03 May 2013

#59 Transport in plants - functions of xylem and phloem

Plants have transport systems to move food, water and minerals around. These systems use continuous tubes called xylem and phloem:

Xylem vessels carry water and minerals from the roots to the leaves.  
- Phloem tubes carry sugar & other organic nutrients made by plant from the leaves to the rest of the plant. 

Structure of the xylem tissue

Xylem vessels consist of dead cells. They have a thick, strengthened cellulose cell wall with a hollow lumen. The end walls of the cells have disappeared, so a long, open tube is formed. The walls of the xylem vessel contains holes called pits which water enters through.

The xylem vessel is specialised to transport water and dissolved minerals from the root up to all the other parts of the plant, and also to helps supporting the stem and strengthening it. 

Scanning electron micrograph of xylem vessels (x1800)

Structure of the phloem tissue

This is a long tube that runs alongside the xylem tissue. They are made of long narrow tubes with perforated sieve plates along the thin length.

Scanning electron micrograph of a sieve plate
 in a phloem tube (x1300)
The function of the phloem tissue is to transport food nutrients such as sucrose and amino acids from the leaves and to all other cells of the plant, this is called translocation.

Unlike the xylem, the phloem tissue is made of columns of living cells, swhich  contains a cytoplasm but no nucleus, and its activities are controlled by a companion cell next to it which has a nucleus, but companion cells have no function in translocation.

Additional resourcextremepapers.com

Related post: Cell functions

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