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Carbohydrate structure and function

One small sugar, glucose, joined to itself a few thousand times becomes the starch in a potato, the glycogen in the liver or the cellulose in a plant cell wall. Which of the three forms depends on which isomer of glucose is used and how the units are joined.

Before this GCSE food tests and the idea of a large molecule · Covalent bonding and molecular formulae

COMMON MISCONCEPTION

Starch and cellulose are made of different sugars, which is why one is food and the other is not.

Same sugar, different bond. Both are chains of glucose: starch of α-glucose, cellulose of β-glucose, which differ only in where one hydroxyl group sits on carbon 1. That flips every second glucose in cellulose, straightens the chain, and leaves the enzymes that hydrolyse starch unable to fit it.

What you should be able to do

Draw or recognise alpha and beta glucose, and say where the two differ.
Explain how a glycosidic bond forms, and say what is released when it does.
Name the two monosaccharides in maltose, sucrose and lactose.
Relate the structure of amylose, amylopectin, glycogen and cellulose to what each one does.
Carry out and interpret the Benedict's and iodine tests, including a quantitative Benedict's method.
Explain how the arrangement of microfibrils and lignin makes plant fibres strong, and measure and calculate the tensile strength of a fibre.
Evaluate the use of plant fibres and starch in place of oil-based plastics as a contribution to sustainability.

One sugar, one ring, and two versions of it

Carbohydrates contain only carbon, hydrogen and oxygen, and so do triglycerides. Proteins also contain nitrogen, and sulfur in cysteine and methionine; nucleic acids contain nitrogen and phosphorus as well. A monosaccharide is a single sugar unit, and most of the ones an A-level course asks for fit the general formula (CH₂O)_n. Glucose is a hexose, six carbons, C₆H₁₂O₆. Ribose fits too: it is a pentose, five carbons, C₅H₁₀O₅. Deoxyribose is the exception on the list. As a deoxy sugar it has lost one oxygen, so its formula C₅H₁₀O₄ breaks the pattern. Both pentoses are worth learning here rather than in the nucleic acid unit, because the sugar in RNA and the sugar in DNA are ordinary monosaccharides doing an unusual job.

In solution glucose closes into a ring, and it can close in two ways. The hydroxyl group on carbon 1 ends up either below the ring or above it. Below gives α-glucose, above gives β-glucose, and that is the entire difference between them: same atoms, same formula, one group moved.

Alpha and beta glucose drawn as rings, differing only in whether the hydroxyl group on carbon 1 points below or above the ring, with ribose and deoxyribose beneath them — The difference is one hydroxyl group on one carbon. Everything a storage polysaccharide and a structural one do differently begins here.

Deoxyribose is ribose with one oxygen fewer, on carbon 2, which is what the name says. Both are sugars in their own right and both are reducing sugars, so a Benedict's test shows only that a reducing sugar is present, never which one it is.

Monosaccharide: A single sugar unit, the monomer from which disaccharides and polysaccharides are built.
Monomer: A small molecule from which a larger one is built by joining many of them together.
Polymer: A large molecule made of many similar monomers joined together.
Macromolecule: A very large molecule with a high relative molecular mass, such as a polysaccharide, a protein, a nucleic acid or a triglyceride.
Condensation: A reaction that joins two molecules with a covalent bond and releases a molecule of water.
Hydrolysis: A reaction that breaks a covalent bond between two molecules using a molecule of water.

Joining sugars: one bond, one water

Two monosaccharides join when a hydroxyl group on one reacts with a hydroxyl group on the other. A hydrogen comes off one and a hydroxyl off the other, the two leave together as water, and the sugars are left joined through an oxygen atom. That link is a glycosidic bond, and the reaction is condensation, exactly as it is when amino acids join into a polypeptide.

Two glucose molecules joined by condensation into maltose, with a glycosidic bond formed and one water released — One bond made, one water released. The bond is named after the two carbons it joins, so a bond between carbon 1 of one glucose and carbon 4 of the next is a 1,4 glycosidic bond.

The three disaccharides worth knowing by name are maltose (glucose and glucose), sucrose (glucose and fructose) and lactose (glucose and galactose). Sucrose is the sugar plants move in phloem and the one in a bag on a kitchen shelf; lactose is the sugar in milk.

Adding water across the bond runs the reaction backwards. That is hydrolysis, and it is what maltase, sucrase and lactase do in the small intestine, and what amylase does to starch before them. A person who makes little or no lactase does not hydrolyse lactose, so it reaches the large intestine intact. There it lowers the water potential of the gut contents, so water is drawn in by osmosis, and bacteria ferment it, releasing gas.

