Osmosis and water potentialAQA A-Level Biology: Subtopic test

1. 1
   Cylinders of potato tissue were placed in sucrose solutions of different concentrations. Pure water has a water potential of 0 kPa and the potato cells have a water potential of −400 kPa.

   (a)

   Which solution would cause water to leave the potato cells by osmosis?

   [1 mark]

   • APure water
   • BA sucrose solution with a water potential of −700 kPa
   • CA sucrose solution with a water potential of −400 kPa
   • DA sucrose solution with a water potential of −100 kPa

   (b)

   Which statement defines osmosis?

   [1 mark]

   • AThe net movement of water from a higher to a lower water potential across a partially permeable membrane
   • BThe net movement of solute molecules from a higher to a lower concentration across a membrane
   • CThe net movement of water from a lower to a higher water potential across a partially permeable membrane
   • DThe movement of water against a water potential gradient using ATP

   (c)

   Explain why the mass of a potato cylinder placed in pure water increased.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student has a stock solution of 1.0 mol dm⁻³ sucrose and distilled water. She makes a dilution series and measures the mass of potato cylinders before and after 30 minutes in each solution.

   (a)

   What volume of the stock solution is needed to make 50 cm³ of a 0.2 mol dm⁻³ solution?

   [1 mark]

   • A5 cm³
   • B20 cm³
   • C40 cm³
   • D10 cm³

   (b)

   Why does she calculate the percentage change in mass rather than the change in mass?

   [1 mark]

   • ATo remove water from the surface of the cylinders
   • BTo make the results statistically significant
   • CThe cylinders may have different starting masses, so percentages allow a fair comparison
   • DTo cancel out the effect of osmosis

   (c)

   Explain how a calibration curve of percentage change in mass against sucrose concentration can be used to find the water potential of the potato tissue.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student recorded the percentage change in mass of potato cylinders after 30 minutes in sucrose solutions. In 0.0 mol dm⁻³ the change was +14.0%, in 0.2 mol dm⁻³ it was +6.0%, in 0.4 mol dm⁻³ it was −4.0%, in 0.6 mol dm⁻³ it was −11.0% and in 0.8 mol dm⁻³ it was −17.0%.

   (a)

   Explain why the cylinder in 0.0 mol dm⁻³ sucrose gained mass and the cylinder in 0.8 mol dm⁻³ sucrose lost mass.

   [3 marks]

   (b)

   Use the data to determine the concentration of sucrose solution that has the same water potential as the potato tissue. Show your working and explain your answer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Plant tissues gain or lose water by osmosis depending on the water potential of the surrounding solution. A student has distilled water, a 1.0 mol dm⁻³ sucrose stock solution, a potato, an onion, a cork borer, a balance and a microscope.

   (a)

   The student places pieces of onion epidermis in distilled water, in a solution with the same water potential as the cell sap, and in a very concentrated sucrose solution. Describe and explain what she would see in each, in terms of water potential.

   [6 marks]

   (b)

   Describe how the student could produce a dilution series and a calibration curve to find the water potential of the potato tissue.

   [6 marks]

   Total for question 4: 12 marks

End of questions