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Pearson CP6.5 uses algal balls or similar at varying distances from a light source. Keep ball number, size and condition, medium and measurement interval comparable. Use the centre’s approved gas-exchange indicator or measurement method and establish what its signal means before calculating a rate.
Video by Science and Plants for Schools. The SAPS demonstration develops algal-ball gas-exchange methods. Apply Pearson CP6.5’s light-distance comparison and this lesson’s controls and signal limits.
Viewing focus: Control an algal-ball light-distance investigation
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Why must an algal-ball indicator result be interpreted as a net signal?
Photosynthesis and respiration both affect carbon dioxide in opposite directions. A colour response does not isolate one process without additional evidence.
After watching 2: Which conditions must be comparable before using reciprocal endpoint time?
Use a defined shared threshold with comparable initial indicator state, medium, organism amount and temperature, along with the intended light comparison.
What you'll be able to do
Pearson CP6.5 uses algal balls or similar at varying distances from a light source. Keep ball number, size and condition, medium and measurement interval comparable. Use the centre’s approved gas-exchange indicator or measurement method and establish what its signal means before calculating a rate.
Hydrogencarbonate-indicator methods can show changes associated with dissolved carbon dioxide. Photosynthesis removes carbon dioxide while respiration produces it, so the result reflects their balance under the conditions. Do not call an indicator colour a direct count of oxygen bubbles or an exact sugar-production measurement.
Vary lamp distance while controlling temperature and other relevant conditions. Use appropriate adjustment time, independent repeats and a suitable no-algae or medium reference for the chosen method. The initial indicator state, medium volume and ball amount can affect a fixed-colour endpoint.
Record actual colour changes, quantitative readings or endpoint times according to the approved method. Colour judgement can be subjective; a fixed endpoint is comparable only when starting conditions and the threshold are defined. A small change may reflect photosynthesis and respiration balancing rather than no biological activity.
For comparable initial conditions and the same small endpoint change, reciprocal time can be used as a relative signal-rate proxy. It is not automatically an amount-of-carbon-dioxide rate. pH-related indicators also need interpretation because pH is not a linear concentration scale.
Relate the signal to distance from the source as the specification requires. An inverse-square intensity model may be useful for a suitable source but must not be assumed exact for every lamp arrangement. At high light another factor can limit net photosynthesis, so explain plateaus using the system and controls.
Attempt these comparisons before opening the reasoning.
Common mistakes to avoid
Key takeaways
Question 1: An indicator shows no net change with living algal balls. Why is “no respiration” not justified?
No detected net change does not establish absence of either process.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.