question_answer 74)

Explain the mass flow hypothesis of transport in phloem.  

Answer:

                  Mass flow hypothesis was first proposed by Ernst Munch (1930). It explains the transportof solute or sugars, i.e., sucrose in the phloem tissue. It involves the following three steps (a) Phloem loading          (b) Translocation of solutes  (c) Phloem unloading

	Loading of sieve tubes takes place in leaves. Photosynthetic cells make sugars, particularly sucrose,  and  other organic solutes.
	Companion cells use energy to collect solutes by active transport. As soluteconcentration increases in the cell   companion cells, water enters by osmosis.
	A pressure is created, which pushes the solutes through plasmodesmata into the sieve tubes. It is an active process.
	Translocation of sucrose takes place through phloem column. Increase in hydrostatic pressure inside sieve tubes is greatest at the source and lowest at the sink. It pushes sucrose etc., from source to sink. Unloading of the sieve tubes takes place at the sink sites.
	It results in active transport of sugars out of the phloem converting them into complex sugars. Loss of solutes from the phloem create a high water potential, and water moves out of phloem, returning back to xylem.
	Sinks are the regions where solutes i.e., sugars or food are being used, e.g., roots, fruits, storage organs and regions of growth.