Mass Transport

• Organisms Exchange Substances with their environment

  • The internal environment of a cell or organism is different from its external environment
  • The exchange of substances between the internal and external environments takes place at exchange surfaces
  • To truly enter or leave an organism, most substances must cross cell plasma membranes
• Large multicellular organisms


  • The immediate environment of cells is some form of tissue fluid
  • Most 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 are 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
  • It also helps to maintain the relatively stable environment that is tissue fluid
• Surface Area to Volume Ratio

  The 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

  • Adaptations that facilitate exchange as this ratio reduces
• Students should be able to appreciate the relationship between surface area to volume ratio and metabolic rate
• Exchange


  • Why do organisms do it?
  • Cells need to take in oxygen and glucose for respiration
  • They need to excrete wastes; carbon dioxide and urea
  • They need to allow heat to escape too
  • The organism's surface area: volume ratio determines how quickly substances can be exchanged
• Learn how surface area:volume affects organisms

  1. surface area

  2. diffusion

  3. adaptation
• Surface Area: Volume Ratio

  • Which has bigger surface area?
  • Which has bigger volume?
  • Which has bigger surface area:volume ratio?
• Make a 'mouse' that has 1cm long sides<|>Make a 'hippo' that is 2cm x 4cm x 4cm


  • Surface Area: 6cm2
  • Volume: 1cm3
  • Surface Area: 64cm2
  • Volume: 32cm3
  • The mouse has a bigger ratio
• • Small organisms have a higher SA: volume ratio
  • Large organisms have a lower SA: volume ratio
• • Gangly organisms have a higher SA: volume ratio
  • Blobby organisms have a lower SA: volume ratio
• Practical


  1. Set up cubes of agar jelly and see how far liquid penetrates them by diffusion over five minutes

  2. Calculate surface area to volume ratio for cubes of different sizes

  3. Consider the problems faced by large organisms
• • As length doubles, surface area multiplies by 4
  • As length doubles, volume multiplies by 8
  • As length doubles, the surface area to volume ratio halves
  • The rate of diffusion is the same for each cube
  • Larger organisms would not be able to get all the nutrients they need, or get rid of all their waste products by diffusion as there is not enough surface area for each cell
  • The rate of diffusion cannot be increased so this will limit the size of the organism unless it has specialised gas exchange surfaces or other mechanisms
• Adaptations of gas exchange surfaces

  • 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
• The gross structure of the human gas exchange system

  • Limited to the alveoli, bronchioles, bronchi, trachea and lungs
• The essential features of the alveolar epithelium


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

  The 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
• Animal Adaptations - SA:Volume

  • teachea
  • spiracles
  • alveolar
• Animal Adaptations - Water Loss

  • Animals living with high SA:volume ratio tend to lose more water via evaporation from their many surfaces
  • To combat this, they have kidney adaptations to produce less urine to compensate
• Animal Adaptations - Metabolic Rate

  • Small animals that have a high metabolic rate need to eat energy rich foods like seeds and nuts
• Animal Adaptations - Hibernation


  • Some smaller mammals may have thick fur and hibernate during cold months of the year
• Animal Adaptations - Being large


  • Larger animal in hot climates have adaptations to combat their small SA:volume ratio
• Diffusion rate is determined by

  • Concentration
  • Temperature
  • Distance
• • If you're a small organism (single-celled) e.g a bacterium, there is a very short distance for substances to travel
  • Therefore, these single-celled organisms can diffuse substance from and into their environment directly
• Gas exchange surfaces


  • Large surface area
  • Thin
  • Maintain concentration gradient
• Gas exchange in fish gills

  1. Water enters mouth

  2. Passes through gill filaments

  3. Lamellae covered in blood

  4. Water and blood flow in opposite directions (counter-current)

  5. Oxygen moves from water into blood down concentration gradient

  6. Circulation and ventilation maintain concentration gradient
• Fish breathe by gulping water through their mouth, then close their mouth and throat. The water is forced though the opening in the back of their throat that is lined with gills.
• Gills


  • Very thin, branched structures like a Christmas tree
  • Filled with blood
  • More efficient than lungs at extracting oxygen
• Dissecting fish gills


  1. Remove gills on one side

  2. Cut through bone at top and bottom where gills are joined to head

  3. Lift back edge and cut away from skin

  4. Each pair of gills has 4 arches, each with a row of gill rakers
• Gills are red because they are filled with blood. Oxygen in the water passes into the blood and is carried through their body.
• Gills are more efficient than lungs at extracting oxygen.
• Insect gas exchange system

  • Tracheae
  • Tracheoles
  • Spiracles
• Insect gas exchange


  1. Tracheae carry air

  2. Spiracles allow air to enter and leave the tracheal system
• Insects have a tracheal system for gas exchange, with tracheae, tracheoles and spiracles.
• Leaf gas exchange


  • Short diffusion distance
  • Large surface area
• Mesophyll cells


  • Main gas exchange surface in leaves
  • Large surface area
  • Lots of gaps for gas to enter and leave through stomata
• Stomata


  • Tiny pores surrounded by guard cells
  • Control gas movement down concentration gradient
  • Control water loss
• Opening stomata


  1. Guard cells fill with ions

  2. Water moves into guard cells

  3. Guard cells become more turgid

  4. Pore opens
• Closing stomata


  1. Guard cells empty of ions

  2. Water moves out of guard cells

  3. Guard cells become less turgid

  4. Pore closes