Gas Exchange

• Respiratory gases


  Oxygen and carbon dioxide
• Materials that need to be interchanged between an organism and its environment

  • Respiratory gases (oxygen and carbon dioxide)
  • Nutrients (Glucose, amino acids & lipids)
  • Excretory products (CO2 & Urea)
  • Heat

  
• Exchange between organisms and their environment


  • Surface area to volume ratio
• For survival an organism must exchange materials effectively with its environment
• Exchange takes place at the surface of an organism, and the materials absorbed are used (assimilated) by the cell
• The cell or cells of the organism make up its volume
• For exchange to be effective, the surface area of the organism must be large compared to its volume

  
• Surface area to volume ratio

  Decreases as an organism gets larger

  
• As surface area to volume ratio decreases as an organism gets larger


  A larger organism loses heat more slowly across its surface than a smaller organism (and vice versa)

  
• Single celled organisms have large enough surface area to volume ratios to meet their gas exchange needs by diffusion across their cell membranes
• Larger organisms (humans, other mammals, insects and fish) have relatively small surface area to volume ratios (by comparison) and can not rely on diffusion (alone) to meet the oxygen demands to all of their cells
• Larger organisms have developed specialised gas exchange surfaces and systems which have adaptations to ensure the rapid diffusion of gases

  
• Diffusion


  The net movement of molecules from an area of higher concentration to an area of lower concentration
• What makes a good exchange surface


  • Large surface area
  • Large concentration gradients
  • Thin exchange surface (few membrane or thin walls)
• Gas exchange in insects

  1. Oxygen enters the insect through spiracles and into the tracheae

  2. Spiracles close

  3. Oxygen diffuses through the tracheae into the tracheoles (down a conc. gradient)

  4. Oxygen is delivered directly to the respiring tissues
• Carbon dioxide produced by (aerobically) respiring tissues moves in the opposite direction and exits the insect when spiracles open
• Why diffusion happens in the gas exchange systems (tracheal system) of insects

  1. Oxygen diffusion:

  2. Tissues respire using oxygen, which reduces the concentration of oxygen at the tissue

  3. Oxygen moves from an area of higher concentration to lower concentration so moves from the tracheae to the tissue

  4. This lowers the oxygen concentration in the tracheae so oxygen moves into the tracheae from outside the insect via the spiracles

  5. Carbon dioxide:

  6. Respiration produces CO2, increasing the concentration at the tissue

  7. CO2 moves from an area of high concentration at the tissue to the low concentration in the tracheae

  8. CO2 then moves from high concentration in tracheae to low concentration outside the insect via the spiracles

  
• Tracheae


  • Network of tubes supported by strengthened rings
  • Provides tubes full of air so that diffusion is fast
• Tracheoles


  • Small tubes with THIN walls so that the diffusion distance is reduced, which extend throughout the body tissues
  • Highly branched so that there is a large surface area
• Ventilation (Abdominal pumping)


  1. Movement of the insects muscles creates a mass movement of air in and out the trachea (Like a bellow action), thus increasing the rate of gaseous exchange

  2. They also have small air sacs in their trachea

  3. Muscles around the trachea contract and pumps the air in the sacs deeper into the tracheoles
• Getting additional Oxygen during flight

  1. When an insect is at rest, water can build up in the tracheoles

  2. During flight, the insect may partly respire anaerobically and produce some lactate (lactic acid)

  3. This lowers the water potential of the muscle cells

  4. As the lactate builds up, water passes via osmosis from the tracheoles into the muscle cells

  5. This adaptation draws air into the tracheoles closer to the muscle cells and therefore reduces the diffusion distance for oxygen when its most needed
• Adaptations = Fick's Law & must have "SO THAT" sentences
• Gills of fish


  • Large surface area
  • Finger-like projections called gill filaments
  • Each filament has MANY lamellae (at 90°C to increase the surface area)
  • Water carrying oxygen enters through the fish's mouth, passes through the lamellae on the gill filaments where most of the oxygen is removed
  • Water containing little oxygen leaves through gill opening

  
• Lamellae


  • Each gill filament has gill lamellae
  • Gill lamellae are positioned at right angles to the filaments
  • Lamellae contain capillaries
  • Thin epithelium (for short distance between water and blood)

  
• Water and blood flow in opposite directions in the counter current system
• This maintains a concentration / diffusion gradient across the whole length of the gill lamellae

  
• Countercurrent flow is the most efficient for oxygen absorption
• Adaptations of leaf for gaseous exchange

  • Flat – gives larger surface area to volume ratio
  • Many Stomata – pores to allow air to move in and out of leaf
  • Air spaces in leaf so short distance between mesophyll cells and air
• Diffusion of CO2 for photosynthesis

  1. Mesophyll cells photosynthesise and this reduces the concentration of CO2 in the cells

  2. CO2 diffuses from the air spaces into the cells

  3. This in turn reduces the CO2 concentration in the air spaces causing CO2 to move into the air spaces from the air outside the leaf, through the stomata
• Diffusion of O2


  1. Mesophyll cells produce O2 as a result of photosynthesis

  2. O2 diffuses into the air spaces from the cells

  3. This increases the concentration of O2 in the air spaces, causing O2 to move from the air spaces to outside the leaf via the stomata
• Adaptations to reduce water loss in leaves

  • Air spaces are saturated with water vapour from the xylem and water diffuses out of the stomata as it evaporates
  • At night, the guard cells close the stomata to prevent water loss
  • Less CO2 is required at this time of day due to the lack of available sunlight for photosynthesis
  • Upper & lower surfaces have a waxy cuticle
  • Most stomata are found / distributed on the lower surface
• How carbon dioxide in the air outside a leaf reaches mesophyll cells inside the leaf

  1. (Carbon dioxide enters) via stomata

  2. (Stomata opened by) guard cells

  3. Diffuses through air spaces

  4. Down diffusion gradient
• Xerophytic plants live in dry/arid environments and have leaf adaptations to reduce water loss
• Adaptations of xerophytic plants to reduce water loss

  • Reduced number of stomata
  • Stomata in pits
  • Hairs to trap water vapour
  • Rolled leaves
  • Leaves reduced to spines
  • Thick waxy cuticles