure, Instead air is drawn in which is consistent with it being under tension

Transpiration pull is a passive process and therefore doesn't require metabolic energy to take place. Indeed the xylem vessels through which the water passes are dead and so cannot actively more the water. As they are dead their walls can break down. This means that xylem forms a series of continuous unbroken tubes from root to leaves which is essential to the cohension-tension theory of water flow up the stem. Energy is nevertheless needed to drive the process of transpiration, This energy is in the form of heat that ultimately comes from the sun

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Role of transpiration

Transpiration is sometimes referred to as 'necessary evil'. This is because although transpiration is universal in floweringplants, it is the unavoidable result of plants having leaves adapted for photosynthesis. Leaves have a large surface area to absorb light, and stomata to allow inward diffusion of carbon dioxide. Both deatures result in an immense loss of water: up to 700dm3 per day in a large tress. Although transpitation helps bring water to the leaves, it is not essential because osmotic processes could achieve this. Less than 1 per cent of water moved in the transpiration streak is used by the plant

Materials such as minerals ions, sugars and hormones are moved around the plant dissolved in water. This water is carried up the plant by the transpiration pull. Without transpiration, water woudl not be so plentiful and the transport of materials would not be rapid

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Factors affecting transpiration

A number of factors affect the rate of transpiration. These include the following:

Light: Stomata are the openings in leaves through which the carbon dioxide needed for photosynthesis diffuses. Photosynthesis only occurs in the light. It follows that the stomata of most plants open in the light and close in the dark. When stomata are open, water moves out of the leaf into the atmosphere. Consequently and increase in light intensity causes an increase in the rate of transpiration

Temperature: Temperature changes affect two factors that influence the rate of transpiration. 1. how much water the air can hold i.e hte water potential of air 2. the speed at which the molecules move. A rise in temperature increases the kinetic energy and hense the speed of movement of water molecules. This increased movement of water molecules increases the rate of evaporation of water. This means that water evaporates more rapidly from the leaves and so the rate of transpiration increases and it decreases the amount of water air can hold i.e it decreases its water potential. Both these changes lead to an increase in transpiration rate. A reduction in temperature has the reverse effect; it reduces transpiration rate.

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Factors affecting transpiration (2)

Humidity - humidity is the measure of the number of water molecules in the air. The humidity of the air effects the water potential gradient between the air outside the leaf and the air inside the leaf. When the air outside the leaf has a high humidity, the gradient is reduced and the rate of transpiration is lower. Conversely, low humidity increases the transpiration rate.

Air movement - As water diffuses through stomata it accumulates as water vapour around the stomata on the ouside of the leaf. The water potential around the stomata is therefore increased. This reduces the water potential gradient between the moist atmosphere in the air spaces within the leaf and the frier air outside. The transpirationrate is therefore reduced. Any movement of air around the leaf will disperse the humid layer at the leaf surface and so decrease the water potential of the air. This increases the water potential gradient and hence the rate of transpiration. The faster the air movement, the move rapidly the humid air is removed and the greater the rate of transpiration.

The energy of transpiration comes from the evaporation of water from the leaves. All these factors are directly or indirectly the result of the sun's energy. Therefore it is the sun that ultimately drives transpiration

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Xerophytic plants

Plants that are adapted to live in very hot places and do not have a plentiful water supply Without these adaptations these plants would be come desiccated and die. Xerophytic plants have modifications designed to increase water uptake, to store water and reduce transpiration

Modifications to the leaves are needed to reduce the rate of transpiration:

  • a thick cuticle - the thicker the cuticle the less water can escape
  • rolling up of leave - a way that protects the lower epidermis from the outside which helps to tap a region of still air in the rolled leaf. This regions then becomes saturated with water vapour and so there is no water poterntial gradient between the inside and the ouside of the leaf. As there is not water potential gradient, transpiration is considerably reduced
  • hairy leaves - traps moist air next to the leaf surface, reducing the water potential gradient between the inside and outside of the leaf therefore less water is lost by transpiration.
  • Stomata pits/grooves - trap moist air next to the leaf and reduce the water potential gradient.
  • A reduced surface area to volume ration of the leaves - reduces rate of water loss. must always be balanced for sufficient area for photsynthesis
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Rachel Forster


is slide 8 abiut the countercurrent exchange system the wrong way round or do i not understand? because it says that water with high O2 meets blood with high O2 and O2 diffuses into the blood....?

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