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Photosynthesis and respiration are examples of mechanisms in the cell which involve the transfer of energy through space from one macromolecular component to another in the form of electrons. Whilst the aromatic side amino acid side chains of proteins have delocalised electron fields and can accept and transfer excess charge, most electron transfer groups utilise prosthetic groups such as haem and iron-sulphur clusters, which contain many more atoms with delocalised electrons. Delocalisation and electron transfer also requires the exclusion of water, and this is provided by the surrounding protein structure.