🧪 AP Biology Lab: Testing for Biological Molecules

Unit Coverage: 1.3 – 1.5 | Estimated Time: 60–90 minutes

Skill Focus: Experimental design, data collection, analysis, and application of macromolecule structure–function relationships.

👤 Student Information

🎯 Learning Objectives

By the end of this lab, students will be able to:

  • Identify the major types of biological macromolecules based on chemical tests.
  • Explain how the structure of biological molecules determines their function.
  • Design and interpret controlled experiments involving indicators.
  • Relate test results to real biological samples (e.g., food composition).

🧠 Background Information

Living organisms are composed of four main types of macromolecules:

  • Carbohydrates – energy storage and structure
  • Lipids – long-term energy storage and insulation
  • Proteins – enzymes, structure, transport, signaling
  • Nucleic Acids – store and transmit genetic information

Each has unique chemical properties that allow specific indicator tests to detect their presence.

🧩 Key Tests and Indicators

Molecule Test Reagent Positive Result Negative Result
Reducing sugars Benedict's Test Benedict's solution (CuSOâ‚„) Brick-red/orange precipitate after heating Blue solution
Starch Iodine Test Iodine-potassium iodide (IKI) Blue-black color Yellow-brown color
Protein Biuret Test Biuret reagent (NaOH + CuSOâ‚„) Violet color Light blue
Lipids Sudan III Test or Brown Paper Test Sudan III stain or paper Red-orange layer or translucent spot No color change
Nucleic acids (DNA) Dische Diphenylamine Test (optional extension) Diphenylamine reagent Blue color No color change

🔬 Materials

  • Test tubes and rack
  • Pipettes or droppers
  • Hot water bath (~80°C)
  • Benedict's solution
  • Iodine solution (IKI)
  • Biuret reagent
  • Sudan III solution or brown paper
  • Distilled water
  • Samples: glucose solution, starch solution, egg white, vegetable oil, banana extract, milk, DNA extract (optional)

⚙️ Procedure

Part A: Prepare and Label Samples

Label test tubes for each sample (water = control).

Add ~2 mL of each test substance to its respective test tube.

Part B: Conduct the Tests

  1. Benedict's Test (Reducing Sugars)
    • Add 10 drops of Benedict's reagent to each test tube.
    • Heat in a hot water bath for 3–5 minutes.
    • Record color change.
  2. Iodine Test (Starch)
    • Add 2 drops of iodine solution to each test tube.
    • Observe color change immediately.
  3. Biuret Test (Proteins)
    • Add 1 mL of Biuret reagent to each sample.
    • Mix gently and record the color change.
  4. Lipid Test (Sudan III or Brown Paper)
    • For Sudan III: Add 2–3 drops to each sample, shake gently.
    • For Brown Paper: Place a small drop of each sample on brown paper, allow to dry, observe translucency.
  5. (Optional) Diphenylamine Test (DNA)
    • Add 2 mL of diphenylamine reagent and heat for 10 minutes in water bath.
    • Record color change.

đź“Š Data Collection

Record your observations for each test and sample. For each test, select whether the result was positive or negative, and note any specific observations in the notes column.

Sample Benedict's Test (Reducing Sugars) Iodine Test (Starch) Biuret Test (Proteins) Sudan III / Paper Test (Lipids) Notes
Water (control)
Glucose
Starch
Egg white
Oil
Milk
Banana
DNA extract (opt.)

đź’¬ Analysis Questions