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Compare The Structure And Function Of The Xylem And Phloem

1486 words - 6 pages

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...

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