WaterAQA A-Level Biology: Subtopic test

1. 1
   The small intestine digests a meal containing starch, protein and lipid, and the products are absorbed into cells where they are built into new polymers. All of these reactions take place in aqueous solution.

   (a)

   Which statement about the role of water in the reactions that digest and rebuild these polymers is correct?

   [1 mark]

   • AWater is a product of hydrolysis reactions
   • BWater is a reactant in condensation reactions
   • CWater is a reactant in hydrolysis reactions and a product of condensation reactions
   • DWater is a product of both hydrolysis and condensation reactions

   (b)

   Which property of water allows sodium ions and glucose to dissolve so that metabolic reactions can occur in the cytoplasm?

   [1 mark]

   • AIts molecules are polar, with partial charges that attract ions and polar molecules
   • BIt has a relatively high specific heat capacity
   • CIts molecules show strong cohesion
   • DIt has a relatively large latent heat of vaporisation

   (c)

   Maltose is hydrolysed by the enzyme maltase in the small intestine. Describe what happens to the maltose molecule in this reaction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A pond in summer contains fish, algae and pond skaters (insects that walk across the surface of the water without sinking). During one day the air temperature above the pond rises by 15 °C but the temperature of the water rises by only 3 °C.

   (a)

   Which property of water explains why pond skaters can stand on the surface of the pond without sinking?

   [1 mark]

   • AIts relatively high specific heat capacity
   • BIts relatively large latent heat of vaporisation
   • CIts ability to act as a solvent
   • DStrong cohesion between water molecules, which produces surface tension at the air boundary

   (b)

   Which property of water best explains why the pond water warms by much less than the air?

   [1 mark]

   • AWater molecules are polar
   • BWater has a relatively high specific heat capacity
   • CWater molecules show strong cohesion
   • DWater acts as a metabolite in condensation reactions

   (c)

   Explain why the temperature of the pond water changes less than the temperature of the air, and why this is an advantage to the organisms living in the pond.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A tall oak tree transports water from its roots to its leaves in xylem vessels. In full sunlight each leaf absorbs 1.20 kJ of energy per minute from the Sun, which would otherwise heat it. The energy needed to evaporate 1 g of water from a leaf is 2.4 kJ.

   (a)

   Explain how the strong cohesion between water molecules helps the tree to transport water up to its leaves.

   [3 marks]

   (b)

   Calculate the mass of water that must evaporate from one leaf each minute to remove all the energy the leaf absorbs. Hence explain why the large latent heat of vaporisation of water is an advantage to the tree.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   During a marathon on a hot day a runner's muscle cells carry out many metabolic reactions, including the hydrolysis of ATP and the synthesis of glycogen, while the runner's body temperature must stay close to 37 °C.

   (a)

   Describe and explain the importance of water as a solvent and as a metabolite in the runner's muscle cells.

   [6 marks]

   (b)

   Explain how the properties of water help to keep the runner's body temperature close to 37 °C during the race.

   [6 marks]

   Total for question 4: 12 marks

End of questions