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The Basics Sunlight plays a much larger role in our sustenance than we may expect all the food we eat and all the fossil fuel we use is a product of photosynthesis which is the process that converts energy in sunlight to chemical forms of energy that can be used by biological systems Photosynthesis is carried out by many different organisms ranging from plants to bacteria Figure 1 The best known form of photosynthesis is the one carried out by higher plants and algae as well as by cyanobacteria and their relatives which are responsible for a major part of photosynthesis in oceans All these organisms convert CO2 carbon dioxide to organic material by reducing this gas to carbohydrates in a rather complex set of reactions Electrons for this reduction reaction ultimately come from water which is then converted to oxygen and protons Energy for this process is provided by light which is absorbed by pigments primarily chlorophylls and carotenoids Chlorophylls absorb blue and red light and carotenoids absorb blue-green light Figure 2 but green and yellow light are not effectively absorbed by photosynthetic pigments in plants therefore light of these colors is either reflected by leaves or passes through the Other photosynthetic organisms such as cyanobacteria formerly known as blue-green algae and red algae have additional pigments called phycobilins that are red or blue and that absorb the colors of visible light that are not effectively absorbed by chlorophyll and carotenoids Yet other organisms such as the purple and green bacteria which by the way look fairly brown under many growth conditions contain bacteriochlorophyll that absorbs in the infrared in addition to in the blue part of the spectrum These bacteria do not evolve oxygen but perform photosynthesis under anaerobic oxygen-less conditions These bacteria efficiently use infrared light for photosynthesis Infrared is light with wavelengths above 700 nm that cannot be seen by the human eye some bacterial species can use infrared light
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