What are the differences between cyclic and non-cyclic photophosphorylation?

Cyclic photophosphorylation only involves photosystem I and produces ATP, while non-cyclic involves both photosystems and produces ATP, NADPH and oxygen.

Cyclic and non-cyclic photophosphorylation are two types of light-dependent reactions in photosynthesis, which occur in the thylakoid membranes of chloroplasts in plants. The main difference between them lies in the number of photosystems involved and the products they generate.

Cyclic photophosphorylation, as the name suggests, is a cycle that only involves photosystem I. When light energy is absorbed by the chlorophyll in photosystem I, it excites electrons to a higher energy level. These electrons are then passed along a series of electron carriers, releasing energy which is used to synthesise ATP from ADP and inorganic phosphate. This process is called photophosphorylation. The electrons eventually return to photosystem I, hence the term 'cyclic'.

On the other hand, non-cyclic photophosphorylation involves both photosystem I and photosystem II. In this process, light energy absorbed by photosystem II causes the release of electrons, which are then passed along an electron transport chain, producing ATP. These electrons do not return to photosystem II; instead, they are taken up by photosystem I, which has also been excited by light energy. The electrons from photosystem I are then used to reduce NADP+ to NADPH. To replace the lost electrons in photosystem II, water molecules are split in a process called photolysis, releasing oxygen as a by-product. This is why non-cyclic photophosphorylation also contributes to the production of oxygen, unlike cyclic photophosphorylation.

In summary, cyclic photophosphorylation is a simpler process that only involves photosystem I and produces ATP. Non-cyclic photophosphorylation is a more complex process that involves both photosystems and produces ATP, NADPH and oxygen. Both processes are crucial for the energy needs of the plant, but non-cyclic photophosphorylation also plays a vital role in producing the oxygen we breathe.