Question

What is Q cycle in photosynthesis?

Answer 1

Hint: Photosynthesis is the process of creating oxygen for the plants. Q cycle is the sequential oxidation and reduction of plastoquinone in the electron transport chain occurring in the thylakoid membrane. The Q cycle elaborates a series of reactions to describe how the oxidation and reduction in plants takes place. It occurs in two forms- ubiquinol and ubiquinone forms.

Complete answer-
First Q cycle was proposed by Peter D. Mitchell. He modified a version called the original scheme and accepted it as a mechanism of complex III which contributes to the biochemical generation of proton and Ph gradient.
The operation of Q cycle is – reduction of Cytochrome C, oxidation of ubiquinol to ubiquinone and transfer of four protons into the intermembrane space, two cycle process. One process is Ubiquinol binds to Q site of complex III of cytochrome b. Ubiquinol is oxidized to one electron each and reaction produces transient semiquinone before complete oxidation to ubiquinone.
getting one electron from ubiquinol, the protein donor is free from electron donor and can migrate from Cytochrome C1 subunit. The electron is transferred from an oxidized molecule of cytochrome C externally to complex III which dissociates from complex III.
The other electron transfers the ubiquinone bound at the Q site. The movement of electrons is moving towards negatively charged side of the membrane which is favorable and attached ubiquinone reducing to semiquinone radical.
               

Note:
Photosynthesis uses all the elements essential for creating oxygen and energy in the form of sugar and transforms it to light energy during photosynthesis in green plants, light energy is captured and converted to water and minerals and energy rich organic compounds. There are stages in photosynthesis for conversion into chemical energy.