<P> The energy in sunlight is captured by plants, cyanobacteria, purple bacteria, green sulfur bacteria and some protists . This process is often coupled to the conversion of carbon dioxide into organic compounds, as part of photosynthesis, which is discussed below . The energy capture and carbon fixation systems can however operate separately in prokaryotes, as purple bacteria and green sulfur bacteria can use sunlight as a source of energy, while switching between carbon fixation and the fermentation of organic compounds . </P> <P> In many organisms the capture of solar energy is similar in principle to oxidative phosphorylation, as it involves the storage of energy as a proton concentration gradient . This proton motive force then drives ATP synthesis . The electrons needed to drive this electron transport chain come from light - gathering proteins called photosynthetic reaction centres or rhodopsins . Reaction centers are classed into two types depending on the type of photosynthetic pigment present, with most photosynthetic bacteria only having one type, while plants and cyanobacteria have two . </P> <P> In plants, algae, and cyanobacteria, photosystem II uses light energy to remove electrons from water, releasing oxygen as a waste product . The electrons then flow to the cytochrome b6f complex, which uses their energy to pump protons across the thylakoid membrane in the chloroplast . These protons move back through the membrane as they drive the ATP synthase, as before . The electrons then flow through photosystem I and can then either be used to reduce the coenzyme NADP, for use in the Calvin cycle, which is discussed below, or recycled for further ATP generation . </P> <P> Anabolism is the set of constructive metabolic processes where the energy released by catabolism is used to synthesize complex molecules . In general, the complex molecules that make up cellular structures are constructed step - by - step from small and simple precursors . Anabolism involves three basic stages . First, the production of precursors such as amino acids, monosaccharides, isoprenoids and nucleotides, secondly, their activation into reactive forms using energy from ATP, and thirdly, the assembly of these precursors into complex molecules such as proteins, polysaccharides, lipids and nucleic acids . </P>

Where do most of the metabolic activities take place within the cell