Photophosphorylation
The third type of phosphorylation to make ATP is found only in cells that carry out photosynthesis. This process is similar to oxidative phosphorylation in several ways. A primary difference is the ultimate source of the energy for ATP synthesis. In oxidative phosphorylation, the energy comes from electrons produced by oxidation of biological molecules. In photosynthesis, the energy comes from the light of the sun. Photons from the sun interact with chlorophyll molecules in reaction centers in the chloroplasts (Figures \PageIndex{1} and \PageIndex{2}) of plants or membranes of photosynthetic bacteria.
The similarities of photophosphorylation to oxidative phosphorylation include:
a membrane associated electron transport chain
creation of a proton gradient
harvesting energy of the proton gradient by making ATP with the help of an ATP synthase.
Some of the differences include :
the source of the electrons - H2O for photosynthesis versus NADH/FADH2 for oxidative phosphorylation
direction of proton pumping - into the thylakoid space of the chloroplasts versus outside the matrix of the mitochondrion
movement of protons during ATP synthesis - out of the thylakoid space in photosynthesis versus into the mitochondrial matrix in oxidative phosphorylation
nature of the terminal electron acceptor - NADP+ in photosynthesis versus O2 in oxidative phosphorylation.
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