Chapter 9


The need for plant transport systems

Plants have transport systems that move substances between leaves, stems and roots at tremendously high pressures confined in very small spaces. There are 3 mains reasons why multicellular plants need transport systems:

  • Metabolic demands: The cells of the green parts of the plant make their own glucose and oxygen by photosynthesis, but many internal and underground parts of the plant do not photosynthesise. They need for oxygen and glucose transported to them and the waste products of cell metabolism removed, hormones made in one part of the plant need transporting to the area where they have an effect, and mineral ions absorbed by the roots need to be transported to all cells to make the proteins required for enzymes and the structure of the cell.
  • Size: Some plants are very small but becuase plants continue to grow throughout their lives, many perrenial plants vevome enourmous, meaning they need a very effective transport system to move substances both up and down from the tip of the roots to the leaves and stems.
  • Surface area: Leaves are adapted to have a large SA:V ratio for the exchange of gases with air, but taking into account the stems trunks and roots means they still have quite a small SA:V ratio, so they cannot rely on diffusion alone to meet the demands of their cells.
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Transport systems in dicotyledonous plants

Dicotyledonous plants (dicots) make seeds that contain 2 cotyledons, organs that act as food stores for the developing embryo and form the first leaves when the seed germinates. Herbaceous dicots ahve soft tissues and a short life cycle (leaves and stems that dye down in winter to soil level) and woody (arborescent) dicots which have hard lignified tissues and a long life cycle. Dicots have a series of transport vessels running through them stem, roots and leaves known as the vascular system, in herbaceous dicots this is made up of two main transport vessels, the xylem and the phloem, which are arranged together in vascular bundles in the leaves, stems and roots. 

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The structure and functions of the xylem

The xylem is largely non-living tissue that has 2 main functions in a plant - the transport of water and mineral ions, and structural support. The flow of materials in the xylem is up from the roots to the shoots and leaves, xylem is made up of several types of cells, most of which are dead when they are functioning in the plant. They are long hollow structures made by severla columns of cells dusing together end to end, these are the xylem vessles and are the main structures. Thick walled xylem parenchyma packs around the xylem vessels, storing food and containing tannin deposits, which is a bitter tasting chemical that protects plants from attack from herbivores. Xylem fibres are long cells with lignified secondary walls which provide extra mechanical strength but don't transport water. Lignin can be layed down in several different ways, forming spirals, rings or tubles with small unlignified areas called bordered pits where water leaves the xylem and moves into other cells, these available forms are called tracheids.

Xylem vessles can be seen clearly stained in cross sections of the plant, you can soak a plant in dye for 24 hours and then make a transverse cut which will show the coloured spots which will be the xylem vessles.

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The structure and functions of the phloem

Phloem is a living tissue that transports food in the form of organic solutes around the plant from the leaves where they are made by photosynthesis. The phloem supplies the cells with sugars and amino acids they need for cellular respiration and for synthesis of other useful molecules. The flow of materials in the phloem can go up and down the plant. The main transporting vessel of the phloem are the sieve tube elements. Like xylem, the phloem sieve tubes are made up of many cells joined end to end to form long, hollow structures, but the phloem tubes are not lignified. In the areas between the cells, the walls become perforated to form sieve plates, which let the phloem contents flow through. As the large pores appear in these cell walls, the vacuole membrane, nucelus and some other organelles break down, so the phloem becomes a tube filled with sap and the matured phloem cells have no nucleus. 

Closely linked to the sieve tube elements are the companion cells, which form with them. These cells are linked the sieve tube elements by many plasmodesmata - microscopic channe;s through the cellulose cell walls linking the cytoplasm of adjacent cells. The companion cells maintain their nucleus and organelles and are very active cells and act as a life support system for the sieve tube cells that have lost most of their normal cell functions. Phloem tissue also contains supporting tissue including fibres and sclereids, cells with extremely thick cell walls. 

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Water transport in plants

Water is key both in the structure and in the metabolism of plants: Turgour pressure as a result of osmosis in plant cells provides a hydrostatic skeleton to support the stems and leaves, turgour also druves cell expansion and is the force that enables plant roots to force their way through tarmacm the loss of water by evapouratuib helps to keep plants cool, mineral ions and the products of photosynthesis are transported in aqeuous solutions, and water us a raw material for photosynthesis. 

