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3.3.5Additional lessons

Limiting Factors of Photosynthesis — IB Biology SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 60 of 142

📚 IB Biology SL 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 6Limiting Factors

Light intensity
  • Light intensity At low light the rate rises with intensity, since the light-dependent reactions are limited.
Carbon dioxide
  • Carbon dioxide The rate increases with carbon dioxide concentration until another factor becomes limiting.
Temperature
  • Temperature The rate rises with temperature as enzymes work faster, then falls when they denature.
Limiting factor
  • Limiting factor The factor in shortest supply at a given time controls the overall rate.
Plateau
  • Plateau On a graph the rate levels off when another factor becomes the one in shortest supply.

Part 2 of 6Investigation with Elodea

1
Place a piece of the pondweed Elodea in water containing sodium hydrogencarbonate as a source of carbon dioxide.
2
Position a lamp at a measured distance and wait for the plant to adjust.
3
Count the bubbles of oxygen released per minute, or measure their volume.
4
Change the distance of the lamp and repeat, keeping the temperature steady with a water bath.
5
Calculate the mean rate for each distance.

Part 3 of 6Interpreting Results

Interpreting Results
Light intensity
  • Light intensity Light intensity is proportional to 1 divided by the distance squared from the lamp.
Controls
  • Controls Keep temperature and carbon dioxide constant when light is the independent variable.
Errors
  • Errors Bubble size varies, and heat from the lamp may change the temperature, which affects results.
Greenhouses
  • Greenhouses Growers control light, carbon dioxide and temperature to increase crop yield.

Part 4 of 6Graph Shapes

Graph Shapes
Light
  • Light Rate rises linearly with intensity at first, then flattens when carbon dioxide or temperature becomes limiting.
Temperature
  • Temperature The rate rises to an optimum, then drops sharply as enzymes denature.
Combined
  • Combined Raising the limiting factor increases the rate, whereas raising a non-limiting factor has no effect.

Part 5 of 6Worked Example

Question

  • An Elodea plant 10 cm from a lamp is moved to 20 cm.
  • Compare the light intensity at the two positions.

  1. Intensity is proportional to 1 ÷ d².
  2. Relative intensity = (10 ÷ 20)² = 0.25.

Answer The intensity at 20 cm is one quarter of that at 10 cm.

Part 6 of 6Exam Checklist & Tips

3.3.5 – Key Points for the Exam

  • Limiting factor The factor in shortest supply that limits the rate of a process.
  • Light intensity The amount of light energy falling on a surface in unit time.
  • Plateau A level section of a graph where the rate is constant.
  • Elodea A freshwater plant used to study photosynthesis.

Common Mistakes

  • Saying that all factors limit the rate at the same time, when only the one in shortest supply does.
  • Ignoring the effect of the heat from the lamp on temperature.
  • Believing the rate keeps rising with temperature, when enzymes denature at high temperature.
  • Using the distance as light intensity directly, without the inverse square relationship.

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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