Pearson Edexcel International GCSE in Biology · 4BI1 · Free · no account needed
What Limits Photosynthesis
Why brightening the light eventually stops working, how to read that off a graph, and the experiments that prove what a plant needs.
Topic 2 · Structures and functions in living organisms — one of 34 lessons in this topic, and one of 68 in Biology.
This lesson ends with Jan Ingenhousz. Learn what Limits Photosynthesis, teach it, then defend it against Jan Ingenhousz.
The definition this lesson teaches, word for word — every Biology definition.
- Limiting factor
- The factor that is in shortest supply at a given moment, and which therefore determines the rate of photosynthesis. Increasing any other factor has no effect until the limiting factor itself is increased.
One of the 16 Biology required practicals.
What this lesson covers in the specification
Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.
- 2.20 — How carbon dioxide concentration, light intensity and temperature affect the rate of photosynthesis
- 2.23 — Practical: investigate photosynthesis — oxygen evolution, starch production, and the requirements for light, carbon dioxide and chlorophyll (required practical)
Specification section 2(e) Nutrition — also taught in Photosynthesis, The Leaf: Structure and Gas Exchange, A Balanced Diet, The Alimentary Canal.
1 · Understand it
No exam language yet. The only question this section answers is: do I actually understand what is happening?
Put a piece of pondweed in water, shine a lamp on it, and it releases a steady stream of oxygen bubbles you can count. Move the lamp closer and the bubbles come faster. Closer still, faster still. It looks as though brightening the light will speed the plant up without limit. It will not. Keep going and the bubble rate settles and stops responding altogether — and the reason why is the single most useful idea in this part of the course.
Think of it like a sandwich shop
A shop makes sandwiches from bread, cheese and one worker. Deliver twice as much bread and nothing whatsoever changes — bread was never what was holding you up, because you ran out of cheese an hour ago. Deliver more cheese and output leaps. The rate is set by whichever ingredient is in shortest supply, and by that one alone; everything else is already in surplus and adding to a surplus achieves nothing. Photosynthesis behaves in exactly this way, and the ingredient currently holding it back is called the limiting factor.
Reading the graph — the whole skill, in four steps
- While the line is still RISING, the factor on the x-axis is the limiting one: give the plant more of it and you get more photosynthesis.
- Where the line FLATTENS, that factor has stopped being limiting. The plant now has more of it than it can use, so the surplus does nothing.
- Something else has taken over as the limiting factor — one of the other two.
- So on a plateau, the answer to 'how would you increase the rate?' is never 'more light'. It is 'more of whichever factor has now taken over'. Name it, and you have the mark.
Why temperature does not behave like the other two
- Light and carbon dioxide are supplies. Give a plant more than it can use and the extra simply sits there unused, so the graph levels off and stays level.
- Temperature is not a supply at all. Photosynthesis is controlled by enzymes, and temperature changes how well those enzymes work.
- Warming towards the optimum makes molecules move faster, so enzymes and substrates collide more often and the rate rises.
- Past the optimum the enzymes begin to denature: the active site changes shape and the substrate no longer fits.
- So a temperature graph rises to a peak and then FALLS. Light and carbon dioxide graphs rise and then level off. Drawing the wrong one of those two shapes is among the most common errors in the whole topic.
One trap in the practical is worth seeing before you meet it. Light intensity is usually changed by moving the lamp — but distance and intensity are not the same variable, and they are not even proportional. Move a lamp twice as far away and you do not halve the light reaching the plant; you cut it to a quarter. This is why a small movement of the lamp changes the bubble count so dramatically, and why writing 'the rate is proportional to the distance' in a conclusion is wrong twice over — the relationship runs the other way, and it is far steeper than proportional. (Where the quarter comes from: light spreads over the surface of an expanding sphere, so intensity falls with the SQUARE of the distance. You are not asked for that rule at International GCSE — only for the fact that moving the lamp changes the light far more than it looks.)
2 · Grade 9 Notes
A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.
Learn this definition · Limiting factor
The factor that is in shortest supply at a given moment, and which therefore determines the rate of photosynthesis. Increasing any other factor has no effect until the limiting factor itself is increased.
The three factors, and exactly what raising each one does
- Light intensity
- raises the rate until light is no longer the factor in shortest supply; the graph then levels off at a plateau
- Carbon dioxide concentration
- raises the rate until carbon dioxide is no longer in shortest supply; the graph then levels off at a plateau
- Temperature
- raises the rate to an optimum, after which the rate FALLS as the enzymes controlling photosynthesis denature
- Distance and intensity — a way to think, NOT an Edexcel equation
- intensity is proportional to 1 ÷ distance², so halving the lamp distance gives four times the intensity rather than twice
Deciding which factor is limiting, from any graph
- Find the point on the curve the question is asking about.
- If the line is still rising there, the factor on the x-axis is the limiting factor.
