AQA A-level Biology: Monomers, Polymers and Carbohydrates (7402) – Revision Notes

These notes condense sections 3.1.1 (Monomers and polymers) and 3.1.2 (Carbohydrates) of the AQA AS and A-level Biology specification, version 1.5, for exams from June 2016 onwards. This is AS content, which is also first-year A-level content. For full explanations and worked examples, read the study guide first, then test yourself with the practice questions.

Use the AQA A-level Biology hub and the printable checklist to track progress, and the free diagnostics to find weak spots. The wider topic is in the biological molecules revision notes.

Definitions to learn word for word

Term	Definition
Monomer	A smaller unit from which larger molecules are made
Polymer	A molecule made from a large number of monomers joined together
Condensation	Joins two molecules with the formation of a chemical bond; eliminates a molecule of water
Hydrolysis	Breaks a chemical bond between two molecules; uses a water molecule
Glycosidic bond	The bond formed between two monosaccharides by condensation
Isomers	Molecules with the same molecular formula but different structures

Monomers at a glance

Monomer	Polymer
Monosaccharide	Polysaccharide
Amino acid	Polypeptide
Nucleotide	Polynucleotide

Rule: n monomers joined in a chain or branched molecule need n − 1 condensations, which release n − 1 water molecules. Complete hydrolysis uses n − 1 water molecules.

Sugars

Monosaccharides: glucose, galactose, fructose. All are C₆H₁₂O₆.

Disaccharides (C₁₂H₂₂O₁₁):

Disaccharide	Made from	Reducing?
Maltose	Glucose + glucose	Yes
Sucrose	Glucose + fructose	No
Lactose	Glucose + galactose	Yes

All monosaccharides are reducing sugars.

Memory hook: sucrose is the odd one out, the only non-reducing disaccharide here, and the only one containing fructose.

Alpha and beta glucose

	α-glucose	β-glucose
OH on carbon 1	Below ring	Above ring
Forms	Starch, glycogen	Cellulose

Ring: five carbons and one oxygen. Carbon 6 is outside the ring.

Polysaccharides: structure to function

	Starch	Glycogen	Cellulose
Monomer	α-glucose	α-glucose	β-glucose
Where	Plant cells (grains in chloroplasts, storage organs)	Animal cells (liver and muscle granules)	Plant cell wall
Structure	Amylose: unbranched helix. Amylopectin: branched	Highly branched, short chains	Straight unbranched chains, alternate units rotated 180°; hydrogen bonds between chains form microfibrils
Role	Energy store	Energy store	Structural strength

Link each feature to a function

• Compact (helix or branched): a lot of glucose in a small space.
• Insoluble: no effect on water potential, so no osmotic entry of water; the molecule cannot diffuse out of the cell.
• Branched: many ends for enzymes to hydrolyse quickly. Glycogen has more branches than starch, so it releases glucose faster, which suits the higher activity of animals.
• Large: cannot cross the membrane.
• Cellulose: many hydrogen bonds between parallel chains give high tensile strength, so the wall resists the pressure from water entering by osmosis.

Biochemical tests

Test	Reagent	Method	Positive	Negative
Reducing sugar	Benedict’s solution	Add to sample, heat	Green, yellow, orange, then brick-red precipitate	Stays blue
Non-reducing sugar	Dilute HCl, then Benedict’s	Benedict’s first (stays blue); heat a fresh sample with acid; neutralise; add Benedict’s and heat	Brick-red precipitate after hydrolysis	Stays blue
Starch	Iodine in potassium iodide	Add to sample	Blue-black	Yellow-brown

Method in steps: identifying sucrose

1. Benedict’s on the sample stays blue, so there is no reducing sugar.
2. Boil a fresh sample with dilute hydrochloric acid to hydrolyse the glycosidic bond.
3. Neutralise (for example sodium hydrogencarbonate), since Benedict’s needs alkaline conditions.
4. Benedict’s and heat. Brick-red means a non-reducing sugar was present.

Skills linked to this section

The specification suggests you could use these techniques, so be ready for data on them.

• Colorimetry and calibration: make a dilution series, react with Benedict’s, measure absorbance, plot a calibration curve, read an unknown from it.
• Chromatography: R_f = spot distance ÷ solvent-front distance (both from the origin line). R_f has no units and is always less than 1. Compare with known standards.

Inside starch

	Amylose	Amylopectin
Chain	Unbranched	Branched
Shape	Coiled helix	Branched
Advantage	Very compact	Many ends for rapid hydrolysis

Method in steps: starch test

1. Place a small sample on a spotting tile or in a tube.
2. Drop in some iodine dissolved in potassium iodide.
3. Note the colour straight away. Blue-black means starch. Yellow-brown means none.

