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Biology · 4. Photosynthesis and respiration

Energy flow in living cells

Connect photosynthesis, respiration and limiting factors through controlled investigation.

40 minutesContent version: 2.1
Start the interactive pathwayOpen the virtual experiment

Driving question

Which factor limits photosynthesis, and how can evidence show it?

Curriculum coverage

Vietnam Biology 7, 10 and 11; Cambridge IGCSE/AS bioenergetics.

Model scope and limitations

Bubble counting only approximates O₂ production because bubble sizes vary and gas may dissolve.

Learning objectives

  • Write word and balanced equations for photosynthesis and aerobic respiration.
  • Explain limiting factors.
  • Design a rate measurement with controls.

Prerequisite knowledge

  • Chloroplasts, mitochondria, enzymes and gas exchange.

Core knowledge

Photosynthesis

Light energy becomes chemical energy in organic molecules; released O₂ comes from water.

Respiration

Cellular respiration releases free energy for ATP synthesis; it is not simply breathing.

Limiting factor

Increasing a factor raises rate only when that factor is limiting.

Worked example

If rate is 20 bubbles/min at 40 cm and 45 at 20 cm, it is consistent with stronger light nearer the source; it does not prove an inverse-square law because bubble volume was not measured.

Misconceptions to avoid

  • Plants respire day and night; net photosynthesis is positive only when photosynthesis exceeds respiration.
  • Unlimited light does not cause unlimited rate; another factor limits or photoinhibition occurs.

Virtual experiment procedure: Rate of photosynthesis

  1. Choose one independent variable: light distance or CO₂ concentration.
  2. Control pondweed species/length, temperature and timing; repeat at least three times.
  3. Calculate mean rate, graph it and identify any plateau.

Safety and cautions

  • Lamps can heat and water near electricity is hazardous; use low voltage, a heat shield and dry spills.

Evidence to collect

  • A table with independent variable, three repeats, mean and a conclusion noting bubble-count limits.
Open the virtual experiment

Diagnostic check

1. Which gas is produced net in photosynthesis?

  1. O₂
  2. CO₂
  3. N₂
Show answer and explanation

Correct answer: O₂

Photosynthesis releases oxygen when water is oxidised.

2. Why place a water bath between lamp and pondweed?

  1. Reduce temperature change
  2. Make more CO₂
  3. Block all light
Show answer and explanation

Correct answer: Reduce temperature change

The bath absorbs heat, helping isolate light intensity.

Application practice

1. Balanced photosynthesis uses how many CO₂ per glucose?

  1. 6
  2. 1
  3. 12
Show answer and explanation

Correct answer: 6

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

2. Rate no longer rises as light increases. Best inference?

  1. Light is no longer limiting
  2. Photosynthesis has certainly stopped
  3. No enzymes exist
Show answer and explanation

Correct answer: Light is no longer limiting

Another factor such as CO₂ or temperature may now limit.

3. A better O₂ measure than bubble count?

  1. Gas volume per time
  2. Bench colour
  3. Cable length
Show answer and explanation

Correct answer: Gas volume per time

Gas volume reduces bias from unequal bubble sizes.

Academic references

  1. OpenStax Biology 2e — peer-reviewed foundational biology

This lesson is maintained against the cited sources below. Simulations are learning models and do not replace supervised physical-laboratory safety procedures.

Content publisher
WebLab STEM
Technical maintainer
Đức Tiến — Control & Automation Engineer