Limiting Factors in photosynthesis
Laura Armstrong
Teacher
Recall Questions
This topic requires prior knowledge of the light dependent and the light independent reactions. You can test your knowledge on these below.
What are the products of the light-dependent reaction?
ATP, reduced NADP (NADPH), and oxygen.
What enzyme fixes carbon dioxide in the light-independent reaction?
RuBisCO (Ribulose bisphosphate carboxylase).
Which two products of the LDR are required for the LIR and why are they needed?
ATP and reduced NADP. Reduced NADP provides the hydrogen and ATP provides the energy to reduce GP to triose phosphate
Topic Explainer Video
Check out this @JoeDoesBiology video that explains limiting Factors in photosynthesis or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What Are Limiting Factors?
A limiting factor is a variable that directly affects the rate of photosynthesis. If this factor is in short supply, it will limit how fast photosynthesis can proceed — even if other conditions are optimal.
If this factor is increased, it will increase the rate of photosynthesis.
The Three Main Limiting Factors of Photosynthesis
|
Factor |
How It Limits Photosynthesis |
|---|---|
|
Light intensity |
Needed for photoionisation of chlorophyll in the LDR. Less light = less ATP and NADPH produced. |
|
Carbon dioxide |
Required for carbon fixation in the LIR. Less CO₂ = less GP and Triose Phosphate made. |
|
Temperature |
Affects enzymes (especially RuBisCO). Low temp = slower enzyme activity. High temp = denaturation. |
Graphical Representations
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Light Intensity:
Increases the rate until it plateaus — once another factor becomes limiting (e.g. CO₂ or temperature). -
Increasing light intensity means there is more light energy to excite electrons in chlorophyll during the light dependent reaction.
-
This leads to the production of more ATP and NADPH
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CO₂ Concentration:
Increases the rate until another factor becomes limiting. -
Increasing CO₂ concentration means there is more CO₂ to join with RuBP. This means more GP and triose phosphate will form.
-
Where the graph plateaus, another factor, such as light intensity or temperature has become the limiting factor.
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Temperature:
Shows an optimum temperature. -
Above this, enzyme denaturation causes a sharp fall in photosynthesis rate.
-
Below this, enzyme activity is reduced as there is less kinetic energy and fewer successful collisions, e.g. Between RuBiCO, RuBP and CO₂ leading to fewer enzyme substrate complexes and less GP produced. This results in less triose phosphate forming.
Interactions Between Factors
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Limiting factors interact — e.g. increasing CO₂ only increases rate if light intensity is also high.
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In greenhouse management, manipulating multiple factors simultaneously can maximise plant growth and yield.
Light Compensation Point
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The Light Compensation Point is the light intensity at which the rate of photosynthesis equals the rate of respiration.
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At this point: O₂ produced in photosynthesis = O₂ used in respiration CO₂ absorbed in photosynthesis = CO₂ released in respiration
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Net gas exchange = zero
→ No net O₂ released
→ No net CO₂ absorbed -
Below the light compensation point, the plant respires at a higher rate than it photosynthesises, leading to net CO₂ release.
-
Above the light compensation point, the plant photosynthesises at a higher rate than it respires, leading to net CO₂ uptake.
Key Terms
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Limiting Factor: The factor in shortest supply that limits the rate of photosynthesis. Increasing a limiting factor will increase the rate of a reaction.
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Photoionisation: The excitation and release of electrons from chlorophyll due to light energy.
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Carbon Fixation: The incorporation of CO₂ into organic molecules (catalysed by RuBisCO).
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Enzyme Denaturation: A loss of enzyme activity due to breakdown of tertiary structure at high temperatures.
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Light Compensation Point: is the light intensity at which the rate of photosynthesis equals the rate of respiration.
No answer provided.
Exam Tip
When interpreting graphs involving limiting factors, always identify which variable is limiting and explain how it affects either the LDR or LIR.
Wherever the gradient of the graph is increasing, it is whatever is on the X-axis that is limiting the rate. Wherever the gradient has plateaued, it is a different factor limiting the rate- name one!
No answer provided.
Explain how carbon dioxide concentration and temperature can act as limiting factors in photosynthesis. (6 marks)
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Carbon dioxide is required for carbon fixation in the light independent reaction.
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It combines with RuBP to form GP, catalysed by RuBisCO.
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Low CO₂ concentration = less GP and triose phosphate formed = slower LIR.
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Temperature affects enzymes, especially RuBisCO in the LIR and ATP synthase in the LDR.
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Low temperatures = reduced kinetic energy = slower enzyme activity.
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High temperatures = enzymes denature = active site changes shape = reduced rate of reaction.
Iron deficiency reduces electron transport. Use this information and your knowledge of photosynthesis to explain the decrease in production of triose phosphate in the iron-deficient plants. (4 marks)
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Less ATP produced
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Less reduced NADP / NADPH produced
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During light-dependent reaction
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Less GP to triose phosphate
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!