OCR A Level Biology A: Water, carbohydrates and lipids (H420) – Revision Notes

These notes condense learning outcomes 2.1.2(a) to 2.1.2(j) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), first assessment 2025. For full explanations read the study guide, then test yourself with the practice set. The course sits on the OCR A Level Biology hub; tick off outcomes on the checklist. The wider module is in the Foundations revision notes, and the free diagnostics show gaps quickly.

Key definitions

Term	Meaning
Polar molecule	Has regions of small positive (δ+) and negative (δ−) charge
Hydrogen bond	Weak electrostatic attraction linking a δ+ hydrogen on one molecule to a δ− oxygen or nitrogen on another
Monomer	Small unit that joins with others to make a polymer
Polymer	Long chain of many monomers
Condensation	Joins two molecules, releases water
Hydrolysis	Breaks a bond using water
Glycosidic bond	Covalent bond between two monosaccharides
Ester bond	Covalent bond between glycerol and a fatty acid
Hydrophilic	Attracted to water
Hydrophobic	Repelled by water

2.1.2(a) Water

Method in steps for any water question: name the property, give the cause (hydrogen bonds), then the role.

Role	Property	Link
Solvent	Polar	Ions and polar molecules dissolve, so reactions occur in solution
Transport medium	Cohesion, solvent	Continuous column in xylem; plasma carries dissolved substances
Coolant	High latent heat of vaporisation	Evaporation removes much energy: sweat, transpiration
Habitat	High specific heat capacity; ice less dense than water; surface tension	Stable temperature; ice floats and insulates; surface supports small animals

Prokaryote examples: bacterial cytoplasm is a solvent; aquatic bacteria live in a thermally stable habitat. Eukaryote examples: sweating in mammals, xylem in plants, ponds as habitats.

Reminder: 144 kJ removed by 60 g of sweat at 2.4 kJ per gram.

2.1.2(b), (c) Monomers, polymers, elements

Bond	Joins	Chain produced
Glycosidic	Sugar units	Polysaccharide
Peptide	Amino acids	Polypeptide
Phosphodiester	Nucleotides	Polynucleotide

Chain of n monomers: n − 1 bonds, n − 1 water molecules released or used.

Molecule	Elements
Carbohydrate, lipid	C, H, O
Protein	C, H, O, N, S
Nucleic acid	C, H, O, N, P

2.1.2(d) Monosaccharides

• Glucose C6H12O6: hexose, ring of 5 C plus 1 O, CH2OH outside the ring.
• Carbon 1 –OH: down in α-glucose, up in β-glucose.
• Ribose C5H10O5: pentose, ring of 4 C plus 1 O.
• Hexose has six carbons; pentose has five.

2.1.2(e) Disaccharides

Disaccharide	Made from
Maltose	two glucose units
Sucrose	glucose and fructose
Lactose	glucose and galactose

All are C12H22O11, Mr 342. Condensation forms the glycosidic bond; hydrolysis breaks it.

2.1.2(f), (g) Polysaccharides

	Amylose	Amylopectin	Glycogen	Cellulose
Unit	α-glucose	α-glucose	α-glucose	β-glucose
Links	α-1,4	α-1,4 plus α-1,6 branches	α-1,4 plus α-1,6 branches	β-1,4
Shape	Helix	Branched	Very branched	Straight chains
Role	Plant energy store	Plant energy store	Animal and fungal energy store	Cell wall

Function in a sentence each:

• Glucose: soluble, so transported; substrate for respiration.
• Starch: insoluble, so no osmotic effect; compact; hydrolysed to glucose when needed.
• Glycogen: many ends, so fast hydrolysis; compact; found in liver and muscle, also in some bacteria.
• Cellulose: hydrogen bonds between straight chains form microfibrils; strong wall resists bursting.

Must-know distinctions: α against β (carbon 1 –OH, and β chains alternate in orientation); amylose against amylopectin (branching); starch against glycogen (branching frequency, and plant against animal).

2.1.2(h), (i), (j) Lipids

Molecule	Structure	Key property	Function
Triglyceride	Glycerol + 3 fatty acids, 3 ester bonds	Hydrophobic, high energy per gram	Energy store, insulation, protection
Phospholipid	Glycerol + 2 fatty acids + phosphate	Hydrophilic head, hydrophobic tails	Bilayer of membranes
Cholesterol	Four-ring steroid with –OH	Small, mostly hydrophobic	Regulates fluidity and stability of eukaryotic membranes

• Saturated: no C=C; straight chain. Unsaturated: one or more C=C; kinked chain.
• Condensation to make a triglyceride releases 3 water molecules.
• About 38 kJ g⁻¹ for lipid and 17 kJ g⁻¹ for carbohydrate (give as data when asked).
• Tristearin: 92 + 3 × 284 − 54 = 890.

