AQA A-level Biology: Lipids and Protein Structure (7402) – Revision Notes

These notes condense sections 3.1.3 (Lipids) and 3.1.4.1 (General properties of proteins) of the AQA AS and A-level Biology specification, for first teaching 2015, used for A-level Biology 7402. Read the study guide when you need the reasoning behind a point, then attempt the practice questions. Other links: AQA A-level Biology hub, printable checklist, free diagnostics and the wider Biological Molecules revision notes.

Key definitions

Term	Meaning
Condensation	Joining two molecules with the loss of one water molecule
Hydrolysis	Breaking a bond by adding one water molecule
Ester bond	Forms between an OH of glycerol and the COOH of a fatty acid
Saturated	Fatty acid chain with no C=C double bonds
Unsaturated	Fatty acid chain with at least one C=C double bond
Hydrophilic	Attracted to water; polar or charged
Hydrophobic	Repelled by water; non-polar
Peptide bond	C-N bond between the carboxyl of one amino acid and the amine of the next
Primary structure	Sequence of amino acids in a polypeptide
Quaternary structure	Arrangement of two or more polypeptides in one protein

Lipids in steps

Triglyceride formation

1. One glycerol plus three fatty acids.
2. Three condensation reactions, each releasing one water.
3. Three ester bonds in the product.

Spotting saturation in a diagram

1. Follow the hydrocarbon chain from the COOH end.
2. Any C=C means unsaturated. Only single bonds means saturated.
3. A bend in the drawn chain usually marks the double bond.

Triglyceride versus phospholipid

Feature	Triglyceride	Phospholipid
Parts	Glycerol and three fatty acids	Glycerol, two fatty acids and a phosphate-containing group
Polarity	Non-polar overall	Hydrophilic head, hydrophobic tails
Behaviour in water	Does not mix; forms separate droplets or a layer	Forms a bilayer, heads out and tails in
Main roles	Energy store, insulation, protection, metabolic water	Structure of cell-surface membranes

Why the properties follow from the structure

• Many C-H bonds in long chains: more energy per gram than carbohydrate.
• Non-polar: insoluble, so no effect on water potential inside a cell.
• High hydrogen content: more metabolic water when oxidised.
• Poor heat conductor and low density: insulation, cushioning and buoyancy.
• Saturated chains are straight and pack closely, so they tend to be solid. Unsaturated chains bend, pack loosely and tend to be liquid at room temperature.
• Phospholipid heads face water, tails face inward: the bilayer core blocks water-soluble substances.

Proteins in steps

Amino acid: one central carbon carrying four attachments: an amine group (NH2), a carboxyl group (COOH), a single hydrogen and an R group. The twenty amino acids found in all organisms differ only in R.

Peptide formation: carboxyl of amino acid 1 reacts with amine of amino acid 2, forming a peptide bond and one water molecule. A chain of n amino acids contains one bond fewer than n.

Counting possibilities: a chain of length n can be built in 20 to the power n different sequences.

The four levels

Level	What it is	Held by
Primary	Order of amino acids	Peptide bonds
Secondary	Alpha helix or beta pleated sheet	Hydrogen bonds along the backbone
Tertiary	Specific three-dimensional fold of one polypeptide	Hydrogen bonds, ionic bonds, disulfide bridges between R groups
Quaternary	Two or more polypeptides together	Same types of bond between chains

Bonds at a glance

Bond	Strength	Sensitive to
Hydrogen	Weak individually	Heat
Ionic	Stronger	Changes in pH
Disulfide	Strongest; covalent, between cysteine sulfur atoms	Only harsher treatment

Structure to function: the sequence fixes the folding, the folding gives the shape, and the shape allows the job. Globular proteins are compact and soluble. Fibrous proteins, such as collagen, are long, cross-linked, insoluble and strong.

Food tests

Test	Method	Positive	Negative
Emulsion (lipid)	Shake sample with ethanol, pour into water	Cloudy white emulsion	Stays clear
Biuret (protein)	Add sodium hydroxide, then dilute copper(II) sulfate	Lilac or purple	Stays blue

Small worked reminders

Formula of a triglyceride. Oleic acid is C18H34O2 and glycerol is C3H8O3.

1. Add: C3H8O3 + 3 C18H34O2 gives C57H110O9.
2. Subtract three waters (H6O3): C57H104O6.
3. Relative formula mass with Ar values C 12, H 1, O 16: (57 × 12) + 104 + (6 × 16) = 884.
4. Check: reactants 92 + (3 × 282) = 938; products 884 + 54 = 938.

