Discribe the photosynthesis in higher plants

Photosynthesis in higher plants involves the following processes:

• Light Reaction
• Dark Reaction

Light Reaction

• This phenomenon occurs in the presence of light.
• The pigment absorbs light and produces energy in the form of ATP.
• The process involves- absorption of light, water splitting, the release of oxygen, and formation of ATP and NADPH.
• The protein-bound pigment molecules form the light-harvesting complexes present within two photosystems- PS-I and PS-II. Each photosystem has a reaction centre consisting of chlorophyll a molecule, and antennae containing accessory pigments.
• The reaction centre for PS-I is P-700 because the absorption peak for chlorophyll a is at 700 nm while that for PS-II is P-680 because the absorption peak for chlorophyll a is at 680 nm.

Photophosphorylation

The formation of ATP in the presence of sunlight is called photophosphorylation. It is of two types:

• Non-cyclic photophosphorylation
• Cyclic photophosphorylation

Non-cyclic Photophosphorylation

• PS-II absorbs light at a wavelength of 680 nm and causes excitation in the electrons. These excited electrons are accepted by an electron acceptor and transferred to the electron transport system.
• The electrons from the electron transport system are transferred to the PS-I. At the same time, the electrons at PS-I receive a wavelength of 700 nm and get excited.
• An electron from the electron acceptor is added to NADP+, which is then reduced to NADPH+ H+.
• The electrons lost by PS-II does not return to it and hence named non-cyclic photophosphorylation.
• In this, both the photosystems are involved.

Cyclic Photophosphorylation

• In cyclic photophosphorylation, only PS-I is involved.
• The electrons circulate within the photosystem which results in a cyclic flow of electrons.
• This only forms ATP and not NADPH+ H+.

Water Splitting

The light-dependent splitting of water is called photolysis. This process is associated with PS-II in which manganese and chlorine play an important role. The electrons lost from P680 are replaced by the electrons formed in this process. A molecule of water splits to release oxygen upon the absorption of light by P680.

Dark Reaction

This process occurs in the absence of light in the stroma of the chloroplasts. The following cycles are involved in the process:

Calvin Cycle (C3 Cycle)

This cycle involves the following steps:

1. Carbon-fixation: Ribulose-1, 5-bisphosphate combines with carbon dioxide to fix it to a 3 carbon compound 3-phosphoglyceric acid. The enzyme RuBisCO is involved in the process.
2. Reduction: 2 molecules of ATP and NADPH fixes one molecule of carbon dioxide to form glyceraldehyde-3-phosphate.
3. Regeneration: Some glyceraldehyde-3-phosphate molecules undergo a series of reactions to form glucose while the RuBP regenerates to continue the cycle.