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BIOLOGY · Stage 9 · Chapter 9: Plants and Photosynthesis

Biology 9.3 Evidence for photosynthesis: testing for starch

5 min read 7 parts Lesson 3 of 9

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Lesson summary

9.3 Evidence for photosynthesis: testing for starch — Alhussan International Academy — Biology (Stages 7–9) notes: Describe how to test a leaf for starch and use the test to show that light, chlorophyll and carbon dioxide are needed for photosynthesis.Starch — glucose made in pho…

  • Why preparation is needed
  • Starch stores
  • Limitation
  • Iodine solution
  • De-starched
  • Variegated leaf

Section 1 of 7Why test for starch

  • Starch
    • glucose made in photosynthesis is soon converted to starch, an insoluble store.
  • Detection
    • glucose is hard to detect in a leaf, but starch turns iodine solution from yellow-brown to blue-black.
  • Meaning
    • if starch is present, the leaf has been photosynthesising.
  • Why preparation is needed
    • leaves have a waxy, waterproof layer and tough cell walls, so iodine cannot reach starch in a living leaf.

Section 2 of 7Starch test method

  1. Take a leaf from a plant that has been in the light for at least 12 hours.
  2. Dip the leaf in boiling water for about 30 seconds to remove the waterproof layer and break open the cells.
  3. Turn off the Bunsen burner, because ethanol is flammable.
  4. Place the leaf in a boiling tube of ethanol and stand it in a beaker of hot water, so the green colour comes out of the leaf.
  5. Remove the pale, stiff leaf and dip it in hot water to soften it.
  6. Spread the leaf on a white tile and add drops of iodine solution.
  7. A blue-black colour shows starch; yellow-brown means no starch.

Section 3 of 7Starting with a de-starched plant

  • Starch stores
    • a leaf in the dark uses its starch for respiration, and it has enough to keep respiring for 2-3 days.
  • Preparation
    • always start with a plant kept in the dark for at least 3 days so that no old starch remains and false results are avoided.

Section 4 of 7Light is needed

  • Method
    • cover part of a leaf of a de-starched plant with silver foil that blocks light, then leave the plant in the light for 24 hours.
  • Result
    • the covered area stays yellow-brown and the uncovered area turns blue-black.

Section 5 of 7Chlorophyll is needed

  • Method
    • use a plant with two-coloured (variegated) leaves, such as geranium or ivy, kept in the dark for 3 days and then in the light for 24 hours.
  • Result
    • only the green areas, which contain chlorophyll, turn blue-black.

Section 6 of 7Carbon dioxide is needed

  • Method
    • place a bag containing a chemical that absorbs carbon dioxide, such as soda lime, around one leaf and leave the plant in the light for 24 hours.
  • Control
    • test a leaf that was in normal air.
  • Result
    • the leaf without carbon dioxide has no starch.

Section 7 of 7Interpreting results and safety

TreatmentResult with iodineConclusion
Leaf in lightBlue-blackStarch made
Covered part of leafYellow-brownLight needed
White part of variegated leafYellow-brownChlorophyll needed
  • Control
    • each experiment needs a leaf from the same plant treated identically except for the factor being tested.
  • Limitation
    • the test shows starch, which is a product of photosynthesis, and a plant can also use stored starch.
  • Safety
    • wear eye protection, take care with hot water and hot ethanol, and keep ethanol away from flames.

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