Cambridge International AS & A Level Biology 9700: Carbohydrates and lipids – Study Guide

This study guide teaches section 2.2 Carbohydrates and lipids from Topic 2, Biological molecules, in the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. It covers all eleven learning outcomes in section 2.2. This is AS Level content: it is examined on Paper 1 (Multiple Choice) and Paper 2 (AS Level Structured Questions), and Paper 4 can draw on it because A Level questions require knowledge of the AS content. Calculators are allowed in all 9700 papers.

For an overview of all of Topic 2, read the Biological molecules study guide. Afterwards, use the Carbohydrates and lipids revision notes for recall and the Carbohydrates and lipids practice questions to test yourself.

What this section covers

Outcome	What you must be able to do
2.2.1	Describe and draw the ring forms of α-glucose and β-glucose
2.2.2	Define six key terms (monomer to polysaccharide)
2.2.3	State the role of covalent bonds in joining smaller molecules into polymers
2.2.4	State that glucose, fructose and maltose are reducing sugars and sucrose is non-reducing
2.2.5	Describe glycosidic bond formation by condensation
2.2.6	Describe hydrolysis and the non-reducing sugar test
2.2.7	Describe amylose, amylopectin and glycogen and relate structure to function
2.2.8	Describe cellulose and how the arrangement of its molecules suits plant cell walls
2.2.9	Describe triglyceride structure and ester bonds
2.2.10	Relate triglyceride structure to function
2.2.11	Describe phospholipids: hydrophilic phosphate head, hydrophobic fatty acid tails

Key terms (outcome 2)

• Monomer: a relatively small molecule that can join to other similar molecules to form a polymer. Glucose is the monomer of starch, glycogen and cellulose.
• Polymer: a large molecule made of many repeating monomer units joined in a chain.
• Macromolecule: a very large biological molecule. Polysaccharides, proteins and nucleic acids are macromolecules.
• Monosaccharide: a single sugar unit, such as glucose or fructose. Monosaccharides are sweet and soluble in water.
• Disaccharide: a sugar made of two monosaccharides joined by a glycosidic bond, such as maltose or sucrose.
• Polysaccharide: a polymer made of many monosaccharides joined by glycosidic bonds, such as starch, glycogen or cellulose.

A triglyceride is not a polymer: it is one glycerol plus three fatty acids, not a chain of repeating units.

Covalent bonds (outcome 3). Monomers are held together in a polymer by covalent bonds. In carbohydrates the covalent bond is a glycosidic bond; in lipids, fatty acids are joined to glycerol by covalent ester bonds. Covalent bonds are strong, so breaking a polymer needs a chemical reaction (hydrolysis).

α-glucose and β-glucose (outcome 1)

Glucose (C₆H₁₂O₆) exists mainly as a ring in water. The ring has six atoms: five carbon atoms and one oxygen atom. The carbons are numbered 1 to 6. Carbon 6 is not part of the ring; it sits above the ring as a CH₂OH group attached to carbon 5.

The two ring forms differ only at carbon 1:

Feature	α-glucose	β-glucose
OH on carbon 1	Below the ring	Above the ring
H on carbon 1	Above the ring	Below the ring
Everything else	Identical	Identical

How to draw them. Draw a hexagon with the oxygen at the top right corner. Number the carbons clockwise from the right-hand corner (carbon 1) round to carbon 5 at the top left. Put CH₂OH above carbon 5. Then add H and OH groups: on carbon 2 OH below, on carbon 3 OH above, on carbon 4 OH below. Finally, on carbon 1, put OH below for α or above for β.

Reducing and non-reducing sugars (outcome 4)

You need to state these facts:

• Glucose, fructose and maltose are reducing sugars. They give a positive Benedict’s test (an orange-red or brick-red precipitate on heating).
• Sucrose is a non-reducing sugar. It does not give a positive Benedict’s test unless it has first been hydrolysed.

Background: in sucrose the glycosidic bond uses the carbon atoms that would otherwise make glucose and fructose reducing.

