Conditions for Photosynthesis
Give a plant carbon dioxide, water and light and it will photosynthesise, but how fast? Speed up one condition and the rate climbs; push it further and the rate flattens out, because something else has become the bottleneck. The conditions for photosynthesis behave like a production line, and understanding which factor is limiting is the key question behind every greenhouse, crop yield and climate model.
Orient yourself
Predict, scan the goals and key terms, and clear the misconception this lesson exists to fix.
Before You Read
warm-upTwo identical pot plants are grown side by side. One is given extra light; the other extra CO₂. The extra-CO₂ plant grows noticeably faster; the extra-light plant barely changes.
Before reading: write a question a scientist might ask to explain this result, and your best guess at the answer.
Learning Intentions
goalsKnow
- The conditions required for photosynthesis: light, CO₂, water, suitable temperature, chlorophyll
- That each can act as a limiting factor
- The general shape of a rate-vs-condition graph
Understand
- What a limiting factor is and why a rate plateaus
- Why temperature has an optimum (enzymes)
- Why only one factor limits the rate at any moment
Can Do
- Discuss the conditions required for photosynthesis
- Identify the limiting factor from a graph or scenario
- Pose a testable question about photosynthesis conditions
Scan these before reading
vocabMisconceptions To Fix
watch outOdd one out: Three of these statements about the conditions for photosynthesis are correct. Which one is wrong?
The Required Conditions
What photosynthesis needs to occur at all
Core Content
The Required Conditions
+5 XPWhat photosynthesis needs to occur at all
Photosynthesis can only proceed when light energy, raw materials, a working enzyme system and the light-absorbing pigment are all present together.
The conditions required for photosynthesis are: light (the energy source), carbon dioxide and water (the raw materials), chlorophyll (the pigment that absorbs light) and a suitable temperature (so the enzymes that catalyse the reactions can work).
Remove any one of these and photosynthesis stops: no light at night, no CO₂ if stomata close, no chlorophyll in a white leaf patch, and no reaction at all if it is too cold for the enzymes. This is why a healthy green leaf in sunlight on a warm day photosynthesises rapidly, while the same leaf in a dark cupboard does not.
Pause, copy the highlighted list of conditions into your book.
The green pigment that absorbs light energy for photosynthesis, and is a required condition for it, is _____.
Limiting Factors
Why one condition at a time sets the rate
Limiting Factors
+5 XPWhy one condition at a time sets the rate
We just listed the conditions photosynthesis needs. That raises a question: if all are present, what actually controls how fast it goes? This card answers it → whichever is in shortest supply, the limiting factor.
At any moment, photosynthesis runs only as fast as its most scarce condition allows, so increasing any other condition has no effect until the scarce one is topped up.
A limiting factor is the condition in shortest supply that holds back the rate of photosynthesis. As you increase a limiting factor (e.g. light), the rate rises; once that factor is plentiful the rate plateaus, because a different factor (e.g. CO₂) has now become limiting.
Text description
- Rate of photosynthesis (up) against light intensity (across).
- Rising part: light is the limiting factor, so more light gives a faster rate.
- Plateau: light is plentiful and another factor, CO₂ or temperature, now limits the rate.
Pause, copy the highlighted definition of a limiting factor and sketch the rising-then-plateau graph.
A limiting factor is the condition in shortest supply that holds back the rate of photosynthesis.
Increasing temperature without limit always increases the rate of photosynthesis.
On a rate-vs-light-intensity graph, the plateau occurs because a factor other than light has become limiting.
Pose and Answer a Limiting-Factor Question
Do the activity in your book, then check what you have understood.
Activity 1
Pose and Answer a Limiting-Factor Question
Pattern D, Questioning (WS01)
For the hook scenario (extra CO₂ helped, extra light did not), write a precise, testable scientific question, state the variable you would change, and predict the result. Then explain, using the idea of limiting factors, why the plant on a bright day responds to CO₂ but not to more light. Finally, name one condition that, if you cooled the plant too far, would change which factor is limiting.
Match each condition to the reason photosynthesis requires it:
- Light
- Carbon dioxide
- Chlorophyll
- Suitable temperature
- The pigment that absorbs the light energy
- The energy source the reaction runs on
- So the enzymes that catalyse the reactions can work
- The raw material the carbon in glucose comes from
Temperature, A Condition with an Optimum
Why heat helps, until it doesn't
Temperature, A Condition with an Optimum
+5 XPWhy heat helps, until it doesn't
We just saw light and CO₂ rise then plateau. That raises a question: does temperature behave the same way? This card answers it → no, temperature rises to an optimum, then the rate falls.