Counting the water

A molecule of amylose is 1,600 glucose units long. How many glycosidic bonds does it contain, and how many molecules of water were released while it was assembled?

Show the working

The bonds sit between the units and not on them, so 1,600 units give 1,600 − 1 = 1,599 glycosidic bonds.

Each bond was made by one condensation, and each condensation released one water, so 1,599 molecules of water were released.

Hydrolysing the whole chain back to glucose would require the same 1,599 molecules of water. A chain of n monomers contains n − 1 glycosidic bonds: draw three circles in a row and count the gaps to confirm it.

Starch and glycogen: two stores of the same sugar

Starch is the plant store and is not one molecule but two. Amylose is an unbranched chain of α-glucose joined by 1,4 glycosidic bonds, and because of the angle those bonds make, the chain coils into a helix. Amylopectin has the same 1,4 backbone with extra 1,6 bonds along it, and each of those is a branch point.

Glycogen is the animal store, held in liver and muscle cells. It is built the same way as amylopectin and branched more often, with shorter chains between the branches.

Amylose, amylopectin, glycogen and cellulose drawn side by side, showing how branching and the type of glucose differ — Each branch terminates in a free end, and the hydrolysing enzyme acts from the ends, so a more branched molecule releases glucose faster.

State the consequence of each structural feature rather than the feature alone. The molecules are large and insoluble, so they do not move out of the cell and do not affect the water potential of the cytoplasm. They coil or branch into a compact shape, so a great deal of glucose is held in a small space. They are hydrolysed back to α-glucose, which is the respiratory substrate the cell actually uses.

	Amylose	Amylopectin	Glycogen	Cellulose
Monomer	α-glucose	α-glucose	α-glucose	β-glucose
Bonds	1,4 only	1,4 and 1,6	1,4 and 1,6	1,4 only
Shape	Coiled helix	Branched	Highly branched	Straight chains
Found in	Plants	Plants	Animals	Plant cell walls
Role	Storage	Storage	Storage	Structural

Cellulose and the plant cell wall

Cellulose is a chain of β-glucose joined by 1,4 glycosidic bonds, and the geometry of β-glucose will not allow that unless every second molecule is rotated through 180 degrees. It is the hydroxyl group on carbon 1 sitting above the ring that forces the flip: only then are the two groups that must react close enough to each other.

A chain built that way does not coil. It runs straight, and straight chains lie alongside each other, so hydroxyl groups on neighbouring chains sit close enough to form hydrogen bonds between them. Each of those bonds is weak. There are so many of them along a chain thousands of units long that the bundle they hold together, a microfibril, has a high tensile strength.

The wall resists the outward push of a cell taking in water, so the cell becomes turgid rather than bursting, and turgid cells are what hold a non-woody plant up.

Humans produce no cellulase, no enzyme whose active site fits the β-1,4 bonds between the glucose monomers of cellulose, so cellulose passes through the human gut as fibre. A cow does not produce one either; cellulose is hydrolysed by bacteria in its rumen, so the digestion of cellulose in a ruminant depends on a mutualistic relationship rather than on the animal's own enzymes.

Glycosidic bond: The covalent bond formed by condensation between two monosaccharides, joining them through an oxygen atom.
Disaccharide: Two monosaccharides joined by a glycosidic bond.
Polysaccharide: Many monosaccharides joined by glycosidic bonds into one large molecule.

Testing for sugars and for starch

Benedict's solution contains copper(II) ions in alkaline solution, and it is blue because of them. A reducing sugar reduces those copper(II) ions to copper(I), which comes out of solution as a brick-red precipitate of copper(I) oxide. Add Benedict's solution to the sample and heat it in a water bath at 80 °C or above for five minutes: blue means no reducing sugar, and green, yellow, orange and brick red mean progressively more.

The colours Benedict's solution turns with increasing amounts of reducing sugar, and the colour iodine solution turns with starch — All the monosaccharides are reducing sugars, and so are maltose and lactose. Sucrose is the one that is not.

A negative result therefore does not mean no sugar. To test for a non-reducing sugar such as sucrose, take a fresh sample, boil it with dilute hydrochloric acid to hydrolyse the glycosidic bond, neutralise it with sodium hydrogencarbonate, and only then add Benedict's solution and heat it again. Include the neutralisation step: Benedict's solution requires alkaline conditions.

For starch, use iodine dissolved in potassium iodide solution. It is orange-brown on its own and turns blue-black in the presence of starch, at room temperature and with no heating. Iodine on its own barely dissolves in water, so name the reagent in full.

