3.2 Gas exchange

• Explain how the body surface of a single-celled organism is adapted for gas exchange

  ● Thin, flat shape and large surface area to volume ratio

  ● Short diffusion distance to all parts of cell → rapid diffusion eg. of O2 / CO2
• Describe the 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
• Explain how an insect’s tracheal system is adapted for gas exchange (part 1)

  • ● Tracheoles have thin walls
  • ○ So short diffusion distance to cells
  • ● High numbers of highly branched tracheoles
  • ○ So short diffusion distance to cells
  • ○ So large surface area
  • ● Tracheae provide tubes full of air
  • ○ So fast diffusion
• Explain how an insect’s tracheal system is adapted for gas exchange (part 2)

  • ● Contraction of abdominal muscles (abdominal pumping) changes pressure in body, causing air to move in / out
  • ○ Maintains 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)
  • ○ Diffusion is faster through air (rather than fluid) to gas exchange surface
• Explain structural and functional compromises in terrestrial insects that allow efficient gas exchange while limiting water loss

  ● Thick waxy cuticle / exoskeleton → Increases diffusion distance so less water loss (evaporation)

  ● Spiracles can open to allow gas exchange AND close to reduce water loss (evaporation)

  ● Hairs around spiracles → trap moist air, reducing ψ gradient so less water loss (evaporation)
• Explain how the gills of fish are adapted for gas exchange

  • ● Gills made of many filaments covered with many lamellae
  • ○ Increase surface area for diffusion
  • ● Thin lamellae wall / epithelium
  • ○ So short diffusion distance between water / blood
  • ● Lamellae have a large number of capillaries
  • ○ Remove O2 and bring CO2 quickly so maintains concentration gradient

  
• Explain the process of counter current flow in fish gills to allow gas exchange

  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
• Why would parallel flow in fish gills not result in gas exchange?

  equilibrium would be reached so oxygen wouldn’t diffuse into blood along the whole gill plate.
• Explain how the leaves of dicotyledonous plants are adapted for gas exchange

  ● Many stomata (high density) → large surface area for gas exchange (when opened by guard cells)

  ● Spongy mesophyll contains air spaces → large surface area for gases to diffuse through

  ● Thin → short diffusion distance
• Explain structural and functional compromises in xerophytic plants that allow efficient gas exchange while limiting water loss

  • ● Thicker waxy cuticle
  • ○ Increases diffusion distance so less evaporation
  • ● Sunken stomata in pits / rolled leaves / hairs
  • ○ ‘Trap’ water vapour / protect stomata from wind
  • ○ So reduced water potential gradient between leaf / air
  • ○ So less evaporation
  • ● Spines / needles
  • ○ Reduces surface area to volume ratio
• What is a xerophyte?


  plant adapted to live in very dry conditions eg. Cacti and marram grass
• Describe the gross structure of the human gas exchange system

  Trachea

  Bronchi

  Bronchioles

  Capillary network surrounding alveoli

  Alveoli

  
• Explain the essential features of the alveolar epithelium that make it adapted as a surface for gas exchange

  • ● Flattened cells / 1 cell thick → short diffusion distance
  • ● Folded → large surface area
  • ● Permeable → allows diffusion of O2 / CO2
  • ● Moist → gases can dissolve for diffusion
  • ● Good blood supply from large network of capillaries → maintains concentration gradient

  
• Describe how gas exchange occurs in the lungs

  • ● Oxygen diffuses from alveolar air space into blood down its concentration gradient
  • ● Across alveolar epithelium then across capillary endothelium

  Carbon dioxide = opposite
• Explain the importance of ventilation

  • ● Brings in air containing higher conc. of oxygen & removes air with lower conc. of oxygen
  • ● Maintaining concentration gradients
• Explain how humans breathe in

  1. Diaphragm muscles contract → flattens
  2. External intercostal muscles contract, internal intercostal muscles relax (antagonistic) → ribcage pulled up / out
  3. Increasing volume and decreasing pressure (below atmospheric) in thoracic cavity
  4. Air moves into lungs down pressure gradient

  
• Explain how humans breathe out

  1. Diaphragm relaxes → moves upwards
  2. External intercostal muscles relax, internal intercostal muscles may contract → ribcage moves down / in
  3. Decreasing volume and increasing pressure (above atmospheric) in thoracic cavity
  4. Air moves out of lungs down pressure gradient

  
• Suggest why expiration is normally passive at rest


  ● Internal intercostal muscles do not normally need to contract

  ● Expiration aided by elastic recoil in alveoli
• Suggest how different lung diseases reduce the rate of gas exchange

  • ● Thickened alveolar tissue (eg. fibrosis) → increases diffusion distance
  • ● Alveolar wall breakdown → reduces surface area
  • ● Reduce lung elasticity → lungs expand / recoil less → reduces concentration gradients of O2 / CO2
• Suggest how different lung diseases affect ventilation

  • ● Reduce lung elasticity (eg. fibrosis - build-up of scar tissue) → lungs expand / recoil less
  • ○ Reducing volume of air in each breath (tidal volume)
  • ○ Reducing maximum volume of air breathed out in one breath (forced vital capacity)
  • ● Narrow airways / reduce airflow in & out of lungs (eg. asthma - inflamed bronchi)
  • ○ Reducing maximum volume of air breathed out in 1 second (forced expiratory volume)
  • ● Reduced rate of gas exchange → increased ventilation rate to compensate for reduced oxygen in blood
• Suggest why people with lung disease experience fatigue

  Cells receive less oxygen → rate of aerobic respiration reduced → less ATP made
• Suggest how you can analyse and interpret data to the effects of pollution, smoking and other risk factors on the incidence of lung disease

  • ● Describe overall trend → eg. positive / negative correlation between risk factor and incidence of disease
  • ● Manipulate data → eg. calculate percentage change
  • ● Interpret standard deviations → overlap suggests differences in means are likely to be due to chance
• Suggest how you can analyse and interpret data to the effects of pollution, smoking and other risk factors on the incidence of lung disease (using statistical tests)

  • ● Use statistical tests → identify whether difference / 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 → for categorical data
• Suggest how you can evaluate the way in which experimental data led to statutory restrictions on the sources of risk factors

  • ● Analyse and interpret data as above and identify what does and doesn’t support statement
  • ● Evaluate context → has a broad generalisation been made from a specific set of data?
  • ● Other risk factors that could have affected results?
• Suggest how you can evaluate the way in which experimental data led to statutory restrictions on the sources of risk factors - evaluate method of collecting data

  • ● Evaluate method of collecting data
  • ○ Sample size → large enough to be representative of population?
  • ○ Participant diversity eg. age, sex, ethnicity and health status → representative of population?
  • ○ Control groups → used to enable comparison?
  • ○ Control variables eg. health, previous medications → valid?
  • ○ Duration of study → long enough to show long-term effects?
• Explain the difference between correlations and causal relationships

  • ● Correlation = change in one variable reflected by a change in another - identified on a scatter diagram
  • ● Causation = change in one variable causes a change in another variable
  • ● Correlation does not mean causation → may be other factors involved