Revision:Xylem and Phloem Tubes

TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Biology > Xylem and Phloem Tubes

Xylem

This is a conducting tissue that has two main functions: Conduction of water and minerals, and support. It's made up of four cell types.

Xylem vessels - heavily lignified to withstand the pressure of carrying water. Dead and hollow so minimum resistance for water flow. Stacked on end to end, with no end walls. Perforated with holes where there used to be plasmodesmata, for sideways/radial transport. Transport of water and mineral ions dissolved in water.

Tracheids - similar to xylem vessels, heavily lignified, dead, but with tapering ends so water only passed sideways through the holes.

Fibres - heavily lignified, only for support.

Xylem parenchyma - normal metabolic activity of the cell, packing tissue. Involved in radial transport. Alive, with cellulose cell walls. May store starch.

Phloem

Phloem sieve tube element - alive, but with only a thin cytoplasmic strand along the sides with no Golgi Apparatus, nucleus or ribosomes. Stacked on end to end, end walls are modified to form sieve tube plates with pores in them. Transport of organic solutes such as sucrose, amino acids.

Companion cells - usually one per sieve tube element, fully functional cell with lots of mitochondria. Involved in translocation by unloading/loading sucrose into the sieve tube element and for metabolic support. Linked to the sieve tube element by plasmodesmata.

Phloem fibre - same as xylem fibre

Phloem parenchyma - same as xylem parenchyma.

Comments

These notes were originally written by Excalibur in this post on TSR Forums.