Because photosynthesis is run by enzymes, temperature speeds the reaction up to an optimum and then sharply slows it as the enzymes are destroyed.
Temperature is a required condition because the reactions of photosynthesis are catalysed by enzymes. As temperature rises, the rate increases up to an optimum (about 25–35 °C for many plants); above the optimum the enzymes denature and the rate falls steeply. Too cold, and the enzymes are too slow to work.
This is why temperature does not simply plateau like light: its graph rises to a peak and then drops. A scientist discussing the conditions for photosynthesis must treat temperature differently, it has a "best" value, not just a "more is better" trend.
Add the highlighted temperature-optimum point to your notes before the check below.
Why does the rate of photosynthesis fall when temperature rises well above the optimum?
Why greenhouses pump in carbon dioxide
Where this biology is doing real work outside the classroom.
Why greenhouses pump in carbon dioxide
Commercial greenhouse growers often burn gas or release bottled CO₂ to raise the carbon dioxide level to two or three times that of normal air. They do this because, with plenty of light and a controlled warm temperature, CO₂ is usually the limiting factor, so topping it up directly increases the rate of photosynthesis and the crop yield. The same logic, in reverse, sits behind concerns about climate change and crop temperatures: push the temperature past the optimum and the world's staple crops photosynthesise less, not more. Knowing which condition is limiting is worth billions of dollars to agriculture.
Discuss the conditions required for photosynthesis. In your answer, explain how light, carbon dioxide and temperature can each limit the rate, and how temperature differs from the other two.
Discuss asks you to identify issues and provide points for and against — here, to go beyond listing the conditions and consider how they interact and where the simple "more is better" picture breaks down.
7 marks. Name the conditions, define a limiting factor, show the rise-then-plateau behaviour of light and carbon dioxide, then set temperature against them as the condition that behaves differently, and close on why only one factor limits at a time.
Photosynthesis requires light as the energy source, carbon dioxide and water as the raw materials, chlorophyll as the pigment that absorbs the light energy, and a suitable temperature so the enzymes catalysing the reactions can work. Remove any one and photosynthesis stops: there is no light at night, no carbon dioxide if the stomata close, no chlorophyll in a white leaf patch, and no useful reaction rate if it is too cold.
A limiting factor is the condition in shortest supply, which is the one holding back the rate of photosynthesis at that moment. Only the limiting factor controls the rate: increasing any condition that is already plentiful produces no change until the scarce one is topped up.
As light intensity is increased from low levels the rate rises, because light is the factor in shortest supply. Beyond a certain intensity the curve flattens into a plateau: light is now plentiful and something else — carbon dioxide concentration or temperature — has become limiting, so further light makes no difference. Carbon dioxide behaves in the same way, raising the rate while it is scarce and then plateauing once another condition takes over as the constraint.
Temperature does not follow the same pattern. Because the reactions of photosynthesis are catalysed by enzymes, raising the temperature increases the rate up to an optimum, around 25 to 35 degrees Celsius for many plants. Above that optimum the enzymes denature — their active sites lose the shape their function depends on — and the rate falls steeply rather than levelling off. Below the optimum the enzymes are too slow. Temperature therefore has a best value, not a "more is better" trend.
The difference matters in practice. Light and carbon dioxide can be raised indefinitely without harming the plant — each simply stops having an effect once it is no longer limiting — whereas temperature can be raised past the point where it damages the enzymes, so that adding more of a required condition actively reduces the rate. Light and carbon dioxide give a rising curve that plateaus; temperature gives a curve that rises to a peak and then falls.
These conditions therefore cannot be considered separately. At any moment the rate is set by whichever is scarcest, so the same plant may be light-limited at dawn, carbon-dioxide-limited at midday with the stomata open in full sun, and temperature-limited on a cold bright morning. Identifying which condition is limiting is what makes it possible to predict the effect of changing any one of them.
