Water, carbohydrates and lipids: exam questions

Answer: B – water molecules are dipoles, with charged ends attracted to ions

• water molecules are dipoles / polar / have slightly positive and slightly negative ends (1)
• ions and polar molecules (such as glucose and amino acids) are attracted to the charged ends of water and are surrounded by water molecules / dissolve (1)
• dissolved substances can be carried in blood plasma / moved around the body (1)
• lipids / non-polar substances do not dissolve (so need other transport) (1)
• Max 3

Answer: C – glucose and galactose (A is maltose, B is sucrose)

• a polymer / polysaccharide of α-glucose, so hydrolysed to glucose for respiration (1)
• insoluble / large molecule, so does not affect water potential / cannot leave the cell (1)
• highly branched / coiled, so compact and stores a lot of glucose in a small space (1)
• many branches give many free ends, so enzymes hydrolyse glucose rapidly / more quickly (1)
• Max 4

• both are polymers of α-glucose / both joined by α-1,4 glycosidic bonds (1)
• amylose is unbranched but amylopectin is branched (1)
• amylopectin has α-1,6 glycosidic bonds (at branch points), amylose does not (1)
• amylose coils into a helix / amylopectin is less tightly coiled (1)
• Max 3. Answer must include at least one similarity and one difference.

• condensation reaction between two glucose molecules (1)
• –OH and –H removed / water molecule released (1)
• α-1,4 glycosidic bond formed (1)
• broken down by hydrolysis, by adding water, to give two glucose molecules (1)
• Max 3

• glycerol joins with three fatty acids (1)
• condensation reaction (1)
• –OH group of glycerol reacts with the –COOH / carboxyl group of a fatty acid (1)
• ester bond formed (three in total) (1)
• water released (three molecules in total) (1)
• Max 4

• saturated have no C=C double bonds but unsaturated have one or more (1)
• saturated chains are straight whereas unsaturated chains are kinked / bent (1)
• saturated pack more closely, so a higher melting point / solid at room temperature (ora) (1)
• both contain ester bonds / both are triglycerides of fatty acids and glycerol (1)
• Max 3. Answer must include at least one similarity and one difference.

• make a range of glucose solutions of known concentration (standards) by diluting a stock solution (1)
• add the same volume of Benedict’s reagent to the same volume of each standard and of the unknown (1)
• heat all tubes in a water bath at the same temperature for the same time (1)
• compare the colour of the unknown with the colours of the standards (against a white background) (1)
• concentration of the unknown is that of the closest matching standard (1)
• repeat and compare / use a smaller concentration range to improve accuracy (1)
• wear eye protection / use tongs, as Benedict’s reagent is an irritant and the water is hot (1)
• Max 5

• dilution factor = 1.0 ÷ 0.25 = 4 (1)
• volume of stock = 20 ÷ 4 = 5 cm³ (1)
• volume of distilled water = 20 − 5 = 15 cm³ (1)