Peptide bonds in a multi-chain protein. Three polypeptides have 10, 12 and 14 amino acids.

1. Bonds per chain: 9, 11 and 13.
2. Total: 33 peptide bonds, so 33 water molecules released in synthesis.

Reading a fatty acid formula. A saturated acid with 18 carbons is C18H36O2. An unsaturated one with the same chain length and one C=C is C18H34O2. Each double bond removes two hydrogen atoms.

Interpreting test results

Tube	Contents	After emulsion test	Conclusion
A	Vegetable oil	Cloudy white	Lipid present
B	Glucose solution	Clear	No lipid detected
C	Egg white solution after biuret test	Purple	Protein present
D	Water after biuret test	Blue	No protein; a negative control

Always say what the result shows, not only the colour. A control tube of water confirms that the reagents themselves do not cause the colour.

Sentence frames for extended answers

• “The R group contains sulfur, so two of them can form a disulfide bridge, which holds the tertiary structure in place.”
• “The chain is straight because it has no C=C, so molecules pack closely and intermolecular forces are stronger.”
• “The head is hydrophilic so it faces the water; the tails are hydrophobic so they face inward, forming a bilayer.”
• “A change in the sequence changes which R groups meet, so different bonds form and the tertiary shape changes.”

Protein examples to link structure and property

Protein	Structure feature	Property it explains
Haemoglobin	Four polypeptides folded compactly (quaternary)	Soluble in cytoplasm, so it can be carried inside red blood cells
Collagen	Three long polypeptides wound together with cross-links	Strong and insoluble, so it resists stretching in tissues
An enzyme	Tertiary fold gives a specific active site shape	Binds only substrates that fit
Any protein	Heat or extreme pH breaks hydrogen and ionic bonds	The shape changes and the protein stops working

Command words for this topic

• State or name: a short fact, such as the name of a bond. No explanation needed.
• Describe: say what happens or what the structure is, such as how a triglyceride forms.
• Explain: give a reason that links structure to property, using words such as “because” and “so”.
• Compare: give a similarity or difference for both items, such as triglyceride and phospholipid.
• Suggest: apply knowledge to an unfamiliar example, for instance an unfamiliar fatty acid diagram.

Must-know distinctions

• Ester bond (lipid) versus peptide bond (protein).
• Peptide bonds (primary) versus hydrogen bonds (secondary).
• Disulfide bridge (needs sulfur, tertiary) versus hydrogen bond (any level above primary).
• Hydrophilic head versus hydrophobic tails.
• Saturated (no C=C) versus unsaturated (C=C present).

Quick self-test

1. How many water molecules are released when one triglyceride forms?
2. State the name of the bond between glycerol and a fatty acid.
3. A fatty acid chain contains one C=C. Is it saturated or unsaturated?
4. Which part of a phospholipid is hydrophobic?
5. How many peptide bonds join the 80 amino acids of one polypeptide?
6. How many different dipeptides can be made from the 20 common amino acids?
7. Name the two types of secondary structure.
8. Which bond forms between the R groups of two cysteine residues?
9. Which bond type is most affected by a change in pH?
10. A sample turns purple after sodium hydroxide and copper(II) sulfate are added. What does this show?
11. Why is ethanol used in the emulsion test?
12. Why does a protein of four polypeptides have quaternary structure while one polypeptide does not?

Answers

1. Three.
2. An ester bond.
3. Unsaturated.
4. The two fatty acid tails.
6. 20 × 20 = 400.
7. Alpha helix and beta pleated sheet.
8. A disulfide bridge.
9. Ionic bonds, because pH changes alter the charges on R groups.
10. Peptide bonds are present, so protein is present.
11. Lipids do not dissolve in water, so ethanol dissolves the lipid before it is added to water.
12. Quaternary structure is the arrangement of two or more polypeptides; a single chain has only the first three levels.

Where marks are usually lost

• Writing “fats are insoluble because they are big” instead of “non-polar, so hydrophobic”.
• Stating that a phospholipid has three fatty acids.
• Calling any C-H chain “saturated” without checking for a double bond.
• Saying hydrogen bonds hold the primary structure.
• Leaving out the loss of water in condensation, or the use of water in hydrolysis.
• Writing that unsaturated fats pack closely together.
• Describing the biuret test as detecting amino acids rather than peptide bonds.
• Giving the emulsion result as a colour change, when it is cloudiness.
• Writing 80 peptide bonds for a chain of 80 amino acids.

Official syllabus

AQA AS and A-level Biology (7401/7402) specification, for first teaching 2015, published by AQA: sections 3.1.3 and 3.1.4.1.