Question

What are the differences between non-cyclic and cyclic electron transport pathways?

Answer 1

In photosynthesis, plants convert light energy to chemical energy. Using certain pigments called chlorophylls, xanthophyll, etc, light energy is trapped and is used for food production. Pigments are organized into light-harvesting complexes that absorb sunlight.

Complete step by step answer: Process of production of ATP from ADP in the presence of light in chloroplasts is called photophosphorylation.
In chloroplasts, this occurs in two ways. They are cyclic and non-cyclic photophosphorylations.
Photochemical reactions are the light-driven reactions of photosynthesis and are also called an electron transport chain.
We know that two pigment systems are present in this step. They are photosystem 1 or PS1 and photosystem 2 or PS2.
In cyclic photophosphorylation, all the pigment molecules in the PS1 absorb light energy. This light energy absorbed is transferred to the next pigment molecule and finally reaches the chlorophyll a molecule, which is the reaction centre of photosystem 1. In photosystem 1, reaction centre is P700. Electrons are emitted and are accepted by a primary electron acceptor. This electron again comes back to the reaction centre and the energy released is used for ATP synthesis. As the electron reaches back in the reaction centre, it is called a cyclic electron transport system. Here, only PS1 is functional and NADPH is not produced. Oxygen is also not released as there is no photolysis of water.
In non-cyclic photophosphorylation, both PS1 and PS2 are involved. P680 is the reaction centre in photosystem 2. It emits electrons and is accepted by a primary electron acceptor. Photolysis of water takes place and electrons released from this do not come back to P680. The electron from the primary electron acceptor then reaches P700 and then P700 emits electrons. ATP and NADPH are produced in the final step. As the electron travels in a non-cyclic pathway, it is called non-cyclic electron transfer.

Note: Non-cyclic photophosphorylation can be stopped by illuminating isolated chloroplasts with lights of wavelengths greater than 680 nm. Here, only PS1 is activated and PS2 remains inactivated. Under these conditions, cyclic photophosphorylation occurs.