$\mathrm{C}_4$ plants are advantageous in following ways
(i) These plants can carry out photosynthesis even at low concentration of $\mathrm{CO}_2$ in the atmosphere and in the shortage of water.
(ii) These plants can tolerate high $\mathrm{O}_2$ concentration and temperature as enzyme PEP carboxylase in $\mathrm{C}_4$ cycle in insensitive to $\mathrm{O}_2$ and do not show- photorespiration in comparison to the $\mathrm{C}_3$ plants, which start process of photorespiration and lose $\mathrm{CO}_2$ fixation in the form of glucose molecule.
Thus, $\mathrm{C}_4$ plants are superior to $\mathrm{C}_3$ plants.
In the figure given below, the black line (upper) indicates action spectrum for photosynthesis and the lighter line (lower) indicates the absorption spectrum of chlorophyll- $a$, answer the following
(a) What does the action spectrum indicate? How can we plot an action spectrum? Explain with an example.
(b) How can we derive an absorption spectrum for any substance?
(c) If chlorophyll- $a$ is responsible for light reaction of photosynthesis, why do the action spectrum and absorption spectrum not overlap?
(a) The effectiveness of different wavelengths of light on photosynthesis is measured and the rate of photosynthesis is plotted. This is called the action spectrum of photosynthesis.
(b) Absorption of different wavelengths of light by a particular pigment is plotted and the graph is called the absorption spectra of that pigment.
(c) Chlorophyll-a is responsible for light reaction of photosynthesis, but the action spectrum and absorption spectrum do not overlap because, though chlorophyll-a is the main pigment responsible for absorption of light, other thylakoid pigments like chlorophyll-b, xanthophylls, carotenoid, which are accessory pigments, also absorb and transfer the energy to chlorophyll-a.
Indeed they not only enable a wider range of wavelength of incoming light to be utilised for photosynthesis but also protect chlorophyll-a from photooxidation.
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.