Cyanobacteria appeared later the excess oxygen they produced contributed directly to the oxygenation of the Earth, which rendered the evolution of complex life possible. The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons. In other bacteria, different mechanisms such as the reverse Krebs cycle are used to achieve the same end. Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as glucose. In the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate (RuBP). In plants, algae and cyanobacteria, sugars are synthesized by a subsequent sequence of light-independent reactions called the Calvin cycle. The hydrogen freed by the splitting of water is used in the creation of two further compounds that serve as short-term stores of energy, enabling its transfer to drive other reactions: these compounds are reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), the 'energy currency' of cells. In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas.

In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane.

Īlthough photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centers that contain green chlorophyll (and other colored) pigments/chromophores. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies most of the energy necessary for life on Earth. Most plants, algae, and cyanobacteria perform photosynthesis such organisms are called photoautotrophs. Some of this chemical energy is stored in carbohydrate molecules, such as sugars and starches, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek phōs ( φῶς), 'light', and synthesis ( σύνθεσις), 'putting together'. Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities.

Dark red and blue-green indicate regions of high photosynthetic activity in the ocean and on land, respectively. Composite image showing the global distribution of photosynthesis, including both oceanic phytoplankton and terrestrial vegetation.

9.1 cellular respiration an overview