TRY IT: Interpreting a negative Benedict's result

A student tests a solution with Benedict's solution and it stays blue. She concludes that the solution contains no sugar. Explain why her conclusion is not safe, and describe what she should do next.

Check your answer

Benedict's solution detects reducing sugars only. Sucrose is a non-reducing sugar, so a solution of sucrose gives exactly this result.

She should take a fresh sample, boil it with dilute hydrochloric acid to hydrolyse any non-reducing sugar into its monosaccharides, neutralise the mixture with sodium hydrogencarbonate, then add Benedict's solution and heat again.

A brick-red precipitate now would show that a non-reducing sugar was present all along. A result that stays blue after that would be evidence for no sugar at all.

Measuring reducing sugar concentration

The colour series on its own is qualitative. It becomes semi-quantitative when standards of known glucose concentration are tested alongside the sample in exactly the same way: the sample is matched to the standard closest to it in colour, or in the time taken to first change colour. A quantitative result requires one of two methods, and both begin the same way: make up a set of glucose solutions of known concentration, treat every tube identically, and use them to calibrate the method.

The colorimeter method measures the unreacted reagent. After heating, filter off the copper(I) oxide precipitate and measure the absorbance of the filtrate with a red filter. The more reducing sugar there was, the more copper(II) was reduced, so the less blue remains and the lower the reading. Plot reading against known concentration to get a calibration curve, treat the unknown the same way, and read its concentration off the curve.

A calibration curve of colorimeter reading against known glucose concentration, with an unknown read off it — The curve is drawn from the standards, and the unknown is read off it. A reading outside the range of the standards cannot be read off at all: dilute the sample and repeat.

The alternative is to filter the precipitate, dry it and find its mass, which measures the same thing without a colorimeter and takes rather longer.

In both methods the same variables must be controlled, or the calibration does not apply to the unknown: the same volume and concentration of Benedict's solution, the same volume of sample, the same temperature, and the same time in the water bath. Benedict's solution must be in excess as well, or the tubes with the most sugar all run out of copper(II) ions and give the same reading as each other.

Plant fibres and their tensile strength

Pearson Edexcel Biology A (Salters-Nuffield) follows cellulose out of the plant and into the things people make from it. A plant fibre in the textile sense is a strand of long, thick-walled cells, mostly sclerenchyma fibres, sometimes with xylem vessels alongside, held together along their length. Flax stems give linen, hemp gives rope and canvas, jute gives the hessian used for sacking, and sisal, from the leaves of an agave, gives twine. Cotton is the odd one out: each cotton fibre is a single hair on the seed, with a thick wall of almost pure cellulose and no lignin.

Stem fibres are freed by retting. The stems are soaked in water or left lying on damp ground, microorganisms digest the pectins that bind the fibre strands to the softer tissue around them, and the strands can then be beaten and combed out.

What makes a fibre worth extracting is the arrangement of the cellulose in its wall. A fibre cell lays down a secondary wall inside its primary wall once it has stopped growing, in layers. In the thickest layer the microfibrils are wound in a steep helix, close to parallel with the long axis of the cell, so a pull along the fibre is taken by the microfibrils along their length. To break the fibre that way means breaking covalent glycosidic bonds along the cellulose chains, or the enormous number of hydrogen bonds between chains, at once. Lignin cements the microfibrils and the neighbouring cells together, and because the tapering ends of the cells overlap, the load is passed from cell to cell along the whole strand. Twisting many strands into a yarn or a rope adds friction between them as well. The result is a material that is very strong in tension, which is how rope and sacking are loaded. It is far weaker in compression: a single fibre pushed end to end simply buckles.

Apparatus for finding the breaking load of a plant fibre: a fibre clamped at the top, masses added below it one at a time, and a padded tray underneath — The breaking load is the mass hanging on the fibre when it snaps. The true value lies somewhere between the last load the fibre held and the one that broke it.

Core practical 8 measures this directly. Clamp one end of a fibre to a stand, hang a mass holder from the other end, and add masses in equal small steps, waiting a few seconds after each, until the fibre breaks. Record the total mass at which it broke. The independent variable is the kind of fibre, or a treatment applied to it; the dependent variable is the breaking load. Keep the length of fibre between the clamp and the hook the same, and use fibres of similar thickness, or measure the diameter of each with a micrometer or an eyepiece graticule. Water content changes the result, so store all the fibres in the same conditions. Test several fibres of each kind and take a mean, because two fibres from the same plant rarely break at the same load.

