Bachelor’s Degree Programme (BSCG) – Plant Physiology and Metabolism Assig. Answer | BBYCT–137(Q-7)
7. a) Describe the Munch Mass Flow model of translocation of solutes.
b) Discuss the ‘sink’ to ‘source’ transition during phloem unloading.
a) Munch’s Mass Flow Hypothesis is the theory that explains the flow of food materials through the phloem from the concentration regions to lower concentration regions. The force is needed to move organic food materials downwards in the phloem. This is caused due to differences in osmotic potential. There is a turgor pressure gradient exists between the supplying and receiving tissues. The experiment was performed using two osmometers with differences in concentrations. This theory proves that there is the movement of food from the leaves to roots. The leaves are the regions where the food is produced and thus, serve as the source end. The roots are the regions that require food to perform absorption and respiration. Thus, the roots serve as sink end. The flow is from the source end to the sink end.
b) 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 :
(i) Phloem loading is an active transport mechanism. It is carried out by 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.
(ii) Long-distance transport of sucrose in the stem and root phloem.
(iii) 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, which is used by sink organs for respiration and biosynthetic reactions.