Photosynthesis requires light as the energy source, carbon dioxide and water as the raw materials, chlorophyll as the pigment that absorbs light energy, and a suitable temperature so that the enzymes catalysing the reactions can work; removing any one stops the process, as at night, when the stomata close, in a white leaf patch, or in the cold. Which condition controls the rate at any given moment is decided by the limiting factor, the condition in shortest supply, and increasing any other condition has no effect until the scarce one is topped up. As light intensity rises from low levels the rate increases because light is limiting, then flattens into a plateau once light is plentiful and carbon dioxide or temperature has become the constraint instead; carbon dioxide behaves the same way, raising the rate while scarce and plateauing thereafter. Temperature does not follow this pattern. Because photosynthesis is enzyme-catalysed, raising the temperature increases the rate only up to an optimum of roughly 25 to 35 degrees Celsius for many plants; above it the enzymes denature, losing the active-site shape their function depends on, and the rate falls steeply rather than levelling off, while below it the enzymes are simply too slow. The practical difference is that light and carbon dioxide can be raised indefinitely and will merely stop having an effect, whereas temperature can be raised past the point at which more of a required condition actively reduces the rate: light and carbon dioxide give a rising curve that plateaus, temperature a curve that peaks and then falls. The conditions therefore cannot be treated separately, since the same plant may be light-limited at dawn, carbon-dioxide-limited at midday and temperature-limited on a cold bright morning, and identifying which is limiting is what makes the effect of any change predictable.
Two identical pot plants stand side by side in bright light on a warm day. One is given additional light; the other is given additional carbon dioxide. Predict which plant will show the greater increase in the rate of photosynthesis, and justify your prediction.
Predict asks for a stated outcome together with the reasoning that produces it. The reasoning here is the limiting-factor principle, so the justification must name which factor is limiting and why.
5 marks. Identify the limiting factor from the conditions described, state the prediction plainly, justify it both ways — why extra carbon dioxide helps and why extra light does not — and note what would change the answer.
The plants are already in bright light on a warm day, so light intensity is high and temperature is near the optimum for enzyme activity. Both plants stand in ordinary air, in which carbon dioxide makes up only about 0.04 per cent. Of the conditions photosynthesis requires, carbon dioxide is therefore the one in shortest supply.
The plant given additional carbon dioxide will show the greater increase in the rate of photosynthesis. The plant given additional light will show little or no increase.
Because carbon dioxide is the limiting factor, raising its concentration relieves the constraint that is currently holding the rate down. Carbon dioxide is the raw material supplying the carbon from which glucose is built, so making more of it available allows the reaction to proceed faster, and the rate rises.
The plant given more light is already light-saturated: it sits on the plateau of the rate-against-light-intensity curve, where light is plentiful and something else limits the rate. Adding more of a condition that is already in surplus cannot raise the rate, so this plant is effectively unchanged. The prediction would reverse in dim light — at low intensity light would be the limiting factor, extra light would raise the rate, and extra carbon dioxide would then have little effect.
Both plants stand in bright light on a warm day, so light intensity is high and temperature is near the optimum for the enzymes, while the surrounding air contains only about 0.04 per cent carbon dioxide; of the conditions photosynthesis requires, carbon dioxide is therefore the one in shortest supply and is the limiting factor. I predict that the plant given additional carbon dioxide will show the greater increase in the rate of photosynthesis, and that the plant given additional light will show little or no increase. Raising the carbon dioxide concentration relieves the constraint currently holding the rate down, and because carbon dioxide is the raw material supplying the carbon from which glucose is built, making more of it available lets the reaction proceed faster and the rate rises. The plant given more light is already light-saturated and sits on the plateau of the rate-against-light-intensity curve, where light is plentiful and another factor limits the rate, so adding a condition already in surplus cannot increase it. The prediction would reverse in dim light: at low intensity light would be the limiting factor, so extra light would raise the rate while extra carbon dioxide would have little effect.
Practise independently
Answer in your own words first. The model answers are for checking, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 3 questions · 14 marks
+5 XPAnalyseBand 5(5 marks) 1. Discuss the conditions required for photosynthesis. In your answer, explain how light, carbon dioxide and temperature can each limit the rate, and how temperature differs from the other two.
1 mark: required conditions named (light, CO₂, water, chlorophyll, suitable temperature) · 1 mark: defines limiting factor · 1 mark: light/CO₂ raise rate then plateau · 1 mark: temperature has an optimum then falls due to denaturation · 1 mark: explicitly contrasts temperature with light/CO₂
AnalyseBand 5(4 marks) 2. A graph plots the rate of photosynthesis against light intensity for a plant at 20 °C and again at 30 °C. Both curves rise and then flatten, but the 30 °C curve flattens at a higher rate. Analyse what is limiting the rate on the steep part of each curve and on each plateau, and explain what the difference between the two plateaus shows.
