Unit 3.1 the importance of ATP

• ATP is described as the universal energy currency.
• It is used in all cells to drive their reactions
• It is made when energy becomes available e.g. in respiration and in the light- dependant reactions of photosynthesis.
• It is broken down when the cell needs energy e.g. in biosynthesis, muscle contraction and powering membrane pumps.

ATP is ideally suited to its role because it:

• is inert (not chemically reactive)
• can pass out of mitochondria into the cytoplasm
• releases energy efficiently
• releases energy in useable quantities, so little is wasted as heat
• is easily hydrolysed to release energy
• is readily reformed by phosphorylation

Chemiosmosis: The flow of protons down an electrochemical gradient, through ATP synthetase, coupled with the synthesis of ATP from ADP and a phosphate ion.

The proton gradient

• Proton gradients occur in non-living systems, such as in oceanic alkaline hydrothermal vents.
• Proton gradients are a fundamental characteristic of all living things.
• In the light dependent stage of photosynthesis, electrons are excited by energy from light.
• These electrons move through a series of carriers in the thylakoid membranes.
• Their energy pumps protons from the stroma into the spaces between the thylakoid membranes.
• The energy is released in chemiosmosis, in which protons flow back down an electrochemical gradient into the stroma, through ATP synthetase. 
• The energy is incorporated into ATP.
• This ATP drives the light independent reactions of photosynthesis and energy is incorporated into macromolecules made by the cell.
• In respiration, electrons are excited by energy derived from food molecules.
• Their energy is made available as they move through a series of carriers on the inner mitochondrial membrane.
• The energy pumps protons across the membrane, from the matrix into the inter-membrane space, setting up a proton gradient. 
• Energy is released in…