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Explain why xylem transport is unidirectional and phloem transport  bi-directional.


The leaves carry out photosynthesis and act as the source of the food. The phloem conducts the food from the source to the sink (the part of the plant requiring or storing food).  But, the source and sink may be reversed depending on the season, or the plant’s needs.  During spring, the food stored in the sink is mobilised toward the growing buds of the plant, through the phloem. Thus, the movement of food in the phloem is bidirectional (i.e., upward and downward) as the realtion of source and sink is variable.

However, the transport of water in the xylem takes place only from the roots to the leaves. Therefore, the movement of water and nutrients in the xylem is unidirectional.

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Difference between the following
(c) Osmotic Pressure and Osmotic Potential

Osmotic Pressure  Osmotic Potential
1.Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane.   1.Osmotic potential is defined as the ability of a solution to draw water in when separated from another solution by a semi-permeable
2. It is expressed in bars with a positive sign. 2. It is expressed in bars with a negative sign.









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Explain pressure flow hypothesis of translocation of sugars in plants.

The pressure flow hypothesis states that food is prepared in the plant leaves in the form of glucose. For translocation to the other parts the glucose is converted to sucrose a disaccharide. The sucrose is moved into the companion cells and then into the living sieve tube cells by active transport. This process is called loading and creates a hypertonic condition in the phloem. Water from the adjacent xylem moves into the phloem by osmosis. Increasing osmotic pressure results in the movement of phloem sap to areas of lower pressure. To reduce the osmotic pressure at the sink the sucrose is moved out of the phloem by active transport. The sucrose is converted to energy, starch or cellulose. The removal of the sugars lowers the osmotic pressure resulting in the movement of water out of the phloem. 

 

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Describe the role played by protein pumps during active transport in plants.

The protein pumps are involved in the active transport of substances like minerals. They are present in the membrane of the cell or organelles. The proteins pumps help in the transfer of molecules from their lower concentration to the region of their higher concentration by using energy.

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What cause the opening and closing of guard cells of stomata during transpiration ?


The change in the turgidity of guard cells results in closing or opening of stomata. The inner and outer walls of the guard cells differ in thickness, the inner walls are thick while the outer are thin. When the guard cells absorb water their turgidity increases, thus the thin outer walls bulge out and force the inner walls into a crescent shape. The change in shape is aided due to the radial orientation of the microfibrils in the cell walls of the guard cells. These microfibrills help the guard cells to bulge out thus opening the stomata. During tranpiration the loss of water makes the guard cells lose turgidity, the elastic inner walls regain their original shape, the guard cells become flaccid and the stomata closes.

 

 

 

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