Demonstration of the digestion of starch during germination by a seed (LC 2026) (Leaving Cert Biology): Revision Notes

Digestion of starch during seed germination

Background:

Embryos in seeds use the enzyme amylase to digest starch for respiration during germination. This experiment uses nutrient agar with added starch, a medium that provides all the nutrients for growth.

Materials needed:

• Broad bean seeds
• Water
• Disinfectant spray/wipes
• Boiling apparatus
• Methylated spirits/disinfectant
• Flamed forceps
• Sterile blade
• Starch agar plates
• Incubator set to 27°C
• Iodine solution

Method:

1. Soak two broad bean seeds overnight in water. This softens the testa coat and starts germination by activating enzymes.
2. Disinfect work surfaces with disinfectant spray or wipes.
3. Boil one of the soaked seeds. Boiling the seed will denature the enzymes and stop the germination process. This seed will act as a control.
4. Sterilise both seeds by dipping them in methylated spirits or disinfectant using flamed forceps. This prevents any contamination by microorganisms.
5. Cut both seeds lengthwise with a sterile blade and place them cut side down on separate starch agar plates. Keep the agar plate lid open for the shortest time and at an angle to avoid contamination.
6. Seal the plates and incubate at 27°C for 48 hours. This is the optimal temperature for plant enzymes.
7. After 48 hours, examine the plates.
8. Flood both plates with iodine to test for starch and record the results.
9. Dispose of plates by soaking them in disinfectant.

Results:

	Dish A (experiment)	Dish B (control)
Appearance	Clear ring in starch agar around bean.	No change in the appearance of the starch agar.
Addition of iodine	Stays brown. No starch present as it has been digested by the seed during germination.	Turns from brown to blue/black as no starch has been digested by the dead seed (boiled seed).

Explanation of results:

When a seed absorbs water from the starch agar, it starts digesting the starch for nourishment. After 24 hours, a clear ring around the bean indicates where the starch was used.

To confirm starch digestion, test with iodine. The area around the bean will remain clear, while the rest of the plate turns blue/black, showing that starch has been digested.

The control seed, which has been boiled, is placed on the agar similarly. No clear ring appears, and the entire plate turns blue/black with iodine, confirming no starch digestion.