Lecture Notes: Biochemical Tests in A Level Biology

Introduction

• Focus on biochemical tests related to biological molecules for A Level Biology.
• Covers tests for carbohydrates, proteins, and lipids.
• Includes a question to pause and think about during the lecture.

Carbohydrates

• Types of Carbohydrates:
  • Polysaccharides: Test for starch
  • Reducing sugars: Test you've learned at GCSE
  • Non-reducing sugars: New test for sucrose

Test for Starch

• Reagent: Iodine
• Procedure:
  • Add iodine to the sample.
  • Observes color change from brown-orange to blue-black if starch is present.
• Positive Result: Blue-black color

Test for Reducing Sugars

• Example: Glucose
• Reagent: Benedict’s solution (bright blue)
• Procedure:
  • Add Benedict's reagent to the sample.
  • Heat the sample.
  • Observe color change.
• Positive Result: Color change from blue to orange/brick red
  • Green/yellow indicates low concentration; orange/red indicates higher concentration.
• Physics Link: Convection currents cause reaction to start at the solution top due to kinetic energy and particle movement.

Test for Non-Reducing Sugars

• Example: Sucrose
• Initial Step: Confirm it is not a reducing sugar using Benedict’s test (negative result remains blue).
• Procedure:
  • Add acid for hydrolysis (boiling required).
  • Cool, neutralize, heat again with Benedict’s.
  • Resulting color change confirms reducing sugars after hydrolysis.
• Summary:
  • Reducing sugars include glucose, fructose, galactose, lactose, maltose.
  • Non-reducing sugar: Sucrose

Proteins

• Reagent: Biuret’s solution (bright blue)
• Procedure: Add Biuret to protein sample (e.g., albumin).
• Positive Result: Purple/lilac color change
• Note: Correct pronunciation is "Biuret," not "burette."

Lipids

• Example: Oil
• Procedure:
  • Dissolve lipid in ethanol.
  • Add distilled water.
  • Shake and observe.
• Positive Result: White emulsion (no precipitate formed)

Summary of Tests

• Starch: Iodine test (positive = blue-black)
• Reducing sugars: Benedict’s test (positive = green/yellow/orange/red)
• Non-reducing sugars (sucrose): Initial negative Benedict’s, then acid hydrolysis and re-test.
• Proteins: Biuret test (positive = purple/lilac)
• Lipids: Emulsion test (positive = white emulsion)

Conclusion

• Know the reagent, conditions, and color change for each test.
• Positive test results summary:
  • Lipids: White emulsion
  • Proteins: Purple/lilac
  • Starch: Blue-black
  • Reducing sugars: Green to brick red, dependent on concentration

Additional Note

• Always include chemical, reagent, conditions, and results in exam answers.