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 e.g. 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 insects tracheal system is adapted 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 - maintains concentration gradient for diffusion.

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

  • as fluid is removed, air fills tracheoles.

  • so rate of diffusion to gas exchange surface increases as diffusion is faster through air.

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 water potential gradient so less water loss (evaporation).

Explain how the gills of a 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.

  • They remove O₂ and bring CO₂ quickly, so maintains concentration gradient.

• Counter-current flow

What is counter current flow in fish?

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 the flow was parallel (rather than counter current), equilibrium would be reached so oxygen wouldn’t diffuse into blood along the whole gill plate.

Explain how the leaves of dicotyledonous plants (ordinary 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 (plants adapted to live in fry conditions e.g. cacti and marram grass) 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. → to ‘trap’ water vapour/ protect stomata from wind.

  • so reduced water potential gradient between leaf/ air → so less evaporation and water loss.

• Spines/ needles reduce surface area to volume ratio.

Describe the gross structure of the human gas exchange system

Trachea → Bronchi → Bronchioles → Alveoli/ capillary network.

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/ CO_2.

• 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 is the opposite.

Explain importance of ventilation.

Brings in air containing high concentration of oxygen and removes air with lower concentration of oxygen - maintaining the concentration gradients.

Explain how humans breathe in and out (ventilation). → breathing in

Inspiration (breathing in)

1. Diaphragm muscles contract → diaphragm flattens.

2. External intercostal muscles contract, internal intercostal muscles relax → ribcage pulled up/ out.

3. Increasing volume and decreasing pressure in thoracic cavity.

4. Air moves into lungs down pressure gradient.

Explain how humans breathe in and out (ventilation). → breathing out

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 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 (e.g. 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 (e.g. 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 and out of lungs (e.g. asthma - inflamed bronchi).

  • reducing maximum volume of air breathed out in 1 second (forced expiratory volume).

• Reduced rate of gas exchange → increased ventilation to compensate for reduced oxygen in blood.

Suggest why people with lung disease experience fatigue.

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