IB Biology HLPhotosynthesisPaper 1 & 2~13 min read
Limiting Factors of Photosynthesis (Skills)
Three things can hold photosynthesis back, but only one of them is holding it back at any given moment. Learning to spot which one — from the shape of a graph — is the whole skill on this page.
📚 What you need to know
A limiting factor is the factor in shortest supply, which therefore determines the rate.
The three main limiting factors are light intensity, carbon dioxide concentration and temperature.
On a graph, the rising part means the factor on the x-axis is limiting; the plateau means something else has become limiting.
Increasing a factor that is not limiting has no effect on the rate.
Light intensity affects the light-dependent reactions, so it controls the supply of ATP and reduced NADP.
Carbon dioxide affects the light-independent reactions, since it is the substrate for fixation.
Temperature affects enzyme activity, so its curve rises to an optimum and then falls as enzymes denature.
The compensation point is the light intensity at which photosynthesis and respiration proceed at the same rate, so there is no net gas exchange.
Rate is usually measured as oxygen produced or carbon dioxide taken up per unit time.
The idea in one sentence
Definition
A limiting factor is the factor whose shortage is restricting the rate of a process at a particular moment.
The consequence is not obvious until you say it out loud: if a factor is not the one in short supply, adding more of it changes nothing. Doubling the light on a plant that has run out of carbon dioxide gains you nothing at all.
🧠
The production line
A factory line runs at the speed of its slowest station. Hiring more staff anywhere else changes nothing. Photosynthesis works the same way — find the slow station before you spend money.
Reading a limiting factor graph
The three curves are deliberately identical at the left-hand end. At low light, nothing else matters — and that is exactly how you know light is the limiting factor there.
The two-part answer examiners want
Any question on this graph splits the same way:
On the rising section — the rate increases as light intensity increases, so light intensity is the limiting factor. More light means more photons absorbed, so more ATP and reduced NADP are made for the Calvin cycle.
On the plateau — increasing light intensity no longer increases the rate, so light is no longer limiting. Something else is: either the carbon dioxide concentration or the temperature.
To identify which — compare curves. Raising CO2 lifts the plateau, so CO2 was limiting. Raising temperature lifts it too, so temperature was limiting. Whichever change lifts the plateau was the bottleneck.
Never write “light is the limiting factor” about a whole graph. It is limiting on the slope and not limiting on the plateau. Always say where on the curve you mean, and you will pick up marks other students drop.
Each factor in turn
Factor
Which stage it affects
Shape of the graph
Light intensity
Light-dependent reactions — controls the supply of ATP and reduced NADP
Rises, then plateaus when another factor takes over
CO2 concentration
Light-independent reactions — it is the substrate that is fixed
Rises, then plateaus; usually the limiting factor outdoors on a bright day
Temperature
All enzyme-catalysed steps, especially the Calvin cycle
Rises to an optimum then falls sharply as enzymes denature
The temperature curve is the odd one out and that is exactly why it is examined. Light and carbon dioxide curves plateau; the temperature curve peaks and then falls, because too much of it damages the enzymes rather than simply failing to help.
The compensation point
Plants respire constantly. At very low light, respiration releases more carbon dioxide than photosynthesis consumes, so the plant is a net producer of CO2. As light intensity rises, photosynthesis catches up.
Definition
The compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration, so there is no net exchange of gases.
Below it the plant is losing carbohydrate; above it, it is gaining. Shade-adapted plants have a lower compensation point, which is what lets them survive on a woodland floor.
Measuring the rate
🧩 Investigating light intensity with pondweed
Place a piece of pondweed in a test tube of hydrogencarbonate solution, which supplies a constant, plentiful source of CO2.
Stand the tube in a water bath to keep temperature constant — the lamp itself is a heat source, so this matters.
Change light intensity by moving the lamp to measured distances, and allow a few minutes to equilibrate at each one.
Measure the rate as bubbles of oxygen per minute, or more accurately as the volume of gas collected in a set time using a capillary tube.
Repeat each distance and calculate a mean.
Light intensity falls with the square of the distance, so plot against 1 ÷ distance2 rather than against distance, or use a light meter.
Why counting bubbles is a weak method. Bubbles are not all the same size, they are easy to miscount, and not all the oxygen produced actually leaves as gas. Collecting and measuring the volume of gas is more valid and more precise. If a question asks you to improve a method, this is almost always an available mark.
Worked examples
WE 1
Explain the shape of a curve
Explain the shape of a graph of the rate of photosynthesis against light intensity. (4 marks)
Point 1: the rising section
The rate increases with light intensity, so light intensity is the limiting factor here.
Point 2: why
More light is absorbed by the pigments, so more ATP and reduced NADP are produced for the Calvin cycle.
Point 3: the plateau
Beyond a certain intensity the rate stops increasing, so light is no longer limiting.
Point 4: what is limiting instead
Another factor — carbon dioxide concentration or temperature — has become the limiting factor.
Light limits the slope; something else limits the plateauname the two possible alternative factors; a vague “something else” may not score
WE 2
Identify the limiting factor
Two curves are plotted for the same plant. Both rise identically at low light, but one plateaus at a higher rate when the CO2 concentration is raised. Explain what this shows. (3 marks)
Point 1: the identical section
At low light intensity both curves are the same, so light was limiting in that region and CO2 made no difference.
Point 2: the higher plateau
Raising CO2 raised the plateau, so at high light intensity CO2 concentration was the limiting factor.
Point 3: the conclusion
Which factor is limiting depends on the conditions — it changes as light intensity changes.
Light limits at low intensity; CO₂ limits at high intensity“the curves are identical at low light” is itself a marking point — do not skip it
WE 3
Explain a temperature result
A grower raises the greenhouse temperature from 25°C to 45°C and the rate of photosynthesis falls. Explain why, and state how this graph differs from the light intensity graph. (3 marks)
Point 1: the cause
Above the optimum, the enzymes of the Calvin cycle begin to denature as the bonds holding their tertiary structure break.
Point 2: the consequence
The active sites change shape, substrates can no longer bind, and the rate falls.
Point 3: the difference in shape
The light intensity graph levels off at a plateau, whereas the temperature graph peaks at an optimum and then falls.
Too much light does nothing; too much heat does damagethe word denature is essential here
💡 Exam tips
Always say where on the curve a factor is limiting.
Link light to the light-dependent reactions and CO2 to the Calvin cycle.
Remember the temperature curve falls after its optimum; the others plateau.
To compare curves, look at where they separate, not just their heights.
Use hydrogencarbonate solution as the CO2 source and a water bath to control temperature.
Mention the inverse square relationship if a lamp distance is used.
⚠ Common mistakes
Saying light is limiting across the whole graph. It stops being limiting at the plateau.
Saying the plateau means the plant is “full”. Another factor has become limiting.
Drawing the temperature curve as a plateau. It must come back down.
Forgetting the lamp heats the water. Without a heat shield or water bath, temperature is not controlled.
Ignoring respiration. Measured gas exchange is the net figure, which is why the compensation point exists.
Treating bubble counts as precise data. Gas volume is the better measure.
Up next: CO2 Enrichment Experiments. If carbon dioxide is so often the factor holding plants back, the obvious move is to add more of it — which is exactly what growers and climate scientists have been testing.
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