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3.3.5Inquiry Process

Limiting Factors of Photosynthesis- Skills — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 120 of 160

📚 IB Biology HL Hub / Topical Revision
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Curriculum Equation Key معادلة البناء الضوئي الموزونة - Balanced Photosynthesis EquationFast Exam Solution
6CO₂ + 6H₂O ➔ C₆H₁₂O₆ + 6O₂
Word Equation & Conditions: Carbon Dioxide + Water (+ Light & Chlorophyll) -> Glucose + Oxygen | ثاني أكسيد الكربون + ماء (في وجود الضوء والكلوروفيل) ← جلوكوز + أكسجين
Location & Specifications: البلاستيدات الخضراء (Chloroplast): التفاعلات الضوئية في الثايلاكويد، ودورة كالفن في الستروما.
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Part 1 of 5Limiting Factors

The rate of photosynthesis is controlled by whichever factor is in shortest supply.

Light intensity
  • Light intensity At low light the rate increases in proportion to light intensity,
    • because the light-dependent reactions are limited by the supply of photons.
Carbon dioxide
  • Carbon dioxide At low carbon dioxide concentration the rate rises with concentration, because the Calvin cycle has too little carbon dioxide to fix.
Temperature
  • Temperature The rate rises with temperature as enzymes such as rubisco work faster, then falls at high temperature as enzymes denature.
Principle
  • Principle The factor in shortest supply sets the rate, so changing a factor that is not limiting has no effect.
Plateau
  • Plateau When a graph levels off, another factor has become limiting, or all of the pigments or enzymes are working at their maximum.

Part 2 of 5Investigating with Pondweed

1
Place a cutting of aquatic plant such as Elodea in water containing dissolved sodium hydrogencarbonate as a source of carbon dioxide.
2
Set a lamp at a measured distance and wait for the rate of bubbling to become steady.
3
Count the bubbles released in a fixed time, such as three minutes, or collect the gas in a syringe.
4
Repeat at several distances, remembering that light intensity is proportional to 1 divided by distance squared.
5
Control temperature, for example with a water bath or by placing a heat shield between lamp and plant, and repeat to calculate means.

Part 3 of 5Improving and Evaluating

Improving and Evaluating
Hypothesis
  • Hypothesis A testable, provisional explanation, such as: raising light intensity will increase the rate of photosynthesis.
Variables
  • Variables Light intensity is independent, bubble count is dependent, and temperature, carbon dioxide and plant size are controlled.
Improvements
  • Improvements Use a gas syringe to measure volume, a datalogger for continuous readings, and repeat to obtain a reliable mean.
Limitations
  • Limitations Bubble size varies and some oxygen dissolves, so bubble counts give only an approximate rate.
Light intensity (arbitrary units)Bubbles per minute
412
821
1221

Part 4 of 5Worked Example

Question

  • In an Elodea investigation the lamp was moved so that the light intensity increased from 4 to 8 arbitrary units.
  • The bubble count rose from 12 to 21 bubbles per minute.
  • Calculate the percentage increase in the rate, and say what the identical rates at 8 and 12 units suggest.

  1. Increase = 21 - 12 = 9 bubbles per minute.
  2. Percentage increase = 9 / 12 x 100 = 75%.
  3. The rate is the same at 8 and 12 units, so light is no longer the limiting factor.

Answer

  • The rate increased by 75%;
    • above 8 units another factor, such as carbon dioxide or temperature, limits the rate.

Part 5 of 5Exam Checklist & Tips

3.3.5 – Key Points for the Exam

  • Limiting factor A factor whose shortage restricts the rate of a process.
  • Hypothesis A proposed explanation that can be tested by experiment.
  • Rubisco The enzyme that fixes carbon dioxide in the Calvin cycle.
  • Plateau The part of a graph where the rate stays constant.

Common Mistakes

  • Saying that increasing light always increases the rate;
    • once another factor limits, there is no further effect.
  • Forgetting to control temperature, because lamps give out heat as well as light.
  • Calling the plateau an effect of denaturation when it is caused by another limiting factor.
  • Using a terrestrial plant, when oxygen bubbles cannot be observed.

Last updated:

Biology 0610 & 9700 Exam Model Limiting Factors Master

Photosynthesis: Limiting Factors & Graph Plateau

6CO2 + 6H2O → C6H12O6 + 6O2 (Light & Chlorophyll required)

  • Limiting Factor Definition: Something present in the environment in such short supply that it restricts life processes.
  • Graph Plateau: When rate levels off as light intensity increases, light is no longer the limiting factor; CO2 concentration or temperature has become limiting.
  • High Temperature Drop: Beyond optimum temp (~40°C), enzymes (e.g. RuBisCO) denature, permanently decreasing rate.
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