Interpret a supervised leaf starch test
Separate starch detection from destarching and rate measurement.
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
Which iodine observation is positive for starch?
What you will learn
- Separate starch detection from destarching and rate measurement.
- Use the stated evidence to explain the method and its limits.
Prepare an observation that can be seen
A leaf starch test uses iodine solution: blue-black indicates starch detection, while a yellow-brown result is negative under the test conditions. The leaf’s green pigment can obscure the observation, so the supervised preparation removes chlorophyll before iodine is applied.
The teacher-approved sequence uses brief hot-water treatment, ethanol in a hot-water bath to remove pigment, rinsing to soften the leaf and then iodine. Ethanol is flammable: keep it away from naked flames and use the approved indirect heating arrangement. Use eye protection and teacher supervision for chemicals, heat and handling.
Distinguish preparation and evidence
Destarching happens before the photosynthesis investigation, by keeping a suitable plant in darkness long enough for stored starch to be used, then checking an initial leaf sample. Boiling the test leaf is not the same as destarching the living plant.
Working iodine controls with known starch and a starch-free comparison help confirm the observation. New starch in an initially destarched, controlled leaf supports carbohydrate formation, but a final starch test does not directly give an instantaneous photosynthetic rate or establish supervised practical competence.
Sources for this lesson
Pause and explain
Why remove chlorophyll before reading the iodine test?
Worked example
An initially starch-negative plant is treated in light. Its prepared leaf turns blue-black with iodine; known-starch and negative controls work. State the supported observation.
Show the worked solution
- Identify blue-black as starch detection.
- Use the initial negative and working controls.
- Do not turn a presence test into a measured rate.
Answer Starch was detected after the treatment; the evidence does not give an exact instantaneous photosynthesis rate.
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.
A prepared leaf remains yellow-brown with iodine while known starch turns blue-black. State the negative observation carefully.
Give me a hint
Report the detection result.
Compare my reasoning
- The positive comparison works.
- No blue-black develops in the tested leaf.
- Limit the conclusion to the test conditions.
Starch was not detected in the tested leaf under these conditions.
Look for these in your work
- Use not detected.
- Do not claim all carbohydrate is absent.
Put the supervised preparation in order: iodine, hot water, rinse, ethanol in a hot-water bath.
Give me a hint
Prepare and decolourise before applying the indicator.
Compare my reasoning
- Treat the leaf with hot water.
- Remove pigment using ethanol in an approved hot-water bath.
- Rinse and then apply iodine.
Hot water → ethanol in a hot-water bath → rinse → iodine.
Look for these in your work
- Correct order.
- Retain indirect heating and supervision.
A student skips the initial destarching check and later finds starch in a covered leaf. Why is “photosynthesis happened without light” not established?
Give me a hint
The starch may already have been present.
Compare my reasoning
- Prior starch has not been excluded.
- A final test cannot identify when it formed.
- Verify initial starch depletion and working controls.
Stored starch may predate the treatment; check destarching before interpreting new starch formation.
Look for these in your work
- Name the prior-store alternative.
- Specify the missing initial check.
Common mistakes
- Hot-water preparation is not destarching.
- A written plan does not certify safe laboratory skill.
What can you explain now?
Why remove chlorophyll before reading the iodine test?
Compare with the explanation
To make the starch colour observation easier to see.
The green pigment can obscure the endpoint.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain the preparation, positive observation and limits of a leaf starch test.
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.