Biological Molecules: Revision Notes

Condensed for the final weeks. For the full explanation, use the Biological Molecules study guide.

The building blocks

Molecule	Elements	Monomer	Example
Carbohydrate	C, H, O	Simple sugar (glucose)	Starch, glycogen, cellulose
Lipid	C, H, O	Fatty acids + glycerol	Fats, oils
Protein	C, H, O, N (+ S)	Amino acids	Enzymes, haemoglobin
DNA	C, H, O, N, P	Nucleotides	Chromosomes

Nitrogen distinguishes proteins from carbohydrates and lipids. Phosphorus additionally marks DNA. That single fact answers a lot of one-mark questions — given an unfamiliar molecule’s elemental composition, checking for nitrogen and phosphorus alone is often enough to place it in the right category without needing to know its full structure.

Structures

• Starch and glycogen — storage polysaccharides of glucose. Compact, insoluble, so they do not affect water potential. Glycogen is more branched (animals, faster release).
• Cellulose — structural polysaccharide of glucose; straight chains hydrogen-bonded into fibres, giving plant cell walls strength. Starch, cellulose and glycogen are all built from the same monomer, glucose, joined in different arrangements — a reminder that it is the arrangement, not the building block, that gives each its distinct role.
• Lipid — one glycerol + three fatty acids, joined by ester bonds; lipids contain a much smaller proportion of oxygen relative to carbon and hydrogen than carbohydrates do, which is part of why they release more energy per gram when respired.
• Protein — a chain of amino acids joined by peptide bonds, folded into a specific 3D shape. The sequence determines the shape, and the shape determines the function — enzymes and antibodies both rely on a precisely-shaped binding site that a wrong sequence would destroy.
• DNA — two strands in a double helix, held by complementary base pairing: A–T and C–G, with the sequence of bases along one strand storing the genetic information that the cell reads to build proteins.

Food tests — learn all four

Test for	Reagent	Method	Positive result
Starch	Iodine solution	Add drops	Orange-brown → blue-black
Reducing sugar (glucose, maltose)	Benedict’s	Heat in water bath	Blue → green → yellow → brick-red
Protein	Biuret	Add, no heat	Blue → purple/lilac
Fat	Ethanol emulsion	Shake with ethanol, add water	White/cloudy emulsion

Benedict’s must be heated, in a water bath. Iodine, biuret and the emulsion test are all done at room temperature. Stating the method is often worth a mark by itself.

Biuret reagent is either a ready-made biuret solution, or sodium hydroxide solution followed by a few drops of copper sulfate solution — either version gives the same purple/lilac positive result, so both are acceptable answers.

Worked example — describing a procedure. To test an unknown solution for starch: add a few drops of iodine solution directly to the sample at room temperature. A colour change from orange-brown to blue-black shows starch is present; no colour change shows it is absent.

The brick-red endpoint is progressive — the more sugar, the further along the colour sequence, which lets Benedict’s be used semi-quantitatively.

Why each matters

Molecule	Function
Carbohydrate	Main energy source; structural (cellulose)
Lipid	Energy store (more per gram), insulation, protection, membranes
Protein	Enzymes, antibodies, haemoglobin, growth and repair
DNA	Stores genetic information

See the Biological Molecules study guide for the full syllabus coverage and worked descriptions behind each table above.

Exam traps

• Proteins contain nitrogen — this is the key distinguishing element.
• Benedict’s requires heating; biuret does not.
• A lipid is three fatty acids and one glycerol, not three of each.
• Cellulose and starch are both glucose polymers — the difference is the arrangement, not the monomer.
• “Brick-red” not “orange”; “blue-black” not “dark”.
• Forgetting biuret can be a single ready-made solution or sodium hydroxide followed by copper sulfate — either is creditable.
• Assuming every food test needs heating — only Benedict’s does; iodine, biuret and the emulsion test are all room-temperature tests.

Beyond the syllabus: what a negative Benedict’s result does not show

Not required for 5090. Syllabus 4.1.3 lists four food tests only: starch (iodine solution), glucose and maltose (Benedict’s solution), protein (biuret test) and lipids (ethanol emulsion test). The extra procedure below is not one of them and is included only as background.

An unknown solution gives a negative Benedict’s test result, but it might still contain a sugar that Benedict’s solution does not detect.

Step 1: Boil a fresh sample with dilute hydrochloric acid, then
        neutralise with an alkali (e.g. sodium hydrogencarbonate).
Step 2: Retest with Benedict's solution, heating in a water bath as
        normal.
If this second test now turns brick-red, the original sample
contained a non-reducing sugar (e.g. sucrose), which the acid
hydrolysed into reducing sugars the standard Benedict's test can
detect.

The point to keep for 5090: a negative Benedict’s result shows that no glucose or maltose (no reducing sugar) is present, not that the sample contains no sugar at all. For the exam, learn the four tests in the table above.

Self-test

1. Which element distinguishes protein from carbohydrate?
2. Give the reagent, method and positive result for reducing sugar.
3. Name the bond joining amino acids.
4. State the complementary base pairs in DNA.
5. Why is starch a good storage molecule?
6. Which of the four food tests require heating, and which are done at room temperature?
7. Describe how you would test an unknown solution for starch, including the result for a positive test.

Answers: 1. Nitrogen. 2. Benedict’s solution, heated in a water bath; blue → brick-red precipitate. 3. Peptide bond. 4. Adenine with thymine; cytosine with guanine. 5. It is compact and insoluble, so large quantities can be stored without affecting the water potential of the cell, and it is readily hydrolysed back to glucose when needed. 6. Only Benedict’s requires heating (in a water bath); iodine, biuret and the emulsion test are all carried out at room temperature. 7. Add a few drops of iodine solution directly to the sample at room temperature; a colour change from orange-brown to blue-black indicates starch is present.