Use covered-leaf evidence for the light requirement
Plan a matched comparison with verified initial starch depletion.
Section B6.1 · Photosynthesis
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall chloroplasts, enzymes and the difference between photosynthesis and respiration.
- Use the stated observations, quantities and controls; diagrams are schematic.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why should the plant be checked for starch before treatment?
What you will learn
- Plan a matched comparison with verified initial starch depletion.
- Use the stated evidence to explain the method and its limits.
Keep a useful internal comparison
Start with a suitable plant whose destarching has been checked. Cover part of a leaf on both sides with an opaque barrier, leaving a comparable green part exposed. Give the plant suitable light while keeping water, temperature and carbon dioxide conditions appropriate.
Record the covered and exposed pattern before removing the cover. After the teacher-supervised leaf starch preparation, compare the iodine observations with that pattern. The exposed part provides an internal comparison under much of the same leaf environment.
Interpret and improve the design
If starch is detected in the exposed region but not in the properly covered region, the result supports a light requirement for new starch formation under these controlled conditions. Verify working iodine controls and initial depletion rather than assuming every negative means the same thing.
A cover can also alter local conditions, including gas exchange or temperature. Fit an opaque light barrier carefully, use repeated leaves and comparable handling, and discuss these limitations. A small gap allowing light beneath a loose cover weakens the intended no-light treatment.
Sources for this lesson
Pause and explain
What does a loose cover that lets light in do?
Worked example
An initially starch-negative leaf has an opaque strip. After exposure, the uncovered green areas test blue-black and the covered area remains yellow-brown. Interpret.
Show the worked solution
- Relate each observation to its recorded light treatment.
- Use the initial negative and working test controls.
- State that new starch formation is supported in the lit region.
Answer The controlled comparison supports light being required for new starch formation; the covered region had no detected starch.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Name the dependent observation in the covered-leaf investigation.
Give me a hint
State what is read after treatment.
Compare my reasoning
- Prepare the leaf under supervision.
- Observe iodine colour in covered and exposed regions.
Starch detection, shown by the iodine colour in each recorded region.
Look for these in your work
- Use the starch test.
- Distinguish observation from light treatment.
Both covered and uncovered areas turn blue-black, but initial destarching was not checked. Give one supported reason the light-requirement conclusion is uncertain.
Give me a hint
Think of a store existing before exposure.
Compare my reasoning
- Initial starch may remain.
- The final positive does not date its formation.
Prior starch could remain, so the observations do not cleanly isolate new light-dependent formation.
Look for these in your work
- Identify the missing initial check.
- Avoid claiming the requirement is disproved.
Suggest two improvements if covered regions repeatedly show weak positives.
Give me a hint
Check the barrier and the test starting condition.
Compare my reasoning
- Verify an opaque cover on both sides with no gaps.
- Check initial starch depletion and working iodine controls.
- Repeat matched leaves and consider altered local conditions.
Improve light exclusion and verify the initial starch-negative condition, with working controls and repeats.
Look for these in your work
- Name two relevant improvements.
- Connect them to plausible alternative causes.
Common mistakes
- Cover both sides and check for light leakage.
- Retain possible cover-related confounders.
What can you explain now?
What does a loose cover that lets light in do?
Compare with the explanation
It weakens the intended no-light comparison.
The treatment no longer reliably excludes light.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain how the exposed region helps interpret the covered region.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.