Condensation: making glycosidic bonds (outcome 5)

A condensation reaction joins two molecules and releases one molecule of water. When two monosaccharides join, an OH group on each reacts: one loses H, the other loses OH, forming H₂O. The sugars are now linked through an oxygen atom. This C–O–C link is the glycosidic bond.

• Maltose = α-glucose + α-glucose, joined by an α-1,4 glycosidic bond (carbon 1 of one glucose to carbon 4 of the next).
• Sucrose = α-glucose + fructose, joined by a 1,2 glycosidic bond (carbon 1 of glucose to carbon 2 of fructose).
• Polysaccharides form by many repeated condensations. Each new glucose added releases one more water molecule.

Worked example 1: water released and molecular mass

A chain of 1,500 glucose molecules is built by condensation. Glucose has Mr 180 and water has Mr 18. How many water molecules are released, and what is the Mr of the chain?

1. Joining n monomers in a single unbranched chain needs n − 1 bonds. So 1,500 glucose units need 1,499 glycosidic bonds.
2. Each bond releases one water molecule: 1,499 water molecules.
3. Mr of chain = (1,500 × 180) − (1,499 × 18) = 270,000 − 26,982 = 243,018.

Likewise maltose and sucrose: (2 × 180) − 18 = 342 (C₁₂H₂₂O₁₁).

Hydrolysis: breaking glycosidic bonds (outcome 6)

Hydrolysis is the reverse of condensation. A water molecule is added across the glycosidic bond, which breaks it. The H goes to one sugar and the OH to the other, restoring two OH groups. In cells, enzymes catalyse hydrolysis; in the laboratory, heating with dilute hydrochloric acid does it.

This is the basis of the non-reducing sugar test:

1. Test the sample with Benedict’s solution. If it stays blue, no reducing sugar is present.
2. Heat a fresh portion of the sample with dilute hydrochloric acid in a boiling water bath. This hydrolyses the glycosidic bond in sucrose, producing glucose and fructose.
3. Cool, then neutralise with sodium hydrogencarbonate until fizzing stops (Benedict’s solution needs alkaline conditions).
4. Add Benedict’s solution and heat. An orange-red or brick-red precipitate now shows that a non-reducing sugar was present, because its hydrolysis released reducing sugars.

The tests themselves are section 2.1, summarised in the Biological molecules revision notes.

Starch and glycogen (outcome 7)

Starch is the energy store in plants. It is a mixture of two polysaccharides, both made of α-glucose.

Feature	Amylose	Amylopectin	Glycogen
Monomer	α-glucose	α-glucose	α-glucose
Bonds	α-1,4 only	α-1,4 and α-1,6	α-1,4 and α-1,6
Shape	Unbranched chain that coils into a helix	Branched	More branched than amylopectin; very compact
Found in	Plants (starch grains)	Plants (starch grains)	Animals and fungi; in humans mainly liver and skeletal muscle cells

Linking structure to function:

• Compact. Helices and branches pack many glucose units into a small space.
• Insoluble. Large polysaccharides do not dissolve, so they do not lower the water potential of the cell. Storing the same glucose as free glucose would draw water in by osmosis.
• Branching gives many ends. Enzymes remove glucose from the ends of chains. More branches mean more ends, so glucose can be released quickly. Glycogen, the most branched, suits animals, which have a higher demand for rapid energy release than plants.

Cellulose (outcome 8)

Cellulose is a polysaccharide of β-glucose joined by β-1,4 glycosidic bonds. Because the OH on carbon 1 of β-glucose is above the ring, every second glucose molecule must be rotated 180° to form the bond. This produces a straight, unbranched chain, not a helix.

How the arrangement of molecules gives a strong wall:

1. Neighbouring chains lie parallel and form many hydrogen bonds between them.
2. Groups of chains held this way form microfibrils; microfibrils group into fibres.
3. Fibres are laid down in layers, often at different angles, in a matrix.

The result: cellulose has high tensile strength. The wall can resist the pressure of water entering the cell by osmosis, so plant cells become turgid without bursting. Spaces between fibres make the wall freely permeable.

