Gas exchange surfacesEdexcel A-Level Biology A: Mind map

Exam questions on Gas exchange surfaces

1. Single-celled organisms such as Amoeba obtain oxygen by diffusion across their cell surface membrane, but large animals have specialised gas exchange surfaces such as lungs. Students use agar cubes to model how the surface area to volume ratio changes as an organism gets larger.
   Explain why a large mammal needs a specialised gas exchange surface, but an Amoeba does not.2 marks
2. Fick's law states that the rate of diffusion is proportional to (surface area × concentration difference) ÷ thickness of the exchange surface. In a healthy adult the alveolar walls have a thickness of 0.5 μm. In a disease called pulmonary fibrosis, scar tissue thickens the alveolar walls.
   During vigorous exercise, a person's breathing rate and heart rate both increase. Explain how this increases the rate of diffusion of oxygen into the blood.2 marks
3. A student models diffusion across an exchange surface using Fick's law, where rate of diffusion is proportional to (surface area × concentration difference) ÷ thickness. Surface A has an area of 12 cm², a concentration difference of 8 arbitrary units and a thickness of 0.2 mm. Surface B has an area of 12 cm², a concentration difference of 6 arbitrary units and a thickness of 0.1 mm.
   Calculate the relative rate of diffusion for surface A and for surface B, and state how many times faster diffusion is across B than across A.3 marks