Transport in plants Dr. Sunjukta Ahsan Different types of cells in plants Cross-section of plant stem Cross-section of root Xylem-Structure and Function Xylem tissue • They are made from cells joined end to end to form tubes • The cells are dead • The walls of the cells are thickened with a hard, strong material called lignin. • Xylem tissue has two functions, namely support and transport. • It contains several different types of cell. Xylem tissue • In flowering plants, xylem tissue contains vessel elements, tracheids, fibres and parenchyma cells. • ■■ Vessel elements and tracheids are the cells that are involved with the transport of water. • Unlike other plants, flowering plants rely mostly on the vessel elements for their water transport. • ■■ Sclerenchyma fibres are elongated cells with lignified walls that help to support the plant. They are dead cells; they have no living contents at all. • ■■ Parenchyma cells Xylem vessels and vessel elements • Vessels are made up of many elongated cells called vessel elements, arranged end to end. • Each vessel element begins life as a normal plant cell in whose wall lignin is laid down. • Lignin is a very hard, strong substance, which is impermeable to water. • As lignin builds up around the cell, the contents of the cell die, leaving a completely empty space, or lumen, inside. Xylem vessels and vessel elements • Groups of plasmodesmata are found where no lignin is laid down. • These non-lignified areas can be seen as ‘gaps’ in the thick walls of the xylem vessel, and are called pits. • Pits are not open pores; they are crossed by permeable, unthickened cellulose cell wall. • The pits in one cell link with those in the neighbouring cells, so water can pass freely from one cell to the next. • The end walls of neighbouring vessel elements break down completely, to form a continuous tube rather like a drainpipe running through the plant. • This long, non-living tube is a xylem vessel. The transport of water • Water moves from a region of higher to lower water potential. • The movement of water is passive as it is driven by evaporation from the leaves. • The process starts in the leaves. The energy of the Sun causes water to evaporate from the leaves, a process called transpiration. • This reduces the water potential in the leaves and sets up a water potential gradient throughout the plant. • Water moves down this gradient from the soil into the plant – for example, through its root hairs. From leaf to atmosphere – transpiration • Water then moves across the root into the xylem tissue in the centre. • The walls of the mesophyll cells are wet, and some of this water evaporates into the air spaces so that the air inside the leaf is usually saturated with water vapour. • The air in the internal spaces of the leaf has direct contact with the air outside the leaf, through small pores called stomata. • If there is a water potential gradient between the air inside the leaf and the air outside then water vapour will diffuse out of the leaf down this gradient. From soil into root hair • The tip of a young root is covered by a tough, protective root cap and is not permeable to water. • However, just behind the tip some of the cells in the outer layer, or epidermis, are drawn out into long, thin extensions called root hairs. • These reach into spaces between the soil particles, from where they absorb water. From soil into root hair (contd.) • Water moves into the root hairs by osmosis down a water potential gradient. • Although soil water contains some inorganic ions in solution, it is a relatively dilute solution and so has a relatively high water potential. • However, the cytoplasm and cell sap inside the root hairs have considerable quantities of inorganic ions and organic substances such as proteins and sugars dissolved in them, and so have a relatively low water potential. • Water, therefore, diffuses down this water potential gradient, through the partially permeable cell surface membrane and into the cytoplasm and vacuole of the root hair cell. From soil into root hair (contd.) • The large number of very fine root hairs provides a large surface area in contact with the soil surrounding the root, thus increasing the rate at which water can be absorbed. • However, these root hairs are very delicate and often only function for a few days before being replaced by new ones as the root grows. • Root hairs are also important for the absorption of mineral ions such as nitrate and magnesium. From soil into root hair (contd.) • Many plants, especially trees, have fungi located in or on their roots, forming associations called mycorrhizas, which serve a similar function to root hairs. • The mycorrhizas act like a mass of fine roots which absorb water and nutrients, especially phosphate, from the soil and transport them into the plant. • Some trees, if growing on poor soils, are unable to survive without these fungi. • In return, the fungi receive organic nutrients from the plant. • The name given to a