3.2 - Gas exchange

Question 1

Describe and explain one feature of the alveolar epithelium that makes the epithelium well adapted as a surface for gas exchange. Do not refer to surface area or moisture in your answer. (2)

Answer 1

• Flattened cells / Single layer of cells
  ‘Reject thin cell wall/membrane but accept thin cells /one cell thick’
• Reduces diffusion distance/pathway
• Permeable
• Allows diffusion of oxygen/carbon dioxide

Question 2

Tidal volume is the volume of air inhaled and exhaled during a single breath when a person is resting. The tidal volume in a person with emphysema is reduced compared with the tidal volume in a healthy person. Suggest and explain how a reduced tidal volume affects the exchange of carbon dioxide between the blood and the alveoli. (3)

Answer 2

• Less carbon dioxide exhaled/moves out (of lung) / More carbon dioxide remains (in lung)
• (So) reduced diffusion/concentration gradient (between blood and alveoli)
• Less/slower movement of carbon dioxide out of blood / More carbon dioxide stays in blood

Question 3

Explain how the counter-current principle allows efficient oxygen uptake in the fish gas exchange system. (2)

Answer 3

• Blood and water flow in opposite directions
• Diffusion/concentration gradient (maintained) along (length of) lamella/filament

Question 4

Describe and explain the mechanism that causes lungs to fill with air. (3)

Answer 4

• Diaphragm (muscle) contracts and external intercostal muscles contract
• Ribs move up and out and diaphragm flattens
• (Causes volume increase and) pressure decrease
• Air moves down a pressure gradient / Air enters from higher atmospheric pressure

Question 5

If alveolar epithelium cells die inside the human body they are replaced by non-specialised, thickened tissue. Explain why death of alveolar epithelium cells reduces gas exchange in human lungs. (3)

Answer 5

• Reduced surface area
• Increased distance for diffusion
• Reduced rate of gas exchange

Question 6

Describe and explain the advantage of the counter-current principle in gas exchange across a fish gill. (3)

Answer 6

• Water and blood flow in opposite directions
• Maintains diffusion/concentration gradient of oxygen / Oxygen concentration always higher (in water)
• (Diffusion) along length of lamellae/ filament/gill/capillary

Question 7

Use your knowledge of gas exchange in leaves to explain why plants grown in soil with very little water grow only slowly. (2)

Answer 7

• Stomata close
• Less carbon dioxide (uptake) for less photosynthesis/glucose production

Question 8

Describe the pathway taken by an oxygen molecule from an alveolus to the blood. (2)

Answer 8

• (Across) alveolar epithelium
• Endothelium / epithelium of capillary

Question 9

Explain how one feature of an alveolus allows efficient gas exchange to occur. (2)

Answer 9

• (The alveolar epithelium) is one cell thick
• Creating a short diffusion pathway / reduces the diffusion distance

Question 10

Describe the gross structure of the human gas exchange system and how we breathe in and out. (5)

Answer 10

• Named structures – trachea, bronchi, bronchioles, alveoli
• Breathing in – diaphragm contracts and external intercostal muscles contract
• (Causes) volume increase and pressure decrease in thoracic cavity
  (to below atmospheric, resulting in air moving in)
• Breathing out - Diaphragm relaxes and internal intercostal muscles contract
• (Causes) volume decrease and pressure increase in thoracic cavity (to above atmospheric, resulting in air moving out)

Question 11

Explain three ways in which an insect’s tracheal system is adapted for efficient gas exchange. (3)

Answer 11

• Tracheoles have thin walls so short diffusion distance to cells
• Highly branched / large number of tracheoles so short diffusion distance to cells
• Highly branched / large number of tracheoles so large surface area (for gas exchange)
• Tracheae provide tubes full of air so fast diffusion (into insect tissues)
• Fluid in the end of the tracheoles that moves out (into tissues) during exercise so faster diffusion through the air to the gas exchange surface
• Fluid in the end of the tracheoles that moves out (into tissues) during exercise so larger surface area (for gas exchange)
• Body can be moved (by muscles) to move air so maintains diffusion / concentration gradient for oxygen / carbon dioxide

Question 12

Explain two ways in which the structure of fish gills is adapted for efficient gas exchange. (2)

Answer 12

• Many lamellae / filaments so large surface area
• Thin (surface) so short diffusion pathway