1. 1
   A student places three identical strips of potato tissue into three beakers of sucrose solution: beaker X (0% sucrose, i.e. pure water), beaker Y (20% sucrose), and beaker Z (a sucrose concentration equal to the concentration of the potato cell cytoplasm). Each strip is weighed before and after 30 minutes.

   (a)

   Which row correctly describes the direction of net water movement in beaker X?

   [1 mark]

   • AWater moves into the potato cells by osmosis, so the strip gains mass
   • BWater moves out of the potato cells by osmosis, so the strip loses mass
   • CWater moves into the potato cells by active transport, so the strip gains mass
   • DThere is no net movement of water because the solution is dilute

   (b)

   Which statement best explains why the potato strip in beaker Z shows no change in mass?

   [1 mark]

   • AActive transport pumps water out of the cells at the same rate that osmosis brings it in
   • BThe sucrose solution in Z is isotonic to the cell cytoplasm, so the water potential inside and outside the cells is equal and there is no net movement of water
   • CThe cell membrane in beaker Z becomes impermeable to water
   • DDiffusion of sucrose into the cells balances the movement of water

   (c)

   The potato strip in beaker Y had a starting mass of 5.20 g and a final mass of 4.68 g. Calculate the percentage change in mass, showing your working, and state whether this is a gain or a loss.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A perfume is sprayed at one end of a still, sealed room. After several minutes, a person at the far end of the room can smell it, even though there are no air currents.

   (a)

   Which process explains this and what is its cause at a particle level?

   [1 mark]

   • AActive transport; particles moved against a concentration gradient using energy from respiration
   • BOsmosis; the movement of water molecules from a high to a low concentration
   • CDiffusion; the random movement of perfume particles from a high to a low concentration
   • DDiffusion; particles are pushed by air pressure differences created by the spray

   (b)

   The same room is then heated so its temperature rises by 15 degrees Celsius. A second spray of the same perfume is detected by the person at the far end more quickly than before. Which explanation for this observation is correct?

   [1 mark]

   • AThe higher temperature makes the air particles heavier, forcing the perfume across the room
   • BThe higher temperature increases the concentration gradient, which is the only factor affecting diffusion rate
   • CHeating converts diffusion into active transport, which is a faster process
   • DThe higher temperature gives the perfume particles more kinetic energy, so they move faster and diffuse more quickly

   (c)

   Give two other factors, besides temperature, that would increase the rate of diffusion of the perfume particles across the room, and explain how each factor has this effect.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Root hair cells absorb mineral ions, such as nitrate ions, from the soil. The concentration of nitrate ions inside a root hair cell is often much higher than the concentration of nitrate ions in the surrounding soil water.

   (a)

   Explain how root hair cells are able to absorb nitrate ions against this concentration gradient, and explain why root hair cells contain unusually large numbers of mitochondria.

   [3 marks]

   (b)

   Root hair cells have long, thin extensions called root hairs that project into the soil. Explain, in terms of surface area and diffusion/osmosis/active transport, how the shape of a root hair cell adapts it for the efficient absorption of water and mineral ions from the soil.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A gardener wants to determine the concentration of sucrose solution that has the same water potential as the cells inside a carrot, so that carrots can be stored in a solution that keeps them fresh without gaining or losing mass. She cuts identical cylinders of carrot tissue, blots them dry, and weighs each one. She then places one cylinder into each of six beakers containing sucrose solutions of 0%, 5%, 10%, 15%, 20% and 25% concentration, leaving the rest of the method identical to the potato investigation described earlier (same volume of solution, same time, same temperature). After 40 minutes she removes each cylinder, blots it dry again, and reweighs it, then calculates the percentage change in mass for each concentration. One student in the class suggests that, instead of measuring mass, the concentration matching the carrot's water potential could instead be found by measuring the length of each carrot cylinder before and after the investigation, since cells that gain water become more turgid and longer.

   (a)

   Analyse how the gardener could use her results to estimate the sucrose concentration that matches the water potential of the carrot cells, and evaluate two ways in which her method could be improved to make this estimate more reliable and accurate.

   [6 marks]

   (b)

   One student in the gardener's class suggests that, instead of measuring mass, the concentration matching the carrot's water potential could instead be found by measuring the length of each carrot cylinder before and after the investigation, since cells that gain water become more turgid and longer. Evaluate whether measuring length would be a more or less reliable measurement than measuring mass in this investigation, and explain one source of random error and one source of systematic error that could affect the results using either method.

   [6 marks]

   Total for question 4: 12 marks

End of questions