Hatch-Slack cycle of photosynthesis or C4 cycle


Do you know how the reduction of carbon dioxide takes place in C4 plants? The below article will explain to you the detailed mechanism of the Hatch-Slack cycle. You will also find the significance of C4 cycle and the differences between C3 and C4 plants.

Introduction

In addition to Calvin cycle, Kortschak discovered an alternative pathway for carbon dioxide fixation in photosynthesis. He observed that C4 dicarboxylic acid is formed as a primary or first stable product of photosynthesis. When carbon dioxide was supplied to sugarcane leaves for a short period, three C4 dicarboxylic acids (OAA) malic acid and aspartic acid) were initially produced. When the period of the exposure was extended, label appeared in 3 PGA. M. D. Hatch and C. R. Slack (1966) studied several plants like tropical grasses like sugarcane, maize, etc. and presented an alternative pathway known as C4 cycle. It is called C4 cycle because the first stable product of this cycle is a C4 dicarboxylic acid.

Characteristics of C4 plants

C4 plants generally grow at high light intensities and at day temperature of 30 degree Celsius to 35 degree Celsius. They have high photosynthetic rate (40-80 mg carbon dioxide /hour). An unusual leaf structure known as Kranz type of anatomy is found in C4 plants. In Kranz anatomy two types of Chloroplast are found in the plants. The chloroplast found in mesophyll are smaller in size but grana is present in it but the chloroplast found in bundle sheath are larger in size but grana are absent in it. In these plants, malic acid and aspartic acid is formed from pyruvic acid within mesophyll cells. Mesophyll chloroplast contains an efficient enzyme phosphoenol pyruvic carboxylase (PEP-C) while bundle sheath chloroplast contains RuDP-C.

Mechanism of C4 cycle


c4 cycle
(Image courtesy: www.wikipedia.org)

The C4 cycle starts in the mesophyll cells of leaves and completes in the bundle sheath cells of the leaves. Hence, C4 cycle takes place in two different types of cells in the leaves:

  • In mesophyll cells: First of all phosphoenol pyruvic acid (PEP) combines with carbon dioxide to form a four carbon compound oxaloacetic acid (OAA) in the presence of enzyme PEP-carboxylase (PEP-C). This oxaloacetic acid (OAA ) is then reduced into malic acid in the presence of NADPH2. The malic acid formed within mesophyll cells is diffused into bundle sheath cells through the plasmodesmata.

  • In Bundle sheath cells: In bundle sheath cells malic acid is decarboxylated, in the presence of NADP+ into pyruvic acid and release carbon dioxide. This internally produced carbon dioxide is used in C3 cycle in bundle sheath's chloroplast and form fructose-6-P and other form of carbohydrates like sucrose, starch and others. This pathway is known as double decarboxylation. Pyruvic acid formed within bundle sheath's chloroplast is diffused into mesophyll cells through plasmodesmata. In mesophyll cells pyruvic acid reacts with ATP and inorganic phosphate to regenerate phosphoenol pyruvic acid (PEP). The PEP produced in mesophyll chloroplast diffused into the cytoplasm of the mesophyll cells and become the substrate for PEP-carboxylase. Thus the C4 cycle completed.


  • Biological significance of C4 cycle

    The biological significance of C4 cycle is as below:

    1. Production of carbohydrate in C4 plants is 2-3 times higher than that of C3 plants.

    2. Very less quantity of carbon dioxide can be used in C4 cycle. It has special importance in tropics where the concentration of carbon dioxide in environment is comparatively low. C4 plants have efficient enzyme system, phosphoenol pyruvic carboxylase. It is found inside the chloroplast of mesophyll cell.

    3. The rate of photorespiration is much low in C4 plants. Therefore, rate of photosynthesis is higher in C4 plants.

    4. C4 plants utilizes maximum amount of light energy because 5 molecules of ATP are used for reduction of 1 molecule of carbon dioxide.

    5. C4 plants are more resistant to gaseous diffusion through which plants are benefitted in two ways. The concentration of carbon dioxide is increased and loss of water through transpiration is minimized. In such a condition, C4 plants can easily grow in dry condition.


    Differences between C3 and C4 plants


    The differences between C3 and C4 plants are as below:

    1. In C3 plants, carbon dioxide is accepted by RuDP whereas in C4 and C4 plants, carbon dioxide is accepted by phophoenol pyrvate.

    2. Only one type of chloroplast is found in the cells of C3 plants but two types of chloroplasts are found in C4 plants.

    3. The first stable product of C3 plant is PTA whereas the first stable product of C4 plants is oxaloacetate.

    4. Excess of atmospheric oxygen inhibits photosynthetic activity in C3 plants whereas excess of atmospheric oxygen has no effect on photosynthesis in C4 plants.

    5. Bundle sheath cells are absent in C3 plants but bundle sheath cells are present in C4 plants.

    6. C3 plants have only Calvin-Benson pathway but C4 plants have both Calvin-Benson and Hatch-Slack pathway.

    7. The rate of photorespiration is high in C3 plants whereas the rate of photorespiration is negligible in C4 plants.

    8. The assimilation of one molecule of carbon dioxide requires 2NADPH2 and 3 ATP molecules in C3 plants but the assimilation of one molecule of carbon dioxide requires 2NADPH2 and 5 ATP molecules in C4 plants.

    9. The optimum temperature for photosynthesis is 10-250C in C3 plants whereas the optimum temperature for photosynthesis is 30-450C in C4 plants.

    10. Carbon dioxide compensation point is 50 ppm in C3plants but carbon dioxide compensation point is 2-5 ppm in C4 plants.

    11. Higher amount of carbon dioxide is released outside of plant body in C3 plants whereas very less amount of carbon dioxide is released outside of plant body in C4 plants.

    12. 18 molecules of ATP are required for production of one glucose molecule in C3 plants but 30 ATP molecules are consumed for the production of one glucose molecule in C4 plants.

    13. PS-I and PS-II are found in all chloroplast of C3 plants whereas PS-II is absent in bundle sheath chloroplast of C plants.


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