Carbohydrates
AQA
A-Level
Monosaccharides are single sugar units that act as the monomers from which larger carbohydrates are made. Glucose, galactose and fructose are the common monosaccharides named in this specification. Monosaccharides can join in condensation reactions to form disaccharides and polysaccharides, while hydrolysis separates larger carbohydrates into smaller sugar units. This monomer relationship is the key distinction: a monosaccharide is one building block, whereas a larger carbohydrate contains sugar units joined by bonds.
24
Objectives
21
Exam Tips
36
Pitfalls
36
Key Terms
38
Mark Points
Subtopics in this area
Monosaccharides are the monomers from which larger carbohydrates are made. Glucose, galactose and fructose are common monosaccharides.
A condensation reaction between two monosaccharides forms a glycosidic bond.
Disaccharides are formed by the condensation of two monosaccharides: maltose is a disaccharide formed by condensation of two glucose molecules sucrose is a disaccharide formed by condensation of a glucose molecule and a fructose molecule lactose is a disaccharide formed by condensation of a glucose molecule and a galactose molecule.
Glucose has two isomers, α-glucose and β-glucose, with structures: Polysaccharides are formed by the condensation of many glucose units.
Glycogen and starch are formed by the condensation of α- glucose. Cellulose is formed by the condensation of β-glucose.
The basic structure and functions of glycogen, starch and cellulose.
The relationship of structure to function of these substances in animal cells and plant cells.
Biochemical tests using Benedict's solution for reducing sugars and non-reducing sugars and iodine/potassium iodide for starch.
Carbohydrates Revision Guide
Learning Objectives
What you need to know and understand
- Define monosaccharides as the monomers of larger carbohydrates.
- Name glucose, galactose and fructose as common monosaccharides.
- Explain how monosaccharide monomers relate to the formation of larger carbohydrates.
- Describe the formation of a glycosidic bond between two monosaccharides, naming the reaction and the molecule released.
- Identify from a diagram whether the glycosidic bonds shown are 1,4 or 1,6, and say what that means for the shape of the chain.
- Explain why a glycosidic bond between two beta-glucose molecules requires one of them to be rotated through 180 degrees.
- State which two monosaccharides condense to form maltose, sucrose and lactose.
- Describe how a disaccharide is formed and broken down, naming the bond, the reaction and the enzyme involved.
- Suggest why an organism supplied with a disaccharide shows a delay before its respiration rate rises.
- Explain the structural difference between alpha-glucose and beta-glucose, naming the carbon atom involved.
- Draw or label a diagram to show the inverted hydrogen and hydroxyl groups on carbon 1 of beta-glucose.
- Explain how the isomer used determines whether the resulting polysaccharide chain is coiled, branched or straight.
- State which glucose isomer forms starch, glycogen and cellulose, and give the bond type in each.
- Compare and contrast starch and cellulose in linked sentences covering monomer, bond, branching and chain shape.
- Explain why glycogen has more 1,6 branch points than amylopectin and what that means for its chain structure.
- Describe the basic structure of glycogen, starch and cellulose in terms of monomer, bond type and branching.
- State where each of the three polysaccharides is found and what job it does there.
- Explain why cellulose is described as a structural polysaccharide while starch and glycogen are stores of glucose.
- Explain why an insoluble polysaccharide is a better store of glucose in a cell than the equivalent mass of soluble glucose.
- Explain how branching in glycogen allows glucose to be released more rapidly than from amylose.
- Explain how hydrogen bonding between cellulose chains produces microfibrils and gives the plant cell wall tensile strength.
- Describe the method for testing a solution for reducing sugars, including the reagent, heating, and the positive result.
- Describe how to test for a non-reducing sugar, explicitly including boiling with dilute acid, neutralisation, and retesting.
- Explain how the Benedict's test can be made quantitative using a colorimeter and a calibration curve.
Marking Points
Key points examiners look for in your answers
- State that monosaccharides are the monomers from which larger carbohydrates are made, bonding via condensation reactions to form disaccharides and polysaccharides.
- Name glucose, galactose and fructose as common monosaccharides.
- Explain that a monosaccharide is one sugar unit and is therefore a monomer rather than a carbohydrate polymer.
- Link monosaccharides to the formation of larger carbohydrates through condensation reactions.
