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?

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

  Tracheae = large tubes full of air that allow diffusion

  Tracheoles = smaller branches from trachede, permeable to allow gas exchange with cells

  
• Explain how an insect’s tracheal system is adapted for gas exchange? (1/3) - refer to tracheoles and tracheae only ?

  • Tracheae provide tubes full of ai rSo fast diffusionance to cells
  • High numbers of highly branched tracheoles So short diffusion distance to cells So large surface area
  • Tracheae provide tubes full of air o fast diffusion

  
• Explain how an insect’s tracheal system is adapted for gas exchange? (2/3) - refer to concentration Gradient only ?

  • Contraction of abdominal muscles (abdominal pumping) changes pressure in body, causing air to move in / out
  • Maintains concentration gradient for diffusion

  
• Explain how the body surface of a single-celled organism is adapted for gas exchange (3/3)- refer to fluid in the end of tracheoles?

  • Fluid in end of tracheoles drawn into tissues by osmosis during exercise (lactate produced in anaerobic respiration lowers water potential 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)

  • trachea = have rings of a substance called chitin for support
  • tracheoles= fluid at the end moves out during exercise increasing SA

  
• • Rhytmic abdominal movement move air in and out through spiracles = maintains CONC GRADIENT
• Explain how the gills of fish are adapted for gas exchange?

  • Gills made of many filaments covered with many lamellaeIncrease surface area for diffusion
  • Thin lamellae wall / epitheliumSo short diffusion distance between water / blood
  • Lamellae have a large number of capillariesRemove 02 and bring CO2, quickly so maintains concentration gradient

  
• What is the counter-current flow?

  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 0, between water and blood
  4. For diffusion along whole length of lamellae
  5. if parallel flow, 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

  
• What are xerophytic plants?

  Plants adapted to dry environments.

  e.g Cacti and marram grass
• 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.

  • fewer stomata

  
• Describe the gross structure of the human gas exchange system?

  Trachea- windpipe from mouth

  Brochi- Braches off from trachea, one bronchus to each lung

  Bronchioles- branch off from bronchi, smaller tubes

  alveoli- tiny air sacs at the end of bronchioles where gas exchange occurs

  • surrounded by ribcage and intercostal muscles
• 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 it’s concentration gradient

  -across alveolar epithelium then across capillary endothelium

  
• 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? Refer to describing trends?

  -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? Refer to using statistical test

  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?

  Evaluate method:

  • 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?
  • → has a broad generalisation been made from a specific set of data?Other risk factors?
• 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
• Exam insight: common mistakes ❌ (1/9)?

  ❌-“Insects are adapted for gas exchange by having thin tracheoles, or tracheoles with thin membranes.”

  ✅- It is the epithelial cell layer of the tracheoles that is thin.
• Exam insight: common mistakes❌- (2/9)?

  ❌-[Named surface] is adapted for gas exchange because it has a short diffusion pathway or large surface area.”

  ✅-Most gas exchange surfaces have these features, but to get the mark you need to identify the feature that results in these to get the mark.
• Exam insight: common mistakes❌- (3/9)?

  ❌-“In xerophytes, hairs trap water.”

  ✅-Hairs trap water vapour which reduces the water potential gradient between the leaf and air—have to say water vapour
• Exam insight: common mistakes ❌- (4/9)?

  
  

  ❌ “In plants, a thick waxy cuticle reduces water loss.”

  ✅-This is insufficient. It increases diffusion distance which reduces water loss.
• Exam insight: common mistakes ❌- (5/9)?

  ❌-In counter current flow, blood and oxygen flow in opposite directions.”

  ✅-Blood and water flow in opposite directions.
• Exam insight: common mistakes ❌- (6/9)?

  
  

  ❌ “Xerophytes have small leaves to reduce their SA:V.”

  ✅They have needles or spines to reduce their SA:V.
• Exam insight: common mistakes ❌- (7/9)?

  ❌-

  *Not mentioning that counter current flow allows diffusion along the whole length of lamellae.*

  ✅-If water and blood flowed in the same direction, equilibrium would be reached and oxygen would only diffuse into blood along part of the gill plate
• Exam insight: common mistakes ❌- (8/9)?

  ❌-*Not linking the contraction / relaxation of the diaphragm to the change in its position.*

  ✅-During inspiration, when the diagram muscles contract, the diaphragm flattens.
• Exam insight: common mistakes ❌- (9/9)?

  ❌-“Ventilation in insects / humans enables efficient gas exchange.”

  ✅-This is insufficient. You need to explain why, in terms of maintaining a concentration gradient.