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 |
Complete the flow chart for cyclic photophosphorylation of the photosystem-I.
The following flow chart show cyclic photophosphorylation and the missing part of this flow chart are
Kranz anatomy refers to the dimorphism in the chloroplast structure. It is found in $\mathrm{C}_4$ plants. The cells of leaves have two types of chloroplast in them.
Granal Chloroplast It is found in the mesophyll cells of leaves. Chloroplast have well developed grana in them. These chloroplast effectively fix $\mathrm{CO}_2$ even if it is present in lower concentrations. PEP carboxylase is present which fix $\mathrm{CO}_2$ and to form oxaloacetic acid (4 carbon compound).
Agranal Chloroplast Present in bundle sheath cells of the leaves. $\mathrm{C}_3$ cycle occurs in these cells with the presence of RuBisCo enzyme.
The $\mathrm{C}_4$ plants are well adapted to high $\mathrm{O}_2$ concentrations and high temperature.
$\mathrm{C}_4$ plants can absorb $\mathrm{CO}_2$ even when $\mathrm{CO}_2$ concentration in much low thus $\mathrm{C}_4$ plants can perform high rate of photosynthesis even the stomata are closed or there is the shortage of water thus, they can conserve water.
Since, PEP-carboxylase is insensitive to $\mathrm{O}_2$ thus excess $\mathrm{O}_2$ has us inhibitory effect in $\mathrm{C}_4$ pathway and there is no photosynthesis in $\mathrm{C}_4$ plant.
Thus, $\mathrm{C}_4$ plants are better adapted to tropical and desert (hot acid habitats) areas than the plants, that lack this anatomy.