RuBisCo is an enzyme which has dual nature. It acts as carboxylase, when $\mathrm{CO}_2$ concentration is good enough in atmosphere. But if $\mathrm{O}_2$ concentration increases, its nature changes and it binds with $\mathrm{O}_2$ and acts as oxygenase enzyme, which forces $\mathrm{CO}_2$ to enter $\mathrm{C}_2$ cycle thus leading to photorespiration and loss of $\mathrm{CO}_2$.

Photosynthesis is an enzyme specific process. All enzymes act at an otptimum temperature (i.e., $25-35^{\circ} \mathrm{C}$ ). If temperature increases, enzyme get denature, thus leading to fall in the rate of photosynthesis.

In light reaction plants trap solar radiation by photosynthetic pigments which convert light energy into chemical energy. Main event of light reaction is photophosphorylation, i.e., formation of ATP from ADP + Pi by using energy of excited electron movement through electron transport chain, present in thylakoid membrane.

Chemiosmosis is the movement of ions across a selectively permeable membrane, down the electrochemical/ proton gradient.

Chemiosmosis hypothesis of ATP formation was first proposed by Mitchell (1961) according to which the enzyme ATP synthase generales ATP via a membrane, proton pump and proton gradient. ATP synthase allows ions $\mathrm{O}_2$ protons to pass through membrane and proton pump. This creales a high cocentration of protons ( $\mathrm{H}^{+}$) in the lumen and hence diffuses across the membrane to activale ATPase, releasing ATP molecules. One molecule of ATP is released for every two $\left(\mathrm{H}^{+}\right)$ions passing Through ATPase.

Melvin Calvin used Chlorella as an experimental material and discovered the first stable compound of photosynthesis, i.e., 3 phosphoglyceric acid so as to trace the path of carbon by using a radioactive isotope of carbon $\left(\mathrm{C}^{14}\right)$ and autoradiography technique. He then, compared the radioactive compounds on the chromatogram as a result of which he found and concluded that the PGA (phosphoglyceric acid), as the first stable product of photosynthesis and gradually the other sugars including hexoses, tetroses and pentoses etc.

Thus, he derived the pathway of $\mathrm{CO}_2$ fixation from these radioactive products (sugars) formed.

Ribulose 5 phosphate is a five carbon compound which accepts atmospheric $\mathrm{CO}_2$ in presence of RuBisCo and form 2 molecules of 3PGA, a ${ }^{30}$ carbon compound. It uses 3 ÃTP and 2 NADPH to fix one molecule of $\mathrm{CO}_2$ per cycle of Calvin.

So, to fix $6 \mathrm{CO}_2$ molecules to form 6 carbon compound glucose 6 cycles are required as mentioned below.

In	Out
$\mathrm{Six} \mathrm{CO}_2$	One glucose molecule $\left(\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6\right)$
18 ATP	18 ADP
12 NADPH	12 NADP