Photosynthesis: the process, the equations and the rate
What photosynthesis actually does
Photosynthesis is the process in which plants convert light energy into chemical energy. The light energy absorbed by chlorophyll in the chloroplasts is transferred to the chemical bonds of glucose.
That glucose is then used to:
- be respired to release energy
- be converted to starch for storage (insoluble, so it does not affect water potential)
- be converted to cellulose for cell walls
- be combined with nitrate ions to make amino acids, and hence proteins
Photosynthesis matters because it is the entry point of energy into almost every food chain, and it removes carbon dioxide from the air and releases oxygen.
The two equations you must be able to write
Word equation
carbon dioxide + water → glucose + oxygen
(with light and chlorophyll written above/below the arrow — they are not reactants)
Balanced symbol equation
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Learn C₆H₁₂O₆ by heart. Then the balancing follows: 6 carbons on the left means 6 CO₂; 12 hydrogens means 6 H₂O; that gives 18 oxygens on the left, 6 are locked into glucose, so 12 are left = 6O₂.
Common mistake: examiners repeatedly report candidates who cannot balance the equation, or who do not know the formula for glucose. Write the numbers as subscripts and count the atoms on both sides before you move on.
Photosynthesis is not respiration in reverse — don't muddle them
| Photosynthesis | Respiration | |
|---|---|---|
| Where | Cells with chloroplasts | All living cells |
| When | Only in the light | All the time |
| Gases | Takes in CO₂, gives out O₂ | Takes in O₂, gives out CO₂ |
| Energy | Light energy → chemical energy | Releases energy from glucose |
Common mistake: candidates have written that respiration requires the absorption of light. Respiration never involves light. A plant respires 24 hours a day; it only photosynthesises when illuminated.
Limiting factors (2.20)
The rate of photosynthesis is controlled by whichever factor is in shortest supply — the limiting factor. Increasing any other factor then has no effect.
Light intensity
As light intensity increases, rate increases in direct proportion, then levels off (plateaus) when something else — usually carbon dioxide concentration or temperature — becomes limiting. In the dark the rate is zero.
Carbon dioxide concentration
Same shape of graph. CO₂ is only about 0.04% of air, so it is very often the limiting factor in a field or greenhouse on a bright day. Growers burn fuel or add CO₂ to glasshouses for this reason.
Temperature
Different shape. Rate increases as temperature rises because the enzymes controlling photosynthesis and the substrate molecules have more kinetic energy, so there are more successful collisions. Above an optimum (around 35–40 °C in many plants) the rate falls sharply because the enzymes are denatured — the shape of the active site changes so the substrate no longer fits.
Answering limiting-factor questions well
Always name the factor that has become limiting: "beyond point X, increasing light intensity has no further effect because carbon dioxide concentration is now the limiting factor."
For temperature above the optimum, do not stop at "the enzymes are denatured". Examiners note that candidates lose the final mark by not continuing to explain the effect on enzyme structure and function — the active site changes shape, the substrate no longer fits, so fewer enzyme–substrate complexes form and the rate falls.
Never write that heat "kills" an enzyme. Enzymes are not alive. Use denatured.
Examiner tip. Two marks are thrown away here every year: not knowing C₆H₁₂O₆ / not balancing 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, and saying high temperature 'kills the enzymes'. Say denatured, then always add what that does — active site changes shape, substrate no longer fits, rate falls.