Questions on carbohydrate structure and function

Question 1

Describe how a student could compare the breaking loads of fibres from two different plants, including how to make the comparison valid and safe.

Accepted Answer

B1 clamp one end of a fibre to a stand and hang a mass holder from the other end B1 add masses in equal small steps until the fibre breaks, and record the total mass at which it breaks B1 use the same length of fibre between clamp and hook each time, and fibres of similar diameter, or measure the diameter so tensile strength can be calculated B1 test several fibres of each type and calculate a mean breaking load for each plant B1 a safety precaution: eye protection in case the fibre whips back; or a padded tray close beneath the masses with feet kept clear; or the stand clamped to the bench so it cannot tip

Question 2

A supermarket plans to replace its polythene carrier bags, made from crude oil, with bags made from a starch-based plastic. Evaluate this plan as a contribution to sustainability.

Accepted Answer

B1 for: starch comes from crops that are grown again every year, so the raw material is renewable, while crude oil is a finite resource B1 for, a second point: the bag is biodegradable, so fewer bags persist as litter or in landfill; or the carbon in starch was taken from the air by photosynthesis, so decomposing or burning the bag returns roughly what the crop absorbed B1 against: growing the crop uses land that could grow food or remain as habitat, and fertiliser and pesticides can pollute water B1 against, a second point: fuel and fertiliser manufacture still release carbon dioxide, so the bag is not carbon neutral; or a bag buried in landfill decays anaerobically, releasing methane; or starch plastic absorbs water and softens, so the bag may fail when wet B1 a judgement supported by the points made, for example that the plan is more sustainable if the bags are composted rather than sent to landfill

Question 3

Describe the structure of a molecule of starch.

Accepted Answer

B1 a polysaccharide made of many α-glucose monomers joined by glycosidic bonds B1 it is a mixture of two polysaccharides, amylose and amylopectin B1 amylose has 1,4 glycosidic bonds only and coils into a helix B1 amylopectin has 1,4 glycosidic bonds along the chain and 1,6 glycosidic bonds at the branch points

Question 4

Cellulose and amylose are both unbranched polysaccharides of glucose. Explain how the structure of cellulose makes it suitable for its role in a plant cell wall.

Accepted Answer

B1 cellulose is made of β-glucose, so every other glucose molecule is rotated through 180° B1 this makes the chains straight rather than coiled, so they lie parallel to one another B1 hydrogen bonds form between hydroxyl groups on neighbouring chains, holding them together as microfibrils B1 the many hydrogen bonds give high tensile strength, so the wall resists the pressure of a cell taking in water and the cell does not burst

Question 5

A colorimeter is available. Describe how you would find the concentration of reducing sugar in a fruit juice.

Accepted Answer

B1 make up a series of glucose solutions of known concentration B1 add the same volume of Benedict's solution to each, in excess, and heat all the tubes in a water bath at the same temperature for the same time B1 filter off the precipitate and measure the absorbance of each filtrate in the colorimeter, then plot absorbance against known concentration as a calibration curve B1 treat the fruit juice in exactly the same way and read its concentration from the curve, diluting the sample and repeating if its reading falls outside the range of the standards

Question 6

Explain why a person who makes very little lactase experiences bloating and diarrhoea after drinking milk, referring to what happens to the lactose that reaches the large intestine.

Accepted Answer

B1 lactase is the enzyme that hydrolyses the glycosidic bond in lactose, breaking it into glucose and galactose so they can be absorbed B1 with too little lactase, much of the lactose in milk is not hydrolysed in the small intestine and passes undigested into the large intestine B1 bacteria in the large intestine ferment the undigested lactose, producing gas, which distends the gut and causes bloating B1 the undigested lactose, and the products of its fermentation, lower the water potential of the gut contents, so water moves into the lumen by osmosis and diarrhoea follows

Question 7

A hemp fibre with a diameter of 0.15 mm breaks when the total load hanging from it reaches 0.90 kg. Calculate the tensile strength of the fibre in N mm⁻². Take g as 9.81 N kg⁻¹.

Accepted Answer

M1 breaking force = 0.90 × 9.81 = 8.83 N M1 radius = 0.15 ÷ 2 = 0.075 mm M1 cross-sectional area = π × 0.075² = 0.0177 mm² A1 tensile strength = 8.83 ÷ 0.0177 = 500 N mm⁻², accepting 499 to 500 from rounded working

Question 8

Ropes have been made from plant fibres such as hemp and sisal for thousands of years. Explain why plant fibres are strong when they are pulled along their length.

Accepted Answer

B1 the fibres are cells with thick secondary walls made of many cellulose microfibrils B1 in the main layer of the wall the microfibrils run close to parallel with the length of the fibre, so a pull along the fibre is taken by the microfibrils along their length B1 breaking them would mean breaking the covalent glycosidic bonds along the chains or very many hydrogen bonds between neighbouring chains at once B1 lignin binds the microfibrils and neighbouring cells together, and the overlapping ends of the cells pass the load from one cell to the next

Question 9

A student measured the mass needed to break five fibres from each of two plants, P and Q. The results are idealised. Calculate the mean breaking mass for each plant, and give two reasons why these results alone do not show that fibres from P have the greater tensile strength.

