The Principles of Exchange and Transport

• Exchange surface


  Demonstrate knowledge and understanding of the features of exchange surface that aid passive and active transport
• Living organisms need to obtain external substances (such as oxygen and glucose for respiration, or nitrate ions in plants to provide the nitrogen needed for amino acid synthesis) from their surroundings and also be able to remove waste products (such as carbon dioxide from respiration, or urea from protein metabolism)
• In unicellular organisms this exchange takes place through the cell surface membrane
• Multicellular organisms often have specialised exchange surfaces that take in or remove materials
• In the more complex organisms, there is a transport system that links the specialised exchange surface (such as gills in fish) to the cells throughout the organism
• Surface area


  The total number of surfaces in direct contact with the environment
• Volume


  The three-dimensional space occupied by metabolically active tissue
• As the size of an organism increases


  The surface area to volume ratio decreases
• Small organisms are able to gain all their requirements (metabolites) through their body surface and remove all waste and toxic products by the same route
• In large organisms the surface area is simply not large enough to meet the metabolic needs of the large number of cells that occupy the proportionally larger volume within the organism's body
• The majority of cells in large multicellular organisms are not in direct contact with the surrounding environment
• Large organisms must have specialised exchange surfaces in order that the rate of exchange of substances into and out of the organism can be increased to meet its greater metabolic needs
• The importance of specialised exchange surfaces is even greater in organisms that are very active and have high metabolic rates
• Features of exchange surfaces that aid passive and active transport


  • Increasing the surface area of the exchange surface
  • Having a thin separating surface
  • Maintaining large concentration gradients
• Examples of exchange surfaces


  • Root hair cells
  • Red blood cells
• Mass flow


  The transport of substances within large organisms by pressure differences
• Mass flow systems in flowering plants and mammals


  • Xylem tissue (flowering plant)
  • Phloem tissue (flowering plant)
  • Circulation in mammals
  • Ventilation (breathing) in mammals
• Gaseous exchange


  Necessary to ensure that sufficient oxygen reaches all the cells in an organism and that carbon dioxide is excreted
• Features of gas exchange (respiratory) surfaces


  • Have a large surface area
  • Have a moist surface
  • Have diffusion gradients for oxygen and carbon dioxide
  • Are permeable to oxygen and carbon dioxide
  • Have a short diffusion path
• Respiration and photosynthesis are the two processes involving gas exchange in plants
• When light energy is available

  The rate of photosynthesis in photosynthesising cells is much greater than the rate of respiration
• When respiration only is occurring


  Oxygen will diffuse into the cells and carbon dioxide will diffuse out
• At the compensation point


  The rate of carbon dioxide used in photosynthesis is the same as the rate of carbon dioxide produced in respiration
• For a plant to grow, the production of carbohydrate in photosynthesis must exceed the loss of carbohydrate as a respiratory substrate
• Leaf adaptations for gas exchange


  • Leaves are usually thin to ensure a high surface area to volume ratio and short diffusion distance
  • Large and moist gas exchange surfaces of the spongy mesophyll cells
• Leaf adaptations for gas exchange


  • The leaf is an organ highly adapted for the process of photosynthesis in general
  • It is also very well adapted for gas exchange-necessary for both respiration and photosynthesis
• Leaf adaptations for gas exchange


  • Leaves are usually thin to ensure high surface area to volume ratio and short diffusion distance
  • Large and moist gas exchange surfaces of the spongy mesophyll cells
  • Loose arrangement of spongy mesophyll cells to provide large surface area for gas diffusion
  • Intercellular air spaces of the spongy mesophyll facilitate diffusion within the leaf
• Stomata


  Pores in the leaf surface that allow respiratory gases to diffuse in and out of the leaf easily
• Most stomata are found in the lower leaf surface
• In most plants the stomata are open during the daytime, when the diffusion of gases into and out of the leaf is at its greatest
• The stomata may be closed during the night when there is no photosynthesis and there will be enough oxygen inside the leaf for respiratory needs
• Closing the stomata during the night ensures that water loss from the leaf is reduced
• Opening and closing of a stoma


  Controlled by two guard cells that can change shape
• Stomata are open when the guard cells are turgid and closed when they are not
• Guard cells, unlike other epidermal cells, contain chloroplasts
• Most water loss from leaves (transpiration) is during the day, when it is usually warmer
• The stomata are unable to close then without significantly compromising the rate of photosynthesis (and growth)
• Mammals


  • Have relatively small surface area to volume ratios
  • Have an impermeable body covering
• Highly specialised exchange surfaces have evolved in mammals to ensure that each of the cells in the body receives sufficient oxygen and that carbon dioxide is eliminated
• Mammalian respiratory system


  • Palmonary (lung) respiratory system
  • Gas exchange surface is the alveolar wall