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5.1.8Unit 5: Respiration, Internal Environment, Coordination and Gene Technology

Core Practical 10: Rate of Photosynthesis — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 6 parts Lesson 15 of 36 Unit quiz

📚 IAL Edexcel Linear 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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Section 1 of 6Limiting Factors

Photosynthesis is limited by whichever factor is nearest its minimum.

Light intensity
  • Light intensity At low intensity, the rate rises with more light because ATP and reduced NADP production increases.
Carbon dioxide
  • Carbon dioxide A higher concentration increases the rate until another factor becomes limiting.
Temperature
  • Temperature Rate rises with temperature until the enzymes, especially rubisco, begin to denature.
Plateau
  • Plateau On a graph the curve levels off when a different factor becomes the limit.

Section 2 of 6Method with an Aquatic Plant

A pondweed such as Elodea releases oxygen bubbles that can be measured.

1
Place a piece of pondweed in a boiling tube of water containing sodium hydrogencarbonate as a source of carbon dioxide.
2
Set a lamp at a measured distance and keep the tube in a water bath to prevent heating effects.
3
Allow the plant to adjust, then count bubbles, or collect the gas in a capillary tube or syringe over a set time.
4
Repeat at several distances, changing only the distance and keeping the other variables the same.

Section 3 of 6Interpreting Light Intensity

Interpreting Light Intensity
Inverse square
  • Inverse square Light intensity is proportional to 1/d², so doubling the distance gives one quarter of the intensity.
Rate
  • Rate Rate can be expressed as volume of oxygen per unit time, such as cm³ min⁻¹.
Control
  • Control A control without the plant or in darkness shows no photosynthetic oxygen release.
Heat
  • Heat LED lamps or a heat shield avoid heating the water and changing the temperature.

Section 4 of 6Improving Accuracy

Measuring gas
  • Measuring gas Collecting gas in a syringe is more accurate than counting bubbles of different sizes.
Repeats
  • Repeats Repeat each distance and calculate a mean to improve reliability.
Same plant
  • Same plant Use the same piece of plant for each distance to avoid variation between samples.
Alternative
  • Alternative Isolated chloroplasts with DCPIP can be used; the dye loses colour as it is reduced.

Section 5 of 6Worked Example

Question

  • A pondweed releases 0.36 cm³ of oxygen in 6 minutes at 10 cm from a lamp.
  • Calculate the rate, and the relative light intensity at 20 cm compared with 10 cm.

  1. Rate = 0.36 / 6 = 0.06 cm³ min⁻¹.
  2. Intensity is proportional to 1/d².
  3. At 10 cm: 1/100; at 20 cm: 1/400.
  4. Ratio = (1/400) / (1/100) = 0.25.

Answer

  • Rate = 0.06 cm³ min⁻¹;
    • intensity at 20 cm is one quarter of that at 10 cm.

Section 6 of 6Exam Checklist & Tips

5.1.8 – Key Points for the Exam

  • Limiting factor The factor in shortest supply, which restricts the rate of a process.
  • Light intensity The amount of light energy falling on a surface.
  • Control variable A variable kept constant so that it does not affect the results.
  • Rate The quantity changing per unit time.

Common Mistakes

  • Counting bubbles and assuming each has the same volume.
  • Letting the lamp heat the water so temperature also changes.
  • Forgetting a source of carbon dioxide so that it limits the rate.
  • Taking the first reading immediately without allowing the plant to adjust.

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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