Photosynthesis: What is it and how does it work?

Photosynthesis is essential for almost all life, and it’s the primary source of oxygen in the atmosphere.

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What is photosynthesis?

Photosynthesis is the process by which carbohydrate molecules are synthesized. It’s used by plants, algae and certain bacteria to turn sunlight, water and carbon dioxide into oxygen and energy, in the form of sugar. It’s probably the most important biochemical process on the planet.

Essentially, it takes the carbon dioxide expelled by all breathing organisms and reintroduces it into the atmosphere as oxygen.

How does photosynthesis work?

Plants require light energy, carbon dioxide, water and nutrients. These ingredients come from both the adjacent atmosphere and the soil.

Phase 1

Plants absorb sunlight through the two top layers of their leaves, the cuticle and epidermis. These layers are thin, so light can travel through them easily. Carbon dioxide is brought in from the atmosphere, and at the same time, water is drawn up from the soil, into the body of the living plant.

Phase 2

Just beneath the cuticle and epidermis are the palisade mesophyll cells. These specialised cells are vertically elongated and arranged closely together to maximise light absorption.

Below the palisade mesophyll cells is the spongy mesophyll tissue, which is loosely packed for efficient gas exchange. As gases move in and out of these cells, they dissolve in a thin layer of water that covers the cells.

Phase 3

Inside the palisade mesophyll cells are the chloroplasts, lots of them. They contain chlorophyll, molecules that don’t absorb green wavelengths of white light. Instead, they reflect it back to us, giving plants their green colour.

Phase 4

Inside the chloroplast is where the magic happens. A light-dependent reaction takes place, where energy from the light waves is absorbed and stored in energy-carrying ATP molecules.

Then, in a light-independent reaction (the Calvin Cycle), ATP is used to make glucose, a source of energy. Water is oxidised, carbon dioxide is reduced, and oxygen is released into the atmosphere.

Oxygen is released via stomata in the leaves, microscopic pores that open to both let in the carbon dioxide, and release oxygen (and water vapour).

What is the equation for photosynthesis?

Photosynthesising organisms form the base of the food chain.

Carbon dioxide + water (with light energy) = glucose + oxygen

As well as the light energy, carbon dioxide and water, plants also need nutrients, which they get from the soil. These nutrients are released again, or recycled, when the plant tissue dies and begins decomposing in the soil.

Oxygen in the form of gas molecules (O2) is actually a by-product of photosynthesis, but it’s responsible for the oxygen in the air that keeps us alive. Plants also release energy and water to the atmosphere through respiration.

6CO2 + 6H2O → C6H12O6 + 6O2

The balanced equation takes it a little further. Six carbon dioxide molecules and six water molecules (the reactants) are converted into one sugar molecule (C6H12O6) and six oxygen molecules, via the light energy captured by the chlorophyll.

Source: BBC News