Root hair cells are the exchange surface in plants where the water is taken into the body of the plant from the soil, a root hair is a long thin extension which is a specialsed epidermal cell found near the root tips. Their microscopic size means they can penetrate easily through soil particles, each one has a large SA:V ratio and there are thousands on each root tip, each hair has a thin surface area through which osmosis and diffusioncan take place quickly, and the concentration of solutes in the cytoplasm of root hair cellsn maintains a water potential gradient between the soil water and the cell. Soil water has a low solute potential and a high water potential, whereas the cytoplasm abd vacuolar sap of root hair, and root cells contain many different solutes including sugars, mineral ions and amino acids so the water potential in the cel is lower, and as a result water moves into the root hair cells by osmosis. 

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Movement of water across the root

Once the water has moved into the root hair cell it continues to move across the root to the xylem through 2 different pathways:

The symplast pathway: Water moves through the symplast - the continuous cytoplasm of the living plant cells that is connected through the plasmodesmata - by osmosis. The root hair cell has a higher water potential than the next cell along, which is the result of water diffusing in from the soil making the cytoplasm more dilute. So water moves from the root hair cell to the next cell along by osmosis and this process continues from cell to cell across the root until the xylem is reached. As water leaves the root hair cell by osmosis, the water potential of the cytoplasm falls again, maintaining a steep water potential gradient to ensure as much water as possible continues to move into the cell from the soil.

Apoplast pathway: This is the movement of water through the apoplast - the cell walls and intercellular spaces. Water fills the spaces between the loose, open network of fibres in the cellulose cell wall.As water molecules move into the xylem, mpre are pulled thriugh the apoplast due to the cohesive forces between the water molecules creating a tension that means there is a continuous flow of water through the open structure of the cellulose cell wall, which offers little or no resistance. 

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Movement of water into the xylem

Water moves across the root in the apoplast and symplast pathways until it reaches the endodermins - the layer of cells surrounding the vascular tissue of the roots. The endodermis is noticeable in the roots because of the effect of the Casparian *****, which is a band of waxy material called suberin that runs around each of the endodermal cells forming a waterproof layer. At this point wtaer in the apoplast pathway cna go no further and is forced into the cytoplasm of the cell, joining the water in the symplast pathway. This diversion is significant as to get there it must mast through selectively permeable cell surface membranes that can exclude any toxic solutes from the soil reaching the living tissues, as the membranes will have no carrier proteins to admit them. Once forced into the cytoplasm the water joins the symplast pathway. The solute potential in the cytoplasm of the endodernal cells is dilute compard to the cells in the xylem, and it appears tgat the endodermal cells move mineralmions into the xylem by active transport. As a result the water potenial in the xylem is much lower than that of the endodermal cells, increasing the rate of water moving into the xylem down a water potential gradient from the endodermis via the symplast pathway. Once inside the vascular bundle, water returns to the apoplast pathway to enter the xylem itself and move up the plant. The active pumping of minerals unto the xylem results in root pressure and is independant of the effects of transpiration. Root pressure gives wtaer a push up the xylem, but under most circumstances is not the main factor in the movement of water. 

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Evidence for the role of active transport in root

  • Some poisons such as cyanide affect the mitochondria and prevent the production of ATP, if cyanide is applied to root cells so there is no energy supply, the root pressure disappears.
  • Root pressure increases with a rise in temperature and falls at lower temperatures, suggesting chemical reactions are involved.
  • If levels of oxygen or respiratory substrates fall, so does root pressure.
  • Xylem sap mat exclude from the cut end of stems at certain times, such as out of special pores at the end of leaves when transpiration is low, this is called guttation.
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Leaves have a very large surface area for capturinf sunlight for photosynthesis, their surfaces are covered in a waxy layer that makes them waterproof so the leaf doesn't lose water rapidly and constantly by evapouration from their surfaces. Carbon dioxide moves from the air into the leaf anf oxygen moves out of the leaf down concentration gradients through microscopic pores on the underisde of leaves called stomata, which can be opened or closed by guard cells which surround the stomatal opening. When the stomata are open tyo allow an exchange of carbon dioxide and oxygen between the air inside the leaf and the external air, water alos moves out by diffusion and is lost, this loss of water vapour from the leaves and stems of plants is called transpiration and is an inevitable consequence of gaseous exchange. Stomata open and close to control the amount of water lost by a plant, but during the day a plant needs tyo take in carbon dioxide for pphotosynthesis and at night when no oxygen is being produced it needs to take in oxyegn for cellular respiration, so some need to be open at all times.

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The transpiration stream

Once at the leaves, water moves by osmosis across membranes and by difusion in the apoplst pathway from the xylem through the cells the leaf where it evapourates from the freely permeabe cellulose cell walls of the mesophyll cells in the leaves into the air spaces, the water vapour then moves into the external air through the stomata. This is the transpiration stream. 

