1. 1
   A species of marine worm lives in sea water, which contains about 0.5 mol dm⁻³ of dissolved sodium and chloride ions. The worm's body fluids are isotonic with sea water. It has no cell walls and cannot regulate the solute concentration of its body fluids. Worms carried by a storm into an estuary, where the water is half the concentration of sea water, swelled and many died.

   (a)

   How are the chloride ions (Cl⁻) in sea water kept in solution?

   [1 mark]

   • AThey are attracted to the δ+ hydrogen atoms of surrounding water molecules
   • BThey are attracted to the δ− oxygen atoms of surrounding water molecules
   • CThey form covalent bonds with water molecules
   • DThey are non-polar, so water molecules surround them

   (b)

   Compared with the worm's body fluids, the estuary water is:

   [1 mark]

   • Aisotonic
   • Bhypertonic
   • Cat the same water potential
   • Dhypotonic

   (c)

   Explain why the worms swelled in the estuary.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Two neighbouring cells in a leaf are compared. Cell P has a solute potential of −800 kPa and a pressure potential of +300 kPa. Cell Q has a solute potential of −600 kPa and a pressure potential of +200 kPa. Water potential is given by ψw = ψs + ψp.

   (a)

   What is the water potential of cell P?

   [1 mark]

   • A−1100 kPa
   • B−500 kPa
   • C+500 kPa
   • D+300 kPa

   (b)

   In which direction is there a net movement of water, and why?

   [1 mark]

   • AFrom P to Q, because P has the higher pressure potential
   • BFrom P to Q, because P has the more negative solute potential
   • CFrom Q to P, because Q has the higher water potential
   • DThere is no net movement, because both cells are turgid

   (c)

   Explain why water potentials are stated relative to pure water, and why the water potential of cell P is negative.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Discs of carrot of equal thickness were cut, their diameters measured, and groups of discs placed in mannitol solutions with water potentials of 0 (distilled water), −300, −600, −900 and −1200 kPa. The solutions were in open beakers at atmospheric pressure. After 3 hours the mean percentage changes in diameter were: 0 kPa, +6.0 %; −300 kPa, +2.4 %; −600 kPa, −0.8 %; −900 kPa, −3.6 %; −1200 kPa, −6.2 %. Microscopic examination of discs from the −1200 kPa solution showed that most cells had their membranes pulled away from their walls.

   (a)

   Determine the water potential of the carrot tissue. Show your working.

   [3 marks]

   (b)

   Explain, in terms of solute potential and pressure potential, the changes in the discs placed in distilled water and in the −1200 kPa solution.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   On a hot afternoon, measurements were made along the water pathway of a tall oak tree. The water potential of the soil water was −30 kPa. Root cortex cells had a solute potential of −700 kPa and a pressure potential of +400 kPa. Xylem sap in the upper trunk had a solute potential of −100 kPa and a water potential of −1500 kPa. Leaf mesophyll cells had a water potential of −1800 kPa and the air around the leaves −95 000 kPa. A nearby orchard is irrigated with salty water, which lowers the water potential of its soil water to −900 kPa; before irrigation, the root cortex cells of the orchard trees had the same solute and pressure potentials as the oak's.

   (a)

   Using the data, explain in terms of water potential and its components why water moves from the soil, through the oak tree, to the air.

   [6 marks]

   (b)

   Discuss the likely effects of irrigation with salty water on the root cortex cells of the orchard trees and on their water uptake.

   [6 marks]

   Total for question 4: 12 marks

End of questions