Recognising what a question wants

Command word	What to do
State	Give the fact only, such as “sucrose” or “hydrogen bonds”
Describe	Say what happens or what you would see, in order
Explain	Give a feature and then its consequence, joined by “so” or “because”
Calculate	Show each step and finish with a unit
Compare	Write a similarity or difference about the same feature in both

Worked example of “explain”: Cellulose has hydrogen bonds between chains (feature), so microfibrils have high tensile strength (consequence), so the wall withstands osmotic pressure (function).

Reading data on sugar tests

• A reducing sugar gives a colour sequence: blue, green, yellow, orange, brick-red. A faint green means a little reducing sugar; brick-red means a lot.
• If a sample is blue before hydrolysis and brick-red after, the sample holds a non-reducing sugar.
• If a sample is brick-red straight away, you cannot tell from these tests which reducing sugar it is. Chromatography with known standards can separate them.
• A sample positive for both starch and reducing sugar contains more than one carbohydrate. Do not assume the starch has been broken down.

Short worked reminders

Water count. A branched polysaccharide of 60 glucose units: 59 glycosidic bonds, 59 water molecules released in making it, 59 used in complete hydrolysis.

Relative molecular mass. Glucose 180, water 18. A disaccharide: 2 × 180 − 18 = 342. A chain of 12 glucose units: 12 × 180 − 11 × 18 = 2160 − 198 = 1962.

R_f. Spot 2.4 cm, solvent front 6.0 cm: 2.4 ÷ 6.0 = 0.40.

Interpolation. A calibration curve gives absorbance 0.50 at 4 g dm⁻³ and 0.30 at 8 g dm⁻³. An unknown at 0.40 lies halfway: 6 g dm⁻³.

Must-know distinctions

Pair	Key difference
Condensation and hydrolysis	Water released versus water used
α- and β-glucose	OH below versus above carbon 1
Starch and glycogen	Glycogen has more branching and shorter chains
Starch and cellulose	α-glucose, coiled or branched versus β-glucose, straight, hydrogen-bonded microfibrils
Glycosidic bonds and hydrogen bonds	Bonds within a chain versus bonds between cellulose chains
Reducing and non-reducing	Direct positive Benedict’s versus positive only after acid hydrolysis

Quick self-test

1. State the monomer of a polypeptide.
2. How many water molecules are released when 30 monosaccharides join to form one molecule?
3. Name the two monosaccharides in lactose.
4. Which disaccharide contains fructose, and is it reducing?
5. Give the relative molecular mass of a disaccharide made from two monosaccharides of 180 (water 18).
6. Describe the difference between α-glucose and β-glucose.
7. Why is starch suited to being an energy store? Give two reasons.
8. Why does glycogen release glucose faster than starch?
9. What is the colour change in the iodine test when starch is present?
10. Why must the solution be neutralised before the second Benedict’s test?
11. A spot moves 4.2 cm and the solvent front 7.0 cm. Calculate R_f.
12. Name two features of cellulose that give a plant cell wall its strength.

Answers

1. Amino acid.
2. 29 (n − 1).
3. Glucose and galactose.
4. Sucrose (glucose + fructose); non-reducing.
5. 2 × 180 − 18 = 342.
6. Carbon 1 has the OH group below the ring in α-glucose and above the ring in β-glucose.
7. Compact, so it stores much glucose in a small space; insoluble, so it does not affect water potential. Branched ends for quick hydrolysis is also creditworthy.
8. It is more highly branched, so it has more free ends for enzymes to act on.
9. Yellow-brown to blue-black.
10. Benedict’s solution only works in alkaline conditions, and the acid used for hydrolysis would stop it reacting.
11. 4.2 ÷ 7.0 = 0.60.
12. Straight chains held together by many hydrogen bonds, and microfibrils grouped into fibres.

Where marks are usually lost

• Saying the OH is “in a different position” without naming carbon 1 and above or below the ring.
• Writing “the cell wall is strong because of cellulose” with no mention of hydrogen bonds between chains.
• Giving hydrolysis as releasing water.
• Counting n instead of n − 1 water molecules.
• Saying starch is insoluble but giving no consequence. Link insolubility to no effect on water potential.
• Missing out the first Benedict’s test and neutralisation in the non-reducing sugar sequence.
• Stating a colour result without the starting colour, for example “blue-black” with no mention of the yellow-brown it replaces.
• Treating glycogen and starch as the same molecule instead of comparing branching.

Official syllabus

AQA, AS and A-level Biology specification (7401 and 7402), version 1.5, for exams from June 2016 onwards. Sections 3.1.1 and 3.1.2.