Exchange surfaces

• Rational


  (in classical economic theory) economic agents are able to consider the outcome of their choices and recognise the net benefits of each one
• Consumers act rationally by


  Maximising their utility
• Producers act rationally by


  Selling goods/services in a way that maximises their profits
• Workers act rationally by


  Balancing welfare at work with consideration of both pay and benefits
• Governments act rationally by


  Placing the interests of the people they serve first in order to maximise their welfare
• Rationality in classical economic theory is a flawed assumption as people usually don't act rationally
• When analysing markets, a range of assumptions are made about the rationality of economic agents involved in the transactions
• The Wealth of Nations was written

  1776
• Marginal utility


  The additional utility (satisfaction) gained from the consumption of an additional product
• If you add up marginal utility for each unit you get total utility
• There is much variety of life within and between organisms but the biochemical basis of life is similar for all living things
• Key molecules required to build structures that enable organisms to function

  • Carbohydrates
  • Proteins
  • Lipids
  • Nucleic Acids
  • Water
• Monomers


  Smaller units from which larger molecules are made
• Polymers


  Molecules made from a large number of monomers joined together in a chain
• Synthetic polymers


  • nylon
  • polyethylene
  • polyester
  • Teflon
  • epoxy
• Enzyme catalysis


  1. Substrate binding

  2. Transition state facilitation

  3. Catalysis

  4. Release
• Enzymes


  • They increase the rate of chemical reactions without themselves being consumed or permanently altered by the reaction
  • They increase reaction rates without altering the chemical equilibrium between reactants and products
• As temperature increases


  The rate of reaction increases
• BIOLOGY Topic 3 Organisms and Exchange, Digestion and Absorption and Mass transport.
• Exchange of many substances between an organism and its environment relies on diffusion.
• In very small organisms such as amoeba diffusion through its outer surface will meet all its gas exchange requirements and allows the removal of heat produced during metabolic reactions.
• Single-celled organisms have a large surface area to volume ratio.
• Large surface area to volume ratio

  • Provides a short diffusion pathway to all parts of the cell
  • Satisfies gas exchange requirements for small organisms and ensures removal of heat
• In large cells, diffusion pathways would be too long to satisfy uptake of nutrients and the removal of waste products and heat.
• As the size of organisms increases the surface area to volume ratio decreases.
• Larger organisms


  • Have developed specialised exchange structures such as the lungs and gills to maintain adequate rates of gaseous exchange
  • Changes in body shape or specialised structures have evolved e.g. large ears in elephants to increase the surface area for heat loss
• Elephants are the largest land animals and have a small surface area to volume ratio. As they are mammals they maintain a constant body temperature.
• The large volume of body cells releases a lot of heat from metabolic processes, mainly respiration. Large ears are an adaptation to increase the overall surface area for this heat loss.
• A mouse has a large surface area to volume ratio and therefore loses a large amount of heat relative to its mass i.e. per gram of tissue.
• Due to this large heat loss, mice have to respire at a high rate to help maintain a constant body temperature (respiration produces heat). Therefore, mice have a high metabolic rate.
• This means they have a high uptake of oxygen (for respiration) relative to their mass i.e. per gram of tissue.
• They also have to consume a lot of food, relative to their mass.
• Therefore in mammals and birds there is generally a correlation between the decrease in surface area to volume ratio and a decrease in metabolic rate.
• The gas exchange system in mammals is the lungs which consist of

  • The trachea
  • Two bronchi that repeatedly divide into smaller tubes, the bronchioles
  • The alveoli at the ends of the bronchioles provide a large surface area where gaseous exchange occurs

  
• Alveoli


  • Their shape and large number produce a large surface area
  • Fluid lining the alveolus allows gases to dissolve and diffuse across
  • Only two cell layers separate the blood and air - providing a short diffusion pathway

  
• An extensive network of blood capillaries surrounding each alveolus provides a large surface area for absorbing oxygen and releasing carbon dioxide.

  
• A circulatory system maintains a high diffusion gradient for gaseous exchange by transporting deoxygenated blood to the lungs and removing oxygenated blood from the lungs.

  
• A ventilation mechanism maintains a high diffusion gradient for gaseous exchange by continually providing air with a high oxygen concentration and removing air with a high carbon dioxide concentration.
• Gaseous Exchange in the Alveolus

  1. Oxygen in the alveolar air space dissolves in the fluid lining the epithelium of the alveolus

  2. Oxygen diffuses across the flattened epithelial cells of the alveolus and the endothelial cells of the capillary wall

  3. Oxygen combines with the haemoglobin in the red blood cells to form oxyhaemoglobin and oxygenated blood is transported away from the lungs

  4. Carbon dioxide diffuses from the blood in the opposite direction into the alveolar space

  
• Blood flow ensures that deoxygenated blood (low oxygen and high carbon dioxide concentration) continually reaches the alveoli and that oxygenated blood is removed - this maintains a high diffusion gradient for gaseous exchange.