Describe the structure of the Xylem and Phloem.

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Describe the structure of the Xylem and Phloem (10)

Relate their structures to their functions (10)

Xylem vessels consist of dead hollow cells because the walls are lignified and the cell contents disintegrate. The lignin makes the cell wall impermeable so they are in effect waterproof. It also makes the vessels extremely strong and prevents them from collapsing.  The lignin is deposited as rings or spirals in the cell wall providing rigidity. The vessel elements of the xylem have secondarily thickened and heavily lignified cell walls to aid in transport of solutes across the plasma membrane.

Xylem cells are single cells, which are elongated and lignified. They are also long, tubular structures formed by fusion of several cells end to end in a row, responsible for conduction of water & mineral salts (and support of the plant)

They have a wide lumen and are linked end to end to create a long, hollow tube since the end cell walls have one or many perforations in them. This allows the transport of large volumes of water. The sidewalls have bordered pits (unlignified areas) to allow lateral movement of water. Xylem vessels are found in angiosperms.

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Water enters through the root hair cells and then moves across into the xylem tissue in the centre of the root. Water moves in this direction because the soil water has higher water potential, than the solution inside the root hair cells. This is because the cell sap has organic and inorganic molecules dissolved in it. The root hairs provide a large surface area over which water can be absorbed. Minerals are also absorbed but, their absorption requires energy in the form of ATP because they are absorbed by active transport. They have to be pumped against the concentration ...

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