The chlorophyll pigments are present in the thylakoid membranes. They have the property of excitability and emits $e^{-}$in the excited stage, though this $e^{-}$is replaced and transferred by the $e^{-}$generated from splitting of water molecules. Red and Blue Light have maximum energy which a chlorophyll pigment absorbs and get excited and initiate the process of photosynthesis. Also, its wavelength are ( $400-700 \mathrm{~nm}$ ) i.e., between the Photosynthetic Active Radiation (PAR). Thus, the rate of photosynthesis is higher in blue and red light.

Absorption Spectrum This depicts the absorption of light of different wavelength by chlorophyll-a, b, xanthophyll and carotenoids.

Action Spectrum This shows the rate of photosynthesis in the plant in the light of different wavelengths.

Super Imposed Absorption and Action Spectrum When we superimposed both action and absorption spectrum, it shows that in the region of red and blue light, the chlorophyll-a and $b$ harness the maximum light energy and are the main photosynthetic pigments. So, the rate of photosynthesis is high in these two regions. It shows maximum activity peak at wavelength (red light) i.e., $660-670 \mathrm{~nm}, 430-470 \mathrm{~nm}$ (blue) and $390-430 \mathrm{~nm}$ (violet).

$\mathrm{C}_4$ 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.

(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.