Investigating Photosynthesis — Leaf Discs

Watch first

FreeScienceLessons · AQA BiologyPhotosynthesis Investigations

Key facts

The leaf disc method

Cut small discs from a leaf using a cork borer.
Remove air from discs using a syringe (with dilute sodium hydrogen carbonate solution) — discs sink.
Place sinking discs in different concentrations of NaHCO₃ solution (provides different CO₂ levels).
Shine a light — as photosynthesis occurs, O₂ produced fills the air spaces in the discs.
Discs float as they fill with oxygen. Measure time for all discs to float.
Faster floating = faster rate of photosynthesis.
Variable being tested: usually light intensity or CO₂ concentration.

Evaluation — common exam questions

Why must the discs be used immediately after syringe treatment? — Air spaces refill with oxygen if left.
Why use sodium hydrogen carbonate and not plain water? — Provides CO₂ for photosynthesis.
Control variable: same species of leaf, same size of disc, same temperature, same number of discs.
Reliability: repeat each condition, calculate mean time for discs to float.
Validity: only change one variable at a time.

Exam questions — 3 questions · 9 marks

1 markWhy discs sink initiallyAQA 8461 P1 RPA style

Why are the leaf discs placed in a syringe before the experiment begins?

A To cut the discs into a uniform size

B To remove the air from the air spaces in the leaf so the discs sink — this ensures they will only float when oxygen from photosynthesis fills the spaces

C To add the sodium hydrogen carbonate into the leaf tissue

D To sterilise the discs before use

4 marksEvaluate the leaf disc methodAQA 8461 P1 RPA style

A student uses the leaf disc method to investigate how light intensity affects the rate of photosynthesis. Give two control variables and explain why each must be controlled. (4 marks)

Hint: Give two DIFFERENT control variables. For each: state the variable → explain what would happen if it were not controlled (what other factor would it affect).

+40 XP

1 markWhy sodium hydrogen carbonateAQA 8461 P1 style

Why is sodium hydrogen carbonate solution used rather than plain water in the leaf disc experiment?

A It prevents the leaf discs from being damaged by osmosis

B It provides a source of carbon dioxide for photosynthesis — plain water would have no CO₂ for the leaf to use

C It prevents bacteria from contaminating the experiment

D It makes the leaf discs denser so they sink faster

Investigating Photosynthesis — Leaf Discs

The leaf disc method

Evaluation — common exam questions

Module complete! 🎉