Energy transfers which take place in living organisms
The nucleotide ATP (adenosine triphosphate) maintains both catabolic and anabolic reactions. Catabolic reactions e.g. respiration are where larger molecules are broken down into smaller ones with energy being released, and anabolic reactions e.g. photosynthesis are where smaller molecules are built up into larger ones which require energy. Catabolism provides the energy for organisms to synthesise larger molecules in its anabolic reactions. To release energy, ATP is hydrolysed into ADP and Pi (an inorganic phosphate) which releases energy that can be used for energy requiring reactions such as photosynthesis. However to maintain the organisms anabolic reactions, ATP must be continually synthesised by condensation reactions where ADP is added onto a phosphate molecule. This process is helped by energy transferred from catabolic reactions such as respiration and occurs in three ways: photophosphorylation, oxidative phosphorylation and substrate-level phosphorylation. Photophosphorylation takes place in chlorophyll- containing plant cells during photosynthesis, oxidative phosphorylation occurs in the mitochondria and animal cells during electron transport, and substrate- level phosphorylation occurs in plant and animal cells when phosphate groups are transferred from donor molecules to ADP to make ATP e.g. when pyruvate is formed at the end of glycolysis. Photosynthesis is an anabolic process where plants produce sugars from carbon dioxide, light energy and water. The sugars are used for other anabolic reactions e.g. protein synthesis and the energy required for these reactions comes from ATP which is synthesised from chemical energy into ATP during respiration. The ATP used in plants is made from the conversion of light energy from the sun, to chemical energy by plants, into the form of organic molecules during respiration in all cells. The ATP produced can then be used by cells to perform useful work e.g....
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