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8.2.2Unit 3: Practical Skills in Biology I

Core Practical 18: Amylase in Germinating Cereal Grains — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 6 parts Lesson 27 of 27 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 6The Biology Behind the Investigation

A barley grain contains the tissues needed to see how gibberellin controls enzyme production.

Embryo
  • Embryo This is the part that grows into the new plant and normally produces gibberellin during germination.
Endosperm
  • Endosperm A starch store surrounding the embryo, which amylase breaks down into maltose and then glucose.
Aleurone layer
  • Aleurone layer A protein-rich layer on the outside of the endosperm where gibberellin stimulates amylase synthesis.
Why remove the embryo
  • Why remove the embryo Without the embryo no gibberellin is made by the grain itself, so the investigator controls the concentration.

Part 2 of 6Method

The steps below follow the starch agar assay.

1
Prepare gibberellic acid solutions of different concentrations, with distilled water as the zero concentration.
2
Remove husks, cut each grain across, discard the embryo half, then sterilise the other halves in sodium hypochlorite for 5 minutes and wash them well.
3
Soak groups of halves in each solution for 12 to 48 hours, loosely covered so that oxygen can enter.
4
Place the halves cut-side down on starch agar plates, label each plate and lightly tape the lids on.
5
After 24 to 48 hours, flood each plate with iodine solution and pour off the excess.
6
Measure the diameter of the clear zone around each grain half and record it in a table.

Part 3 of 6Interpreting the Results

Interpreting the Results
Blue-black agar
  • Blue-black agar Iodine turns starch blue-black, so this colour shows that starch is still present.
Clear zone
  • Clear zone Where amylase has hydrolysed the starch, no blue-black colour forms around the grain.
Larger zone
  • Larger zone A bigger diameter means more amylase was released, showing the effect of gibberellin.
Averages
  • Averages Several grains per concentration allow a mean to be calculated and reduce the effect of anomalies.

Part 4 of 6Variables, Controls and Safety

Independent variable
  • Independent variable The concentration of gibberellic acid solution applied to the grain halves.
Dependent variable
  • Dependent variable The diameter of the clear zone on the starch agar.
Controlled variables
  • Controlled variables Use the same grain type and size, soaking time, incubation time and temperature, and volume of iodine.
Control
  • Control A group soaked in distilled water shows how much amylase is made without added gibberellin.
Safety
  • Safety Wear eye protection, take care with the scalpel, and dispose of iodine and hypochlorite waste properly.
ObservationConclusion
Large clear zoneHigh amylase activity
No clear zoneLittle or no amylase released

Part 5 of 6Worked Example

Question

  • Make 10 cm³ of 0.1 g dm⁻³ gibberellic acid from a 1 g dm⁻³ stock.
  • Clear zones for one concentration were 8 mm, 10 mm, 9 mm and 11 mm.
  • Find the volumes needed and the mean diameter.

  1. DilutionC₁V₁ = C₂V₂, so V₁ = (0.1 × 10) / 1 = 1 cm³ of stock.
  2. Water needed = 10 - 1 = 9 cm³.
  3. Mean diameter = (8 + 10 + 9 + 11) / 4 = 38 / 4 = 9.5 mm.

Answer

  • Use 1 cm³ of stock with 9 cm³ of distilled water;
    • the mean clear zone diameter is 9.5 mm.

Part 6 of 6Exam Checklist & Tips

8.2.2 – Key Points for the Exam

  • Starch agar A gel containing starch used to detect the presence of amylase.
  • Clear zone The area around a grain where starch has been digested and iodine gives no colour.
  • Gibberellin A plant growth factor that increases amylase production in the aleurone layer.
  • Aleurone layer The outer layer of the endosperm where amylase is made.

Common Mistakes

  • Leaving the embryo on the grain, so the grain makes its own gibberellin and the results cannot be compared.
  • Measuring the width of the blue-black region; the measurement is of the clear zone around the grain.
  • Not sterilising the grains, so that microorganisms make enzymes that affect the starch agar.
  • Forgetting a distilled water control, which is needed to compare with the gibberellin treatments.

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