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A2 · Practice questions · Limiting factors and investigations

Find what holds it back.

Six original Cambridge-style questions on limiting factors and the practicals that measure them, from reading a plateau to planning a pondweed investigation.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Slope, plateau, control.

01
Recall
[2 marks]

Define the term limiting factor and name three limiting factors of photosynthesis.

  • The factor in shortest supply (furthest from its optimum), which limits the rate of the process; increasing it increases the rate. ✓
  • Light intensity, carbon dioxide concentration and temperature. ✓
02
Explain
[4 marks]

Explain the effects of increasing temperature from 10 °C to 45 °C on the rate of photosynthesis of a plant in bright light and high carbon dioxide concentration.

  • From 10 °C up to the optimum (about 25-30 °C) the rate increases. ✓
  • The light-independent stage is enzyme-controlled; molecules gain kinetic energy, giving more collisions and more enzyme-substrate complexes, for example with rubisco. ✓
  • Above the optimum the rate falls, as hydrogen and ionic bonds in enzymes break and active sites change shape (denaturation). ✓
  • Membrane proteins in the thylakoids may also be damaged, so the rate falls towards zero by about 45 °C. ✓
03
Data
[3 marks]

Pondweed was placed at different distances from a lamp. Bubbles per minute: 10 cm, 48; 20 cm, 13; 30 cm, 6; 40 cm, 3. Calculate the light intensity at 20 cm as a proportion of that at 10 cm, and comment on the relationship between rate and light intensity.

  • Intensity ∝ 1/d2: (10 ÷ 20)2 = 0.25 of the intensity at 10 cm. ✓
  • The rate at 20 cm is 13 ÷ 48 = 0.27 of the rate at 10 cm. ✓
  • The rate is roughly proportional to light intensity, so light is the limiting factor over this range. ✓
04
Analysis
[4 marks]

Describe how you would use a chloroplast suspension and DCPIP to investigate the effect of light wavelength on the rate of the light-dependent stage.

  • Extract chloroplasts by grinding leaves in ice-cold isotonic buffer, filtering and centrifuging; keep the suspension on ice. ✓
  • Mix equal volumes of chloroplast suspension and DCPIP in tubes; place each behind a different coloured filter (for example red, blue, green) at the same distance from the same lamp. ✓
  • Record the time for the blue colour to disappear (or the change in absorbance with a colorimeter); rate = 1 ÷ time. ✓
  • Controls: a tube in the dark (foil) and DCPIP without chloroplasts; keep temperature constant and repeat for a mean. ✓
05
Explain
[2 marks]

Explain why DCPIP changes colour when a chloroplast suspension is illuminated.

  • Light causes photoactivation of chlorophyll, releasing electrons in the light-dependent stage (photolysis supplies electrons too). ✓
  • DCPIP accepts these electrons in place of NADP and is reduced, changing from blue to colourless. ✓
06
Analysis
[3 marks]

A student counts bubbles from pondweed to investigate the effect of carbon dioxide concentration. Suggest three ways to improve the reliability and validity of the results.

  • Collect the gas in a capillary tube / photosynthometer and measure its volume rather than counting bubbles of different sizes. ✓
  • Keep the light intensity and temperature constant (fixed lamp distance, water bath or heat shield, LED lamp). ✓
  • Allow the plant to acclimatise at each concentration before measuring; repeat each reading at least three times and calculate a mean. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.