1 mark: light is the limiting factor on the steep part of both curves · 1 mark: light is no longer limiting at the plateau, some other factor is · 1 mark: identifies temperature as the factor that differs between the two plateaus · 1 mark: concludes that only the factor in shortest supply controls the rate at any moment
EvaluateBand 5–6(5 marks) 3. A greenhouse grower already runs lights at high intensity and keeps the air enriched with carbon dioxide. They propose raising the greenhouse temperature from 28 °C to 42 °C to increase yield further. Discuss whether this proposal will achieve what they expect, referring to the conditions required for photosynthesis.
1 mark: recognises that raising temperature increases rate only up to an optimum · 1 mark: states that photosynthesis is enzyme-catalysed · 1 mark: explains that above the optimum enzymes denature and the rate falls · 1 mark: notes 42 °C is above the typical 25-35 °C optimum, so the rate would be expected to fall rather than rise · 1 mark: reaches a judgement, and identifies what should be tested or changed instead
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Answer
Q1 (5 marks): Photosynthesis requires light (the energy source), carbon dioxide and water (raw materials), chlorophyll (to absorb light) and a suitable temperature (so the enzymes work). At any moment the rate is set by the limiting factor, the condition in shortest supply. As light intensity is increased, the rate rises while light is limiting, then plateaus once light is plentiful and another factor (such as CO₂) becomes limiting; increasing CO₂ behaves the same way. Temperature differs: because photosynthesis is enzyme-controlled, the rate rises with temperature only up to an optimum, then falls steeply above it as the enzymes denature, and is very slow when too cold. So while "more light" or "more CO₂" gives an increase-then-plateau, temperature gives a rise to a peak followed by a decline, there is a best temperature rather than simply "the more the better".
Q2 (4 marks): On the steeply rising part of both curves, light intensity is the limiting factor: it is in shortest supply, so every increase in light produces an increase in rate. At each plateau light is now plentiful and is no longer limiting; some other required condition is holding the rate back, most often carbon dioxide concentration or temperature. The fact that the 30 °C plateau sits higher than the 20 °C plateau shows that temperature was one of the factors limiting the rate at 20 °C: raising it let the enzymes work faster and lifted the ceiling. If carbon dioxide alone had been limiting, warming the plant would not have moved the plateau. This is the principle of limiting factors: at any moment only the condition in shortest supply controls the rate, and increasing any other condition changes nothing until the scarce one is topped up.
Q3 (5 marks): The proposal is unlikely to work and would probably reduce yield. The grower is applying "more is better" reasoning that is valid for light and carbon dioxide, both of which raise the rate and then plateau, but temperature does not behave that way. Photosynthesis is catalysed by enzymes, and enzyme-driven rates rise with temperature only to an optimum, about 25-35 °C for many plants. Above the optimum the enzymes begin to denature: their active sites change shape, they can no longer bind their substrates, and the rate falls steeply rather than plateauing. At 28 °C the crop is already near or inside its optimum band, so moving to 42 °C moves it past the peak and down the falling side of the curve. My judgement is that the grower should not make this change. Because light and CO₂ are already high, the sensible next step is to test small temperature increments and measure the rate, so the optimum for this particular crop is found rather than assumed, and to check whether water supply has become the limiting factor instead.
Retrieve and reflect
Use the spaced-review session, then compare how your thinking has changed since your opening prediction.
Check what actually stuck
A short retrieval session mixing this lesson with earlier ones, so the ideas move into long-term memory instead of fading by the exam.
Lock it in
bossYou can now discuss the conditions for photosynthesis and identify the limiting factor in any scenario. Next lesson turns this into an experiment: predicting how a changed condition affects the rate.
↩ Back to focus-area hubHow did your thinking change?
At the start, extra CO₂ helped your plant but extra light did not. The reason is that the plant was already light-saturated, so light was not the limiting factor, CO₂ was. Adding more of a plentiful factor does nothing; adding more of the limiting factor raises the rate.
If you thought "more of everything always helps", revisit the misconception box and the limiting-factor graph: only the scarcest condition controls the rate.