Question

Differentiate Between.
$C_3$ and $C_4$ cycles.

Answer 1

In $C_3$ cycle (also known as Calvin cycle), $C{O}_2$ is fixed when it is accepted by a 5-C compound RuBP to form two molecules of a 3-C compound, 3- osphoglyceric acid, the first product of photosynthesis. Ultimately RuBP is regenerated. On the other hand, in $C_4$ cycle (also known asHatch and Slack cycle), the first compound in photosynthesis is a 4-C compound, the oxaloacetate. The chloroplasts of mesophyll cells of $C_4$ plants though have typical granal stacking and other usual features of a chloroplast from a green plant, they lack ribulose bisphosphate carboxylase activity which is present in the chloroplasts of mesophyll cells of $C_3$ plants: The $C_4$ cycle is comparatively more energy-expensive than the $C_3$ cycle, as in $C_3$ plants, there is an approximate requirement of 3 ATP and 2 NADPH per molecule of $C{O}_2$ assimilated, whereas in $C_4$ plants, additional ATPs are required for conversion of pyruvate to phosphoenol pyruvate of decarboxylation of oxaloacetate to phosphoenol pyruvate. In general, $C_4$ pathway of photosynthesis in tropical habitats is a device to reduce water loss through transpiration by means of partial stomatal closure so that the plants lose less water for $C{O}_2$ fixation compared to $C_3$ plants and can make more economic use of water for growth.