Destarching, the leaf starch test, testing whether light, chlorophyll and CO₂ are needed, and showing respiration with limewater and yeast.
4 narrated scenes. Press play, listen and read along — the points appear as they are explained. Use ‹ Prev / Next › or the scene bar to jump to any scene.
To prove a plant needs carbon dioxide you must remove it — and keep everything else the same. That’s experimental design.
Narration uses your device’s built-in voice. No sound? Switch to “Captions only” — the captions advance on their own.
Copy these into your notebook in your own words — that is how they stick.
Plant in darkness 24–48 h so starch is used up.
Boil in water → ethanol in water bath (Bunsen off) → rinse → iodine (blue-black = starch).
Foil-covered leaf part stays brown.
Variegated leaf: only green parts make starch.
KOH (or soda lime) absorbs CO₂; control uses water.
Limewater turns milky with CO₂; boiled seeds as control; thermos shows heat.
The kind of support a teacher gives after the lesson: worked examples to attempt first, a board illustration, key terms, common misconceptions and memory aids.
Try it in your notebook first, then reveal the solution one step at a time.
Try it in your notebook first, then reveal the solution one step at a time.
| Step | Reason |
|---|---|
| Boil the leaf in water for about 1 minute | Kills the cells, stops reactions, makes the cells permeable to iodine |
| Turn off the Bunsen; put the leaf in ethanol in a hot water bath | Removes chlorophyll so colour changes can be seen (ethanol is flammable) |
| Dip in warm water | Softens the leaf, which ethanol made brittle |
| Spread on a white tile; add iodine | Blue-black = starch present; yellow-brown = no starch |
Learn the reason with each step; most marks are for the reasons.
Leaf starch test: Boil, Bleach (ethanol), Bathe (warm water), Brown-or-Black (iodine).
Greenhouse growers sometimes raise CO₂ levels and add lighting because they know light, CO₂ and chlorophyll are all needed for the plant to make food.
Cover part of a leaf on a potted plant with foil for two days. Ask your teacher to help you test it for starch and compare the covered and uncovered parts.
3 quick games on this lesson. Earn XP and badges as you play — your best score for each game is saved. Games are for practice and do not change your exam readiness.
6 items modelled on real CSEC Paper 1 questions. Choose one answer for each, then check. Aim for 80% or better.
Write full answers, then reveal the mark scheme and tick the points you earned (8 marks in total). Your answers are saved on this device.
Question 1 [8 marks]
A student wants to show that carbon dioxide is needed for photosynthesis.
(a) Describe the set-up, including the control. [4]
(b) Describe how a leaf is tested for starch and give the expected results. [4]
Everything in this lesson is based on these parts of the CXC syllabus and papers.