Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

3.3.2Unit 3 — Life on Earth

Practical: Factors Affecting Starch Production

5 min read 5 parts Lesson 16 of 31

0%read

Part 1 of 5Why Test a Leaf for Starch?

3.3.2 Practical: Factors Affecting Starch Production
Glucose to starch
  • Glucose made in photosynthesis is quickly changed into starch for storage in the leaf.
Iodine test
  • Iodine solution turns blue-black when starch is present, and stays orange-brown when starch is absent.
Indicator of photosynthesis
  • A leaf containing starch must have been photosynthesising, so the test shows where photosynthesis took place.
Destarching
  • Keeping a plant in darkness for 24 hours uses up any starch already stored, so the result starts from zero.

Part 2 of 5Method for Testing a Leaf for Starch

1
Dip the leaf in boiling water to kill it and soften the tissue.
2
Heat the leaf in ethanol in a water bath until the green colour is removed, with all flames out because ethanol is flammable.
3
Rinse the pale leaf in hot water to remove ethanol and soften it again.
4
Spread the leaf on a white tile and cover it evenly with iodine solution.
5
Record the colour: blue-black means starch is present and orange-brown means no starch.

Part 3 of 5Showing What Photosynthesis Needs

Showing What Photosynthesis Needs
Chlorophyll
  • A variegated leaf has green and white areas, and only the green areas turn blue-black with iodine.
Light
  • A destarched plant has one leaf covered with foil, and only the uncovered leaf from the light shows starch.
Carbon dioxide
  • One plant sits under a bell jar with sodium hydroxide that absorbs carbon dioxide, and its leaf stays orange-brown.
Control
  • A second plant with a beaker of water in its sealed jar shows starch, proving the missing carbon dioxide was the cause.

Part 4 of 5Improving Reliability

Not destarched
  • Leaving the plant in full darkness for at least 24 hours makes sure old starch has gone.
Leaf still green
  • Keep the leaf in hot ethanol until it is pale, so the colour change is easy to see.
Uneven iodine
  • Cover the whole leaf evenly so that a patchy colour does not hide the pattern.
Foil or jar leaks
  • Press the foil flat and seal the bell jar base with petroleum jelly so light or air cannot get in.

Part 5 of 5Exam Checklist & Tips

3.3.2 – Key Points for the Exam

  • Starch A storage carbohydrate made from glucose, which turns iodine blue-black.
  • Variegated leaf A leaf with both green areas containing chlorophyll and white areas without it.
  • Destarch To use up the starch in a plant by keeping it in the dark.
  • Control A set-up that differs only in the factor being tested, used for comparison.
  • Sodium hydroxide A solution that absorbs carbon dioxide from the air in the bell jar.

Common Mistakes

  • Heating ethanol over a naked flame; it must be heated in a hot water bath because it is flammable.
  • Writing that iodine shows glucose; iodine tests for starch only.
  • Forgetting to destarch the plant, which makes the starch result unreliable.
  • Leaving out the control, so the missing carbon dioxide cannot be shown to be the cause.
\n