Biology Practical Concepts

Overview

This lecture reviews key concepts and procedures for the International AEV Biology Unit 3 Alternative to Practical Paper, focusing on core practicals, essential experimental variables, laboratory techniques, and exam strategies.

Experimental Variables & Validity

• The independent variable is changed by the experimenter; the dependent variable is measured as it is affected.
• Controlled variables are factors kept constant to maintain validity.
• Control experiments remove the independent variable to confirm it causes the observed effect.
• Accuracy improves with precise instruments; validity by controlling variables; reliability by repeating experiments and identifying anomalies.
• Reliability is assessed by error bars and standard deviation; repetition increases reliability.

Core Practical 1: Food Tests

• Starch is tested with iodine (brown/yellow to blue-black).
• Reducing sugars are tested with Benedict’s reagent (blue to brick red after heating).
• Protein test uses Biuret reagent (blue to lilac).
• Semi-quantitative tests compare color to standards; quantitative tests use colorimeters and calibration curves.
• Controlled variables in comparisons: same mass/volume of food, same temperature, same filter size.

Core Practical 2: Vitamin C Content

• DCPIP is reduced and decolorized by vitamin C; color change is used to calculate vitamin C content.
• Ensure validity by controlling mass, extraction methods, and DCPIP volume.
• Add extract dropwise, record volume needed to decolorize.

Core Practical 3: Cell Membrane Permeability

• Use beetroot cylinders; test effect of temperature or alcohol on membrane permeability via pigment leakage.
• Use colorimeter to measure absorbance (darker = more membrane damage).
• Control size, temperature, time, solution volume, and pH.
• Wash beetroot to remove excess pigment before testing.

Osmosis & Dilutions

• Water moves into cells by osmosis down the water potential gradient.
• Prepare serial dilutions using C₁V₁ = C₂V₂.
• Cut plant tissues to equal size, measure before/after mass, blot dry, and repeat for each solution.
• Plot graphs to analyze results; discuss equilibrium and water potential changes.

Core Practical 4: Enzyme Activity

• Rate of reaction measures speed at which reactants become products.
• Use a colorimeter or gas syringe to measure reaction rate.
• Control temperature, pH, enzyme/substrate concentration.
• Measure initial rate for accuracy as substrate is depleted over time.

Core Practical 5: Light Microscopy

• Light microscopes have limited resolution and magnification.
• Calibration uses eyepiece graticule and stage micrometer.
• Prepare slides: collect cells, stain, cover slip, and observe under low power first.
• Staining adds contrast for clearer observation; avoid using excess stain.

Core Practical 6: Mitotic Index

• Mitotic index = (number of cells in mitosis)/(total number of cells).
• Use root tips, treat with acid, stain, squash, observe under microscope.
• Count cells in different mitosis stages to represent phase duration.
• Safety: handle scalpels, glass, and acid with care.

Core Practical 7: Plant Structure Observation

• Thin sections, stain, slide preparation, and low-power drawing.
• Label root and stem structures; staining helps differentiate tissues.
• Use micrometer and graticule for size comparison.

Core Practical 8: Tensile Strength of Fibres

• Tensile strength is the force before a fiber breaks.
• Control fiber length, humidity, cross-sectional area, and type.
• Gradually add weights, note breaking force, and repeat for reliability.

Core Practical 9: Antimicrobial Properties

• Prepare plant extracts and inoculate agar with bacteria.
• Diffuse extracts using wells or discs, observe inhibition zones.
• Control variables: disc size, extract volume, incubation time.
• Use aseptic techniques: sterile equipment, minimal exposure, partial covering of Petri dish.

Key Terms & Definitions

• Independent variable — factor deliberately changed in an experiment.
• Dependent variable — factor measured as an outcome.
• Controlled variable — condition kept constant.
• Accuracy — closeness to the true value.
• Validity — correctness; measures what it’s intended to.
• Reliability — consistency; results reproducible on repetition.
• Semi-quantitative — approximate measurement via comparison.
• Colorimeter — device for measuring color intensity for quantitative analysis.
• Mitotic index — proportion of actively dividing cells.
• Aseptic technique — methods to prevent contamination.

Action Items / Next Steps

• Practice past paper questions for each core practical.
• Memorize key equations (C₁V₁ = C₂V₂, mitotic index formula).
• Review organelle identification in micrographs.
• Prepare safety and controlled variable lists for each experiment.