- one mark for stating that the bond forms by a condensation reaction between two monosaccharides
- one mark for stating that a molecule of water is released as the bond forms
- one mark for naming the glycosidic bond, and identifying it as a 1,4 linkage where the diagram shows a straight chain
- one mark for stating that alternate beta-glucose molecules are rotated 180 degrees so the bond can form
- one mark for stating that the bond is broken by hydrolysis, using a water molecule
- one mark for stating that two monosaccharides join by condensation, forming a glycosidic bond and releasing water
- one mark for maltose as two glucose molecules
- one mark for sucrose as glucose and fructose, and lactose as glucose and galactose
- one mark for identifying maltose as a disaccharide that must be hydrolysed into glucose before it is respired
- one mark for naming the specific enzyme that hydrolyses a named disaccharide
- one mark for stating that alpha-glucose and beta-glucose have the same molecular formula but a different arrangement of atoms
- one mark for identifying the difference as the inverted positions of the hydrogen and hydroxyl groups on carbon atom 1
- one mark for stating that many glucose units join by condensation, forming glycosidic bonds, to make a polysaccharide
- one mark for linking alpha-glucose to starch and glycogen and beta-glucose to cellulose
- one mark for stating that alternate beta-glucose molecules are rotated 180 degrees in the chain
- one mark for stating that starch is formed from alpha-glucose but cellulose is formed from beta-glucose, written as a comparison
- one mark for identifying the inverted positions of the hydrogen and hydroxyl groups on carbon 1 as the origin of that difference
- one mark for a shared feature, such as both being polysaccharides, both being glucose polymers or both containing glycosidic bonds
- one mark for starch being helical or branched while cellulose is straight and unbranched
- one mark for starch containing 1,6 glycosidic bonds while cellulose does not, or for starch being a mixture of two polysaccharides while cellulose is one
- one mark for identifying starch as the storage polysaccharide of plants and glycogen as that of animals
- one mark for describing starch as amylose, which is helical and unbranched, and amylopectin, which is branched
- one mark for describing glycogen as alpha-glucose with 1,4 and 1,6 bonds and more branching than amylopectin
- one mark for describing cellulose as straight, unbranched beta-glucose chains held together by hydrogen bonds in microfibrils
- one mark for identifying cellulose as structural, giving the cell wall strength, rather than as a store
- one mark for insoluble, so it has no effect on water potential and water does not enter or leave by osmosis
- one mark for large, so the molecule cannot cross or leak out of the cell-surface membrane
- one mark for helical or compact, so a large amount of glucose is stored in a small space
- one mark for branched with many terminal ends, so glucose is hydrolysed and released rapidly for respiration
- one mark for hydrogen bonds between parallel cellulose chains forming microfibrils, giving tensile strength to the cell wall
- Describe the reducing sugar test: add Benedict's solution and heat in a water bath to observe a green, yellow, orange, or brick-red precipitate.
- Describe the non-reducing sugar test: boil the sample with dilute hydrochloric acid to hydrolyse it, neutralise (e.g., with sodium hydrogencarbonate), and retest with Benedict's solution and heat.
- Describe the starch test: add iodine in potassium iodide solution to the sample, looking for a colour change to blue-black.
- Explain how to make the Benedict's test quantitative: use a colorimeter to measure absorbance and compare against a calibration curve of known glucose concentrations.
Examiner Tips
Expert advice for maximising your marks
- 💡When asked to define a monosaccharide, explicitly use the term 'monomer' and state they form larger carbohydrates.
- 💡If asked for examples, give the three named in the specification: glucose, galactose and fructose.
- 💡If you quote carbon numbers, check the diagram first - 1,4 for a straight chain, 1,6 only at a branch point.
- 💡Pair the two reactions in your answer: condensation forms the glycosidic bond and releases water, hydrolysis breaks it and uses water.
- 💡Spell glycosidic correctly; it is a technical term the mark scheme expects to see in full.
- 💡Memorise the three pairings as a short list; they are recall marks that cost nothing to secure.
- 💡If a question gives an unfamiliar disaccharide such as trehalose, apply the general rule - two monosaccharides, one glycosidic bond, hydrolysis needed before use.
- 💡Link each disaccharide to where it is found: maltose from starch digestion, sucrose in phloem, lactose in milk.
- 💡Number the carbons on any glucose diagram before you answer - carbon 1 is the one to the right of the ring oxygen.
- 💡Use the word inverted or reversed about the hydrogen and hydroxyl groups; it is the phrasing the mark scheme rewards.
- 💡Explain the consequence as well as the difference: alpha allows coiling and branching, beta forces a straight chain.
