How photosynthesis works

What photosynthesis does

A plant cannot eat. It builds its own food out of two simple, low-energy ingredients: carbon dioxide from the air, which enters through tiny pores in the leaves called stomata, and water from the soil, which travels up from the roots. Light provides the energy to rearrange their atoms into glucose, a sugar that stores that energy in its chemical bonds.

The plant uses glucose in three ways: it burns some for energy in cellular respiration, links some into starch to store, and builds some into cellulose and other materials that make up its body. Oxygen is a by-product, released through the stomata.

Almost every food chain on Earth starts here. When you eat bread or a burger, the energy in it was captured from sunlight by a plant.

The photosynthesis equation

In words: carbon dioxide plus water, using light energy, gives glucose plus oxygen. As a balanced chemical equation, 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light energy written above the arrow and chlorophyll often written below it.

A balanced equation has the same number of each kind of atom on both sides, because atoms are rearranged in a reaction, never created or destroyed. Checking it is good practice:

Where it happens: leaves and chloroplasts

Most photosynthesis happens in the leaves, in cells packed with chloroplasts. A chloroplast has an outer membrane, a fluid inside called the stroma, and stacks of flattened sacs called thylakoids. The thylakoid membranes hold chlorophyll and run the light reactions; the stroma is where the Calvin cycle builds sugar.

Chlorophyll absorbs red and blue light strongly and reflects most green light. That reflected green is why leaves look green.

Stage 1: the light reactions

The light reactions capture energy. They need light, so they only run when the plant is lit.

In plain words, here is what happens in the thylakoid membranes:

• Chlorophyll absorbs light, and the energy excites electrons, boosting them to a higher energy level.
• To replace those electrons, the plant splits water molecules. This releases oxygen gas, which is where all the oxygen from photosynthesis comes from.
• The energised electrons pass along a chain of proteins, and their energy is used to make two energy-carrying molecules, ATP and NADPH.
• ATP and NADPH carry the captured energy to the stroma, ready for the second stage.

Stage 2: the Calvin cycle

The Calvin cycle builds sugar. It does not use light directly, which is why it is sometimes called the light-independent reactions, though in most plants it runs during the day because it depends on the ATP and NADPH the light reactions supply.

An enzyme called rubisco attaches carbon dioxide from the air to a five-carbon molecule already in the stroma. This step is called carbon fixation: carbon from a gas becomes part of a solid molecule. Using the energy of ATP and the electrons of NADPH, the cell turns the products into a three-carbon sugar called G3P. Some G3P leaves the cycle to build glucose and other molecules, and the rest is recycled to pick up more CO₂.

It takes six turns of the cycle, fixing six CO₂ molecules, to supply the carbon for one six-carbon glucose molecule.

How photosynthesis links to cellular respiration

Cellular respiration is photosynthesis run in reverse, in terms of the overall equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Photosynthesis stores energy in glucose; respiration, in the mitochondria, releases it as ATP the cell can use.

Plants do both. They photosynthesise only when lit, but they respire all the time, day and night, in every living cell. On a sunny day a plant makes far more oxygen than it uses, so its net effect is to release oxygen and absorb carbon dioxide. Animals respire but cannot photosynthesise, so the two processes form a cycle: the oxygen and sugar plants make feed respiration, and the carbon dioxide and water respiration releases feed photosynthesis.

Common misconceptions

These ideas feel natural but are wrong:

• Plants get their food from the soil. Soil supplies water and small amounts of minerals, but most of a plant's dry mass is built from carbon taken out of the air as CO₂.
• Plants do not respire. Every living plant cell respires to release energy, all the time.
• The oxygen released comes from carbon dioxide. It comes from splitting water in the light reactions; the oxygen atoms in CO₂ end up in glucose and water.
• Plants make energy. Energy cannot be made; plants convert light energy into chemical energy stored in glucose.
• The Calvin cycle happens at night. It does not use light directly, but it needs ATP and NADPH from the light reactions, so it runs mainly in daylight.
• Photosynthesis happens in every plant cell. Only cells with chloroplasts can do it, so root cells, for example, cannot.

What speeds it up or slows it down

The rate of photosynthesis depends on light intensity, carbon dioxide concentration and temperature. Raising any one of them speeds photosynthesis up until something else becomes the limiting factor, the one in shortest supply. A plant in bright light but low CO₂ will not speed up with more light; it needs more CO₂.

Temperature works differently because the Calvin cycle runs on enzymes. Warming speeds it up to an optimum, but too much heat changes the shape of the enzymes and the rate falls sharply. A classic class experiment counts the oxygen bubbles from pondweed at different distances from a lamp: closer light, more bubbles, up to a point.

How to practise and check your understanding

Write the word equation and the balanced equation from memory, then count atoms on each side as in the worked example above. If any element does not match, check the coefficients.

Draw a chloroplast and label where each stage happens, with arrows showing what goes in and comes out: water and light into the thylakoids, oxygen out; CO₂, ATP and NADPH into the stroma, sugar out. Then draw a mitochondrion beside it and connect the two with arrows to show the cycle with respiration.

Finally, test yourself on the misconceptions: explain in one sentence where a tree's wood comes from, and where the oxygen it releases comes from.