Plants, algae and cyanobacteria use a chemical reaction known as photosynthesis to create the materials they need from what’s around them. Plucking carbon dioxide from the air, water from the ground and light from the sun, land plants make sugar and kick out oxygen as a waste product. Which is lucky for us. Without this oxygen supply to counterbalance the carbon dioxide we breathe out, most life on this planet, including us, would suffocate.
In plants, photosynthesis takes place in structures within their cells called chloroplasts. Chloroplasts, like the mitochondria in our own cells that drive our metabolism, are thought to have originated from bacterial cells that came to live in symbiosis inside their host.
Plants can harvest light because their chloroplasts are stuffed full of a pigment called chlorophyll, which absorbs red and blue light. When the sun’s rays hit plants, they absorb these colours but not green, which gets reflected, giving most plants their distinctive hue.
Plants use the sugars they make to fuel their growth and combine them into more complex molecules like cellulose to make material. This process of taking carbon from the air and using it to make large polymers makes plants an extremely useful ally in combating climate change, which is predominantly caused by carbon dioxide emissions. Ironically, photosynthesis is also behind many of the world’s fossil fuels, which formed from decayed prehistoric plants and animals.
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Some organisms depend on pigments other than chlorophyll to photosynthesise, such as carotenoids, which are red, orange or yellow and absorb blue-green light.
While plants, algae and cyanobacteria all use oxygen-based photosynthesis, there is also a version of the reaction called anoxygenic photosynthesis. This typically occurs in bacteria, such as purple bacteria and green sulphur bacteria, in aquatic habitats. These organisms photosynthesise use chemicals like hydrogen sulphide instead of water and produce sulphur as a by-product rather than oxygen.
Some animals also seem to be able to photosynthesise. The emerald green sea slug, for example, consumes algae and uses its chloroplasts. And it is thought that some green aphids can also harness light using their pigments. Chris Simms
