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