1. 1
   A nurse observes that oxygen moves from the air in the alveoli into the blood in the surrounding capillaries, while carbon dioxide moves in the opposite direction, across a very short diffusion distance and a large surface area.

   (a)

   Which process describes the net movement of oxygen from the air in the alveoli, where it is in higher concentration, into the blood, where it is in lower concentration?

   [1 mark]

   • AActive transport
   • BOsmosis
   • CDiffusion
   • DFiltration

   (b)

   Which feature of the alveoli increases the rate of diffusion of gases into and out of the blood?

   [1 mark]

   • AA thick, multi-layered wall
   • BA large surface area and thin, one-cell-thick walls
   • CA low concentration of oxygen in the alveolar air
   • DA slow blood flow through the capillaries

   (c)

   Explain why a large surface area and a short diffusion distance both increase the rate of diffusion of gases in the alveoli.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chef places raw potato chips into two beakers: one containing a concentrated salt solution and one containing distilled water. After an hour, they observe changes in the length and firmness of the chips in each beaker.

   (a)

   Which term describes the movement of water molecules from the distilled water into the potato chip, across the partially permeable cell membranes?

   [1 mark]

   • AOsmosis
   • BDiffusion of solutes
   • CActive transport
   • DTranspiration

   (b)

   What change would be observed in the potato chips placed in the concentrated salt solution?

   [1 mark]

   • AThey become longer and firmer
   • BThey stay exactly the same length and firmness
   • CThey dissolve completely
   • DThey become shorter and softer (flaccid)

   (c)

   Explain, in terms of water concentration, why the potato chips placed in distilled water increased in mass and became firmer.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A plant scientist investigates how root hair cells absorb mineral ions from soil, where the concentration of ions is lower than inside the root hair cell, meaning the ions must be moved against their concentration gradient.

   (a)

   Explain why the root hair cells must use active transport, rather than diffusion, to absorb mineral ions from the soil, and state where in the cell the energy for this process comes from.

   [3 marks]

   (b)

   Describe how the structure of a root hair cell is adapted to increase the rate of both active transport and diffusion of substances from the soil.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A biologist compares the rate of substance exchange in a small single-celled organism living in a pond with that of large multicellular organisms such as humans, to investigate how surface area to volume ratio limits body size and how larger organisms overcome this limitation.

   (a)

   Explain why small, single-celled organisms living in a pond can rely on diffusion alone to exchange substances with their environment, while larger, multicellular organisms such as humans cannot.

   [6 marks]

   (b)

   Describe how factors such as temperature, concentration gradient and distance affect the rate of diffusion, osmosis and active transport of substances into and out of cells.

   [6 marks]

   Total for question 4: 12 marks

End of questions