3.3.2 Gas Exchange

• Gas exchange in single-celled organisms

  Thin, flat shape and large surface area to volume ratio, short diffusion distance to all parts of cell → rapid diffusion eg. of O2 / CO2
• Tracheal system of an insect


  1. Spiracles = pores on surface that can open / close to allow diffusion

  2. Tracheae = large tubes full of air that allow diffusion

  3. Tracheoles = smaller branches from tracheae, permeable to allow gas exchange with cells
• Adaptations of an insect's tracheal system for gas exchange

  • Tracheoles have thin walls so short diffusion distance to cells
  • High numbers of highly branched tracheoles so short diffusion distance to cells and large surface area
  • Tracheae provide tubes full of air so fast diffusion
  • Contraction of abdominal muscles (abdominal pumping) changes pressure in body, causing air to move in / out to maintain concentration gradient for diffusion
  • Fluid in end of tracheoles drawn into tissues by osmosis during exercise (lactate produced in anaerobic respiration lowers ψ of cells) so diffusion is faster through air (rather than fluid) to gas exchange surface
• Structural and functional compromises in terrestrial insects that allow efficient gas exchange while limiting water loss

  • Thick waxy cuticle / exoskeleton to increase diffusion distance so less water loss (evaporation)
  • Spiracles can open to allow gas exchange AND close to reduce water loss (evaporation)
  • Hairs around spiracles to trap moist air, reducing ψ gradient so less water loss (evaporation)
• Adaptations of fish gills for gas exchange

  • Gills made of many filaments covered with many lamellae to increase surface area for diffusion
  • Thin lamellae wall / epithelium so short diffusion distance between water / blood
  • Lamellae have a large number of capillaries to remove O2 and bring CO2 quickly so maintains concentration gradient
• Counter current flow in fish gills

  1. Blood and water flow in opposite directions through/over lamellae

  2. So oxygen concentration always higher in water (than blood near)

  3. So maintains a concentration gradient of O2 between water and blood

  4. For diffusion along whole length of lamellae
• Adaptations of dicotyledonous plant leaves for gas exchange

  • Many stomata (high density) to provide large surface area for gas exchange (when opened by guard cells)
  • Spongy mesophyll contains air spaces to provide large surface area for gases to diffuse through
  • Thin to provide short diffusion distance
• Structural and functional compromises in xerophytic plants that allow efficient gas exchange while limiting water loss

  • Thicker waxy cuticle to increase diffusion distance so less evaporation
  • Sunken stomata in pits / rolled leaves / hairs to 'trap' water vapour / protect stomata from wind, so reduced water potential gradient between leaf / air and less evaporation
  • Spines / needles to reduce surface area to volume ratio
• Essential features of the alveolar epithelium adapted for gas exchange

  • Flattened cells / 1 cell thick to provide short diffusion distance
  • Folded to provide large surface area
  • Permeable to allow diffusion of O2 / CO2
  • Moist to allow gases to dissolve for diffusion
  • Good blood supply from large network of capillaries to maintain concentration gradient
• Ventilation

  Brings in air containing higher conc. of oxygen & removes air with lower conc. of oxygen, maintaining concentration gradients
• How humans breathe in and out (ventilation)

  Inspiration (breathing in): Diaphragm muscles contract → flattens, external intercostal muscles contract, internal intercostal muscles relax → ribcage pulled up / out, increasing volume and decreasing pressure (below atmospheric) in thoracic cavity, air moves into lungs down pressure gradient

  Expiration (breathing out): Diaphragm relaxes → moves upwards, external intercostal muscles relax, internal intercostal muscles may contract → ribcage moves down / in, decreasing volume and increasing pressure (above atmospheric) in thoracic cavity, air moves out of lungs down pressure gradient
• Expiration is passive at rest

  Internal intercostal muscles do not normally need to contract so expiration is aided by elastic recoil in alveoli
• Different lung diseases reduced the rate of gas exchange

  Thickened alveolar tissue increases the diffusion distance

  The alveolar wall break down so it reduces the surface area

  Reduces lung elasticity so lungs expand and recoil less which reduces concentration gradient
• Diffrerent lung diseases affect ventilation

  • Reduce lung elasticity so lungs expand and recoil less, which reduces the volume of air in each breath and reduces the maximum volume of air breathed out in one breath
  • Narrow airways so it reduces airflow in and out of the lungs, which reduces the maximum volume of air breathed out in 1 second
  • Reduced rate of gas exchange so increased ventilation rate to compensate for reduced oxygen in blood
• People with ling disease experience fatigue because their cells recieve less oxygen, so rate of aerobic respiration is reduced, so less ATP is made.
• Analysing and interpreting data to the effects of pollution, smoking and other risk factors on the incidence of lung disease

  1. Describing the overall trend, e.g, positive or negative correlation between risk factors and incidence of disease

  2. Manipulate data by calculating percentage change

  3. Interpret standard deviations, if there is an overlap, this suggests that differences in means are likely to be due to chance

  4. Use statistical tests to identify whether the difference or correlation is significant or due to chance
• Correlation coefficient

  Examining an association between 2 sets of data
• Student's T test

  Comparing means of 2 sets of data
• Chi-squared test

  Analysing categorical data
• Evaluating how experimental data led to statutory restrictions on the sources of risk factors

  1. Analyse and interpret data

  2. Identify what does and doesn't support the statement

  3. Evaluate the method of collecting data

  4. Assess sample size - large enough to be representative of the population?

  5. Assess participant diversity eg. age, sex, ethnicity and health status - representative of population?

  6. Assess control groups - used to enable comparison?

  7. Assess control variables eg. health, previous medications - valid?

  8. Assess duration of study - long enough to show long-term effects

  9. Evaluate if a broad generalisation has been made from a specific set of data

  10. Identify other risk factors that could have affected results
• The difference between correlations and causal relationships is that:

  • Correlation = change in 1 variable reflected by a change in another - shown on a scatter diagram
  • Causation = change in 1 variable causes a change in another variable
  • Correlation doesn't mean causation -> may be other factors involved