Xylem and phloem
Xylem and phloem
Plants have two separate transport systems. A network of xylem vessels transports water and mineral ions from the roots to all other parts of the plant. Phloem tubes transport food made in the leaves to all other parts of the plant. Neither of these systems has a pump, this is because they are not as active as animals and do not need such rapid supplies of food. Neither xylem nor phloem transports oxygen as oxygen gets to a plants cell by diffusion. Both stems and roots contain xylem vessels and phloem tubes. In a stem these are grouped into vascular bundles arranged in a ring. In a root these are arranged in the centre forming a structure called the stele.
Xylem tissue has the dual functions of support and transport. It contains several different types of cells these are vessel elements, traceids, fibres and parenchyma cells. In contrast to this phloem tissue is living and comprises of sieve tubes, phloem parenchyma (also known as companion cells) and phloem fibres.
In the xylem tissue the vessel elements and tracheids are the cells that are involved with the transport of water. Fibres are elongated with lignified walls that help to support the plant. They are dead cells; they have no living contents at all. Parenchyma cells are plant cells they have unthickened cellulose cell walls and contain all the organelles you would expect to see. However the parenchyma cells in xylem tissue do not usually have chloroplasts as they are not exposed to light. They can vary in shape, however most of them are isodiametric that is approximetly the same size in all directions.
In contrast in the phloem, the sieve tubes are made up of many elongated sieve elements, joined end to end vertically to form a continuous column- this also has all the organelles you would expect to see- such as a cellulose cell wall and a plasma membrane. However there is only a small amount of cytoplasm, there is no nucleus or ribosomes in the sieve tube. Each sieve element has at least one companion cell lying close beside it. Companion cells have the structure of a normal plant cell however the number of mitochondria and ribosomes is larger than normal and the cells are metabollically very active.
In the xylem, vessels are made up of many elongated vessel elements arranged end to end. Each began as a normal plant cell in whose wall a substance called lignin was laid down. Lignin is a very hard, strong substance, which is impermeable to water. As it built up around the cell, the contents of the cell died, leaving a completely empty space or lumen. However in several parts of the original cell walls, where groups of plasmodesmata were no lignin were laid down. These non-lignin areas can be seen as gaps in the thick walls of the xylem vessels, and are called pits. Pits are not open pores; they are crossed by permeable, unthickened cellulose cell wall.
The end walls of neighbouring vessel elements break down completely, to form a continuous tube running through the plant. This long, non living tube is a xylem vessel. Tracheids like vessel elements are dead cells with lignified walls, but they do not have open ends they are elongated cells with tapering ends. They have pits in their walls so water can pass from one tracheid to the next.
The evaporation of water from plants is called transpiration. When the water reaches the top of the xylem vessels it goes into the leaves. Leaves contain large air spaces because the cells in the mesophyll (middle leaf) layer are not tightly packed. 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 or 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 cell down this gradient. The gas diffuses out through the air spaces and stomata into the air. This loss of water vapour from the leaves of a plant is called transpiration.
As water evaporates from the cell walls of mesophyll cells, more water is drawn into them to replace it. The source of this water is the xylem vessels in the leaf. Water constantly moves out of these vessels, down a water potential gradient either into the mesophyll cells or along their cell walls. The removal of water from the top of xylem reduces the hydrostatic pressure. 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, causing a pressure difference between the top and bottom. The water in the xylem vessels is under tension; its walls may collapse inwards as a result of the pressure differences. Xylem vessels have strong lignified walls to stop them from collapsing in this way.
The movement of water up through xylem vessels is by mass flow. This means that all the water molecules move together, as a body of liquid.
In contrast to the structure of the xylem vessels, the sieve tubes in the phloem have end walls which when next to each other a sieve plate is formed. This is made up of the walls of both elements, perforated by large pores. Companion cells are closely associated with their neighbouring sieve elements. Numerous plasmodesmata pass through their cell walls, making direct contact between the cytoplasms of the companion cell and sieve element. The liquid inside the phloem sieve tubes is called phloem sap containing sucrose, potassium ions, amino acids, chloride ions, phosphate ions, magnesium ions, sodium ions, ATP, nitrate ions and plant group substances e.g. auxin and cytokinin.
Translocation is the term used to describe the transport of soluble organic substances within a plant. These are substances which the plant itself has made, for example sugars made by photosynthesis in the leaves, these substances are called assimilates. Assimilates are transported in phloem tissue, along with several other types of cells including companion cells, parenchyma and fibres. Phloem sap, like the contents of xylem vessels moves by mass flow.
However whereas in xylem vessels differences in pressures are produced by a water potential gradient between the soil and the air, requiring no energy input from the plant, this is not so in phloem transport. To create the pressure differences needed for mass flow in phloem, the plant has to use energy. Phloem transport can therefore be considered an active process, in contrast to the passive transport in xylem.
The pressure difference is produced by active loading of sucrose into the sieve elements at the place from which sucrose is to be transported. This is usually in a photosynthesising leaf. As sucrose is loaded onto the sieve element, this decreases the water potential in the sap inside it. Therefore water follows the sucrose into the sieve element, moving down a water potential gradient by osmosis.
There are several similarities with the transport of water, in each case liquid moves by mass flow along a pressure gradient, through tubes formed by cells stacked end to end.
Unlike water transport through xylem, which occurs through dead xylem vessels, translocation through phloem sieve tubes involved active loading of sucrose at sources, thus requiring living cells.
Xylem vessels have lignified cell walls, whereas phloem tubes do not. The presence of lignin in a cell wall prevents the movement of water and solutes across it, and so kills the cell. This does not matter in xylem, as xylem vessels do not need to be alive; indeed, it is a positive advantage to have an entirely empty tube through which water can flow unimpeded and the dead xylem vessels with their strong walls also support the plant. Sieve tubes however must remain alive, and so no lignin is deposited in their cellulose cell walls.
The end wall of xylem elements disappear completely, whereas those of phloem sieve elements form sieve plates. These sieve plates probably act as supporting structures to prevent the phloem sieve tube collapsing; xylem already has sufficient support provided by its lignified walls. The sieve plates also allow the phloem to seal itself up rapidly if damaged, for example by a grazing herbivore, rather as a blood vessel in an animal is sealed by clotting.
Phloem sap has a high turgor pressure because of its high solute content, and would leak out rapidly if the holes in the sieve plate were not quickly sealed. Moreover, phloem sap contains valuable substances such as sucrose, which the plant cannot afford to lose in large quantity. The “clotting” of phloem sap may also help to prevent the entry of micro-organisms which might feed on the nutritious sap or cause disease.