1. 1
   A student places a small piece of red onion tissue into a beaker of pure water at room temperature and leaves it for 30 minutes. The student then repeats the experiment using a second piece of onion tissue, but this time places it in a concentrated sugar solution instead of pure water. Both pieces are weighed before and after the experiment.

   (a)

   After 30 minutes, the tissue has become firmer and slightly heavier. Which process is mainly responsible for the water entering the onion cells?

   [1 mark]

   • AActive transport, because water moves against its concentration gradient
   • BOsmosis, because water moves from a dilute solution into a more concentrated solution through a partially permeable membrane
   • CDiffusion of gases across the cell membrane
   • DSimple diffusion of solute particles out of the cell

   (b)

   The second piece of onion tissue, placed in the concentrated sugar solution, is found to have lost mass after 30 minutes. Which statement correctly explains this result?

   [1 mark]

   • AWater moved out of the onion cells into the sugar solution because the sugar solution was more concentrated than the cell contents
   • BSugar molecules moved into the onion cells by active transport, increasing the cell mass
   • CWater moved into the onion cells because the cell contents were more concentrated than the sugar solution
   • DThe onion tissue respired more quickly in the concentrated solution, reducing its mass

   (c)

   Suggest two variables, other than the concentration of the sugar solution, that the student should control to make this a fair test. For each variable, explain why it must be controlled.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A species of tapeworm lives in the human small intestine and absorbs digested food molecules directly through its body surface, as it has no digestive system of its own. The tapeworm's body is long, flattened, and ribbon-like, with a highly folded outer surface and a very thin body wall only a few cells thick.

   (a)

   Which feature of the tapeworm's body surface most directly increases the rate of diffusion of nutrients into its cells?

   [1 mark]

   • AThe folded surface increases the distance nutrients must travel to reach every cell
   • BThe folded surface increases the volume of the tapeworm's body
   • CThe folded surface reduces the concentration gradient across the membrane
   • DThe folded surface increases the surface area available for diffusion

   (b)

   The tapeworm's body wall is only a few cells thick. Why does this thinness help the tapeworm absorb nutrients efficiently?

   [1 mark]

   • AIt allows the tapeworm to actively transport nutrients using less energy
   • BIt increases the concentration of nutrients in the small intestine
   • CIt provides a shorter diffusion path, so nutrients reach the tapeworm's cells more quickly
   • DIt reduces the surface area in contact with digested food

   (c)

   The tapeworm lives permanently inside the human small intestine, which already contains a high concentration of digested sugar molecules following a meal. Explain how the tapeworm can still absorb sugar molecules from the intestine even after the concentration gradient has become very small, or has reversed.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A gardener notices that seedlings grown in soil with very little water and few dissolved minerals still manage to absorb mineral ions into their root hair cells, even though the soil solution is more dilute than the solution inside the root hair cells. The gardener also observes that the seedlings' roots are covered in many fine, hair-like projections.

   (a)

   Explain how the root hair cells are able to absorb mineral ions under these conditions.

   [3 marks]

   (b)

   The gardener notices that the seedlings' roots are covered in many fine, hair-like projections (root hairs). Explain how the shape of root hair cells helps them to absorb water and mineral ions efficiently, and describe two other features that make an exchange surface effective in general.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A species of small mammal has a body mass of only 3 grams, while a much larger mammal of the same general body plan has a body mass of 3000 grams. Both animals are warm-blooded and must generate enough heat through respiration to maintain their body temperature. The smaller mammal has a much higher surface area to volume ratio than the larger mammal.

   (a)

   Explain, using the idea of surface area to volume ratio, why the smaller mammal must eat proportionally far more food relative to its body mass than the larger mammal, and why multicellular organisms in general require specialised exchange surfaces and transport systems rather than relying on diffusion alone across their outer body surface.

   [6 marks]

   (b)

   The small mammal's lungs, like the lungs of the larger mammal, are made up of millions of tiny air sacs called alveoli, each surrounded by a dense network of capillaries. Explain how the structure of the alveoli and their surrounding capillaries makes the lungs an effective exchange surface for gases, and describe how this structure allows the lungs to meet the higher metabolic demand of the smaller mammal, given its greater surface area to volume ratio.

   [6 marks]

   Total for question 4: 12 marks

End of questions