Photosynthesis in Higher Plants - Photosynthesis as a means of Autotrophic nutrition

Photosynthesis is a physico-chemical process by which green plants use light energy to synthesize organic compounds. It is the basis of autotrophic nutrition in the biosphere.

The overall process occurs in the chloroplasts. The membranous system (grana) is responsible for the light reaction (trapping light energy and synthesizing $ATP$ and $NADPH$), while the stroma is the site for the dark reaction (enzymatic reduction of $CO_{2}$ into sugar).

Photosynthetic pigments include Chlorophyll $a$ (bright or blue-green), Chlorophyll $b$ (yellow-green), Xanthophylls (yellow), and Carotenoids (yellow to yellow-orange). Chlorophyll $a$ is the primary pigment acting as the reaction center.

The 'Absorption Spectrum' shows the ability of pigments to absorb light at different wavelengths, while the 'Action Spectrum' indicates the rate of photosynthesis at different wavelengths of light.

Photosynthesis is a redox process where water ($H_{2}O$) is oxidized to oxygen ($O_{2}$) and carbon dioxide ($CO_{2}$) is reduced to carbohydrates ($C_{6}H_{12}O_{6}$).

The Light Harvesting Complex (LHC) consists of hundreds of pigment molecules bound to proteins, forming Photosystem I ($PS\ I$) and Photosystem II ($PS\ II$), named in the sequence of their discovery.