Limiting factors of photosynthesis: light, carbon dioxide and temperature
The limiting factors of photosynthesis are light intensity, carbon dioxide concentration and temperature. AQA adds the amount of chlorophyll. A limiting factor is whatever is in shortest supply, so it holds the rate back. Give the plant more of it and the rate goes up. Once something else runs short, adding more of the first factor makes no difference, and the graph levels off.
The three graph shapes
Light intensity
Light gives the energy for photosynthesis. In dim light the rate is low, and brighter light speeds it up. The line climbs, then bends and goes flat. On the flat part light is no longer the limiting factor. Carbon dioxide or temperature is holding the rate back instead.
Carbon dioxide concentration
Carbon dioxide is one of the reactants in the photosynthesis equation. More of it means a faster rate, up to a point. This graph has the same climb and plateau as the light graph. A plateau is the flat top of a curve.
Temperature
This graph has a different shape. The rate rises as the plant warms up, reaches a peak, then falls as it gets hotter. OCR Twenty First Century explains the temperature effect through enzymes, the proteins that control the reactions. Edexcel notes that enzymes can be denatured. That means the active site, the part of the enzyme where the reaction happens, changes shape.
Drag or step the marker along the interactive graph. The line under the graph tells you which factor is limiting at that point.
Higher tier: when factors work together
Every board that covers this topic makes two ideas Higher tier only. The first is how the three factors interact. Graphs for this have two or more lines, such as one at a low carbon dioxide level and one at a high level. If the high line levels off at a faster rate, carbon dioxide was limiting the low line.
The second idea is the inverse square law for light. OCR Twenty First Century states it: light intensity is inversely proportional to the square of the distance from the light source. Move a lamp twice as far away and the light intensity drops to a quarter. Move it three times as far and it drops to a ninth.
What each board includes
AQA (8461 and 8464) lists four factors: temperature, light intensity, carbon dioxide concentration and the amount of chlorophyll. Edexcel (1BI0 and 1SC0), OCR Gateway (J247) and OCR Twenty First Century (J257) list only the first three.
The KS3 curriculum does not mention limiting factors. They start at GCSE.
AQA, Edexcel and OCR Twenty First Century all include practical work on this. AQA required practical 6 (practical 5 in Combined Science) uses pondweed to test the effect of light intensity, and Edexcel has a core practical on the same question. J257 suggests collecting gas or counting bubbles from pondweed.
Common mistakes
- Naming the factor along the bottom axis as limiting on the flat part of the graph. Once the line is flat, a different factor is limiting.
- Saying a higher temperature always speeds up photosynthesis. Past the peak, the rate falls.
- Expecting chlorophyll on every board's list. Only AQA names it as a factor. Chlorophyll sits in the chloroplasts on the plant cell diagram.
- Halving the light intensity when a lamp moves twice as far away. The inverse square law makes it a quarter.
Read rate graphs and pick the limiting factor in Photosynthesis Lab, with a Higher tier option for the inverse square law. The biology hub links the rest of this set.
Questions people ask
What does limiting factor mean in biology?
It is the one condition that is in shortest supply and so sets the rate. Improving any other condition will not speed the reaction up.
Is the inverse square law on Foundation papers?
No. AQA, Edexcel and both OCR specifications mark it as Higher tier only.
How do limiting factors matter to growers?
AQA links them to greenhouses in its Higher tier content. A grower can add heat, light or carbon dioxide, and each one costs money. The aim is the fastest rate of photosynthesis that still makes a profit.