Valofosforilaatio

Valofosforilaatio, also known as photophosphorylation, is a metabolic process that converts light energy into chemical energy in the form of adenosine triphosphate (ATP). This process occurs in the thylakoid membranes of chloroplasts in plants, algae, and cyanobacteria, as well as in the plasma membranes of photosynthetic bacteria.

The core mechanism of valofosforilaatio involves the absorption of light energy by pigments, primarily chlorophylls. This absorbed energy excites electrons within the pigment molecules. These energized electrons then move through a series of electron carriers embedded in the membrane, forming an electron transport chain. As electrons are passed along this chain, their energy is gradually released.

This released energy is used to pump protons (H+) from the stroma of the chloroplast (or the cytoplasm of bacteria) into the thylakoid lumen (or periplasmic space in bacteria). This creates an electrochemical gradient across the membrane, with a higher concentration of protons on one side. This proton gradient represents stored potential energy.

The flow of protons back across the membrane, driven by this gradient, occurs through a specific enzyme complex called ATP synthase. As protons pass through ATP synthase, they provide the energy for the enzyme to catalyze the phosphorylation of adenosine diphosphate (ADP) to ATP, a process that stores the captured light energy in the high-energy phosphate bonds of ATP.

Valofosforilaatio is a crucial step in photosynthesis, providing the ATP required for the Calvin cycle (or other carbon fixation pathways), where carbon dioxide is converted into sugars. There are two main types: cyclic and non-cyclic valofosforilaatio, differing in the path of electrons and whether NADPH is also produced.