The important events of light reaction are
(i) Excitation of chlorophyll molecule to emit a pair of electrons and use of their energy in the formation of ATP from ADP + Pi. This process is called photophosphorylation.
(ii) Splitting of water molecule
(a) $2 \mathrm{H}_2 \mathrm{O} \longrightarrow 4 \mathrm{H}^{+}+4 \mathrm{e}^{-}+\mathrm{O}_2 \uparrow$
(b) NADP $+2 \mathrm{H}^{+} \longrightarrow \mathrm{NADPH}_2$
End products of light reaction are NADPH and ATP.
Reducing power is produced in the light reaction i.e., ATP and NADPH ${ }_2$ molecules which are used up in dark reaction, $\mathrm{O}_2$ is evolved as a by product by the splitting of water.
In the diagram shown below label A, B, C. What type of phosphorylation is possible in this?
A-Electron acceptor
C-Chlorophyll (photosystem I) $\mathrm{P}_{700}$
The cyclic photophosphorylation is shown in the above figure.
RuBP carboxylase and oxygenase has dual nature. It has affinity for both $\mathrm{CO}_2$ and $\mathrm{O}_2$ but has more affinity for $\mathrm{CO}_2$ than $\mathrm{O}_2$. Thus, the concentrations of two determines which of the two will bind to the enzyme.
Consider the following two situations
(i) In a normal condition when $\mathrm{CO}_2$ and $\mathrm{O}_2$ concentrations are normal, it acts as carboxylase and fix $\mathrm{CO}_2$ by combining with ribulose bisphosphate and $\mathrm{C}_3$ cycle operates normally, producing glucose molecule as an first product of photosynthesis.
(ii) If $\mathrm{O}_2$ concentration goes up and $\mathrm{CO}_2$ goes down, it starts acting as an oxygenase enzyme and $\mathrm{C}_2$ cycle (photorespiration) starts where RuBP binds with $\mathrm{O}_2$ to from phosphoglycolate.
(iii) $\mathrm{C}_4$ plants have mechanisms to increase the concentration of $\mathrm{CO}_2$ at enzyme site, and increasing the intracellular concentration of $\mathrm{CO}_2$. Thus, here RuB is Co acts as carboxylase, minimising the affect of oxygenase.
Kranz Anatomy, is the feature exhibited by $\mathrm{C}_4$ plant. These possess two types of chloroplast in their leaves. Agranal chloroplast found in bundle sheath cells whereas granal chloroplast is found in the mesophyll cells.
Bundle sheath cell perform $\mathrm{C}_3$ cycle (dark reaction) where as mesophyll cell perform $\mathrm{C}_4$ cycle. $\mathrm{C}_4$ plants are more efficient even in high $\mathrm{O}_2$ concentration and temperature as compared to $\mathrm{C}_3$ plants. Many important crop plants (monocots) show $\mathrm{C}_4$ cycle like maize, sorghum, sugarcane and millet.
The important enzyme of $C_3$ cycle is RuBP carboxylase oxygenase which catalyses reaction of carboxylation of ribulose bis-phosphate, which is 5-carbon compound. to form PGA, the first stable product in $\mathrm{C}_3$ cycle.
In $\mathrm{C}_4$ cycle, the important enzyme is phosphoenol pyruvate carboxylase (PEP carboxylase) which help in fixing $\mathrm{CO}_2$ to form oxaloecetate (4-carbon compound), the first stable product of dark reaction is $\mathrm{C}_4$ cycle.