relationship such as this, in which two organisms of different species both benefit, is mutualism. Root pressure • Plants may also increase the pressure difference between the top and bottom by raising the water pressure at the base of the vessels • The pressure is raised by the active secretion of solutes into the water in the xylem vessels in the root. • Cells surrounding the xylem vessels use energy to pump solutes across their membranes and into the xylem by active transport. • The presence of the solutes lowers the water potential of the solution in the xylem, thus drawing in water from the surrounding root cells. • This influx of water increases the water pressure at the base of the xylem vessel. From root hair to xylem • Water transport in plants is largely a passive process, driven by transpiration from the leaves. • Water taken up by root hairs crosses the cortex of the root and enters the xylem in the centre of the root. • It does this because the water potential inside the xylem vessels is lower than the water potential in the root hairs. • Therefore, the water moves down this water potential gradient across the root. From root hair to xylem • The water takes two routes through the cortex. Individual molecules can switch from one route to the other at any time. • The cells of the cortex, like all plant cells, are surrounded by cell walls made of several layers of cellulose fibres criss-crossing one another. • Water can soak into these walls, rather as it would soak into blotting paper, and can seep across the root from cell wall to cell wall without ever entering the cytoplasm of the cortical cells. This is called the apoplastic pathway. • Another possibility is for the water to move into the cytoplasm or vacuole of a cortical cell by osmosis, and then into adjacent cells through the interconnecting plasmodesmata. This is the symplastic pathway From root hair to xylem • Normally, it is probable that the symplastic pathway is more important but, when transpiration rates are especially high, more water travels by the apoplastic pathway. • Once the water reaches the endodermis, the apoplastic pathway is abruptly blocked. • The cells in the endodermis have a thick, waterproof, waxy band of suberin in their cell walls. This band, called the Casparian strip, forms an impenetrable barrier to water in the walls of the endodermis cells. • The only way for water to cross the endodermis is through the cytoplasm of the endodermal cells. As the endodermal cells get older, the suberin deposits become more extensive, except in certain cells called passage cells, through which water can continue to pass freely. From root hair to xylem • It is thought that this arrangement gives a plant control over what mineral ions pass into its xylem vessels, as everything has to cross cell surface membranes. • It may also help with the generation of root pressure. • Once across the endodermis, water continues to move down the water potential gradient across the pericycle (parenchyma and sclerenchyma) and towards the xylem vessels. • Water moves into the xylem vessels through the pits in their walls. From xylem across the leaf • Water constantly moves out of the xylem vessels through the unlignified parts of the xylem vessel walls. • Symplastic pathway, water moves from cell to cell via the plasmodesmata. • In the other pathway, known as the apoplastic pathway, water moves through the cell walls. From root to stem and leaf in the xylem • From root to stem and leaf in the xylem • The removal of water from xylem vessels in the leaf reduces the hydrostatic pressure in the xylem vessels. (Hydrostatic pressure is pressure exerted by a liquid.) • The hydrostatic pressure at the top of the xylem vessel becomes lower than the pressure at the bottom. • This pressure difference causes water to move up the xylem vessels in continuous columns. From root to stem and leaf in the xylem • The lower the hydrostatic pressure, the lower the water potential, so a hydrostatic pressure gradient is also a water potential gradient. • The water in the xylem vessels, like the liquid in a ‘sucked’ straw, is under tension. • The movement of water up through xylem vessels is by mass flow. This means that all the water molecules (and any dissolved solutes) move together, as a body of liquid, like water in a river. • This is helped by the fact that water molecules are attracted to each other by hydrogen bonding; this attraction is called cohesion. From root to stem and leaf in the xylem • They are also attracted to the cellulose and lignin in the walls of the xylem vessels, and this attraction is called adhesion. • Cohesion and adhesion help to keep the water in a xylem vessel moving as a continuous column. • The vessels are full of water. The fact that the cells are dead is an advantage, because it means there is no protoplasm to get in the way of transport. • The small diameter of xylem vessels helps to prevent breaks in water column due to air bubbles from occurring. • Also, the