Method boxes

Counting bonds and water in a polymer

1. Count the monomers, n.
2. Bonds formed = n − 1, and water released = n − 1.
3. Mr of polymer = (n × Mr of monomer) − ((n − 1) × 18).
4. Check: complete hydrolysis uses the same n − 1 water molecules.

Counting for a triglyceride

1. Mr = glycerol (92) + 3 × fatty acid − 3 × 18.
2. Each of the three ester bonds comes with one water given off.

Comparing two molecules

1. State one similarity (for example, both are glucose polymers).
2. Give the difference in bond type, then in shape, then in the property that follows.
3. Finish with the function that the property allows.

Percentage change

1. (new − old) ÷ old × 100.
2. State whether it is an increase or a decrease.

Worked reminders

Lactose, 1.71 g, hydrolysed completely: 1.71 ÷ 342 = 0.00500 mol; two monosaccharides per lactose gives 0.0100 mol; 0.0100 × 180 = 1.80 g. The extra 0.09 g is the water used. (Galactose also has Mr 180.)

A glycogen molecule of 2 400 residues with a branch every 12 residues has 200 branches and 201 free ends. Amylopectin of the same size with a branch every 30 has 80 branches and 81 free ends. The ratio is about 2.5.

A vesicle membrane leaks dye at 10 units per minute without cholesterol and 6 units with it: (10 − 6) ÷ 10 × 100 = 40% decrease.

Energy: 4 g of lipid at 38 kJ g⁻¹ gives 152 kJ. The mass of carbohydrate at 17 kJ g⁻¹ needed for 152 kJ is 152 ÷ 17 = 8.9 g.

Linking ideas together

• Hydrogen bonds matter in three places: between water molecules, between cellulose chains, and (in other units) inside proteins and between DNA strands.
• Condensation always releases water and hydrolysis always uses it, whether the product is a glycosidic bond or an ester bond.
• Hydrophobic means tails or chains that avoid water: fatty acids and cholesterol rings. Hydrophilic means groups that attract water: the phosphate head, and the –OH groups of sugars.
• Insoluble stores (starch, glycogen, triglyceride) do not alter water potential; soluble transport molecules (glucose, sucrose) can.

Quick self-test

1. State which atom of H2O is δ− and which are δ+.
2. Name the property that makes sweating an effective coolant.
3. A chain is built from 250 glucose residues. State the water released and calculate its Mr.
4. State the structural difference between α-glucose and β-glucose.
5. What is the molecular formula of lactose, and which monosaccharides make it?
6. Why is glycogen hydrolysed faster than amylose?
7. Give two reasons why cellulose is suited to plant cell walls.
8. How many ester bonds are in one phospholipid, and how many in one triglyceride?
9. How much energy is stored in 5 g of lipid and 5 g of carbohydrate, using 38 and 17 kJ g⁻¹?
10. Name the type of cell whose membranes contain cholesterol, and state its effect on membrane fluidity.
11. Name the hydrophilic part of a phospholipid.
12. Explain the lower density of ice compared with liquid water.

Answers

1. Oxygen is δ−; the two hydrogens are δ+.
2. High latent heat of vaporisation: evaporation takes away a large amount of energy for a small mass of water.
3. 249 water molecules; Mr = 250 × 180 − 249 × 18 = 45 000 − 4 482 = 40 518.
4. Carbon 1 has its –OH down (α) or up (β), relative to the ring.
5. C12H22O11; glucose and galactose.
6. Glycogen is much more branched, so it has more free ends where enzymes can act at once.
7. Straight β-1,4 chains form hydrogen bonds with neighbouring chains, making strong microfibrils; the wall resists the pressure of the cell contents.
8. Two ester bonds in a phospholipid; three in a triglyceride.
9. Lipid 5 × 38 = 190 kJ; carbohydrate 5 × 17 = 85 kJ.
10. Eukaryote (especially animal) cells; it regulates fluidity, restricting tail movement when warm and preventing close packing when cold.
11. The phosphate head.
12. In ice, hydrogen bonds hold the molecules in an open, fixed lattice, so they are further apart than in liquid water.

Where marks are usually lost

• Naming a property (such as “high specific heat capacity”) without saying what causes it or what it does for the organism.
• Stating that hydrogen bonds are “between atoms in the molecule”.
• Giving a role for water with only a human example when a prokaryote or plant is asked.
• Confusing condensation and hydrolysis, or forgetting which one uses water.
• Counting n bonds and n water molecules for a polymer of n monomers.
• Describing α and β glucose as different formulae. Both are C6H12O6.
• Saying starch is “branched” without separating amylose from amylopectin.
• Describing cellulose function without hydrogen bonding between chains.
• Explaining the bilayer without naming which region faces water.
• Calling cholesterol a phospholipid or a triglyceride.

Official syllabus

OCR, A Level Biology A (H420) specification, Version 4.1 (April 2026), first assessment 2025: section 2.1.2, learning outcomes (a) to (j).