Biology matters: 2013

· Light energy is converted to chemical energy in the form of ATP.

· Consist of cyclic photophosphorylation involving only photosystem I and

non-cyclic photophosphorylation involving both photosystems II and I.

· Light energy captured by pigments/light harvesting complexes passed to

special chlorophyll a, causing excitation of electrons to

higher energy level, which is subsequently captured by primary electron acceptor.

· For cyclic photophosphorylation, excited electrons are returned back to

special chlorophyll a (final electron acceptor), after passing down chain of electron carriers and proton pumps.

· For non-cyclic photophosphorylation, Photosystem I receives replacement of

electrons from photosystem II,

· photosystem II receives replacement of electrons from photolysis of water with

production of oxygen as byproduct.

· Excited electrons are passed along a chain of electron carriers of progressively

lower energy level/electron transport chain, energy released used by proton pumps to

pump H+ from stroma into thylakoid space.

· Creating a proton gradient across thylakoid membrane.

· ATP synthase harness proton gradient/proton motive force to generate ATP as

H⁺ diffuse back into stroma via ATP synthase.

· Chemiosmosis couples energy released from passage of electrons down

electron transport chain to active transport of H⁺ from stroma into thylakoid space

via proton pumps.

· For non-cyclic photophosphorylation, NADP reductase combines electrons with H⁺

and NADP⁺(final electron acceptor) to form NADPH for use in Calvin cycle.

Biology matters