How Photosynthesis Works: From Sunlight to Sugar

Photosynthesis is the process that green plants, algae and cyanobacteria use to make their own food. They take in carbon dioxide from the air and water from the soil and, using the energy in sunlight, join them into glucose, a sugar that stores chemical energy. Oxygen is released as a by-product.

The whole process can be summed up in one balanced equation:

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

In plants it takes place inside chloroplasts, small green structures packed into the cells of leaves. Their colour comes from chlorophyll, the pigment that absorbs light. Chlorophyll absorbs red and blue light most strongly and reflects green, which is why leaves look green to us.

The first stage happens on the thylakoid membranes, stacks of flattened discs inside each chloroplast. When light hits chlorophyll, it excites electrons and sets off a chain of reactions:

• Water is split into hydrogen ions, electrons and oxygen. This is where the oxygen we breathe comes from.
• The energy is captured in two carrier molecules, ATP and NADPH.
• The oxygen diffuses out of the leaf through the stomata.

This stage needs light, so it only runs while the plant is lit.

The second stage takes place in the stroma, the fluid around the thylakoids. It does not use light directly, but it depends on the ATP and NADPH made in stage 1.

An enzyme called RuBisCO attaches carbon dioxide to a five-carbon molecule called RuBP. Through a cycle of reactions, the carbon ends up in a three-carbon sugar that the plant uses to build glucose, sucrose and starch. Because it fixes carbon from the air into sugar, this stage is also called carbon fixation.

Three factors limit how fast photosynthesis can run:

1. Light intensity: more light means faster reactions, up to a point.
2. Carbon dioxide concentration: more CO₂ speeds up the Calvin cycle until another factor becomes limiting.
3. Temperature: the enzymes work best in a moderate range; very high temperatures slow them down.

Water matters too. On a hot, dry day a plant closes its stomata to save water, which also cuts off its supply of carbon dioxide.

Picture a mango tree in an Indian courtyard at 9 a.m. Sunlight falls on its leaves, and chlorophyll starts absorbing red and blue light. Water drawn up from the soil reaches the leaf cells, and carbon dioxide slips in through the stomata on the underside of each leaf.

Inside the chloroplasts, light splits water and releases oxygen into the air. The Calvin cycle then uses the captured energy to fix carbon dioxide into sugar. Some sugar feeds the tree straight away. The rest is moved around the tree as sucrose, and some ends up stored in the fruit, which is part of why a ripe mango tastes sweet.

By the hot afternoon, if the soil is dry, the leaves may partly close their stomata to save water, and photosynthesis slows down.

Photosynthesis is the entry point for almost all the energy in food chains: the rice and wheat we eat and the grass cattle graze both depend on it. It also removes carbon dioxide from the atmosphere and releases oxygen, which makes it central to the carbon cycle and to life on Earth.