Is there a general mechanism to explain opening and closing of stomata? Justify your answer.

The opening and closing of stomata is operated by turgor changes in guard cells. When the guard cell absorb water they become turgid and the stomata opens. The opening and closing of stomatal pore is aided by the differential thickening of the guard cell's inner and outer membrane and the orientation of the microfibrils in the cell. 
When the guard cell absorb water then the 
thin outer walls bulge out and force the inner walls into a crescent shape, thus opening the stomatal pore. The radial orientation of cellulose microfibrils rather than longitudinally enable the stoma to open. When the lose water they lose turgidity , the elastic inner walls regain their original shape, the guard cells become flaccid and the stoma closes.

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Give the comparison among simple diffusion, facilitated diffusion and active transportation.

Simple Diffusion

Facilitated Diffusion

Active Transport

1. Membrane proteins are not required .

2. It is not selective.

3. Saturation does not occur.

4. It does not occur against concentration gradient.

5. ATP is not required.

1. Membrane proteins are required.

2. It is selective.

3. Saturation occurs.

4. Not against the concentration gradient.

5. ATP is not required.

1 . Membrane proteins are required.

2. It is selective.

3. Saturation occurs

4. Against the concentration gradient. 

5. It needs ATP. 

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Explain transpiration and photosynthesis are complementary to each other.

Transpiration : It is the evaporation of water from aerial parts of plant.

Photosynthesis : It is the process of manufacture of food from C02 and water by the green parts of plant.

It is the transpiration which creates a transpiration pull for absorption of water and minerals The water is raw material for photosynthesis. If water availability is limited then photosynthesis also becomes limited. Actively photosynthetic plant  needs proper supply of water. C4 plants were evolved only to use minimum amount of water and maximum use of C02 and they are double efficient to C3 plants.

C4 plant loses half the amount of water as compared to C3 plants for fraction of same amount of C02.

Thus it appears to be a compromise between transpiration and photosynthesis.

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How does water is absorbed by the roots ? Discuss different pathways for its translocation.


Absorption of water : The water is absorbed mainly by diffusion. 

The root hair absorb water a

Transportation : Once water is absorbed it is translocated into deeper layers of root by different pathways as :

(i) Apoplast pathway (ii) Symplast pathway 

(i) Apoplast pathway : It is the pathway which occurs through cell walls. It is non living pathway.

The apoplastic pathway is continuous through walls as well as through intercellular spaces. It does not involve crossing of cell membrane. It does not provide any barrier. Mass flow of water through apoplastic pathway is mainly due to cohesive and adhesive forces.

The casparian strips of endodermis provide check to apoplastic pathway and water has to cross the cell membrane and pass through cytoplasm. The pathway becomes symplast.

(ii) Symplast Pathway : It occurs through interconnected protoplasts. The protoplasts of adjacent cells are connected via plasmodesmata.

The symplastic pathway is slow because of cell membranes and their organelles Symplastic movement may be aided by streaming movement of cytoplasm as in leaf of Hydrilla.

(iii) Vacuolar symplast pathway :  It is the movement of water in the xylem vessels. 

 

How does water is absorbed by the roots ?

Three pathways of water movement inside the root.

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Does transpiration bring any useful function in plant ?

Transpiration plays following useful functions in plants :

1. Ascent of sap - It is the pulling force that helps in the movement of water up the plant. 

2. Cooling effect - cools the leaf surfaces sometimes upto 10 to 15 degrees. 

3. Transport minerals from the soil to all parts of the plants.

4. Maintains the shape and structure of the plants by keeping the cells turgid. 

5. Supplies water for photosynthesis. 

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