AQA AS biology....gaps in knowledge of exchange and transport

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)

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)

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

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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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.

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

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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Apoplastic:

• As H2O drawn into endodermal cells it pulls more in behind it due to cohesive property of H2O molecules. This creates tension that draws H2O along cell walls of cells of root cortex.
• Mesh-like structure of cell walls has water-filled spaces so there is no resistance.

Symplastic:

• Takes place across cytoplasm of cells of the cortex by osmosis.
• H2O passes through walls along plasmodesmata (tiny openings), each filled with strand of cytoplasm to form a continuous column from root hair to xylem.
• 1. H2O enters by osmosis & increases water potential of root-hair cell.
• 2. Cell now has higher water potential than 1st cell in cortex so H2O moves to 1st cell by osmosis down concentration gradient.
• 3. 1st cell now has higher water potential than neighbour so H2O moves etc.
• 4. At same time, this loss of H2O decreases water potential of cell causing more water to enter and the process repeats. A water potential gradient is set up.

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.

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

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)

• 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. 

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

• 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.

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.

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.

• 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

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

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

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

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

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.

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

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

Out Through Stomata - humidity of atmosphere is less than that of air spaces next to stomata. Water vapour molecules diffuse out of air spaces into surroundings. Water lost from the air is replaced by water evaporating from cell walls of mesophyll cells. Control rate by changing size of pores.

Across the Cells of a Leaf - H2O is replaced to mesophyll cells from xylem by AP or SP. In SP, movement occurs because of cells lose H2O to air spaces so have a lower water potential & H2O enters from neighbouring cells, which in turn take H2O from their neighbour by osmosis.

Up the Stem in the Xylem - 2 main factors are root pressure and cohesion-tension. H2O evaporates due to transpiration. Molecules form H bonds so stick together (cohesion). It forms a continuous, unbroken pathway across cells & down xylem. As H2O evaporates more molecules are drawn up because of cohesion. H2O is pulled up due to transpiration (transpiration pull). This puts the xylem under tension (cohesion-tension). Evidence includes: change in the diameter of tree trunks according to rate, at its greatest there is more tension so trunk shrinks; if a xylem vessel is broken then air enters & it can no longer draw up H2O as mols can't stick together. Air is drawn in not out due to tension.

Transpiration is the main force that pulls water up the stem in evaporation. Diffusion and photosynthesis result in a huge loss in H2O. Transpiration helps bring H2O to leaves but osmosis also does this. Materials such as ions, sugars & hormones are moved dissolved in H2O carried by the transpiration pull. Without it, it would be slow. Factors:

• Light - photosynthesis only occurs in light. Stomata open in the light so H2O moves out, increase in light intensity increases rate of transpiration.
• Temperature - changes affect 2 factors that influence rate: how much H2O the air can hold (i.e. water potential) and speed at which molecules move. A rise in temp increases kinetic energy & so speed of movement of mols, this increases the rate of evaporation & hence the transpiration rate. It also decreases the amount of H2O air can hold which increases rate.
• Humidity - this is a measure of the numberof H2O mols in air. It affects the water potential grad between air inside & outside leaf. When outside air has high humidity the gradient is reduced and rate is lower.
• Air movement - as H2O diffuses out it accumulates outside the leaf, reducing the water potential gradient so the rate is reduced. Movement of air will disperse the humid layer & increase the rate of diffusion.
• Ultimately, it is the sun that drives transpiration.

Xerophytic Plants - plants that do not have a plentiful water supply develop adaptations to limit water loss through transpiration. Increase water uptake & storage.

• Thick cuticle - waterproof barrier. The thicker it is the less that is lost e.g. holly.
• Rolling up leaves - stomata confined largely to lower epidermis. Leaves roll to protect lower epidermis from the outside so trap a region of air. it becomes saturated so there is no water potential gradient & transpiration is reduced. Good in hot/windy conditions e.g. marram grass.
• Hairy leaves - thick layer, especially on lower epidermis trap moist air so the gradient is reduced e.g. heather.
• Stomata in pits/grooves - trap moist air to reduce gradient e.g. pine trees.
• Reduced s.a. to volume ratio - slows diffusion, must be balanced against needs of photosynthesis e.g. pine needles.

Xerophytes found in the desert, sand dunes, places with high rainfall but low temps, salt marshes near the coast and cold regions.