Transport Across Membranes

Question 1

Describe the pathway taken by an oxygen molecule from an alveolus to the blood

Answer 1

1. (Across) alveolar epithelium;
2. Endothelium / epithelium of capillary;

Question 2

Explain the importance of one adaptation of the gas exchange surface in the tracheal system of an insect

Answer 2

1. Tracheole (wall) thin/one cell thick;
2. (So) rapid diffusion (into cells)
   OR
   (So) short diffusion pathway/distance;
   OR
3. Tracheoles enter/supply tissues/muscle fibres;
4. (So) diffusion direct into cells
   OR
   (So) short diffusion pathway/distance
   OR
   (So) rapid diffusion (into cells);
   OR
5. Tracheoles are highly branched;
6. (So) short diffusion distance/pathway
   OR
   (So) large surface area for (rapid) diffusion;

Question 3

Aps damage prokaryotic cells by making channels in them. They do not damage eukaryotic cells
Prokaryotic cell membranes do not contain cholesterol

Assess why the aps do not damage the eukaryotic cells of the organisms that produce them

Answer 3

Eukaryotic cell membranes contain cholesterol which stabilises the cell membrane
So aps cannot make channels in them

Question 4

Suggest how environmental conditions have resulted in adaptations of systems using external gas exchange surfaces (like fish) rather than internal ones (like humans)

Answer 4

1. Water has low(er) oxygen partial
   pressure/concentration (than air);
2. So (system on outside) gives large surface area (in contact with water)
   OR
   So (system on outside) reduces diffusion
   distance (between water and blood);
3. Water is dense(r) (than air);
4. (So) water supports the systems/gills;

Question 5

NHE3 actively transports one sodium ion into the cell in exchange for one proton
(hydrogen ion) out of the cell.
Use your knowledge of transport across cell membranes to suggest how NHE3
does this

Answer 5

1. Co-transport;
2. Uses (hydrolysis of) ATP;
3. Sodium ion and proton bind to the protein;
4. Protein changes shape (to move sodium ion and/or proton across the membrane);

Question 6

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

Answer 6

1. Tracheoles have thin walls so short diffusion distance to cells
2. Highly branched/large number of tracheoles so short diffusion distance to cells;
3. Highly branched/large number of tracheoles so large surface area (for gas exchange);
4. Tracheae provide tubes full of air so fast diffusion (into insect tissues);
5. 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;
   OR
   Fluid in the end of the tracheoles that moves out (into tissues) during exercise so larger surface area (for gas exchange);
6. Body can be moved (by muscles) to move air so maintains diffusion/concentration gradient for oxygen/carbon dioxide;