A: The outside movement of water from the cell during which plasma membrane shrinks is called plasmolysis or exosmosis or negative osmosis or Anosmosis or reverse osmosis. It occurs when a cell is kept in hypertonic solution as follows.
1. Water is first lost from the cytoplasm.
2. Then water from the vacuole is lost.
3. Protoplast shrinks.
4. Vacuole becomes smaller.
5. Plasma membrane separates from the cell wall, first in the corners. It is called incipient plasmolysis. The cell that looses water is said to be plasmolysed or flaccid cell. In such cells cell sap becomes hypertonic.
6. Hypertonic solution is seen between cell wall and plasma membrane.
It occurs when there is water stress or in saline environments. The principle of plasmolysis is useful in making
1) Jellys 2) Pickles 3) Raisins 4) Jams.
Q: Stomata are turgor operated valves. Explain.
A: Stoma is an elliptical pore in the epidermis. It is protected by 2 kidney shaped guard cells. The inner wall of each guard cell, towards the pore is thick and outer wall towards the subsidiary cells is thin, convex
and elastic. Guard cells have chloroplasts. The cell walls have cellulosic microfibrils oriented radially.
According to Levitt's K+ Pump Theory active Efflux of H+ leads to passive influx of K+ and Cl −. Water potential value of guard cells decreases. Endosmosis occurs. Guard cells become turgid. Outer thin walls of guard cells expand outwardly and become more convex. It forces the inner walls into a crescent shape. Stoma opens. At night, Efflux of K+ and Cl− occurs. In turn influx of H+ occurs water potential increases. Exosmosis occurs. Guard cells
become flaccid. Stoma closes. Thus the turgidity of the guard cells controls the movement of stomata. Hence we can say that stoma is a turgor operated valve.
Q: Explain pressure flow hypothesis of translocation of sugars in plants.
A: Girdling experiment proves that food materials are translocated through phloem. Food is conducted in the form of sucrose. It occurs between source (Leaf) and sink (Root). Source is one but sink may be any non-photosynthetic organ. So the conduction of food is multidirectional. Munch (German) proposed Mass flow/ Pressure flow hypothesis to explain translocation of Sugars. It occurs according to Turgor Pressure gradient (from higher T.P. to lower
t.p.).
It is explained in 2 steps:
1. Phloem Loading: Sucrose moves into sieve tube cells through companion cells, by active transport. The solution in phloem becomes hypertonic. Water moves from Xylem moves into Phloem by osmosis. ψ in phloem increases. T.P. becomes more. Then sugars move from source to sink.
2. Phloem Unloading: At sink sucrose is unloaded. It results increase in Mass at sink. Sucrose is converted to starch or utilised in sink. ψ increases. It pushes the water in the Xylem towards source.
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