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.
RuBisCo, now termed as RuBP carboxylase-oxygenase, is widely occurring enzyme because RuBisCo is used in the Calvin cycle to catalyse the first major step of carbon fixation.
RuBisCO is thought to be the most abundant protein in the world since, it is present in every plant that undergoes photosynthesis and molecular synthesis through the Calvin cycle.
It makes about $20-25 \%$ of the soluble protein in leaves and is made on the earth at the rate of about $1000 \mathrm{~kg} / \mathrm{s}$. It is estimated that every person on earth is supported by about 44 kg of RuBisCo.