pits in the vessel walls allow water to move out into neighbouring vessels and so bypass such an air lock. • Air bubbles cannot pass through pits. Pits are also important because they allow water to move out of xylem vessels to surrounding living cells. Factors affecting transpiration • ■■ Humidity. If the water potential gradient between the air spaces in the leaf and the air outside becomes steeper, the rate of transpiration will increase. • ■■ Wind speed and temperature. Transpiration may also be increased by an increase in wind speed or rise in temperature. • ■■ Light intensity. In most plants, stomata open during the day and close at night. The rate of transpiration is almost zero at night. Stomata must be open during the day to allow carbon dioxide to diffuse into the leaf for photosynthesis, which increases the rate of transpiration. Closing at night, when photosynthesis is impossible, reduces unnecessary water loss. Factors affecting transpiration (contd.) • ■■ Very dry conditions. Partially or completely closing its stomata to prevent its leaves drying out, even if this means reducing the rate of photosynthesis. As water evaporates from the cell walls inside the leaf, it absorbs heat energy from these cells, thus reducing their temperature. Xerophytes-Adaptations and Examples Xerophytes • Xerophytes (or xerophytic plants) are plants that live in places where water is in short supply. • Many xerophytes have evolved special adaptations of their leaves that keep water loss down to a minimum. Marram grass • Very long roots to search for water deep down in sand dunes. • Leaves that roll up in dry weather to increase humidity around stomata, reducing transpiration. • Sunken stomata to create high humidity and reduce transpiration. • Fine hairs around stomata, reducing air movement so humidity builds up and transpiration is reduced. • Very long roots to search for water deep down in sand dunes. • Leaves that roll up in dry weather to increase humidity around stomata, reducing transpiration. • Sunken stomata to create high humidity and reduce transpiration. • Fine hairs around stomata, reducing air movement so humidity builds up and transpiration is reduced. Transport of mineral ions • Apart from the carbohydrates made in photosynthesis, plants need a supply of mineral ions to complete their nutrition. Examples are nitrate, phosphate, sulfate, potassium, magnesium and calcium. • Mineral ions in solution are absorbed along with water by the roots, particularly by the root hairs. • Their route through the plant is the same as that for water, crossing the root by apoplastic and symplastic pathways before moving in the mass flow of xylem sap up the xylem to the rest of the plant. • From the xylem they enter the apoplastic and symplastic pathways again. Xylem and Sclerenchyma • Similarities • 1. Both are mechanical tissues. • 2. In both the cell walls are thickened due to the deposition of lignin. • 3. Both the tissues are dead at maturity. • Differences • 1. Sclerenchyma is a simple tissue while xylem is a complex tissue. • 2. Lignin deposition is uniform in sclerenchyma, while it uneven in xylem. • 3. Sclerenchyma offers only mechanical support while xylem is mechanical and it also helps in conduction. Transport in Phloem Transport of mineral ions (contd.) • As well as moving by mass flow through the apoplastic pathway and xylem, mineral ions can also move by diffusion and active transport. For example, they can diffuse into the apoplastic pathway of the root from the soil and once in the apoplastic pathway can diffuse in any direction according to concentration gradients. • They can also enter cells by methods such as diffusion, facilitated diffusion and active transport. • Facilitated diffusion and active transport allow cells to control what ions enter or leave cells. • One important control point is the root endodermis, where the Casparian strip forces ions to pass through living cells before they can enter the xylem. Translocation in phloem Translocation • The term translocation can be applied to transport in both xylem and phloem – it means literally moving from one place to another. • It usually describes the transport of soluble organic substances within a plant. • These are substances which the plant itself has made – for example, sugars which are made by photosynthesis in the leaves. These substances are sometimes called assimilates. Assimilation and Assimilates • Assimilates are the chemical compounds made by the plant itself as a result of assimilation • Assimilation in plants is the range of processes by which the plant converts its inorganic nutrients into organic compounds. An example of assimilation is photosynthesis. During photosynthesis, inorganic carbon dioxide and water are converted, using energy, to organic solutes like sugars. Another example of assimilation is the use of nitrates obtained from the soil to help make amino acids. Sucrose and amino acids are two of the common assimilates that are