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

This guide teaches section 3.1.3 (Lipids) and section 3.1.4.1 (General properties of proteins) of the AQA AS and A-level Biology specification, for first teaching 2015, as used for A-level Biology 7402. Both sections sit in Topic 1, Biological molecules. Revise with the revision notes, then test yourself with the practice questions. Course links: AQA A-level Biology hub, printable checklist and the free diagnostics. For the wider topic see Biological Molecules.

What you must be able to do

Section	Statement in short
3.1.3	Triglycerides form by condensation of one glycerol and three fatty acids, giving ester bonds
3.1.3	Fatty acid R-groups may be saturated or unsaturated; recognise each from a diagram
3.1.3	In a phospholipid, one fatty acid is replaced by a phosphate-containing group
3.1.3	Explain how the properties of triglycerides and phospholipids relate to their structures
3.1.3	Carry out and interpret the emulsion test
3.1.4.1	Draw the general amino acid; 20 common amino acids differ only in the R group
3.1.4.1	Peptide bonds, dipeptides, polypeptides; a functional protein has one or more polypeptides
3.1.4.1	Roles of hydrogen bonds, ionic bonds and disulfide bridges
3.1.4.1	Primary, secondary, tertiary and quaternary structure and function; biuret test

Triglycerides

A lipid is not a polymer, but it is built by the same reaction you met for carbohydrates: condensation. One glycerol (a three-carbon alcohol with three hydroxyl groups) joins to three fatty acids. Each fatty acid is a hydrocarbon chain ending in a carboxyl group, written RCOOH. Each condensation between an OH on glycerol and the COOH of a fatty acid releases one water molecule and forms an ester bond. With three fatty acids attached, the product has three ester bonds and three molecules of water have been lost.

glycerol + 3 fatty acids  ->  triglyceride + 3 H2O

Worked example: building trimyristin

Myristic acid is C14H28O2 and glycerol is C3H8O3.

1. Add the reactants: C3H8O3 + 3 C14H28O2 gives C45H92O9 in total.
2. Remove three water molecules (H6O3): C45H86O6.
3. Relative formula mass, using Ar values C 12, H 1, O 16: glycerol 92, myristic acid 228, so reactants 92 + 3 × 228 = 776.
4. Product: (45 × 12) + 86 + (6 × 16) = 722. Water lost: 3 × 18 = 54. Check: 722 + 54 = 776, so mass is conserved.

Hydrolysis is the reverse: three water molecules are added and the three ester bonds break.

Saturated and unsaturated fatty acids

The R group is a hydrocarbon chain. In a saturated fatty acid every carbon-carbon bond in the chain is single, so the chain carries as many hydrogen atoms as it possibly can. An unsaturated fatty acid contains at least one carbon-carbon double bond (C=C) somewhere along the chain. One double bond is monounsaturated; more than one is polyunsaturated.

Stearic acid (saturated)   CH3-(CH2)16-COOH      C18H36O2
Oleic acid (unsaturated)   CH3-(CH2)7-CH=CH-(CH2)7-COOH   C18H34O2

To recognise them from a diagram, scan the chain for a C=C. If you find one, it is unsaturated. If the chain is only single bonds, it is saturated. Never decide from chain length.

A double bond usually puts a bend in the chain. Bent chains pack together less closely, so intermolecular forces between molecules are weaker. This is why triglycerides rich in unsaturated fatty acids tend to be liquids (oils) at room temperature and those rich in saturated fatty acids tend to be solids (fats).

Phospholipids

A phospholipid is built on the same glycerol backbone as a triglyceride, but a phosphate-containing group takes the place of one fatty acid. It therefore has glycerol, two fatty acids and a phosphate group.

Triglyceride:   glycerol - fatty acid
                         - fatty acid
                         - fatty acid

Phospholipid:   glycerol - fatty acid
                         - fatty acid
                         - phosphate-containing group

The phosphate-containing group is polar and charged, so it is hydrophilic and attracted to water. Each of the two fatty acid tails is non-polar and therefore hydrophobic. A molecule with both properties is amphipathic.

Properties linked to structure

Triglycerides

• Energy store. The long hydrocarbon chains contain many C-H bonds, so oxidation releases more energy per gram than the same mass of carbohydrate.
• Insoluble in water. The molecule is non-polar, so it has no osmotic effect inside cells and can be stored in bulk.
• Metabolic water. Their high proportion of hydrogen gives a large yield of water on oxidation.
• Insulation and protection. Fat is a poor conductor of heat and low in density, so adipose tissue insulates, cushions organs and gives buoyancy.

