Photosynthesis

Photosynthesis is the process by which plants, algae and certain bacteria capture energy from sunlight and use it to build chemical compounds from carbon dioxide and water. In doing so, they release oxygen as a by-product. It is the main way energy enters the living world, feeding almost every food chain and maintaining the oxygen in the atmosphere.

In everyday terms, photosynthesis is how green organisms make their own food. The phrase often taught in schools is that plants take in carbon dioxide and give out oxygen; that captures the headline, but the underlying chemistry is more intricate.

The basic ingredients and products

An organism carrying out photosynthesis needs light, carbon dioxide and water. It uses these to produce sugars, which store energy, and releases oxygen. The overall transformation is often summarised as carbon dioxide plus water, with light energy, yielding glucose and oxygen. The sugars become building blocks and fuel for the organism, and the oxygen escapes into the atmosphere. This is the reverse, in effect, of respiration, in which organisms release energy from sugars and give off carbon dioxide and water.

The two main stages

Photosynthesis is usually divided into two linked stages. The first, the light-dependent reactions, takes place in the membranes of structures called chloroplasts, specifically in stacked membranes known as thylakoids. Here, pigments such as chlorophyll absorb light, and the captured energy is used to split water molecules, producing oxygen and generating energy-carrying molecules that the cell can use. The second stage, the Calvin cycle, occurs in the fluid around the membranes. It does not need light directly; instead it uses the energy carriers from the first stage to fix carbon dioxide into simple sugars. The two stages together turn light into stored chemical energy.

Pigments and colour

Chlorophyll is the main pigment responsible for photosynthesis and gives most plants their green colour because it absorbs red and blue light strongly while reflecting green. Other pigments, such as carotenoids, help capture additional wavelengths and protect the plant from damage. In autumn, as chlorophyll breaks down, the yellows and oranges of other pigments become visible, which is why leaves change colour. Different organisms have evolved variations that suit their environments, including some that use different pigments altogether.

Variations across species

Not all photosynthesis follows the same pattern. Many familiar plants use what is called the C3 pathway, especially in cooler, moist conditions. Plants in hot, dry places often use C4 photosynthesis, which is more efficient at capturing carbon dioxide when water is scarce; maize, sugarcane and many grasses are examples. Cacti and other desert plants use a third variation, CAM, which takes in carbon dioxide at night to reduce water loss. These adaptations show how the same core process can be tuned to different climates.

Why it matters

Photosynthesis shapes the planet in profound ways. It produces the oxygen that animals, including humans, need to breathe, and it removes carbon dioxide from the air, playing a central role in the carbon cycle and the regulation of climate. It forms the base of nearly all food webs, because the energy that plants capture ultimately supports the animals that eat them. Fossil fuels, such as coal and oil, are the remains of ancient organisms that once stored solar energy through photosynthesis. Understanding it also matters for agriculture, forestry and efforts to engineer more productive or resilient crops.

Common misconceptions

Photosynthesis is not the same as respiration, though the two are related and complementary. Plants do not only photosynthesise; they also respire day and night, so they consume some oxygen as well as producing it. Photosynthesis does not happen only in leaves, and not only in green plants — algae and some bacteria perform it too. Nor is it "breathing" for plants in a simple sense; it is a set of chemical reactions powered by light.

Key points