transported over long distances in the phloem. • Assimilates are transported from sources to sinks in phloem. • A source is the place where the assimilate is located. • A sink is the place it has to be moved to and where it is needed for growth and development or for storage. Common sources are leaves and storage organs, such as tubers. Common sinks are buds, flowers, fruits, roots and storage organs. Structure of phloem Sieve Tube Elements and Companion Cells • Sieve tube elements are elongated in shape. The cells are joined end to end vertically to form a continuous tube. Like a typical plant cell, a sieve tube element has a cell wall containing cellulose, a cell surface membrane and cytoplasm containing endoplasmic reticulum and mitochondria. However, the amount of cytoplasm is very reduced and only forms a thin layer lining the inside of the cell wall. There is no nucleus and there are no ribosomes. • The most striking feature of sieve tube elements is their end walls. Where the end walls of two sieve tube elements meet, a sieve plate is formed. This is made up of the walls of both cells, perforated by large pores. These pores are easily visible with a good light microscope. Sieve Tube Elements and Companion Cells • The pores are open, presenting little barrier to the flow of liquids through them. • Each sieve tube element has at least one companion cell lying close beside it. Companion cells have the structure of a typical plant cell, with a cell wall containing cellulose, a cell surface membrane, cytoplasm, a small vacuole and a nucleus. However, the number of mitochondria and ribosomes is greater than normal, and the cells are metabolically very active. • Companion cells are very closely linked with their neighbouring sieve tube elements in terms of their function. In fact, they are regarded as a single functional unit. Numerous plasmodesmata pass through their cell walls, making direct contact between the cytoplasm of the companion cell and that of the sieve tube element. Translocation in Phloem • Phloem sap, like xylem sap, moves by mass flow • In xylem vessels, the movement is passive, meaning it requires no energy input from the plant (only the Sun). This is not the case in phloem. To create the pressure differences needed for mass flow in phloem, the plant has to use energy. • Phloem transport is therefore an active process, in contrast to the passive transport in xylem. The pressure difference in phloem is produced by active loading (moving) of sucrose into the sieve tube elements at the source (the place where sucrose starts its journey). The source is usually a photosynthesising leaf or a storage organ. Translocation in Phloem • Loading a high concentration of sucrose into a sieve tube element greatly decreases the water potential of the sap inside it. Therefore, water enters the sieve tube element, moving down a water potential gradient by osmosis. This causes a build-up of pressure inside the sieve tube element. The pressure is referred to as hydrostatic pressure. A pressure difference is created between the source and the sink. This pressure difference causes a mass flow of water and dissolved solutes through the sieve tubes, from the high pressure area to the low pressure area. At the sink, sucrose is removed, causing the water to follow by osmosis. The loss of water from the tube reduces pressure inside the tube, thus maintaining the hydrostatic pressure gradient. Translocation in Phloem • The sucrose, in solution, moves from the mesophyll cells, across the leaf to the phloem tissue. It may move by the symplast pathway, moving from cell to cell via plasmodesmata. Alternatively, it may move by the apoplast pathway, travelling along cell walls. It is now known that companion cells and sieve tube elements work together. Sucrose is loaded into a companion cell by active transport. • Hydrogen ions (protons, H+) are pumped out of the companion cell into its cell wall by a proton pump using ATP as an energy source. Proton pumps are proteins found in the cell surface membrane. The proton pump creates a large excess of hydrogen ions in the apoplast pathway outside the companion cell. The hydrogen ions can move back into the cell by passive diffusion down their concentration gradient, through a protein which acts as a carrier for both a hydrogen ion and a sucrose molecule at the same time. • Because it carries two substances at the same time, the protein is called a co-transporter. The sucrose molecules are carried through this co-transporter molecule into the companion cell, against the concentration gradient for sucrose, but down the concentration gradient for hydrogen ions. The co-transporter will only work if both a hydrogen ion and a sucrose molecule move through it together. Once inside the companion cell, the sucrose molecules can move by diffusion into the sieve tube. They do this by moving through the plasmodesmata which connect the companion cell to the sieve tube (the symplast pathway). Mass flow in phloem
