Question

Discuss the mechanism of photosynthesis.

Answer 1

Photosynthesis

It is the biochemical process through which an organism can produce carbohydrates from carbon dioxide (CO₂) and water (H₂O) with the help of sunlight and chlorophyll.

$6{\mathrm{CO}}_2+6{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{Sunlight}\&\mathrm{Chlorophyll}}{\to }{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2$

Mechanism of photosynthesis

Photosynthesis occurs in two phases:

• Light reaction or Light-dependent reaction
• Dark reaction or Light independent reaction

Light Reaction

• It takes place in the thylakoid present within the chloroplasts, where there are two types of photosystems that act as the reaction centre.
• Light energy is absorbed, which helps in the excitation of electrons.
• The electrons undergo two electron transport chains, which results in the generation of ATP (Adenosine triphosphate) and NADPH (Nicotinamide adenine dinucleotide phosphate).
• Photolysis of water, i.e. splitting of water molecule follows, which is required to balance the electrons lost during the electron transport chain and thus, oxygen is formed.

The chemical equation of light reaction is:

$2{\mathrm{H}}_2\mathrm{O}+2\mathrm{NADP}++3\mathrm{ADP}+3\mathrm{Pi}\to {\mathrm{O}}_2+2\mathrm{NADPH}+3\mathrm{ATP}$

Dark Reaction

• It occurs in the stroma of the chloroplast. It is a cyclic process where the NADPH and ATP produced during the light reaction is utilized.
• The cycle starts with the addition of 6 molecules of CO₂ to 6 molecules of RuBP (Ribulose bisphosphate) to form 12 molecules of Phospho-glyceric acid (PGA) with the help of an enzyme Rubisco.
• The PGA molecule now gets reduced by NADPH to form 12 molecules of Glyceraldehyde-3-phosphate.
• Two molecules of glyceraldehyde-3-phosphate move out of the cycle to form one glucose molecule.
• The rest of the Glyceraldehyde-3-phosphate continues the cycle to regenerate RuBP.

The ultimate product after the several reactions of photosynthesis is glucose.