- If the line has levelled off, that factor is not limiting; one of the other two is.
- Name the other factor, and state that the plant now has more of the x-axis factor than it can use.
The two graph shapes — do not confuse them
A. Light intensity or carbon dioxide on the x-axis: the rate rises and then LEVELS OFF at a plateau. A supply the plant cannot use simply has no further effect.
B. Temperature on the x-axis: the rate rises to an optimum and then FALLS, because the enzymes controlling photosynthesis denature and their active sites change shape.
Required practical 2.23 (a) — measuring the rate of photosynthesis with pondweed
- Place a piece of pondweed, cut end upwards, in a boiling tube of sodium hydrogencarbonate solution, which supplies carbon dioxide.
- Stand the tube in a large beaker of water. This water bath absorbs the heat from the lamp, so light intensity can be changed without changing the temperature.
- Set the lamp at a measured distance and leave the plant for five minutes to settle to a steady rate before counting anything.
- Count the bubbles released in one minute. Repeat three times at that distance and take a mean.
- Repeat across a range of lamp distances, then plot mean bubbles per minute against distance — or against 1 ÷ distance², which is proportional to light intensity.
Variables
Independent (changed) — light intensity, changed by moving the lamp to measured distances
Dependent (measured) — number of oxygen bubbles released per minute
| Control variable | Why it must be held constant |
|---|---|
| Temperature | photosynthesis is enzyme-controlled, so warming changes the rate |
| Carbon dioxide concentration | otherwise it becomes the limiting factor instead of light |
| The same piece of pondweed | different pieces have different numbers of chloroplasts |
| Counting time | a longer count gives more bubbles whatever the rate |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Judgement | Bubbles of different sizes are counted as though equal. | Collect the gas in a capillary tube and measure its volume instead. |
| Systematic | Room lighting reaches the plant as well as the lamp. | Work in a darkened room so the lamp is the only source. |
| Random | Bubbles are released unevenly, so a single count is unrepresentative. | Take three counts at each distance and use the mean. |
Required practical 2.23 (b) — proving what photosynthesis requires, using the starch test
- Destarch the plant first by leaving it in darkness for 48 hours, so that any starch found afterwards must have been made during the experiment.
- Set up the one condition being tested: foil over part of a leaf removes LIGHT; a variegated leaf tests for CHLOROPHYLL; sealing a leaf in a flask with soda lime removes CARBON DIOXIDE.
- Leave the plant in bright light for several hours.
- Test the leaf for starch: boil in water to kill it, then boil in ethanol in a water bath with the flame off, as ethanol is highly flammable, to remove the chlorophyll.
- Rinse in warm water to soften the brittle leaf, spread it out and add iodine solution.
- Blue-black means starch is present and photosynthesis has happened; orange-brown means it has not.
Variables
Independent (changed) — the single condition removed — light, chlorophyll or carbon dioxide
Dependent (measured) — whether the leaf turns blue-black with iodine solution
| Control variable | Why it must be held constant |
|---|---|
| Destarching time | starch left over from before would give a false positive |
| Time in the light | too short and even a working leaf makes no detectable starch |
| The same plant | different plants photosynthesise at different rates |
| Temperature | it changes the rate at which starch is made |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Systematic | The plant was not fully destarched, so every leaf tests positive. | Test one leaf before starting, to confirm destarching worked. |
| Judgement | Faint blue-black and dark orange-brown are hard to tell apart. | Rinse thoroughly and compare against a known positive leaf. |
| Random | The foil slips and lets light onto the covered area. | Fix the foil on both surfaces and check it before testing. |
What each version of the requirements experiment proves
- Leaf part-covered with foil: only the uncovered part turns blue-black — LIGHT is required
- Variegated leaf: only the green parts turn blue-black — CHLOROPHYLL is required
- Leaf sealed in a flask with soda lime, which absorbs carbon dioxide: no part turns blue-black — CARBON DIOXIDE is required
- A leaf left in normal conditions turns blue-black throughout — the control that proves the method itself works
Model answer [4 marks]
A student increases the light intensity falling on a pond plant. The rate of photosynthesis rises at first and then stops rising. Explain why. [4]
At low light intensity, light is the limiting factor, so increasing it increases the rate of photosynthesis. Above a certain intensity the plant has more light than it can use, so light is no longer limiting and the rate stops rising. Another factor has become limiting instead — either carbon dioxide concentration or temperature. Increasing the light further will now have no effect; only increasing that other factor will raise the rate.
Model answer [2 marks]
Explain why a plant is left in darkness for 48 hours before a leaf is tested for starch. [2]
The plant is destarched, so any starch already present is used up or converted and moved away. Any starch detected at the end must therefore have been made during the experiment itself, which is what makes the result valid evidence.
Mark-losing trap. On a plateau, more light does nothing. Name the factor that has taken over — carbon dioxide or temperature — or the mark is not given.