  • Water molecules evapourate from the surface of the mesophyll cells into the air spaces in the leaf and move out of the stomata into the surrounding air by diffusion down a concentration gradient.
  • The loss of water by evapouration from a mesophyll cell lowers the water potential of the cell, so water moves into the cell from an adjacent cell by osmosis, along both apoplast and symplast pathways, and this is repeated across the leaf to the xylem where water moves out of the xylem by osmosis into the cells of the leaf.
  • Water molecules from hydrogen bonds with the carbohydrates in the walls of the xylem vessels - adhesion, and with each other- cohesion, which results in capillary action where water can rise up a narrow tissue against gravity, and is a continuous stream that replaces the water lost by transpirtaion, this is thr transpiration pull, and results in tension in the xylem which helps water move across the roots from the soil
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The cohesion-tension theory

This mode; of water moving in a continuous stream up the xylem and across the leaf is known as the cohesion-tension theory. Several pieces of evidence suppport this theory:

  • Chages in the diameter of trees. When transpiration is at its height during the day so is the tension is the xylem vessels and as a result the tree shrinks in diameter, and when transpiration is at its lowest, so is the tension in the xylem vessels and the tree diameter increases.
  • When a xylem vessel is broken, air is drawn into the the xylem rather than leaking out, showing cohesive and adhesive charcteristics. 
  • If a xylem vessel is broken and air is pulled in, the plant can no longer move water up the stream as the continuous stream of water held together by cohesive forces has been broken.

In summary transpiration delivers water and the minera ions to the cells where they are needed, the evapouration also helps to cool the leaf and prevent heat damage, but water loss is a problem because the amount of water available is often limited. In high intensity sunlight when the plant is photosynthesising rapidly, there will be a high rate of gaseous exchange and the stomata will always be open and the plant may lose so much water through transpiration that the supply cannot meet the demand.

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Stomata - controlling the rate of transpiration

The main way in which the rate of transpiration is controlled by the plant is by the opening and closing of the stomatal pores, which is a turgor driven process. When turgor is low the asymmetric configuration of the guard cell walls close the pore. When the environmental conditions are favourable the guard cells pump in solutes, increasing their turgor. Cellulose hoops orevent the cells from swelling in width, so they extend lengthways. Because the inner wall of the guard cell is less flexible than the outer wall, the cell becomes bean-shaped and opens the pore. When water becomes scarce, hormonal signals from the roots can trigger turgor loss from the guard cells, which closes the stomatal openigns and conserves water.

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Factors affecting transpiration

Any factor affecting the rate of water loss from the leaves of a plant will affect the rate of transpiration, factors affecting this must either act on the opening/closing of the stomata, the rate of evapouration from the surfaces of the leaf, or the diffusion gradient between the air spaces in the leaf and the air surrounding the leaf:

  • Light is required for photosynthesis and in the light the stomata open for the gas exchange needed. In the dark most of the stomata will close. Increasing light intensity gives increasing amounts of open stomata, increasing the rate of water vapour diffusing out and therefore the increasing evapouration from the surface of the leaf, increasing the rate of transpiration.
  • Relative humidity is a measure of the amount of water vapour in the air compared to a maxiumum concentration. A high relative humidity will lower the rate of transpiration because of the reduced water potential gradient between the inside of the leaf and external air, very dry air has the opposite effect and results in an increased rate of transpiration.
  • An increase in temperature increases the kinetic energy of the water moleculs and therefore increases the rate of evapouration from the spongy mesophyll cells into the air spaces of the leaf. An increase in temperature also increases the concentration of water vapour that the external air can hold, so decreases humidity and its water potential.
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Factors affecting transpiration II

  • Therefore increasing the temperature increases the diffusion gradient between the air inside and outside the leaf, so increasing the rate of transpiration.
  • Air movement - Each leaf has a layer of still air around it trapped by the shape of the leaf and features such as hairs on the surface of the leaf decrease air movement close to the leaf. The water vapour that diffuses out of the leaf accumulates here so that the water potential around the stomata increases, in turn reducing the rate of transpiration. Anything that increases the diffusion gradient will increase the rate of transpiration, so air movement or wind will increase the rate of transpiration, and a long period of still air will reduce the rate of transpiration.
  • Soil-water availabilty - The amount of water available in the soil can affect the transpiration rate, if it is very dry then the plant will be under water stress and the rate of transpiration will be reduced.
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The leaves of a plant produce large amounts of glucose, which is needed for respiration by all the cells of the plant, it is converted to sucrose for transport and once it has reached the cells where it is needed it is converted into eithert glucose for respiration or starch for storage. Plants transport organic compounds from soruces to sinks - tissues that need them, in a process called translocation. In many plants translocation is an active process that requires energy to take place and substances can be moved up or down the plant. The products of photosynthesis that are transported are known as assimilates. Although glucose is made in photosynthesis, the main assimilate transported around the plant is sucrose, and makes up about 20-30% of the sap content.

The main sources of assimilates in a plant are: Green leaves and stems, storage organs like tubers and tap roots that are unloading their food stores at the beginning of a growth period, and food stores in seeds when they germinate.

The main sinks in a plant are: Roots that are gorwing and/or actively absorbing mineral ions, mersitems that are actively dividing in the roots, shoots and vascular cambium, and any parts of the plant that are laying down food stores such as developing seeds, fruits or storage organs.

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Phloem loading

In many plants the soluble producrs of photosynthesis are moved into the phloem from the sources by an active process, sucrose being the main cabrohydrate that is being transported - it is not metabolised as readily as glucose so is less likely to be metabolised during the transport process, phloem laoding occurs passively in th symplast route, and actively in the apoplast route.

The symplast route: In some species of plants the sucrose from the sources moves through the cytoplasm of of the mesophyll cells and on into the sieve tubes by diffusion through the plasmodesmata. This route is largely passive as the sucrose ends up the sieve elements and water follows by osmosis, creating a pressure of water that moves the sucrose through the phloem by mass flow.

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Phloem loading II (Apoplast route)

In many plant species sucruse from the source travles through the cell walls and the inter-cell spaces to the companion cells and the sieve elemends (known as the apoplast route) by diffusion down a concentration gradient, maintained by the removal of surcrose into the phloem vessels. In the companion cells sucrose is moved into the cytoplasm across the cell membrane in an active process. H+ ions are actively pumped out of the companion cells into the surrounding tissues using ATP, and the H+ ions return to the companion cells down a concentration gradient via co-tranport proteins. Sucrose is the molecule that is co-transported. This increases the concentration of sucrose in the companion cells and in the sieve elements through the many plasmodesmata between the two linked cells. Complanion cells have many infoldings in their cell membranes to give an increases surface area for the active transport of sucrose into the cell cytoplasm, and they also have many mitochondria to supply the ATP needed for the transport pumps. As a result of the build up of sucrose in the companion cells and sieve tube elementsm qater alos moves in my osmosis. This leads to a build up of turgor pressure due to the rigid cell walls. The water carrying the assimilates moves into the tubes of the sieve elements reducing the pressure in the companion cells, and moves up or down the plant by mass flow to areas of lower pressure (the sinks). Solute accumilation in the source phloem leads to an increase in turgor pressure that forces sap to regions of lower pressure. These pressure differences are how plants transport solutes and water rapidly over many metres. Solutes are translocated up or down the plant depending on the position of the source.

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Phloem unloading

The sucrose is unloaded from the phloem at any point into the cells that need it. The main mechanism of phloem unloading is by diffusion into the surrounding cells. The sucrose rapidly moves into other cells by diffusion or is converted into other substances so that the concentration gradient of sucrose is maintained between the contents of the phloem and the surrounding cells. The loss of the solutes from the phloem leads to a rise in the water potential of the phloem, water moves out into the surrounding cells by osmosis, some of the water used to carryn the solutes is drawn into the transpiration stream. Evidence for translocation:

  • Advances in microscopy allow us to see the adaptations of the companion cells for active transport, and if the mitochondria of these cells are poisoned, translocation stops.
  • The flow of sugars in the phloem is 10,000 times faster than it would be with diffusion alone, suggesting that an active process is driving the mass flow.
  • Using aphids studies can show that there is a positive pressure in the phloem that forces sap out of the stylet, the pressure therefore the flow rate is lower closer to the sink than near the source, and the concentration of sucrose in phloem sap is higher nearer the source

However some questions remain as not all solutes in the phloem move at the same rate.

