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Read more: 鈥�Instant Expert 30: Photosynthesis鈥�

You have photosynthesis to thank for every lungful of air you breathe. In fact, photosynthesis is probably the most important biochemical process on the planet. Besides pumping oxygen into the atmosphere, it is the energy source behind all our food and almost all the heat and power we use. Without it, the evolution of life on Earth would have followed a very different path. Yet unpicking the molecular details of photosynthetic chemistry, and understanding how the process shapes our environment, remains a key challenge

Photosynthesis: the basics

Photosynthesis is the process by which plants, algae and some bacteria convert carbon dioxide and water into carbohydrates using energy from sunlight. In most cases, they achieve this by splitting apart the hydrogen and oxygen in water (H₂O), giving off oxygen (O₂) as a by-product. In many ways, photosynthesis is the reverse of respiration: when we animals respire, we use O₂ to burn up carbohydrates, releasing CO₂ and producing the energy we need to live.

Photosynthesis consists of a complex series of reactions, but it can be divided into four key stages: light absorption, charge separation, carbon fixation and oxygen evolution. First, a photon of sunlight is absorbed by chlorophyll pigments and passed to a 鈥渞eaction centre鈥�, which contains a specially aligned pair of chlorophyll molecules. Here charge separation occurs: the chlorophyll pair uses the photon鈥檚 energy to spit out an electron. This triggers the final two stages. The ejected electron is passed along a chain of molecules until it is used to convert CO₂ into carbohydrate, a process known as carbon fixation. Meanwhile the reaction centre is 鈥渞eset鈥� with a new electron stripped out of water. This replacement comes from part of the reaction centre complex called the oxygen evolving centre, which splits water molecules into electrons, hydrogen ions and oxygen gas. The complete process can be summarised in a simple equation:

H₂O + CO₂ + light 鈫� C(H₂O) + O₂

All this chemistry, from light absorption to the synthesis of carbohydrates, occurs in a structure called a chloroplast.

Chloroplasts have two membranes. The smooth outer membrane holds the whole structure together. The inner membrane is folded into a series of stacked discs called thylakoids that contain the pigments and protein complexes required to capture solar energy and release oxygen. The enzymes and other components involved in converting CO₂ into sugars are located in the stroma, the fluid-filled space inside the chloroplast (see diagram).

The central role played by chloroplasts was highlighted 75 years ago, when Robert Hill, a biochemist at the University of Cambridge, discovered that these organelles can when illuminated in the absence of CO₂. This finding was a key discovery because it provided one of the first indications that the ultimate source of electrons is water and not CO₂.

The light and dark reactions

In the chloroplast, the photosynthetic reactions that depend on light are physically separated from those that do not. These are called the 鈥渓ight鈥� and 鈥渄ark鈥� reactions, respectively.

All the components of the light reactions are arranged in or on proteins held in the thylakoid membrane. Light harvesting antenna, for instance, are proteins that contain chlorophyll pigments arranged to absorb light and pass the energy to nearby reaction centres.

While some bacteria contain one kind of photosynthetic reaction centre, algae and plants contain two types 鈥� photosystem one and photosystem two. Charge separation in PS2 pulls electrons from the oxygen evolving centre and passes them to PS1. PS1 is activated by a second photon and the electrons it produces are passed out of the thylakoid membrane and onto molecules involved in the dark reactions.

The dark reactions occur in the stroma. Here, enzymes drive a cyclic reaction that converts CO₂ and a sugar containing five carbon atoms into molecules of 3-phosphoglycerate, a 3-carbon sugar. A proportion of these sugars are fed back into the cycle. The rest are used as building blocks to form carbohydrates such as sucrose, cellulose or starch. The , known as the Calvin Cycle, were discovered in 1950 by Melvin Calvin, James Bassham, and Andrew Benson at the University of California in Berkeley.

The enzyme responsible for fixing carbon from CO₂ is called rubisco. It is probably the most abundant protein on the planet. Every atom of carbon in your body has been captured from the atmosphere by rubisco yet remarkably it is a rather inefficient enzyme; it has a low affinity for CO₂, and also reacts with O₂ in a process called photorespiration, with the result that about a third of the carbon it fixes is released back into the atmosphere.

Driven by light energy, photosynthetic chemistry in the thylakoid membrane produces , a molecular source of energy, along with a reducing agent called NADPH. These molecules are then consumed in the dark reactions. NADPH is formed during the final stage of the electron transport chain while ATP is created when energy from photons is used to pump protons across the thylakoid membrane. This sets up an electrochemical gradient that pushes the protons back out, releasing energy and generating ATP.