Wear eye protection, because a fibre can whip back when it snaps. Clamp the base of the stand to the bench, or weigh it down, so the hanging load cannot tip it over. Put a padded tray or a box of crumpled paper close beneath the masses so they drop only a short way, and keep feet and hands out from under them. Check your school's risk assessment before you start.

Comparing breaking loads is fair only between fibres of the same thickness, since a thicker fibre of the same material holds more. Tensile strength removes the thickness: it is the breaking force divided by the cross-sectional area of the fibre, and the force is the breaking mass in kilograms multiplied by 9.81 N kg⁻¹.

From a breaking mass to a tensile strength

A flax fibre 0.20 mm in diameter breaks when the total load reaches 1.50 kg. Calculate its tensile strength in N mm⁻². (Idealised figures, chosen to show the method.)

Show the working

Breaking force = 1.50 kg × 9.81 N kg⁻¹ = 14.7 N.

The fibre is treated as a cylinder, so the cross-section is a circle of radius 0.10 mm. Area = πr² = π × 0.10² = 0.0314 mm².

Tensile strength = 14.7 ÷ 0.0314 = 468 N mm⁻², or about 470 N mm⁻² to two significant figures.

One N mm⁻² is one megapascal, so this is about 470 MPa. Halve the diameter before squaring it. A fibre is not a perfect cylinder, so the area is the least certain number in the calculation.

Tensile strength: The greatest pulling force a material can withstand before it breaks, divided by its cross-sectional area.
Retting: Soaking or leaving plant stems in damp conditions so that microorganisms digest the pectins binding the fibres to the tissue around them, and the fibres can be separated.

Starch and plant fibres in place of oil-based plastics

The same specification asks how plant fibres and starch can contribute to sustainability: using resources to meet present needs in a way that does not leave less for the people who come later. Most plastics, polythene, polypropylene and polystyrene among them, are made from crude oil, which is a finite resource, and most of them are not broken down by microorganisms, so what is thrown away persists in landfill and in the sea for a very long time.

Starch can be made into plastics in two ways. Heated with water and a plasticiser such as glycerol, starch granules lose their structure and form a thermoplastic that can be moulded or blown into film; the loose-fill packaging pieces that dissolve in water are made this way. Alternatively, starch from maize or another crop is hydrolysed to glucose, bacteria ferment the glucose to lactic acid, and the lactic acid is polymerised into polylactic acid, PLA, which is used for food packaging, cups and 3D-printer filament. Plant fibres replace oil-based materials more directly: jute and sisal in place of polypropylene sacks and twine, and flax or hemp in place of glass fibre in moulded composite panels such as the inside panels of car doors, where their lower density makes the part lighter.

For	Against
Renewable: the crop is grown again every year, while oil reserves are finite	Growing the crop takes land that could grow food or remain as habitat
The carbon in the product was taken from the air by photosynthesis, so decay or burning returns roughly what the plant absorbed	Fuel for tractors, fertiliser manufacture, processing and transport still releases carbon dioxide, so the product is not carbon neutral
Biodegradable: microorganisms decompose starch and cellulose, so less waste persists	PLA breaks down quickly only in industrial composting at high temperature, and slowly in soil or sea water
Fibres are strong in tension for their mass	Fertilisers and pesticides used on the crop can pollute water and harm wildlife
Less crude oil is used as the raw material	Starch plastics absorb water and soften, so they suit dry, short-lived uses

Biodegradable material buried in a landfill site decays without oxygen, and anaerobic decay releases methane, a more powerful greenhouse gas than carbon dioxide, so the way a product is disposed of counts in the sustainability case as well as the source of its raw material.

A question on this statement is likely to ask you to evaluate or discuss. Give points on both sides, tie each one to the product in the question, and finish with a judgement that follows from them: for single-use packaging that will be composted, the case for a starch-based material is strong; for a product that must last for years in the wet, it is weak.

Sustainability: Using resources to meet present needs in a way that does not reduce the ability of future generations to meet theirs.
Biodegradable: Able to be broken down by microorganisms into simple substances such as carbon dioxide and water.
Polylactic acid: A plastic made by fermenting glucose from plant starch into lactic acid and polymerising it; abbreviated to PLA.