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Most plants have adaptations to ocnserve water, including a waxy cuticle to reduce transpiration rate and having stomata on the underside of the plant that can open/close through gaurd cells. However in hot, dry and breezy conditions, this is not enough as water evapourates rapidly off leaf surfaces. Plants in dry habitats have evolved a number of adaptations that enable them to live in these harsh conditions are called xerophytes. Cacti is an examples of xerophytes, along with many plants that live in cold and icy conditions. These are some ways xerophytes conserve water:

  • A thick wacy cuticle - this lowers water loss by lowering the rate of transpiration, it is seen commonly in evergreen plants and helps plant survive both hot summers and cold winters when water can be hard to absorb from the frozen or dry ground.
  • Suken stomata - many xerophyes have their stomata located in pits which reduces air movement, producing a microclimate of still, moist air that reduces the water potential gradient and so reduces transpiration, this can be seen in marram grass and cacti etc.
  • Reduced numbers of stomata - Many xerophytes have a reduced number of stomata as this reduces water loss by transpiration, but also reduces their gas exchange capabilities.
  • Reduced leaves - by reducing the leaf area water loss can be greatly reduced as thin leaves have a greatly reduced SA:V ratio, minimising water loss by transpiration.
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Xerophytes II

  • Hairy leaves - some have hairy leaves that like the spines of a cacti, create a microclimate of still humid air reducing the water vapour potential gradient and minimising the loss of water vapour from leaf surfaces, marram grass even have these in the stomatal pits.
  • Curled leaves - another adaptation that greatly reduces water loss when combined with other adaptations is the growth of curled leaves, as it confines all the stomata within a microenvironment of still humid hair, such as marram grass.
  • Succulents - these type of plants store water in specialised parenchyme tissue in their stems and roots, and so have a swollen or fleshy appearance. Water is stored when it is plrentiful in supply and then used in times of drought, aloe vera is a good example.
  • Leaf loss - some plants prevent water loss through their leaves by simply losing their leaves when water is not available and instead the trunk and branches turn green to photosynthesis with miniaml water to keep it alive, such as the Palo verde tree.
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Xerophytes III

  • Root adaptations - many xerophytes have root adaptations by having long tap roots growing deep into the ground so they can access water that is a long way below the surface, or a mass of widespread shallow roots with a large surface area so they can maximise water uptake after rain showers before the waterv evapourates. Many cacti show both of these adaptations. The root system of marram grass consists of long vertical roots that penetrate metres into the sand and a mat of horizontal rhizomes (modified stems) from which many more roots can develop to form an extensive network that can tchange their environment and enable the sand to hold more water.
  • Avoiding the problemts - some plants cope with low water availability by avoiding the situation entirely, plants may lose their leaves and become dormant, or die completely leaving seeds behind to germinate when rain falls again. Others survive as storage organs like bulbs and tubers. A few plants can withstand complete dehydration and recover, thry appear dead but when it rains the cells recover, the ability to do this is linked to the polysaccardie trehalose which appears to enable the cells to survive unharmed. 
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Hydrophytes are plants that live submerged, or on the surface of water and need adaptations to cope with growing in water or permanently saturated soil. Examples include water lillies and water cress. It is important for surface water plants to be able to float so they can photosynthesise. Water logging is a major problem for all hydrophytes, and the air spaces of the plant need to full of air rather than water for the plant to survive. Some adaptations of hydrophytes include:

  • Very thin or no waxy cuticle - hydrophytes do not need to conserve water as their is always plenty available, so water loss by transpiration is not an issue.
  • Open stomata on upper surfaces - maximising the number of stomata maximises the amount of gaseous exchange, and unlike other plants their is no risk of losing turgor as there is always an abundance of water available, so the stomata are usually open all the time and the guard cells are inactive. In plants with floating leaves the stomata need to be on the upper surface of the leaf so that they are in contact with the air.
  • Reduced structure to the plant - the water supports the leaves and flowers so there is no need for strong supporting structures.
  • Wide flat leaves - some hydrophytes including water lillies have wide, flat leaves that spread across the surface of the water to capture as much light as possible.
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Hydrophytes II

  • Small roots - water can diffuse quickly inot the stem and leaf tissue so there is less need for uptake by roots and makes more efficient use of materials.
  • Large suface area of stems and roots underwater - this maximises the area for photosynthesis and for oxygen to diffuse into submerged plants.
  • Aerenchyma - these are specialised paranchyma tissues that form in the stems, roots and leaves of hydrophytes. It has many large air spaces which seem to be formed in part by programmed cell death in normal parenchyma. It has sever different in the plants including that it makes the leaves and stems more buoyant, and that it forms low resistance internal pathways fir the movement of substances like oxygen to tissues below water, and this helps the plant to cope with anoxic conditions in mud.

Aerenchyma is often found in plant species that grow in water lile rice, studies suggest that aerenchyma may provide a low resistance pathway by which methane can be produced and vented into the atmosphere, which is a major problem as it contributes to global warming. In situations where there is plenty of water, such as in swamps, roots can become waterlogged where it is air that is in short supply. Special aerial roots called pneumatophores grow upwards in the air and have many lenticels, which allow the entry of air into the woody tissue.

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