Question

Describe the phloem transport from source to sink

Answer 1

Organic solutes are translocated through the sieve tubes from the supplying end or source to the consumption end or sink. Sucrose transport involves the following three processes :

(A) Phloem loading is an active transport mechanism. It is carried out by a specific carrier protein molecules in the cell surface membrane of companion cells that uses ATP from the mesophyll cells to the sieve tubes in the veins of a leaf. Sugar moves from the photosynthesizing cells into a chain of mesophyll cells, transfer cells, to the phloem of a vascular bundle.

(B) Long distance transport of sucrose in the stem and root phloem.

(C) Phloem unloading is a passive transport mechanism from the sieve tubes to the cells at the root tip. It takes place passively down a concentration gradient of sucrose. The transfer cells are often present at unloading sites. Phloem unloading also requires metabolic energy, that is used by sink organs for respiration and biosynthetic reactions.