The mechanism of Photosynthesis.

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy from the sun into chemical energy stored in the form of glucose. It takes place in the chloroplasts of plant cells. The overall equation of photosynthesis is:

$6CO_2 + 6H_2O + Light\ Energy \rightarrow C_6H_{12}O_6 + 6O_2$

Photosynthesis occurs in two main stages:

1. The Light Reactions (Photochemical Phase)
2. The Dark Reactions (Calvin Cycle)

The Light Reactions (Photochemical Phase)

The light reactions take place in the grana (thylakoid membranes) of the chloroplast.

When light falls on the chlorophyll pigments, electrons become excited and move to a higher energy level. This light energy is used to split water molecules into hydrogen ions (H⁺), electrons (e⁻), and oxygen gas (O₂) through a process known as photolysis of water.

The oxygen produced during this process is released as a by-product into the atmosphere.

The excited electrons then pass through a series of proteins known as the electron transport chain (ETC). As electrons move through the chain, energy is released and used to convert ADP and inorganic phosphate (Pi) into ATP. This process is called photophosphorylation.

At the same time, NADP⁺ accepts hydrogen ions and electrons to form NADPH₂.

Products of the Light Reactions

• ATP
• NADPH₂
• Oxygen (O₂)

The ATP and NADPH₂ produced during the light reactions are utilized in the dark reactions.

The Dark Reactions (Calvin Cycle)

The dark reactions take place in the stroma of the chloroplast and do not directly require light.

In this stage, carbon dioxide from the atmosphere is fixed into organic molecules. Carbon dioxide combines with a five-carbon compound called RuBP (Ribulose Bisphosphate) in the presence of the enzyme RuBisCO.

This reaction forms an unstable six-carbon compound, which immediately breaks down into two molecules of 3-PGA (3-Phosphoglycerate).

Using the ATP and NADPH₂ produced during the light reactions, 3-PGA is reduced to G3P (Glyceraldehyde-3-Phosphate).

Some of the G3P molecules are used to synthesize glucose and other organic compounds, while the remaining G3P molecules regenerate RuBP, allowing the cycle to continue.

Important Facts about the Calvin Cycle

• For every 3 molecules of CO₂ fixed, 1 molecule of G3P is produced.
• 6 turns of the Calvin Cycle are required to produce 1 molecule of glucose.