The structure of monosaccharides, disaccharides and polysaccharides, the formation of triglycerides and phospholipids, and the levels of protein structure, linking each molecule's structure to its function.

What this dot point is asking

Edexcel wants you to describe the structures of carbohydrates, lipids and proteins, explain how condensation and hydrolysis build and break them, and link the structure of each molecule (such as glycogen, a triglyceride or haemoglobin) to its function in the body. The recurring exam skill is structure-to-function reasoning, so always finish a description with why that structure suits the job.

Carbohydrates

Polysaccharides are many monosaccharides joined together.

• Starch (amylose and amylopectin) is the storage carbohydrate in plants; it is compact, insoluble and easily hydrolysed to release glucose.
• Glycogen is the storage carbohydrate in animals; it is more highly branched than starch, so it can be hydrolysed quickly to meet high energy demand.
• Cellulose is made of beta-glucose, giving straight chains that hydrogen-bond into strong microfibrils, ideal for plant cell wall support.

Lipids

A saturated fatty acid has no carbon-to-carbon double bonds, so the chains are straight, pack closely and are solid at room temperature (animal fats). An unsaturated fatty acid has one or more carbon-to-carbon double bonds, which puts a kink in the chain so molecules cannot pack closely; these stay liquid at room temperature (plant oils) and are generally healthier in the diet.

Test reminders

Edexcel expects the food tests: Benedict reagent gives a brick-red precipitate with reducing sugars; iodine in potassium iodide turns blue-black with starch; the emulsion test gives a white emulsion with lipids; and the biuret test turns lilac or purple with proteins. These link the molecule classes to practical identification.

Proteins

Proteins are polymers of amino acids joined by peptide bonds (formed by condensation between an amino group and a carboxyl group). There are four levels of structure.

• Primary structure: the sequence of amino acids in the chain.
• Secondary structure: local folding into alpha helices or beta pleated sheets, held by hydrogen bonds.
• Tertiary structure: the overall three-dimensional shape, held by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions.
• Quaternary structure: two or more polypeptide chains together, as in haemoglobin (four chains plus four haem groups).

Proteins are either globular (rounded, soluble, with hydrophilic groups outwards, such as enzymes and haemoglobin) or fibrous (long, insoluble, structural, such as collagen and keratin). The folding that gives the final shape is the basis of every protein function, from catalysis to oxygen transport.

Examples in context

Example 1. Haemoglobin and oxygen transport. Haemoglobin shows all four levels of structure. Its quaternary structure (four polypeptide chains each holding a haem group with an iron ion) lets it bind four oxygen molecules. Binding of one oxygen slightly changes the tertiary and quaternary shape, making the next bind more easily (cooperative binding). This is a clear example of structure determining function and is a favourite exam link between this topic and gas transport.

Example 2. Cellulose versus starch in plants. Both are glucose polymers, but cellulose is built from beta-glucose, so alternate units flip and the straight chains hydrogen-bond into strong microfibrils for cell wall support. Starch is built from alpha-glucose, so it coils into a compact, easily hydrolysed store. Same monomer, different isomer, completely different function, which examiners use to test whether you understand why bond geometry matters.

Try this

Q1. Describe how a glycosidic bond is formed between two glucose molecules. [2 marks]

• Cue. A condensation reaction joins the two monosaccharides, removing one water molecule and forming a glycosidic bond.

Q2. Explain how the structure of a phospholipid suits its role in the cell membrane. [3 marks]

• Cue. The hydrophilic head faces water and the hydrophobic tails face inwards, so phospholipids form a bilayer that is the barrier of the membrane.