9.3Chapter 9: Plants and Photosynthesis

Evidence for photosynthesis: testing for starch — Alhussan Biology - Stage 9 Revision Notes & Topical Questions

5 min read 9 parts Lesson 3 of 9 2 quizzes

📚 Alhussan Biology - Stage 9 Hub / Topical Revision

0%read

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

Part 1 of 7Why test for starch

1Starch

Starch glucose made in photosynthesis is soon converted to starch, an insoluble store.

2Detection

Detection glucose is hard to detect in a leaf, but starch turns iodine solution from yellow-brown to blue-black.

3Meaning

Meaning if starch is present, the leaf has been photosynthesising.

4Why preparation is needed

Why preparation is needed leaves have a waxy, waterproof layer and tough cell walls, so iodine cannot reach starch in a living leaf.

Part 2 of 7Starch test method

Take a leaf from a plant that has been in the light for at least 12 hours.

Dip the leaf in boiling water for about 30 seconds to remove the waterproof layer and break open the cells.

Turn off the Bunsen burner, because ethanol is flammable.

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.

Remove the pale, stiff leaf and dip it in hot water to soften it.

Spread the leaf on a white tile and add drops of iodine solution.

A blue-black colour shows starch; yellow-brown means no starch.

Part 3 of 7Starting with a de-starched plant

Starch stores

Starch stores a leaf in the dark uses its starch for respiration, and it has enough to keep respiring for 2-3 days.

Preparation

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.

Part 4 of 7Light is needed

Method

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

Result the covered area stays yellow-brown and the uncovered area turns blue-black.

Part 5 of 7Chlorophyll is needed

Method

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

Result only the green areas, which contain chlorophyll, turn blue-black.

Part 6 of 7Carbon dioxide is needed

Method

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

Control test a leaf that was in normal air.

Result

Result the leaf without carbon dioxide has no starch.

Part 7 of 7Interpreting results and safety

Treatment	Result with iodine	Conclusion
Leaf in light	Blue-black	Starch made
Covered part of leaf	Yellow-brown	Light needed
White part of variegated leaf	Yellow-brown	Chlorophyll needed

Control

Control each experiment needs a leaf from the same plant treated identically except for the factor being tested.

Limitation

Limitation the test shows starch, which is a product of photosynthesis, and a plant can also use stored starch.

Safety

Safety wear eye protection, take care with hot water and hot ethanol, and keep ethanol away from flames.

Key terms

Term	Meaning
Starch	An insoluble carbohydrate stored in plants.
Iodine solution	A yellow-brown reagent that turns blue-black with starch.
De-starched	Kept in the dark long enough to use up stored starch.
Variegated leaf	A leaf with green and white areas.
Control	A treatment kept the same except for the factor being tested.
Ethanol	A flammable solvent used to remove chlorophyll from a leaf.