Water and carbohydratesEdexcel International A Level Biology: Subtopic test

1. 1
   Blood plasma is about 90% water. It carries dissolved glucose, amino acids and mineral ions such as hydrogencarbonate ions (HCO₃⁻) around the body of a mammal.

   (a)

   Which statement correctly describes a molecule of water?

   [1 mark]

   • AThe hydrogen atoms carry a slight negative charge and the oxygen atom a slight positive charge
   • BThe oxygen atom carries a slight negative charge and the hydrogen atoms a slight positive charge
   • CThe electrons in the O–H bonds are shared equally, so the molecule has no charged regions
   • DThe oxygen atom carries a full negative charge and each hydrogen atom a full positive charge

   (b)

   Which statement best explains why hydrogencarbonate ions dissolve in the water of blood plasma?

   [1 mark]

   • AThe ions form covalent bonds with the water molecules
   • BWater molecules are non-polar, so they slip between the ions
   • COnly the hydrogen atoms of water molecules are attracted to the ions
   • DWater molecules cluster around each ion, with their oppositely charged ends attracted to it

   (c)

   Explain how the structure of glucose allows it to dissolve in blood plasma so that it can be transported.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A marathon runner stores energy as glycogen in her liver and muscle cells. During a race, the glycogen is broken down to provide glucose for respiration.

   (a)

   Which statement correctly describes glycogen?

   [1 mark]

   • AA polymer of fructose found in plant cells
   • BA branched polymer of glucose found in animal cells
   • CAn unbranched polymer of glucose found in plant cells
   • DA disaccharide of two glucose molecules found in muscle

   (b)

   The runner's glycogen is completely hydrolysed. Which product is released?

   [1 mark]

   • AFructose
   • BGalactose
   • CGlucose
   • DMaltose

   (c)

   Explain why glycogen is a suitable energy store for the runner's muscle cells.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Potato tuber cells store starch over the winter. In spring the tuber must release glucose quickly to supply energy for the growth of new shoots. Starch is a mixture of two polysaccharides, amylose and amylopectin.

   (a)

   Compare the structures of amylose and amylopectin.

   [3 marks]

   (b)

   Explain how the structure of starch makes it suitable both for storing energy over the winter and for releasing glucose rapidly in spring.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student estimated the concentration of reducing sugar in a sample of fruit juice. She made glucose solutions of known concentrations to use as colour standards. She heated equal volumes of each standard and of the juice with Benedict's reagent. She also tested a leaf extract with iodine solution, which turned blue-black. She boiled a second sample of the leaf extract with dilute hydrochloric acid, neutralised it, and heated it with Benedict's reagent. This sample gave a brick-red precipitate, whereas the untreated leaf extract had given no colour change.

   (a)

   Describe how the student could use Benedict's reagent and the glucose solutions to estimate the concentration of reducing sugar in the juice.

   [6 marks]

   (b)

   Explain the results with the leaf extract and evaluate how well the Benedict's method estimates concentration.

   [6 marks]

   Total for question 4: 12 marks

End of questions