Proton gradient in the context of Photosystem II

Light-dependent reactions are the chemical reactions involved in photosynthesis induced by light; all light-dependent reactions occur in thylakoids. There are two light-dependent reactions: the first occurs at photosystem II (PSII) and the second occurs at photosystem I (PSI).

PSII absorbs a photon to produce a so-called high energy electron which transfers via an electron transport chain to cytochrome b₆f and then to PSI. The then-reduced PSI, absorbs another photon producing a more highly reducing electron, which converts NADP to NADPH. In oxygenic photosynthesis, the first electron donor is water, creating oxygen (O₂) as a by-product. In anoxygenic photosynthesis, various electron donors are used.Cytochrome b₆f and ATP synthase work together to produce ATP (photophosphorylation) in two distinct ways. In non-cyclic photophosphorylation, cytochrome b₆f uses electrons from PSII and energy from PSI to pump protons from the cytoplasm (or stroma in chloroplasts), to the lumen of the thylakoid. The resulting proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form ATP. In cyclic photophosphorylation, cytochrome b₆f uses electrons and energy from PSI to create more ATP and to stop the production of NADPH. Cyclic phosphorylation is important to create ATP and maintain NADPH in the right proportion for the light-independent reactions.

In the process of photosynthesis, the phosphorylation of ADP to form ATP using the energy of sunlight is called photophosphorylation. Cyclic photophosphorylation occurs in both aerobic and anaerobic conditions, driven by the main source of energy available to living organisms, which is sunlight. All organisms produce ATP, which is the universal energy currency of life. In photophosphorylation, light energy is used to pump protons across a biological membrane, mediated by flow of electrons through an electron transport chain. This stores energy in a proton gradient. As the protons flow back through an enzyme called ATP synthase, ATP is generated from ADP and inorganic phosphate. ATP is essential in the Calvin cycle to assist in the synthesis of carbohydrates from carbon dioxide and NADPH.

The scientist Charles Barnes first used the word 'photosynthesis' in 1893. This word is taken from two Greek words, photos, which means light, and synthesis, which in chemistry means making a substance by combining simpler substances. So, in the presence of light, synthesis of food is called 'photosynthesis'.

Neurotransmitter transporters are a class of membrane transport proteins that span the cellular membranes of neurons. Their primary function is to carry neurotransmitters across these membranes and to direct their further transport to specific intracellular locations. There are more than twenty types of neurotransmitter transporters.

Vesicular transporters move neurotransmitters into synaptic vesicles, regulating the concentrations of substances within them. Vesicular transporters rely on a proton gradient created by the hydrolysis of adenosine triphosphate (ATP) in order to carry out their work: v-ATPase hydrolyzes ATP, causing protons to be pumped into the synaptic vesicles and creating a proton gradient. Then the efflux of protons from the vesicle provides the energy to bring the neurotransmitter into the vesicle.