Mark-losing trap. Temperature graphs peak and fall; light and carbon dioxide graphs level off. Draw the wrong shape and the answer goes with it.
Mark-losing trap. The lamp heats the water as well as lighting it. Say the beaker of water acts as a heat shield, or temperature is not controlled.
Mark-losing trap. Sodium hydrogencarbonate solution is there to supply carbon dioxide. It is not plant food and it is not a fertiliser.
Mark-losing trap. Counting bubbles measures a rate only when the time is fixed. Write bubbles per minute, never just bubbles.
3 · Prove it — the five questions
The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.
- Grade 6 · State [1 mark] — A graph shows the rate of photosynthesis against light intensity. The line rises steeply at first and then becomes horizontal. At the horizontal part of the graph, what is true?
- Grade 7 · Predict [1 mark] — A pond plant is sitting on the plateau of its light-intensity graph, in water at 20 °C with a normal carbon dioxide concentration. Which single change would increase its rate of photosynthesis?
- Grade 8 · Explain [4 marks] — A grower keeps a greenhouse at a constant 25 °C. Adding extra lighting at night increases the tomato yield, but adding still more lighting after that has no further effect. Select every statement that belongs in a full-mark explanation.
- Grade 9 · Calculate [3 marks] — With the lamp at 20 cm a piece of pondweed released 12 bubbles per minute. With the lamp moved to 10 cm it released 41 bubbles per minute. Calculate the percentage increase in the rate. Give your answer to three significant figures.
- 9+ · Evaluate [5 marks] — In winter a tomato grower can afford one change only. Overnight the greenhouse falls to 8 °C, the carbon dioxide concentration is normal atmospheric, and the lighting is already bright enough that the plants sit on the plateau of their light-intensity graph. The choice is between more lighting, a paraffin heater that both warms the air and releases carbon dioxide, or doing nothing. Select every statement that belongs in a full-mark answer.
The people behind this science
Two ways into the same idea — the one who worked out how light thins as it spreads, and the one who took sunlight apart. Inside Incandio each of them answers knowing exactly which lesson you have just finished.
Johannes Kepler — the one who worked out how light thins as it spreads
This practical changes light intensity by moving a lamp, and every conclusion drawn from it depends on knowing that intensity falls with the SQUARE of the distance rather than in proportion to it. Kepler was the first to state that rule, in 1604, and he reached it not by measuring brightness — which nobody could do then — but by reasoning about light spreading over the surface of an expanding sphere.
- “Why does light get weaker with distance in the particular way it does?”
- “How did you work out the rule without any way to measure brightness?”
- “If I move a lamp from 10 cm to 30 cm, how much of the light is left?”
- “Why is it wrong to treat distance and light intensity as the same variable?”
Isaac Newton — the one who took sunlight apart
A leaf looks green because chlorophyll absorbs some colours of sunlight and reflects that one back at you. That sentence only means anything once you know white light is a mixture rather than a single pure thing — which is what Newton established with a prism in a darkened room in 1666, against the general opinion of the time that the prism was manufacturing the colours itself.
- “How did you show that white light is a mixture rather than one single thing?”
- “What convinced you that the prism was not adding the colour itself?”
- “If a leaf looks green, what has become of the rest of the sunlight?”
- “Could a plant absorb every colour that falls on it, and what would it look like?”
Think you understand it?
Reading is the start. On Incandio a topic counts as mastered only once you have taught it and defended it — the two things a revision guide cannot ask of you.
Incandio this topic → Prove it. Teach it. Defend it. What does “Incandio it” mean?
- Prove it — five questions on what Limits Photosynthesis, marked on the spot, that tell you what was missing rather than just what was wrong.
- Teach it — Explain what Limits Photosynthesis in your own words to Ember, an AI pupil who does not have it yet and asks the awkward question. Understanding is credited; wording is not required.
- Defend it — Argue what Limits Photosynthesis against Jan Ingenhousz — three rounds, marked against descriptors you can read before you begin.
- Then the exam — read all 9 authored exam-style questions on this unit, each with its command word and its tariff, then have them marked on Incandio with the mistake you actually made named.
Completely free. No subscriptions, no paywalls, no account needed to start.
Carry on through the course
- Previous lesson: Photosynthesis
- Next lesson: The Leaf: Structure and Gas Exchange
- Levels of Organisation — From organelle to organism — the five-step ladder that every later topic in biology is built on.
- Comparing Cells, and Stem Cells — What plant and animal cells share and where they differ — then why a cell specialising is one of the most powerful ideas in biology.
- Carbohydrates, Proteins and Lipids — The three big food molecules — what they are made of, what they are built from, and how to test for them.
- Enzymes: Catalysts With a Shape — Why a protein with a hole in it speeds up a reaction, and why heat destroys that ability permanently.
- Enzymes and pH — Why acid ruins one enzyme and is the only condition another will work in — and how to find an enzyme's best pH by experiment.
- All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification