Organisms exchange substances with their environment

• Internal environment


  Different from external environment
• Exchange of substances


  Between internal and external environments takes place at exchange surfaces
• To enter or leave an organism

  Most substances must cross cell plasma membranes
• Immediate environment of cells


  Some form of tissue fluid
• Cells are too far away from exchange surfaces and from each other for simple diffusion alone to maintain the composition of tissue fluid within a suitable metabolic range
• Exchange surfaces


  Associated with mass transport systems that carry substances between the exchange surfaces and the rest of the body and between parts of the body
• Mass transport


  • Maintains the final diffusion gradients that bring substances to and from the cell membranes of individual cells
  • Helps to maintain the relatively stable environment that is tissue fluid
• Surface area to volume ratio

  • Relationship between the size of an organism or structure and its surface area to volume ratio
  • Changes to body shape and the development of systems in larger organisms as adaptations that facilitate exchange as this ratio reduces
  • Relationship between surface area to volume ratio and metabolic rate
• Determining effect of surface area to volume ratio and concentration gradient on diffusion

  Use agar blocks containing indicator to determine the effect
• Calculating surface area to volume ratios of cells with different shapes

  Given the dimensions of cells
• Gas exchange surfaces


  • Adaptations shown by gas exchange across the body surface of a single-celled organism
  • In the tracheal system of an insect (tracheae, tracheoles and spiracles)
  • Across the gills of fish (gill lamellae and filaments including the counter-current principle)
  • By the leaves of dicotyledonous plants (mesophyll and stomata)
• Structural and functional compromises


  Between the opposing needs for efficient gas exchange and the limitation of water loss shown by terrestrial insects and xerophytic plants
• Human gas exchange system

  Alveoli, bronchioles, bronchi, trachea and lungs
• Alveolar epithelium


  Essential features as a surface over which gas exchange takes place
• Ventilation and the exchange of gases in the lungs

  Mechanism of breathing to include the role of the diaphragm and the antagonistic interaction between the external and internal intercostal muscles in bringing about pressure changes in the thoracic cavity
• Lung disease


  Effects on gas exchange and/or ventilation
• Pollution and smoking


  Effects on the incidence of lung disease
• Risk factors


  Incidence of lung disease
• Experimental data led to statutory restrictions on the sources of risk factors
• Correlations and causal relationships can be recognised
• Digestion


  Large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes
• Digestion of carbohydrates, lipids and proteins in mammals


  By amylases and membrane-bound disaccharidases, lipase (including the action of bile salts), and endopeptidases, exopeptidases and membrane-bound dipeptidases respectively
• Absorption of the products of digestion by cells lining the ileum of mammals

  • Co-transport mechanisms for the absorption of amino acids and of monosaccharides
  • The role of micelles in the absorption of lipids
• Mass transport


  Efficient movement of substance to and from exchange surfaces over large distances
• Haemoglobins


  • A group of chemically similar molecules found in many different organisms
  • Haemoglobin is a protein with a quaternary structure
• Role of haemoglobin and red blood cells

  In the transport of oxygen
• Loading, transport and unloading of oxygen


  In relation to the oxyhaemoglobin dissociation curve
• Cooperative nature of oxygen binding to haemoglobin

  The change in shape of haemoglobin caused by binding of the first oxygens makes the binding of further oxygens easier
• Effects of carbon dioxide concentration

  On the dissociation of oxyhaemoglobin (the Bohr effect)
• Adaptations to environment


  Possessing different types of haemoglobin with different oxygen transport properties
• Blood circulation in a mammal


  General pattern, with names required only of the coronary arteries and of the blood vessels entering and leaving the heart, lungs and kidneys
• Structure of the human heart

  Gross structure
• Cardiac cycle


  Pressure and volume changes and associated valve movements that maintain a unidirectional flow of blood
• Structure of arteries, arterioles and veins


  In relation to their function
• Structure of capillaries


  Importance of capillary beds as exchange surfaces
• Tissue fluid


  Formation and return to the circulatory system
• Pressure and volume changes during the cardiac cycle

  Data can be analysed and interpreted
• Specific risk factors


  Incidence of cardiovascular disease
• Conflicting evidence associated with risk factors affecting cardiovascular disease can be evaluated
• Xylem


  Tissue that transports water in the stem and leaves of plants