Accepted Answer

M1 add the five masses for each plant and divide by 5: 6.20 ÷ 5 and 4.70 ÷ 5 A1 mean for P = 1.24 kg and mean for Q = 0.94 kg B1 tensile strength depends on cross-sectional area, and the diameters of the fibres were not measured, so the fibres from P may simply be thicker B1 any one of: the results for Q vary widely, from 0.70 to 1.40 kg, so five fibres may not give a reliable mean; the ranges overlap, since one Q fibre held more than any P fibre; no statistical test was done; the length or water content of the fibres may not have been controlled

Question 10

One molecule of glycogen was built from 2,400 molecules of α-glucose. Taking the relative molecular mass of water as 18, calculate the total relative mass of the water released while that molecule was assembled.

Accepted Answer

M1 number of glycosidic bonds = 2400 − 1 = 2399 M1 one molecule of water is released per bond formed, so 2399 molecules of water are released A1 2399 × 18 = 43 182

Question 11

Compare the structure of glycogen with the structure of amylose.

Accepted Answer

B1 both are polysaccharides of α-glucose joined by 1,4 glycosidic bonds, whereas only glycogen also has 1,6 glycosidic bonds B1 glycogen is branched, whereas amylose is unbranched B1 amylose coils into a helix, whereas glycogen has shorter chains between its branch points and is more compact

Question 12

A student adds Benedict's solution to a solution of a carbohydrate and heats it in a water bath. The mixture stays blue. Suggest why this result does not show that the solution contains no carbohydrate, and suggest what the student should do next.

Accepted Answer

B1 Benedict's solution detects reducing sugars only, and the carbohydrate may be a non-reducing sugar such as sucrose, or a polysaccharide such as starch B1 boil a fresh sample with dilute hydrochloric acid to hydrolyse the glycosidic bonds, then neutralise it with sodium hydrogencarbonate B1 add Benedict's solution and heat again; a brick-red precipitate would now show that a non-reducing sugar was present. Testing a separate sample with iodine in potassium iodide solution would detect starch

Question 13

State the molecular formula of ribose, state the molecular formula of deoxyribose, and state which single atom differs between them.

Accepted Answer

B1 ribose has the molecular formula C₅H₁₀O₅ B1 deoxyribose has the molecular formula C₅H₁₀O₄ B1 deoxyribose has one fewer oxygen atom than ribose, lost from carbon 2, which is why it is called a deoxy sugar

Question 14

Describe how α-glucose and β-glucose differ, and describe the general formula that both fit as monosaccharides.

Accepted Answer

B1 both are ring forms of glucose with the molecular formula C₆H₁₂O₆, fitting the general monosaccharide formula (CH₂O)_n B1 they differ only in the position of the hydroxyl group on carbon 1: below the ring gives α-glucose B1 above the ring gives β-glucose, with every other atom and bond in the two rings identical

Question 15

Describe the procedure and the positive result of the iodine test for starch.

Accepted Answer

B1 iodine dissolved in potassium iodide solution is added directly to the sample at room temperature, with no heating required B1 the reagent is orange-brown on its own B1 a colour change to blue-black indicates that starch is present

Question 16

Explain why Benedict's solution must be used in excess when carrying out a quantitative test for reducing sugar concentration by measuring the mass of precipitate formed.

Accepted Answer

B1 the mass of precipitate formed depends on how much reducing sugar is available to react, provided enough copper(II) is always available to react with it B1 if Benedict's solution were not in excess, a tube with a high sugar concentration could run out of copper(II) ions before all the sugar had reacted, giving the same precipitate mass as a tube with an even higher concentration B1 using an excess in every tube ensures that the amount of precipitate formed is limited by the amount of reducing sugar present, and by nothing else, so the calibration curve is valid

Question 17

Compare a monosaccharide with a polysaccharide, referring to solubility, structure and role as a respiratory substrate.

Accepted Answer

B1 a monosaccharide such as glucose is a single small sugar unit that is soluble in water and can affect the water potential of a cell in solution, whereas a polysaccharide is a large molecule made of many monosaccharides and is generally insoluble B1 a monosaccharide can be used directly as a respiratory substrate, whereas a polysaccharide such as starch or glycogen must first be hydrolysed back to its monosaccharide units before it can be respired B1 a polysaccharide's coiled or branched structure packs a large quantity of stored energy into a small, compact space, which a store of free monosaccharide of the same energy content could not do without drawing in water by osmosis

Question 18

Outline how polylactic acid (PLA) plastic is made from the starch in a crop such as maize.

Accepted Answer

B1 the starch is hydrolysed to glucose B1 bacteria ferment the glucose to lactic acid B1 the lactic acid molecules are joined together, or polymerised, to form polylactic acid

Question 19

State what is meant by a monosaccharide, and state the molecular formula of glucose.

Accepted Answer

B1 a monosaccharide is a single sugar unit, the monomer from which disaccharides and polysaccharides are built B1 the formula of glucose is C₆H₁₂O₆

Question 20

Name the bond formed when two monosaccharides are joined together, and name the two monosaccharides that make up sucrose.

Accepted Answer

A1 a glycosidic bond, formed by condensation A1 glucose and fructose

Question 21

Name the two monosaccharides that make up maltose, and name the two monosaccharides that make up lactose.

Accepted Answer

A1 maltose is made of glucose and glucose A1 lactose is made of glucose and galactose

Question 22

Flax stems are left lying on damp ground for several weeks before the fibres are taken out of them. Explain why.

Accepted Answer

B1 microorganisms digest the pectins that bind the fibres to the cells around them; this is retting B1 the strands of fibre can then be separated from the softer tissue around them without being broken

Fibre	Breaking mass, P / kg	Breaking mass, Q / kg
1	1.20	0.80
2	1.35	1.40
3	1.10	0.70
4	1.25	0.95
5	1.30	0.85