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13.2.113 · Photosynthesis

Limiting Factors of Photosynthesis — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 11 of 13 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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13.2.1 Limiting Factors of Photosynthesis — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how light intensity, carbon dioxide concentration and temperature limit the rate of photosynthesis.Needs — photosynthetic pigments, CO2, water, light energy and a suita…

  • Below optimum
  • Possible limiting factors at the plateau
  • Light-dependent reactions
  • Calvin cycle
  • Rising line
  • Horizontal line

Section 1 of 8Requirements for photosynthesis

Needs
  • Needs photosynthetic pigments, CO2, water, light energy and a suitable temperature.
Shortage
  • Shortage photosynthesis cannot reach its maximum rate.
Limiting factors
  • Limiting factors light intensity, CO2 concentration and temperature.
Below optimum
  • Below optimum one factor below optimum reduces the rate even if the others are optimal.

Section 2 of 8Light intensity

1Effect
  • Effect with CO2 and temperature constant, rate increases as light intensity increases.
2Reason
  • Reason the light-dependent stage is faster, giving more ATP and reduced NADP for the Calvin cycle.
3Plateau
  • Plateau light is no longer limiting, and another factor limits the rate.
4Possible limiting factors at the plateau
  • Possible limiting factors at the plateau temperature too low or too high, or too little CO2.

Section 3 of 8Carbon dioxide concentration

Effect
  • Effect rate increases as CO2 concentration increases.
Reason
  • CO2 is a raw material, combined with RuBP in the light-independent stage.
Plateau
  • Plateau another factor in short supply, such as temperature or light.

Section 4 of 8Temperature

1Effect
  • Effect rate increases as temperature rises because the reactions are enzyme-controlled.
2Optimum
  • Optimum beyond it, enzymes begin to denature and the rate falls.
3Light-dependent reactions
  • Light-dependent reactions temperature has little effect because light supplies the energy.
4Calvin cycle
  • Calvin cycle affected by temperature because rubisco and other enzymes control it.

Section 5 of 8Reading graphs

Rising line
  • Rising line the limiting factor is the x-axis factor.
Horizontal line
  • Horizontal line the limiting factor is a different one.
Water
  • Water a shortage can reduce the rate but usually affects other processes first.

Section 6 of 8More from the specification

1Investigating Photosynthesis Rates
  • Using an aquatic plant such as Elodea or pondweed, the rate of photosynthesis can be investigated by measuring the volume of oxygen gas produced or bubble counts per unit time under varied light intensities.
2Redox Indicators (DCPIP)
  • The Hill reaction can be monitored using redox indicator dyes like DCPIP;
    • in its oxidised state DCPIP is blue,
  • but when reduced by electrons accepted from the light-dependent reactions of chloroplasts, it undergoes a colour change to colourless.
3Light and Carbon Dioxide Limiting Factors
  • A graph of photosynthetic rate against light intensity or carbon dioxide concentration shows an initial proportional increase until another factor becomes limiting,
  • causing the curve to plateau.
4Investigating Photosynthesis Rates
  • Using an aquatic plant such as Elodea or pondweed, the rate of photosynthesis can be investigated by measuring the volume of oxygen gas produced or bubble counts per unit time under varied light intensities.
5Redox Indicators (DCPIP)
  • The Hill reaction can be monitored using redox indicator dyes like DCPIP;
    • in its oxidised state DCPIP is blue,
  • but when reduced by electrons accepted from the light-dependent reactions of chloroplasts, it undergoes a colour change to colourless.
6Light and Carbon Dioxide Limiting Factors
  • A graph of photosynthetic rate against light intensity or carbon dioxide concentration shows an initial proportional increase until another factor becomes limiting,
  • causing the curve to plateau.
7Independent variable
  • The factor deliberately changed in an investigation.
8Redox indicator
  • Dye that changes colour when reduced or oxidised.
9Indicator
  • A chemical that changes colour depending on pH.
10Pondweed
  • Aquatic plant such as Elodea used to collect oxygen.

Section 7 of 8Key terms

TermMeaning
Limiting factorFactor in shortest supply that limits the rate.
Optimum temperatureTemperature giving the fastest enzyme-controlled rate.
PlateauPart of a graph where the rate stops rising.
RubiscoEnzyme fixing CO2 in the Calvin cycle.
DenaturationLoss of an enzyme's shape and function.

Section 8 of 8Exam technique

1
Saying all three factors limit at once — one factor is limiting at a time.
2
Saying temperature affects the light-dependent reactions strongly — it mainly affects the Calvin cycle.
3
Saying CO2 is limiting on a rising light-intensity graph — light is limiting there.
4
Saying enzymes are killed at high temperature — they are denatured.

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