Compare and contrast xylem tissue and phloem tissue, including their respective structures and functions. The stems and roots of plants contain two separate transport systems; xylem vessels and phloem tubes, of which neither transport oxygen as it is transported to cells by diffusion. The network of xylem vessels transports water and mineral ions from the roots to all other parts of the plant whereas phloem tubes transport food made in the leaves to all other parts of the plant.
In the stems the tissue is collectively known as vascular tissue, within the roots they form a structure called the stele. The movement of water from roots to shoots is conducted via the xylem using mass flow. The force of cohesion – a force produced by the xylem, increases the attraction between the molecules which make up the water in the xylem. The xylem is composed of different kinds of cells; tracheids, vessels, fibres and unthickended xylem parenchyma. Both tracheids and vessels form pipes through which liquid can be moved, conducting water and supporting tissues.
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Fibres simply play a support role and parenchyma have a storage function. The xylem pipework in all living plants consists entirely of dead cells; composed of lignin rich tissues that once have reached maturity promptly die via a process known as apoptosis. This cell death creates a hollow tube leaving water to move rapidly, free of obstacles. Tracheids are long and thin with tapered end walls and a narrow lumen. Vessel elements however are broad, short and have wider lumen. These vessel elements are arranged in columns forming the vessels.
Vessels are vital in rapid water transport from roots to shoots as little resistance to water flow is offered by their broad lumen. Pitted tracheids link with adjacent cells to form a series of pipes along the stem, these pipes increase the resistance of the pipe to the flow of water. When water evaporates from the cells via transpiration, more water is drawn into them to replace it, the xylem vessels within the leaf source this water. Water constantly moves out of the vessels, down a water potential gradient along the cell walls, and the removal of water from the top of the xylem reduces the hydrostatic pressure.
The pressure at the top of the xylem vessel becomes lower than the pressure at the bottom and it is this pressure difference that causes water to move up the xylem vessels. The water in the xylem vessel is under tension and these pressure differences could cause the walls to collapse, however, the strong lignified xylem vessels stop them from collapsing in this way. The phloem transports photosynthetic products around the plant and like the xylem it is a tissue. Composed of three types of cells; standard packing parechyma, sieve-tube elements and companion/transfer cells.
In contrast to the structure of the xylem vessels, the sieve-tube elements have end walls which are arranged adjacently to form sieve tubes, the actual conducting element. Companion cells are also linked to sieve elements. Sieve elements contain no nuclei or organelles at maturity however the cytoplasm contains large amounts of fibrous phloem protein known as P-Protein. Plasmodesmata pass through these sieve plates and into the cell walls (apoplast pathway) which make contact with both the cytoplasm (symplast pathway) of companion cells and the sieve elements.
Transfer cells have a similar appearance to parenchyma however they have an absorptive function. The liquid inside these sieve tubes is called phloem sap, this sap contains sucrose, potassium ions, amino acids, chloride ions, phosphate ions, magnesium ions, sodium ions, ATP, and nitrate ions. Phloem sap has a high turgor pressure because of its high solute content and would leak rapidly however the sieve plates act as supporting structures to prevent the phloem sieve tube collapsing and allow the phloem to seal itself up promptly.
This is also a defence mechanism if the plant were to become damaged by a grazing predator. The transport of soluble organic substances within plants is known as translocation, and substances produced by the plant itself – such as sugars made by photosynthesis are known as assimilates. Assimilates are transported through phloem tissue, including companion cells, parenchyma and fibres. Phloem sap, like the contents of xylem vessels moves by mass flow. (See fig 1. ) However whereas in xylem vessels differences in pressure are produced by a water potential gradient, requiring no energy input from the plant, however in phloem transport this is not so and the plant has to use energy to create the pressure differences required for mass flow. The pressure difference is produced by active transport of sucrose into the sieve elements at the site from which sucrose is to be transported i. e. a photosyntesising leaf. Sucrose is loaded into the sieve element, decreasing he water potential in the sap inside it and thus the 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 the phloem’s sieve tubes involves the active loading of sucrose and so requires living cells. (See fig 1. 2)