Compare and Contrast Xylem Tissue and Phloem Tissue, Including Their Respective Structures and Functions.

888 WordsSep 13, 20114 Pages
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…show more content…
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 1.1) 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 the 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

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