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Photosynthesis, step by step
How plants use light energy to build sugars from water and carbon dioxide.
Photosynthesis is how plants, algae and some bacteria use light energy to make glucose from carbon dioxide and water, releasing oxygen. In plants it happens in the chloroplasts.
Covers GCSE, AP and A-level. Exam boards differ slightly, so check your course specification.
6CO2 + 6H2O + light → C6H12O6 + 6O2
Carbon dioxide + water + light energy → glucose + oxygenThe two stages
Both stages happen inside the chloroplast.
- 01
Light-dependent reactions
Thylakoid membranesChlorophyll absorbs light energy. Water is split, which releases oxygen. The energy is captured in ATP and NADPH.
Makes ATP, NADPH and O2
- 02
Calvin cycle
StromaCarbon dioxide is fixed by the enzyme rubisco. ATP and NADPH from the first stage power its conversion into sugar (G3P), which is built into glucose.
Makes G3P, used to build sugars
The Calvin cycle is sometimes called the light-independent stage. It doesn't use light directly, but it depends on the ATP and NADPH the light-dependent stage makes.
Limiting factors
The rate of photosynthesis depends on three main factors: light intensity, carbon dioxide concentration and temperature. A limiting factor is the condition currently holding back the rate.
Increasing light or CO2 raises the rate when that factor is limiting. Warming helps only up to the optimum temperature. At some point it levels off, because something else is now the limiting factor. That's why graphs of photosynthesis plateau.
How each factor affects the rate
| Factor | As it increases | Why |
|---|---|---|
| Light intensity | Rate rises, then levels off | More light means more energy to drive the light-dependent reactions, until another factor limits. |
| Carbon dioxide concentration | Rate rises, then levels off | More CO2 is available for the Calvin cycle, until another factor limits. |
| Temperature | Rate rises to an optimum, then falls sharply | Enzymes work faster when warmer, but denature if it gets too hot. |
Key terms
- Chlorophyll
- The green pigment that absorbs light, mainly red and blue. It reflects green, which is why leaves look green.
- Chloroplast
- The organelle where photosynthesis happens.
- Stomata
- Pores in the leaf that let carbon dioxide in and oxygen out.
- Rubisco
- The enzyme that fixes carbon dioxide in the Calvin cycle.
- Glucose
- The sugar made by photosynthesis. Plants use it for respiration or turn it into starch, cellulose and other molecules.
- Limiting factor
- The condition that currently holds back the rate of a process.
Common mistakes
The oxygen comes from water, not carbon dioxide. Water is split in the light-dependent reactions.
Plants respire as well as photosynthesise. They do both in the light, and only respire in the dark.
Photosynthesis takes in energy. It's an endothermic process: light energy is stored in glucose.
Glucose isn't only used for energy. Plants also turn it into starch for storage, cellulose for cell walls, and other molecules like proteins and lipids.
A worked example
This is the kind of answer SnapBio gives you when you snap a question: the answer first, then the steps.
What is the role of chlorophyll in photosynthesis?
How to get there
- 1
Chlorophyll is a green pigment found in chloroplasts.
- 2
It absorbs light energy, mostly red and blue light.
- 3
That energy drives the conversion of carbon dioxide and water into glucose and oxygen.
Check yourself
Where do the light-dependent reactions happen?
The light-dependent reactions happen on the thylakoid membranes. The Calvin cycle happens in the stroma.
Where does the oxygen released in photosynthesis come from?
Water is split in the light-dependent reactions, releasing oxygen.
A graph of photosynthesis rate against light intensity levels off. Why?
Once light is no longer the limiting factor, the rate is held back by something else, such as CO2 concentration or temperature.
FAQ
What is the equation for photosynthesis?
6CO2 + 6H2O + light energy → C6H12O6 + 6O2. In words: carbon dioxide + water → glucose + oxygen, using light energy.
Where does photosynthesis happen in a plant?
Mainly in the leaves, inside chloroplasts. The light-dependent reactions happen on the thylakoid membranes, and the Calvin cycle in the stroma.
Why are leaves green?
Chlorophyll absorbs mostly red and blue light and reflects green light, so leaves look green.
How is photosynthesis different from respiration?
Photosynthesis stores energy in glucose using light, carbon dioxide and water. Respiration releases that energy as ATP, producing carbon dioxide and water. See cellular respiration explained.
Sources and further reading
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