What is photosynthesis? Definition, equation and the two stages
Photosynthesis is the process in which plants, algae and cyanobacteria convert carbon dioxide and water into glucose and oxygen using light energy. The overall equation is 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂, and it runs in the chloroplast in two stages: the light-dependent reactions in the thylakoid membrane, which capture the energy, and the Calvin cycle in the stroma, which builds the sugar. The oxygen released comes from the water, not from the carbon dioxide.
Key takeaways
- Photosynthesis converts light energy into chemical energy stored in glucose.
- The oxygen comes from splitting water, not from carbon dioxide.
- The light reactions produce ATP and NADPH; the Calvin cycle spends them.
- Photosynthesis and respiration are opposites in the overall equation but not in mechanism.
What is photosynthesis?
Photosynthesis is the process that converts light energy into chemical energy stored in organic molecules. The plant takes in carbon dioxide through stomata in the leaf and water through the roots, and from these builds glucose while releasing oxygen.
The process underpins almost every food chain on Earth. Plants are autotrophs, meaning they manufacture their own organic material; animals are heterotrophs and must eat what something else has made.
What is the equation for photosynthesis?
The overall equation is 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂. Six carbon dioxide molecules and six water molecules yield one glucose molecule and six oxygen molecules.
The equation is an accounting statement, not a description of events. No single reaction in the cell looks like this; the equation sums dozens of steps and hides the fact that water is split early while carbon is fixed much later.
Where in the cell does photosynthesis happen?
Photosynthesis takes place in the chloroplasts, found mainly in the mesophyll cells of the leaf. Inside the chloroplast are the thylakoids, membrane sacs stacked into grana and surrounded by a fluid called the stroma. The light-dependent reactions occur in the thylakoid membrane, where chlorophyll sits; the Calvin cycle occurs in the stroma.
The division is not arbitrary. The thylakoid membrane is needed to build a proton gradient, and a gradient requires an enclosed space to pump protons into.
What happens in the light-dependent reactions?
The light reactions capture light energy and store it as ATP and NADPH while splitting water and releasing oxygen. Chlorophyll a in photosystem II absorbs light and loses an electron, which is replaced by splitting a water molecule – this is where the oxygen comes from. The electron passes down an electron transport chain, pumping protons into the thylakoid lumen, and reaches photosystem I, where a second absorption of light lets it reduce NADP⁺ to NADPH. The proton gradient drives ATP synthase, which makes ATP.
What happens in the Calvin cycle?
The Calvin cycle fixes carbon dioxide into organic molecules using the ATP and NADPH made by the light reactions, and it needs no light directly. The enzyme rubisco attaches a carbon dioxide molecule to the five-carbon sugar RuBP, the product splits into two three-carbon molecules, these are reduced to G3P using ATP and NADPH, and most of the G3P is recycled to regenerate RuBP. Six turns are required before one glucose molecule can be assembled.
Do not call it the dark reaction. The name is common but misleading: the cycle does not run at night, because it stops as soon as ATP and NADPH run out.
How do the two stages differ?
| Light-dependent reactions | Calvin cycle | |
|---|---|---|
| Location | Thylakoid membrane | Stroma |
| Requires light | Yes | No, but requires the products of the light reactions |
| Inputs | Water, light, NADP⁺, ADP | Carbon dioxide, ATP, NADPH |
| Outputs | ATP, NADPH, oxygen | G3P, and from it glucose |
| Key molecule | Chlorophyll a | Rubisco |
What limits the rate of photosynthesis?
Three factors limit the rate: light intensity, carbon dioxide concentration and temperature. Whichever is in shortest supply sets the ceiling – the limiting factor principle in practice. At low light the rate rises linearly with intensity and then plateaus as carbon dioxide supply becomes limiting. Temperature acts on the enzymes: the rate climbs to an optimum and then falls sharply as proteins denature. A plant with closed stomata on a hot day is carbon dioxide limited even in full sun.
What is the difference between C3, C4 and CAM plants?
The difference lies in how the plant handles rubisco's habit of also reacting with oxygen. C3 plants such as wheat and rice fix carbon dioxide directly in the Calvin cycle and lose energy to photorespiration when it is hot. C4 plants such as maize and sugarcane first fix carbon into a four-carbon molecule in an outer cell and deliver it, concentrated, to rubisco in an inner cell. CAM plants such as cacti instead open their stomata at night and store the carbon as an acid until morning. All three are solutions to the same water-loss problem.
How does photosynthesis relate to respiration?
The two processes mirror each other in their overall equations: photosynthesis builds glucose and releases oxygen, while cellular respiration breaks glucose down and consumes oxygen. They are not reversed reactions at the molecular level, though – they run in different organelles, with different enzymes and different electron carriers (NADPH versus NADH). Plants also do both at once; a plant "giving off carbon dioxide at night" simply means photosynthesis has stopped while respiration continues.
How should I revise this for an exam?
Draw the process from memory before you reread it. Start with an empty chloroplast, label the thylakoid and the stroma, and fill in what enters and leaves each compartment – that forces retrieval rather than recognition, which is the effect active recall is built on. Then test yourself on the equation, the products of each stage and the three limiting factors. You can upload the chapter to Memmo and let the flashcards tool build cards from your own material; review is scheduled with FSRS, the same algorithm family Anki moved to, and the principle behind it is explained in our article on spaced repetition.
Frequently asked questions
Where does the oxygen in photosynthesis come from?
The oxygen comes from the water molecules, not from carbon dioxide. This was shown with isotope-labelled water back in the 1930s, and it is why six water molecules are needed for six oxygen molecules in the overall equation.
Does photosynthesis happen at night?
No. The light reactions require light, and once ATP and NADPH run out the Calvin cycle stops shortly after. Respiration, by contrast, continues around the clock.
Why are plants green?
Chlorophyll absorbs mainly blue and red light and reflects green, so green is the part of the spectrum the plant does not use. The colour is a by-product of absorption, not a purpose.
What is photorespiration?
Photorespiration is rubisco binding oxygen instead of carbon dioxide, which costs energy without building sugar. It increases at high temperature and low carbon dioxide, and it explains why C4 and CAM plants evolved their alternative routes.
How much of Earth's oxygen comes from land plants?
Less than most people assume. Marine phytoplankton and cyanobacteria account for a very large share of global oxygen production, with land plants making up the rest. Estimates of the exact split vary, so always cite a source when you use a figure.