topic 3.2 Gas exchange

Question 1

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

Answer 1

● Thin, flat shape and large surface area to volume ratio
● Short diffusion distance to all parts of cell → rapid diffusion eg. of O2 / CO2

Question 2

Describe the tracheal system of an insect

Answer 2

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

Question 3

Explain how an insect’s tracheal system is adapted for gas exchange

Answer 3

● 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
● 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)
○ As fluid is removed, air fills tracheoles
○ So rate of diffusion to gas exchange surface
increases as diffusion is faster through air

Question 4

Explain structural and functional compromises in terrestrial insects that
allow efficient gas exchange while limiting water loss

Answer 4

● 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)

Question 5

Explain how the gills of fish are adapted for gas exchange

Answer 5

● 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

Question 6

Counter current flow:

Answer 6

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

If parallel flow, equilibrium would be reached so oxygen wouldn’t diffuse into blood along the whole gill plate

Question 7

Explain how the leaves of dicotyledonous plants are adapted for gas
exchange

Answer 7

● 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

Question 8

Explain structural and functional compromises in xerophytic plants that
allow efficient gas exchange while limiting water loss

Answer 8

Xerophyte = plant adapted to live in very dry conditions

● 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

Question 9

Describe the gross structure of the human gas exchange system

Answer 9

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Question 10

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

Answer 10

● 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

Question 11

Describe how gas exchange occurs in the lungs

Answer 11

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

Question 12

Explain the importance of ventilation

Answer 12

● Brings in air containing higher conc. of oxygen & removes air with lower conc. of oxygen
● Maintaining concentration gradients

Question 13

Explain how humans breathe in and out (ventilation)

Answer 13

Inspiration (breathing 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

Expiration (breathing 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

Question 14

Suggest why expiration is normally passive at rest

Answer 14

● Internal intercostal muscles do not normally need to contract
● Expiration aided by elastic recoil in alveoli

Question 15

Suggest how different lung diseases reduce the rate of gas exchange

Answer 15

● 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

Question 16

Suggest how different lung diseases affect ventilation

Answer 16

● 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

Question 17

Suggest why people with lung disease experience fatigue

Answer 17

Cells receive less oxygen → rate of aerobic respiration reduced → less ATP made

Question 18

Suggest how you can analyse and interpret data to the effects of pollution,
smoking and other risk factors on the incidence of lung disease

Answer 18

● 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
● 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

Question 19

Suggest how you can evaluate the way in which experimental data led to
statutory restrictions on the sources of risk factors

Answer 19

● Analyse and interpret data as above and identify what does and doesn’t support statement
● 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?
● Evaluate context → has a broad generalisation been made from a specific set of data?
● Other risk factors that could have affected results?

Question 20

Explain the difference between correlations and causal relationships

Answer 20

● 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