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Biology · 6.1 Photosynthesis · Paper 5/6 practical

Rate of Photosynthesis. Count the bubbles.

Pondweed gives off oxygen bubbles as it photosynthesises. Move the lamp closer to raise the light intensity and count the bubbles per minute. The rate rises with light — until carbon dioxide or temperature becomes the limiting factor.

0610 Topic 6.1 — Photosynthesis light intensity · limiting factors Paper 5/6 — Practical
Setup — set the lamp distance, then count bubbles for one minute.
0 s
bubbles 0

Variables

20
100%
25

Live readouts

Light intensity ∝ 1/d²
—
Bubbles / min
—
Limiting factor
—
Light intensity follows the inverse-square law: halve the distance and the intensity goes up four times.

Results table

Record the rate at several lamp distances.

Rate vs light intensity — plateau = limiting factor

📋 Method (Cambridge practical procedure)
  1. Put a piece of pondweed (e.g. Elodea) cut-end-up in water containing sodium hydrogencarbonate (a CO₂ source).
  2. Place a lamp a measured distance away; allow a few minutes to settle.
  3. Count the oxygen bubbles released per minute (or collect and measure the gas volume).
  4. Repeat at several distances; calculate light intensity as proportional to 1/d².
  5. Plot rate against light intensity. It rises, then plateaus when another factor (CO₂ or temperature) becomes limiting.
⚠ Control variables & precautions
  • Keep temperature constant (a heat-absorbing water screen stops the lamp warming the water) and CO₂ supply the same when light is the variable.
  • Use the same piece of pondweed; let it acclimatise at each distance before counting.
  • Count for the same time and repeat for a mean.
  • The lamp gives off heat as well as light — control this so only light intensity changes.
🎯 Syllabus reference (0610)
  • 6.1 Photosynthesis — investigate the effect of light intensity, carbon dioxide and temperature on the rate; identify and explain limiting factors.

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