Triglycerides (outcomes 9 and 10)

A triglyceride is made of one glycerol molecule and three fatty acids.

• Glycerol (C₃H₈O₃) has three carbons, each carrying an OH group.
• A fatty acid has a carboxyl group (–COOH) at one end and a hydrocarbon chain (the “tail”).
  • Saturated: no C=C double bonds; the chain holds the maximum number of hydrogen atoms and is straight.
  • Unsaturated: one or more C=C double bonds; each double bond puts a kink in the chain. One double bond = monounsaturated; more than one = polyunsaturated.

Ester bonds. The –COOH of each fatty acid reacts with one OH of glycerol in a condensation reaction, releasing water and forming an ester bond. Three fatty acids means three ester bonds and three water molecules.

State this exactly: triglycerides are non-polar, hydrophobic molecules. They do not mix with water.

Worked example 2: Mr of a triglyceride

A triglyceride is made from glycerol (Mr 92), palmitic acid (Mr 256), oleic acid (Mr 282) and stearic acid (Mr 284). Calculate its Mr.

1. Add the reactants: 92 + 256 + 282 + 284 = 914.
2. Three ester bonds form, so three water molecules are lost: 3 × 18 = 54.
3. Mr = 914 − 54 = 860.

Oleic acid (C₁₈H₃₄O₂) has two fewer H atoms than stearic acid (C₁₈H₃₆O₂): it has one C=C bond.

Structure related to function (outcome 10):

Structural feature	Function it supports
Many C–H bonds in the fatty acid tails, little oxygen	Energy store: oxidation releases more energy per gram than carbohydrate (roughly 37 kJ per gram for fat compared with 17 kJ per gram for carbohydrate)
Non-polar and insoluble	Stored in large amounts without affecting water potential
Less dense than water	Buoyancy in aquatic animals
Poor conductor of heat	Thermal insulation, for example fat under the skin or blubber
Oxidation releases water	Source of metabolic water, useful to desert animals
Forms cushioning layers	Protects delicate organs such as kidneys

Worked example 3: comparing stores

Using 37 kJ g⁻¹ for fat and 17 kJ g⁻¹ for carbohydrate, what mass of glycogen stores the same energy as 1.0 kg of fat?

1. Energy in 1.0 kg fat = 1,000 × 37 = 37,000 kJ.
2. Mass of glycogen = 37,000 ÷ 17 = 2,176 g = 2.18 kg (3 s.f.).

Carbohydrate storage needs more than twice the mass.

Phospholipids (outcome 11)

A phospholipid is like a triglyceride with one fatty acid replaced by a phosphate group:

• Glycerol, joined by ester bonds to two fatty acids.
• A phosphate group on the third carbon of glycerol.

The phosphate group is charged, so the “head” is polar and hydrophilic: it attracts water. The two fatty acid tails are non-polar and hydrophobic: they repel water. How this makes phospholipids form a bilayer is in Topic 4; see the Cell membranes and transport study guide.

Common errors

• Drawing the glucose ring with six carbons in the ring. The ring has five carbons and one oxygen.
• Swapping α and β. Check carbon 1 only: OH below = α, OH above = β.
• Forgetting the water. Each bond made releases one water; each bond broken uses one.
• Calling sucrose a reducing sugar, or forgetting that maltose is reducing.
• Leaving out neutralisation in the non-reducing sugar test.
• Saying cellulose is made of α-glucose, or that it coils into a helix.
• Saying cellulose chains are joined by glycosidic bonds to each other. Chains are linked by hydrogen bonds; glycosidic bonds are within each chain.
• Saying glycogen is found in plants.
• Not giving the reason lipids store more energy: more C–H bonds per gram.

Next steps

• Recall: Carbohydrates and lipids revision notes
• Test yourself: Carbohydrates and lipids practice questions
• Find your gaps: Free 9700 AS diagnostic
• Course hub: Cambridge A Level Biology
• Printable checklist: 9700 checklist

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027, Cambridge University Press & Assessment – Topic 2, Biological molecules (section 2.2 Carbohydrates and lipids).