Photosynthesis is a process which mainly occurs in leaves of all green plants. The plants have designed their leaf in such a way that it is able to trap solar radiation and effectively convert solar/light energy to chemical energy. But biology is science of exceptions. Some plants carry out photosynthesis in modified plant parts other then leaves.

Few examples are as follows

1. Root as Photosynthetic Organ

When roots develop chlorophyll and start photosynthesis, they are called assimilitory roots. Trapa and Tinospora are the examples of assimilatory roots.

2. Stem as Photosynthetic Organ

In Opuntia, the stem gets modified to take up the function of leaves. It becomes flattened, thick and succulent and perform photosynthesis. Such structures are called phylloclade.

3. Petiole as Photosynthetic Organ In Australian Acasia the petiole takes the shape and function of photosynthesis because leaf lamina soon falls off.

(a) Synthesis of ATP and NADPH takes place in outer side of thylakoid membrane.

(b) Photolysis of water occurs in inner side of thylakoid membrane.

(c) Fixation of $\mathrm{CO}_2$ occurs in stroma of chloroplast.

(d) Synthesis of sugar molecule occurs in chloroplast.

(e) Synthesis of starch occurs in cytoplasm.

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.