- 💡In compare and contrast questions, write each point as one linked sentence - 'starch has alpha-glucose whereas cellulose has beta-glucose' - because separate statements may not be credited as a comparison.
- 💡Include at least one similarity; full marks on a six-mark compare and contrast usually require one of the 'both' points.
- 💡Use branching as your structural marker: glycogen most branched, amylopectin branched, amylose and cellulose unbranched.
- 💡Write 'store of glucose' rather than 'store of energy' - the phrasing is marked.
- 💡Remember that cellulose is the only one of the three with a structural role, so never offer it as a store.
- 💡Learn the location as well as the molecule: chloroplasts and storage organs for starch, liver and muscle for glycogen, cell wall for cellulose.
- 💡Use the phrase 'so that' in every sentence; it forces the structure-to-function link the question is marking.
- 💡Bring water potential into any storage answer - it is the standard credited reason for storing glucose as a polysaccharide.
- 💡While 'iodine solution' is often accepted in exams, it is best practice to write 'iodine in potassium iodide solution' to demonstrate precise biochemical knowledge.
- 💡When describing the non-reducing sugar test, ensure you sequence the steps logically: initial negative Benedict's test, boil with dilute HCl, neutralise, and finally retest with Benedict's and heat.
Common Mistakes
Pitfalls to avoid in your exam answers
- Confusing monosaccharides with disaccharides; correction: sucrose, maltose and lactose are disaccharides, whereas glucose, galactose and fructose are monosaccharides.
- Stating that monosaccharides are polymers; correction: monosaccharides are the individual monomers that bond together to form larger carbohydrates.
- Giving examples of disaccharides when asked for common monosaccharides; correction: use glucose, galactose and fructose.
- referring to 1,6 linkages in cellulose, which is rejected because cellulose chains are unbranched; correction: cellulose contains only 1,4 glycosidic bonds
- calling the bond a peptide or ester bond in a carbohydrate question; correction: the bond between monosaccharides is a glycosidic bond
- describing the bond as forming 'between two hydroxyl groups' without saying that water is released; correction: condensation releases water as the bond forms
- writing glycogenic or glucosidic instead of glycosidic; correction: the correct spelling is glycosidic
- saying the bond is broken by heat rather than by hydrolysis; correction: hydrolysis uses water to break the bond
- swapping the pairings, most often giving sucrose as glucose and galactose
- describing a disaccharide as a monomer of a polysaccharide
- saying a disaccharide is formed from two glucose molecules regardless of which disaccharide is named
- forgetting that sucrose is non-reducing and reporting a brick-red Benedict's result for it
- stating that two water molecules are released when one disaccharide forms
- claiming the two isomers have different molecular formulae or different numbers of carbon atoms
- naming the wrong carbon, most often carbon 4, as the one where the groups are inverted
- answering 'they are different shapes' with no reference to the hydroxyl group position
- stating that beta-glucose cannot form glycosidic bonds at all, rather than needing rotation first
- labelling a diagram without indicating which carbon is carbon 1
- stating that cellulose is made from alpha-glucose
- writing separate paragraphs about starch and cellulose without linking them, so the points are not comparative
- treating starch as a single molecule and never mentioning amylose and amylopectin
- saying glycogen is found in plants or that starch is found in animals
- claiming cellulose contains 1,6 branch points
- writing that starch and glycogen store energy, when they store glucose which releases energy on respiration
- describing cellulose as an energy store in plants
- saying starch is one molecule and never naming amylose and amylopectin
- claiming hydrogen bonds form within a single cellulose chain rather than between neighbouring chains
- giving glycogen as the plant store and starch as the animal store
- saying starch is insoluble so it can be transported, when insolubility is what stops it moving
- placing the hydrogen bonds within one cellulose molecule instead of between neighbouring chains
- describing a single hydrogen bond as strong, rather than many weak bonds acting collectively
- claiming the cell wall stops water entering the cell, when it resists the pressure that entering water creates
- giving structural features with no linked consequence, so the answer describes rather than explains
- Omitting the heating step during acid hydrolysis for non-reducing sugars. Always specify boiling or heating the sample with dilute hydrochloric acid to ensure hydrolysis occurs.
- Forgetting to neutralise the sample after acid hydrolysis. State that the solution must be neutralised because Benedict's reagent requires alkaline conditions to function.
- Describing the Benedict's positive result as a solution colour change rather than a precipitate. Explicitly state that a coloured precipitate (e.g., brick-red) forms.