What photosynthesis does

Almost every bite of food you eat and nearly all the oxygen you breathe trace back to green organisms soaking up sunlight. Photosynthesis is the process in which plants, algae, and some bacteria capture light energy and use it to build carbohydrates, such as sugars, from carbon dioxide and water. Oxygen is released along the way.

Organisms that make food and organisms that eat it

Living things are built from organic molecules: carbon-based molecules such as sugars, fats, and proteins. Small, simple molecules such as CO₂ and water are inorganic. Every organism needs organic molecules, both as building material and as fuel, and there are two ways to get them.

A heterotroph gets organic molecules by eating: it takes them in from its surroundings as food. All animals and fungi are heterotrophs, and so are most bacteria and protists.

An autotroph makes its own organic molecules from inorganic ones, such as CO₂ and H₂O. Autotrophs that use light as their energy source are photoautotrophs. They include plants, algae, and some bacteria, such as cyanobacteria.

An energy cycle runs the biosphere

The biosphere is every place on Earth’s surface and in its atmosphere where living things are found. Most life in it depends on two processes that feed each other:

• Photosynthesis uses light, CO₂, and H₂O to make organic molecules and O₂.
• Cellular respiration breaks organic molecules back down to CO₂ and H₂O. It stores the released energy in ATP, the small molecule cells spend to power their activities, and in NADH, a carrier of energized electrons. In aerobic respiration, O₂ is used up and becomes water.

Each process’s products are the other’s raw materials. The CO₂ released by respiration goes into the air, where photoautotrophs can take it up to build more sugar. The O₂ they release, in turn, supports aerobic respiration.

The overall equation

For plants and algae, the equation for making one molecule of glucose, a six-carbon sugar (C₆H₁₂O₆), is:

6 CO₂ + 12 H₂O + light energy → C₆H₁₂O₆ + 6 O₂ + 6 H₂O

Some photosynthetic bacteria use hydrogen sulfide (H₂S) in place of water and give off sulfur instead of oxygen. One general equation covers every photosynthesizer:

CO₂ + 2 H₂A + light energy → CH₂O + A₂ + H₂O

Here CH₂O is shorthand for one carbon’s worth of carbohydrate, and A stands for oxygen in plants and algae or sulfur in those bacteria. When A is oxygen, A₂ is simply O₂, and the plant version reads:

CO₂ + 2 H₂O + light energy → CH₂O + O₂ + H₂O

Multiply every term by 6 and you get the glucose equation above.

An uphill redox reaction

Photosynthesis is a redox reaction, one in which electrons move from one molecule to another. The molecule that loses electrons is oxidized, and the molecule that gains them is reduced.

In photosynthesis, water is oxidized: it gives up electrons, and its oxygen is released as O₂. Carbon dioxide is reduced: its carbon gains electrons as it becomes part of a carbohydrate. In the general equation, H₂A is oxidized and CO₂ is reduced.

This reaction runs uphill. Its free-energy change (ΔG) compares how much usable energy the products hold with how much the starting materials held. For photosynthesis, ΔG is +685 kcal for every mole of glucose made. (A mole is a chemist’s counting unit, like a dozen but vastly larger.)

A positive ΔG means the reaction is endergonic: it can’t go forward on its own and has to be driven by an energy source. For photosynthesis, that source is sunlight.

Check yourselfIn the overall equation for photosynthesis, which molecule is oxidized and which is reduced?

Water is oxidized: it loses electrons, and its oxygen atoms end up in O₂. Carbon dioxide is reduced: its carbon gains electrons and becomes part of a carbohydrate such as glucose.