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Chemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient.An important example is the formation of adenosine triphosphate (ATP) by the movement of hydrogen ions (H +) across a membrane during cellular respiration or photosynthesis.
This transport chain produces a proton-motive force, pumping H + ions across the membrane and producing a concentration gradient that can be used to power ATP synthase during chemiosmosis. This pathway is known as cyclic photophosphorylation, and it produces neither O 2 nor NADPH.
A proton gradient is created across the thylakoid membrane (6) as protons (3) are transported from the chloroplast stroma (4) to the thylakoid lumen (5). Through chemiosmosis, ATP (9) is produced where ATP synthase (1) binds an inorganic phosphate group (8) to an ADP molecule (7).
An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts: The chemical gradient, or difference in solute concentration across a membrane. The electrical gradient, or difference in charge across a membrane.
During photosynthesis, the lumen becomes acidic, as low as pH 4, compared to pH 8 in the stroma. [30] This represents a 10,000 fold concentration gradient for protons across the thylakoid membrane. Source of proton gradient
The difference between passive transport and active transport is that the active transport requires energy, and moves substances against their respective concentration gradient, whereas passive transport requires no cellular energy and moves substances in the direction of their respective concentration gradient. [10]
It has two components: a difference in proton concentration (a H + gradient, ΔpH) and a difference in electric potential, with the N-side having a negative charge. [4] ATP synthase releases this stored energy by completing the circuit and allowing protons to flow down the electrochemical gradient, back to the N-side of the membrane. [5]
The sucrose concentration in sieve tubes is typically 10–30% in the leaves but only 0.5% in the photosynthesis cells. [citation needed] The gradient of sugar from source to sink causes pressure flow through the sieve tube toward the sink. [4]