A LIMITING FACTOR is any factor that, when in short supply, restricts the rate of a reaction — even if all other factors are in abundance.
Imagine a factory with three workers on a production line. If one worker is absent (or working slowly), the whole line slows down — even if the other two are working perfectly.
For photosynthesis, the three main limiting factors are:
1. LIGHT INTENSITY
2. CARBON DIOXIDE CONCENTRATION
3. TEMPERATURE
At any given moment, the rate of photosynthesis is limited by whichever of these three factors is in shortest supply.
Light Intensity
Light provides the ENERGY for photosynthesis.
As light intensity INCREASES:
More energy available → chlorophyll absorbs more light → faster reactions → rate of photosynthesis increases.
But once light is no longer the limiting factor, increasing it further has NO effect — another factor takes over as the bottleneck.
In very LOW light:
Rate is slow — not enough energy to drive the reactions.
This is why plants grow more slowly in winter (shorter days, lower light levels).
EVIDENCE in experiments:
Aquatic plants (e.g. Elodea) produce oxygen bubbles.
Moving a lamp CLOSER increases bubble rate.
Moving it FURTHER decreases bubble rate.
NOTE: Light intensity decreases with distance from a source.
Carbon Dioxide Concentration
Carbon dioxide is a RAW MATERIAL for photosynthesis — needed to build glucose molecules.
As CO₂ concentration INCREASES:
More raw material available → more glucose can be produced → rate increases.
If CO₂ falls too low:
Rate is limited even if light and temperature are optimal.
This is why GREENHOUSES often pump extra CO₂ into the air — raising CO₂ concentration above the normal atmospheric level (~0.04%) can significantly increase crop yields.
Normal atmospheric CO₂ is quite low — for many plants, CO₂ is often the limiting factor during a warm, sunny day.
Temperature
Temperature affects the ENZYMES that control the reactions of photosynthesis.
As temperature INCREASES (below optimum):
Enzyme molecules and substrate molecules have more kinetic energy.
More frequent and more successful collisions.
Rate of photosynthesis increases.
At OPTIMUM TEMPERATURE (~25–40°C depending on the plant):
Maximum rate of photosynthesis.
ABOVE OPTIMUM TEMPERATURE:
Enzymes in the chloroplast BEGIN TO DENATURE.
Active site shape changes permanently.
Enzyme-substrate complexes can no longer form.
Rate of photosynthesis FALLS SHARPLY — eventually to zero.
This explains why very hot conditions reduce photosynthesis even in good light with plenty of CO₂ — the enzyme system has broken down.
Investigating Photosynthesis Rate
The rate of photosynthesis can be measured by:
Counting the number of O₂ bubbles produced per minute by an aquatic plant.
Measuring the volume of O₂ collected over a set time.
Using a CO₂ sensor to measure how fast CO₂ is being absorbed.
In RP5 you investigate the effect of LIGHT INTENSITY:
Set up a beaker of water with an aquatic plant (Elodea or Cabomba).
Place a lamp at a measured distance.
Count bubbles per minute (or collect gas).
Move the lamp to different distances and repeat.
Plot a graph of rate (bubbles/min) against light intensity.
Key controls:
Keep temperature constant (use a water bath or ice).
Keep CO₂ constant (add sodium bicarbonate solution to the water).
This ensures only light intensity is the independent variable.
⚠️ Common Mistake
When a graph of photosynthesis rate levels off (plateaus), students often say 'the plant has run out of chlorophyll' or 'there is no more light'. The correct answer is that ANOTHER FACTOR has become limiting — usually CO₂ or temperature. The plant hasn't run out of anything — a different factor is now the bottleneck.
📌 Key Note
Three limiting factors: light intensity, CO₂ concentration, temperature. Rate increases with each up to a point, then another factor limits. Above optimum temperature — enzymes denature, rate falls.
🎯 Matching Activity — Match the Limiting Factor to its Effect
Match each condition to what happens to the rate of photosynthesis. — drag the symbols on the right to match the component names on the left.
Low light intensity
Drop here
Low CO₂ concentration
Drop here
Low temperature
Drop here
Temperature above optimum
Drop here
Rate levels off (plateau)
Drop here
Enzymes denature permanently — rate falls sharply to zero
Enzymes have less kinetic energy — fewer successful collisions, slow rate
Not enough raw material to build glucose — rate is slow
Not enough energy — rate is slow even if CO₂ and temperature are optimal
Another factor has become limiting — not that the plant ran out of chlorophyll
⭐ Higher Tier Only
Inverse square law: light intensity is proportional to 1/distance². Doubling the distance from the lamp quarters the light intensity. If a plant is 10 cm from the lamp and moved to 20 cm, light intensity drops to 1/4. Students should be able to apply this to RP5 data and interpret rate vs light intensity graphs — explaining plateau regions as evidence of another limiting factor.
🧪 Required Practical
🔬 RP5 — Investigate the effect of light intensity on rate of photosynthesis using aquatic plants. Count bubbles per minute. Move lamp to change light intensity. Control CO₂ (add sodium bicarbonate) and temperature.
Know the method, variables, equipment and how to analyse results.
🎯 Test Yourself
Question 1 of 3
1. A plant is given very bright light and high CO₂. The rate of photosynthesis stops increasing. What is now the most likely limiting factor?
2. In an experiment, moving a lamp further from an aquatic plant reduces the bubble count. What does this show?
3. Why does photosynthesis rate decrease at temperatures above the optimum?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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