Biology Lecture: Factors Affecting Membrane Permeability

Topic Overview

• Focus: Experiments on membrane permeability (OCR A-Level Biology, spec 2.1.5)
• Common experiment: Beetroot investigation

Beetroot Investigation

• Experimental Setup:
  • Samples of uncooked beetroot placed in water at various temperatures
  • Observe and compare color changes in water

Key Variables to Control

• Mass and Length: All beetroot pieces must have the same mass and length (not size).
• Species of Beetroot: Use the same species or ideally the same beetroot.
• Volume of Water: Each beetroot sample should be placed in the same volume of water.
• Immersion Time: All beetroot cylinders should be in the water for the exact same length of time.
• Drying Samples: Dry beetroot samples before placing them in water to remove excess pigment due to membrane damage from cutting.

Observations and Measurements

• Temperature Effect: As temperature increases, color change in water becomes darker.
  • Qualitative Data: Subjective (e.g. dark pink vs. medium pink).
  • Quantitative Data: Use of biosensors and colorimeters to measure absorbance.
    • Colorimeter Calibration: Standard lab equipment.
      • Steps:
        1. Calibrate using a blank cuvette with water:
           • Insert blank, close the lid, press 'calibrate' to set absorbance to zero.
        2. Set Filters and Readings: Ensure filter is red and absorbance is measured.
        3. Measure Absorbance: Insert cuvette with sample solution, record absorbance, recalibrate with blank between each sample.

Expected Results and Conclusions

• Trends: Increase in temperature leads to higher absorbance readings.
  • Indicates more pigment released due to membrane damage from increased kinetic energy.

Important Points for Exams

• Emphasis on using precise terminology (mass, length, species, etc.)
• Understanding of equipment setup and calibration steps.

Conclusions: Higher temperatures increase membrane permeability, resulting in more pigment release.