6 - Exchange

• External environment


  Different from the internal environment found within an organism and within its cells
• Tissue fluid


  The environment around the cells of multicellular organisms
• Transfer of materials between external and internal environments

  1. Occurs at exchange surfaces

  2. Involves crossing cell plasma membranes
• Majority of cells are too far from exchange surfaces for diffusion alone to supply or remove their tissue fluid</b>
• Distribution of absorbed materials

  1. Rapidly distributed to the tissue fluid

  2. Waste products returned to the exchange surface for removal
• Mass transport system


  Maintains the diffusion gradients that bring materials to and from the cell surface membranes
• Size and metabolic rate of an organism

  Affect the amount of each material that is exchanged
• Organisms with a high metabolic rate exchange more materials and so require a larger surface area to volume ratio
• Things that need to be interchanged between an organism and its environment

  • Respiratory gases (oxygen and carbon dioxide)
  • Nutrients (glucose, fatty acids, amino acids, vitamins, minerals)
  • Excretory products (urea and carbon dioxide)
  • Heat
• Passive exchange


  No metabolic energy is required, by diffusion and osmosis
• Active exchange


  Metabolic energy is required, by active transport
• Exchange surfaces


  • Large surface area relative to the volume of the organism which increases the rate of exchange
  • Very thin so that the diffusion distance is short and therefore materials cross the exchange surface rapidly
  • Selectively permeable to allow selected materials to cross
  • Movement of the environmental medium
  • A transport system to ensure the movement of the internal movement
• Diffusion


  Rate = surface area x difference in concentration / length of diffusion path
• Gas exchange in single-celled organisms

  Oxygen is absorbed by diffusion across their body surface, which is covered only by a cell-surface membrane. Carbon dioxide from respiration diffuses out across their body surface. Cell wall is no additional barrier to gas diffusion.
• Specialised exchange surfaces are often located inside an organism to avoid damage and dehydration
• Gas exchange in single-celled organisms

  • Small size, large surface area to volume ratio
• Where an exchange surface is located inside the body, the organism needs to have a means of moving the external medium over the surface
• Gas exchange in insects

  Insects have evolved an internal network of tubes called tracheae to conserve water. Tracheae divide into smaller dead end tubes called tracheoles that extend throughout the body tissues.
• Respiratory gases move in and out of the tracheal system

  1. Diffusion gradient

  2. Mass transport

  3. Water movement in tracheoles
• Tracheal system


  • Atmospheric air with oxygen is brought directly to respiring tissues
  • Short diffusion pathway from tracheole to body cell
  • Relies mostly on diffusion to exchange gases
• Gases enter and leave tracheae through tiny pores called spiracles on the body surface
• Insects keep their spiracles closed for much of the time to prevent water loss
• The tracheal system is an efficient method of gas exchange but has limitations - the length of the diffusion pathway limits the size that insects can attain
• Despite being small, insects are one of the most successful groups of organisms on Earth
• Fish


  • Have a waterproof, and therefore a gas-tight, outer covering
  • Have a small surface area to volume ratio
  • Their body surface is not adequate to supply and remove their respiratory gases
  • Have evolved a specialised internal gas exchange surface: the gills

  
• Gills


  The internal gas exchange surface of fish

  
• Structure of the gills

  1. Gill filaments are stacked up in a pile

  2. Gill lamellae are at right angles to the filaments, increasing the surface area

  3. Water is taken in through the mouth and forced over the gills and out through an opening on each side of the body

  4. The flow of water over the gill lamellae and the flow of blood within them are in opposite directions (counter current flow)

  
• Counter current flow


  The flow of water over the gill lamellae and the flow of blood within them are in opposite directions

  
• Gas exchange in the leaf of a plant

  All plant cells require oxygen and produce carbon dioxide during respiration, but some plant cells also carry out photosynthesis which takes in carbon dioxide and produces oxygen
• Counter current exchange system


  • The blood and the water that flow over the gill lamellae do so in opposite directions
  • Blood that is already well loaded with oxygen meets water, which has its maximum concentration of oxygen, allowing diffusion of oxygen from the water to the blood
  • Blood with little oxygen meets water which has had most, but not all, of its oxygen removed, allowing diffusion of oxygen from the water to the blood
  • Maintains a diffusion gradient for oxygen uptake across the entire width of the gill lamellae
  • Allows about 80% of the oxygen available in the water to be absorbed into the blood of the fish

  
• If the water and blood flowed in the same direction (parallel flow)

  Far less gas exchange would take place, with only 50% of the available oxygen being absorbed by the blood

  
• Gas exchange in plants

  1. When photosynthesis is taking place, carbon dioxide is obtained from the external air and oxygen from photosynthesis diffuses out

  2. When photosynthesis is not occurring, oxygen diffuses into the leaf because it is constantly being used by cells during respiration, and carbon dioxide produced during respiration diffuses out
• Gas exchange in plants

  • No living cell is far from the external air, which makes diffusion rapid
  • Diffusion takes place in the gas phase (air), which makes it more rapid than if it were in water
  • There is a short, fast diffusion pathway
  • The air spaces inside a leaf have a very large surface area compared with the volume of living tissue
  • There is no specific transport system for gases, which simply move in and through the plant by diffusion
• Adaptations of leaves for rapid gas diffusion

  • Many small pores, called stomata, so no cell is far from a stoma and the diffusion pathway is short
  • Numerous interconnecting air-spaces that occur throughout the mesophyll so gases can readily come in contact with mesophyll cells
  • Large surface area of mesophyll cells for rapid diffusion
• Stomata


  Minute pores that occur mainly on the underside of leaves, each surrounded by a pair of guard cells that can open and close the pore to control the rate of gaseous exchange
• Terrestrial organisms lose water by evaporation, so plants have evolved to balance the conflicting needs of gas exchange and control of water loss by closing stomata when water loss would be excessive
• Terrestrial organisms


  • Problems arise from the opposing needs of an efficient gas-exchange system and the requirements to conserve water
  • The features that make a good gas exchange system are the same features that increase water loss
  • Terrestrial organisms must limit their water loss without compromising the efficient of their gas-exchange system
• Gas exchange surfaces of terrestrial organisms

  • Inside the body
  • Air at the exchange surface is more or less 100% saturated with water vapour
  • Less evaporation of water from the exchange surface
• Insects


  • Most are terrestrial
  • Water easily evaporates from the surface of their bodies and they can become dehydrated
  • Evolved adaptations to conserve water
• Efficient gas exchange


  Requires a thin, permeable surface with a large area