Bioenergetics · GCSE Biology
Limiting factors of photosynthesis
Read photosynthesis graphs for GCSE Biology: rate rises then plateaus as light, carbon dioxide or temperature limits, and why growers change the factor that is actually capping the yield.
A limiting factor is the condition in shortest supply. The graph rises while you increase that factor, then plateaus when something else takes over. Temperature can fall after the optimum because enzymes denature.
The important bits
What you need to know
- 1
Limiting factors of photosynthesis are light intensity, carbon dioxide concentration and temperature. Water can limit in a drought, but the three named factors dominate exam graphs.
- 2
The factor in shortest supply caps the rate. Increasing a factor that is not limiting does not raise the rate — the line is already flat.
- 3
A light-intensity graph: rate rises as intensity increases (more energy for the chloroplasts), then plateaus when CO₂ or temperature becomes limiting. The plateau is not “photosynthesis stopped”.
- 4
A carbon-dioxide graph has the same shape: rise then plateau when light or temperature limits. Normal air is about 0.04% CO₂; greenhouses may raise this towards 0.1%.
- 5
Temperature affects enzyme-controlled steps in chloroplasts. Rate rises to an optimum as collisions increase, then falls if enzymes denature. This graph can drop; light and CO₂ graphs usually flatten rather than crash (unless light is so low it is zero).
- 6
On Higher tier, light intensity follows an inverse-square relationship: intensity ∝ 1/d², where d is distance from a lamp. Double the distance, intensity falls to a quarter.
- 7
Growers add lamps, burn fuel to add CO₂ and heat, or use glasshouses because extra glucose means extra biomass — but only if they raise the factor that is actually limiting. Wasted energy on more light when CO₂ is limiting does not increase yield.
- 8
At night, light intensity is zero, so light is limiting and net photosynthesis stops even if it is warm and CO₂ is plentiful. Respiration continues.
Quotations worth analysing
Short evidence. Real method.
“The graph rises then levels off because another factor is now limiting.”
Never write “the reaction stops” on a plateau. Name the likely new limiter: CO₂, light or temperature.
“A limiting factor is the factor in shortest supply that stops the rate increasing.”
Use this sentence, then point to the rising section (that factor is limiting) and the flat section (a different factor is limiting).
“light intensity ∝ 1/d²”
If distance doubles, intensity becomes 1/4. Predict fewer bubbles only if light is still the limiter.
Go deeper
How do I read a rise-then-plateau graph in four sentences?
Sentence 1: as light intensity increases, rate of photosynthesis increases because more light energy is absorbed by chlorophyll. Sentence 2: light is the limiting factor on this rising section. Sentence 3: the line then plateaus even though light is still increasing, so light is no longer limiting. Sentence 4: carbon dioxide or temperature is now limiting — there is not enough CO₂ for chloroplasts, or enzymes are working at a fixed temperature. If a second line is drawn at higher CO₂, it plateaus higher: that is evidence CO₂ had been the limiter on the first plateau. If the second line is at higher temperature and it rises then falls, enzymes are denaturing. Practise pointing to regions of the graph rather than writing a memorised blob that never mentions the axes.
Go deeper
Why can the temperature graph fall when the others flatten?
Light and CO₂ are raw materials (energy and a reactant). Extra light cannot break the protein machinery; extra CO₂ cannot denature enzymes by itself at GCSE. Temperature changes collision rate and enzyme shape. Below the optimum, cold means few collisions between CO₂, intermediates and enzymes, so rate is low. At the optimum, rate is highest. Above it, chloroplast enzymes denature, active sites change shape, and rate falls — possibly to zero if the leaf is heat-damaged. That is why a temperature graph can look like an enzyme graph. Do not draw a temperature plateau and call it denaturation; denaturation is the drop. Growers therefore avoid overheating a glasshouse even though they want it warmer than a winter field.
Go deeper
How do inverse-square lamp questions work on Higher tier?
A lamp is a point source. Intensity is proportional to 1/d². From 10 cm to 20 cm, d × 2, intensity × 1/4. From 10 cm to 30 cm, intensity × 1/9. You predict the bubble rate falls in the same ratio only if light is limiting. If moving the lamp barely changes the count, CO₂ or temperature is limiting and the inverse-square law still holds for light — it just is not the factor capping the chloroplasts. Practical design: black cloth to cut side light, a ruler for d, a water bath or heat sink (water tank) so the lamp does not change temperature as well, and sodium hydrogencarbonate to set CO₂. Changing two variables at once (distance and heat) makes the graph invalid.
See the idea in action
A graph of rate against light intensity rises from 0 to 20 arbitrary units, then is flat to 50. On the rising section light is limiting: more light, more photosynthesis. On the plateau another factor is limiting — likely CO₂ or temperature — so adding still more light does not raise the rate. A grower should increase CO₂ or temperature, not buy brighter lamps, until the plateau lifts. If a temperature graph is shown instead and falls after 40 °C, the explanation is denaturation of enzymes, not a second limiting factor in the plateau sense.
Exam technique
Turn knowledge into marks
For every graph: name the factor on the x-axis, say it is limiting while the line rises, and name a different factor on the plateau. Never say photosynthesis has stopped on a plateau. Inverse square is Higher: intensity ∝ 1/d².
Common mistakes
Do not give these marks away
- 01
Describing a plateau as “the reaction stops” rather than “another factor is now limiting”.
- 02
Forgetting that temperature graphs can fall because enzymes denature, unlike typical light and CO₂ plateaus.
- 03
Changing lamp distance and temperature together in the pondweed practical, or ignoring inverse square on Higher papers.
A photosynthesis graph rises with light intensity then flattens. What does the flat part show?
APhotosynthesis has stopped completely
BLight is still the only limiting factor
CAnother factor such as carbon dioxide or temperature is now limiting
DThe plant has run out of chlorophyll permanently
Show the answer
Another factor such as carbon dioxide or temperature is now limiting. The rise means light was limiting. The plateau means extra light is not increasing rate, so a different factor is capping photosynthesis. The reaction has not stopped.
Quick questions
If this is the bit you searched
What are the limiting factors of photosynthesis?
Light intensity, carbon dioxide concentration and temperature. The factor in shortest supply determines the rate.
Why does a photosynthesis graph level off?
The factor you are increasing is no longer limiting. Another factor is now in shortest supply, so the rate cannot rise further.
Why can temperature decrease the rate of photosynthesis?
Enzymes in the chloroplast denature above the optimum. The active site changes shape and the reaction slows or stops.
What is the inverse square law for light intensity?
Higher tier: intensity is proportional to 1/d². If you double the distance from a lamp, light intensity falls to a quarter.