Phospholipids

• In water, the heads face the water and the tails face away from it. Two layers can form a bilayer with tails sandwiched inside, which is the basis of cell-surface membranes.
• The hydrophilic head means a phospholipid interacts with water, unlike a triglyceride. This lets it sit at the boundary between watery and fatty regions.
• Because the bilayer core is hydrophobic, it is a barrier to water-soluble molecules and ions.

Worked example: comparing energy

A diet label gives 37 kJ per gram for fat and 17 kJ per gram for carbohydrate.

1. Energy from 20 g of fat: 20 × 37 = 740 kJ.
2. Carbohydrate giving the same energy: 740 / 17 = 43.5 g.
3. So a fat store needs less than half the mass of a carbohydrate store for the same energy, which suits an organism that must move around.

The emulsion test for lipids

1. Mix the sample with absolute ethanol and shake, so any lipid dissolves.
2. Pour the ethanol into a tube of water.
3. A cloudy white emulsion is a positive result: the lipid is no longer dissolved, so tiny droplets scatter light. A clear solution means no lipid.

Ethanol is needed because lipids do not dissolve in water. Keep ethanol away from flames. The test is qualitative: it tells you lipid is present, not how much.

Amino acids

Proteins are polymers whose monomers are amino acids. Every amino acid has the same general structure around a central carbon.

      R
      |
H2N - C - COOH
      |
      H

NH2 is the amine group, COOH is the carboxyl group and R is the side chain (R group). Twenty amino acids are common to all organisms and they differ only in their R group. One R group is a single hydrogen atom (glycine); another contains sulfur (cysteine); others carry a charge or are hydrophobic.

Peptide bonds and polypeptides

When the carboxyl group of one amino acid reacts with the amine group of its neighbour, a condensation reaction forms a peptide bond (C-N) and one water molecule is released. Two amino acids make a dipeptide. Many amino acids make a polypeptide.

Worked example: glycylalanine

Glycine is C2H5NO2 (Mr 75) and alanine is C3H7NO2 (Mr 89).

1. Dipeptide formula: C5H12N2O4 minus H2O gives C5H10N2O3.
2. Mr = 75 + 89 - 18 = 146.

Worked example: counting bonds

A protein consists of two polypeptides, one of 45 amino acids and one of 62.

1. Each chain has one bond fewer than its number of amino acids: 44 and 61.
2. Total peptide bonds: 44 + 61 = 105.
3. Water released on synthesis: 105 molecules. Complete hydrolysis uses 105.

Because each position can hold any of 20 amino acids, there are 20 × 20 = 400 possible dipeptides and 20³ = 8000 possible tripeptides. This explains the huge variety of proteins.

Protein structure

A functional protein may be one polypeptide or several, and must fold correctly to work. Proteins have a wide variety of functions in organisms, such as catalysis, transport and support, and each function depends on a particular structure.

• Primary structure is the order in which amino acids are arranged along a polypeptide, joined by peptide bonds. This order determines every higher level.
• Secondary structure is the coiling or folding of the chain into an alpha helix or beta pleated sheet. It is held by hydrogen bonds linking C=O groups to N-H groups in the backbone.
• Tertiary structure is the further folding into a specific three-dimensional shape, held by interactions between R groups: hydrogen bonds, ionic bonds and disulfide bridges.
• Quaternary structure is the arrangement of two or more polypeptides in one protein. Haemoglobin has four polypeptides.

Bond	Between	Notes
Hydrogen	Slightly positive H and slightly negative O or N	Weak singly, strong in number; broken by heat
Ionic	Oppositely charged R groups	Stronger than hydrogen; affected by pH
Disulfide	Sulfur atoms of two cysteine R groups	Covalent and strongest; not broken by modest heating or pH change

Structure and properties

The shape of a protein comes from its sequence, so changing one amino acid can change the bonds that form and so the shape. Examples:

• Globular proteins (enzymes, haemoglobin) fold compactly with hydrophilic R groups outside, so they are soluble; the precise shape explains specific binding.
• Fibrous proteins (collagen) have long chains with many cross-links, so they are strong and insoluble, which suits structural roles.
• Heat or extreme pH breaks hydrogen and ionic bonds, the shape changes and the protein stops working (denaturation).

The biuret test

1. Add sodium hydroxide solution to the sample.
2. Add a few drops of dilute copper(II) sulfate solution and mix.
3. Blue stays blue if no protein is present. A lilac or purple colour shows peptide bonds, so protein is present.

The test detects peptide bonds, so it identifies protein, not individual amino acids.

Common errors

• Writing that unsaturated fats have “no hydrogen”. They have fewer hydrogens than saturated chains of the same length.
• Placing disulfide bridges in secondary structure. They belong to tertiary structure.
• Saying hydrogen bonds link amino acids in the primary structure. Peptide bonds do.
• Describing the head of a phospholipid as hydrophobic.
• Saying a positive emulsion test turns purple. That is the biuret result.

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.