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Substance Exchange

Apr 24, 2025

AQA Biology A-Level: Organisms Exchange Substances with their Environment

Introduction

  • Internal vs. external environments
  • Exchange of substances via exchange surfaces
  • Substances must cross cell plasma membranes to enter/leave an organism
  • Importance of tissue fluid and mass transport systems in large organisms

3.3.1 Surface Area to Volume Ratio

  • Relationship between organism size and surface area to volume ratio
  • Adaptations in body shape and systems for efficient exchange
  • Connection between surface area to volume ratio and metabolic rate

Skills Development

  • Experiment with agar blocks to study diffusion and surface area to volume ratio
  • Calculate surface area to volume ratios of various cell shapes

3.3.2 Gas Exchange

  • Adaptations for Gas Exchange
    • Single-celled organisms
    • Tracheal system in insects
    • Fish gills and counter-current principle
    • Leaves of dicotyledonous plants
  • Structural and functional compromises in terrestrial insects and xerophytic plants
  • Human gas exchange system: alveoli, bronchioles, bronchi, trachea, lungs
  • Breathing mechanism: diaphragm and intercostal muscles

Skills Development

  • Dissecting mammalian lungs and fish/insect gas exchange systems
  • Use microscopes to examine gas exchange surfaces
  • Measure volumes of gas exchange using respirometers
  • Calculate pulmonary ventilation rate

3.3.3 Digestion and Absorption

  • Digestion: Hydrolysis of large molecules into absorbable smaller molecules
  • Mammalian digestion processes:
    • Carbohydrates by amylases and disaccharidases
    • Lipids by lipases and bile salts
    • Proteins by endopeptidases, exopeptidases, dipeptidases

Absorption Mechanisms

  • Co-transport of amino acids and monosaccharides
  • Role of micelles in lipid absorption

Skills Development

  • Investigate enzyme reaction rates and effects of pH and bile salts
  • Use Visking tubing models for digestion absorption studies

3.3.4 Mass Transport

3.3.4.1 Mass Transport in Animals

  • Haemoglobin and role in oxygen transport
  • Oxyhaemoglobin dissociation curve and Bohr effect
  • Blood circulation in mammals
  • Structure of heart and arteries, arterioles, veins, capillaries
  • Formation and return of tissue fluid

Skills Development

  • Data analysis of cardiac cycle, cardiovascular disease risk factors
  • Investigate variables affecting human pulse or heart rate

3.3.4.2 Mass Transport in Plants

  • Water transport in xylem: cohesion-tension theory
  • Organic substance transport in phloem: mass flow hypothesis
  • Tracer and ringing experiments to study transport

Skills Development

  • Use potometer to study transpiration rates

Required Practicals

  • Dissection of exchange systems or mass transport systems
  • Practical experiments to develop understanding of exchange and transport mechanisms