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Photosynthesis in Higher Plants

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Give comparison between the following :

(b) Cyclic and non-cyclic photophosphorylation


(b) Distinction between cyclic and non-cyclic photophosphorylation :


Cyclic photophosphorylation

Non-cyclic photophosphorylation


It is performed by PS I independently.

It is performed by both PS I and PS II with the help of each other.


An external source of electrons is not required because the same electrons get recycled.

The process requires an external electron donor, because the same electron is not recycled.


It is not connected with photolysis of water, therefore, no oxygen is evolved.

It is connected with photolysis of water and liberation of oxygen.


It synthesizes only ATP.

ATP synthesis with the production of NADPH.


Electron flow is cyclic.

Electron flow is non-cyclic. 



By looking at which internal structure of a plant can you tell whether a plant is C3 or C4? Explain.

The internal structure of the leaves of C4 plants have a special anatomy called Kranz anatomy. This makes them different from C3 plants. Special cells, known as bundle-sheath cells, surround the vascular bundles are present in several layers around the vascular bndles. These cells have a large number of chloroplasts. They are thick-walled and have no intercellular spaces. They are also impervious to gaseous exchange. All these anatomical features help distinguish C4 plants from C3 plants.

Even though a very few cells in a C4 plant carry out the biosynthetic – Calvin pathway, yet they are highly productive. Can you discuss why?

The productivity of a plant is measured by the rate at which it photosynthesises. The amount of carbon dioxide present in a plant is directly proportional to the rate of photosynthesis. C4 plants have a mechanism for increasing the concentration of carbon dioxide. In C4 plants, the Calvin cycle occurs in the bundle-sheath cells. The C4 compound (malic acid) from the mesophyll cells is broken down in the bundle-sheath cells. As a result, COis released. The increase in COensures that the enzyme RuBisCo does not act as an oxygenase, but only as a carboxylase. This prevents photorespiration. Since there is no photorespiration, thus there is no waste of energy and rate of photosynthesis is increased which in turn increases the productivity. Thus, C4 plants are highly productive.

By looking at a plant externally can you tell whether a plant is C3 or C4? Why and how?

One cannot distinguish between the C3 or C4 by looking at the plants externally. Whether a plant is a C3 plant or C4 plant, the leaves of both are similar in appearance. 

RuBisCo is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCo carries out more carboxylation in C4 plants?

RuBisCo enzymes acts as both carboxylase and oxygenase. It carries out more carboxylation in the C4 plants as these plants have special mechanism that increases the concentration of the CO2 at the enzyme site. There is no photorespiration in the C4 plants and thus oxygen does not bind to the RuBisCo enzyme. All this minimizes the oxygenase activity of the enzyme and increase the carboxylase activity.