In the exam

Specify which glucose. 'Made of glucose' applies to starch, glycogen and cellulose alike; α-glucose or β-glucose distinguishes them.
Link structure to function in the same sentence: 'branched, so there are many ends for enzymes to act on, so glucose is released quickly', rather than 'branched' alone.
n monosaccharides give n − 1 glycosidic bonds and n − 1 molecules of water. Use that relationship for questions quoting a mass of water.
For Benedict's, name the colour change and the conditions: heat in a water bath. For iodine, name the reagent in full as iodine in potassium iodide solution, and state that it requires no heating.
Give both consequences of insolubility: the store does not leave the cell, and the water potential of the cytoplasm is unaffected.
Tensile strength is force over area. Convert the breaking mass to newtons with 9.81 N kg⁻¹, and use the radius of the fibre, not its diameter.
In a sustainability answer, say where the material comes from and where it goes when thrown away, and give the case against as well as the case for.

Check yourself

Starch and cellulose are both polymers of glucose, and a plant makes both. Explain why one of them can be used as an energy store and the other cannot, and why cellulose is strong enough to hold a cell in shape.

Answer

Starch is built from α-glucose and cellulose from β-glucose, which differ only in the position of the hydroxyl group on carbon 1.

In cellulose that position forces every second glucose to be rotated through 180 degrees, so the chains are straight rather than coiled. Straight chains lie alongside one another and form many hydrogen bonds between them, building microfibrils with a high tensile strength.

Starch coils and branches instead, so it is compact and insoluble: useful as a store because it exerts no effect on water potential and is hydrolysed back to α-glucose when the cell needs it.

The plant's starch-digesting enzymes hydrolyse α-1,4 bonds and do not fit the β-1,4 bonds of cellulose, so the wall is not digested by the cell it surrounds.

Questions

Written to the command words the boards use. Try them on paper before opening a scheme: the marks go to points made, not to length.

Question 15 marks

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

Mark scheme

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 25 marks

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.

Mark scheme

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 34 marks

Describe the structure of a molecule of starch.

Mark scheme

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 44 marks

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.

Mark scheme

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 54 marks

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

Mark scheme

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 64 marks

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.

Mark scheme

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 74 marks

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.

Mark scheme

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 84 marks

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.

Mark scheme

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 94 marks

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.

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

Mark scheme

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 103 marks

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.

Mark scheme

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 113 marks

Compare the structure of glycogen with the structure of amylose.

Mark scheme

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 123 marks

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.

Mark scheme

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 133 marks

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

Mark scheme

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 143 marks

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

Mark scheme

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 153 marks

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

Mark scheme

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 163 marks

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.

Mark scheme

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 173 marks

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

Mark scheme

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 183 marks

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

Mark scheme

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 192 marks

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

Mark scheme

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 202 marks

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

Mark scheme

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

Question 212 marks

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

Mark scheme

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

Question 222 marks

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

Mark scheme

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

Worth remembering

α-glucose has the hydroxyl on carbon 1 below the ring; β-glucose has it above.
One glycosidic bond made, one water released; n monosaccharides give n − 1 of each.
Amylose is unbranched and coiled, amylopectin is branched, glycogen is more branched still, cellulose is straight.
Benedict's solution finds reducing sugars and needs heating; iodine in potassium iodide solution finds starch and does not.
Microfibrils running along a fibre, cemented by lignin, make it strong in tension; tensile strength is breaking force divided by cross-sectional area.

CHECK YOUR PROGRESS

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Draw or recognise alpha and beta glucose, and say where the two differ.
Explain how a glycosidic bond forms, and say what is released when it does.
Name the two monosaccharides in maltose, sucrose and lactose.
Relate the structure of amylose, amylopectin, glycogen and cellulose to what each one does.
Carry out and interpret the Benedict's and iodine tests, including a quantitative Benedict's method.
Explain how the arrangement of microfibrils and lignin makes plant fibres strong, and measure and calculate the tensile strength of a fibre.
Evaluate the use of plant fibres and starch in place of oil-based plastics as a contribution to sustainability.

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Practise this lesson

Practise carbohydrate structure and function with 22 original questions and point-by-point mark schemes

WORKBOOK

The written questions from this topic, on paper with room to work, and a separate book of mark schemes. The question that is set on a diagram stays on this page, where the diagram can be drawn. Free to use; please do not redistribute or sell.

Carbohydrate structure and function

What you should be able to do

One sugar, one ring, and two versions of it

Joining sugars: one bond, one water

Starch and glycogen: two stores of the same sugar

Cellulose and the plant cell wall

Testing for sugars and for starch

Measuring reducing sugar concentration

Plant fibres and their tensile strength

Starch and plant fibres in place of oil-based plastics

In the exam

Questions

Practise this lesson

Question workbook

Question workbook

Answer book