A-Level Biology biological molecules: carbohydrates, lipids, proteins and water

Where biological molecules sit in each specification

In Edexcel IAL, cellulose and starch in plant cell structure are revisited in Unit 2, Topic 4.

The key ideas

Carbohydrates: alpha-glucose and beta-glucose differ only in the position of the OH group on carbon 1. Two alpha-glucose join by a 1,4 glycosidic bond to form maltose; glucose plus fructose gives sucrose; glucose plus galactose gives lactose. Amylose is an unbranched helix of alpha-glucose (1,4 bonds), compact for storage. Amylopectin and glycogen also have 1,6 branches, so enzymes can reach many ends at once and release glucose quickly. Cellulose is made of beta-glucose, with alternate molecules flipped, forming straight chains held together by hydrogen bonds into strong microfibrils.

Lipids: a triglyceride forms when glycerol joins three fatty acids by condensation, making three ester bonds. Saturated fatty acids have no C=C double bonds; unsaturated ones have at least one, which puts kinks in the chain. In a phospholipid one fatty acid is replaced by a phosphate group, giving a hydrophilic head and hydrophobic tails, which is why phospholipids form bilayers.

Proteins: amino acids have an amine group, a carboxyl group and a variable R group. The primary structure is the sequence; the secondary structure (alpha-helix and beta-pleated sheet) is held by hydrogen bonds; the tertiary structure is the 3D folding held by hydrogen, ionic and disulfide bonds and hydrophobic interactions; quaternary structure is more than one polypeptide, as in haemoglobin. Globular proteins such as haemoglobin and enzymes are soluble and have specific shapes; fibrous proteins such as collagen are insoluble and strong.

Worked example 1: how many water molecules are released?

1. Each condensation reaction joining two monomers releases one water molecule.
2. A polypeptide of 150 amino acids has 149 peptide bonds, so 149 water molecules are released when it forms.
3. A triglyceride has three ester bonds, so 3 water molecules are released.
4. A glycogen chain of 1,000 glucose units with no branches would have 999 glycosidic bonds and release 999 water molecules. Each branch point adds a 1,6 glycosidic bond and one more water molecule.
5. Hydrolysis runs the other way: to break all 149 peptide bonds, 149 water molecules are used.

Worked example 2: identifying a non-reducing sugar

1. A solution gives a blue colour with Benedict's reagent after heating, so no reducing sugar is present.
2. Boil a fresh sample with dilute hydrochloric acid. This hydrolyses any glycosidic bonds in a non-reducing sugar such as sucrose.
3. Cool, then neutralise with sodium hydrogencarbonate, because Benedict's reagent needs alkaline conditions.
4. Add Benedict's reagent and heat in a water bath. A green, yellow, orange or brick-red precipitate now shows reducing sugars were released.
5. Conclusion: the original solution contained a non-reducing sugar. For a semi-quantitative estimate, compare the colour (or the mass of precipitate, or the colour after filtering with a colorimeter) with standards of known glucose concentration.

Common mistakes that cost marks

• Writing that hydrolysis 'removes' water. Hydrolysis uses water; condensation releases it.
• Saying starch is made of beta-glucose or cellulose of alpha-glucose.
• Describing glycogen as 'more branched' without linking it to faster hydrolysis and release of glucose.
• Saying an enzyme is 'killed' by heat. It is denatured: bonds holding the tertiary structure break and the active site changes shape.
• Forgetting to neutralise after acid hydrolysis in the non-reducing sugar test.
• Naming the bonds in tertiary structure without saying they are between R groups.

Exam technique and how a tutor helps

Many questions on this topic ask you to relate structure to function. Use a two-part sentence every time: a feature, then why it matters. 'Glycogen is highly branched, so there are many ends for enzymes to hydrolyse, releasing glucose quickly for respiration.' Examiners give the mark for the link, not for the feature alone.

Learn the reagents and colour changes precisely, and in practical questions name the control and the variable you keep constant, such as volume of reagent, temperature of the water bath and heating time.

In one-to-one lessons a tutor checks recall with quick questions, then spends most of the time on past-paper questions that ask for explanations, marking the student's sentences against the official mark scheme so the student learns which words earn credit on their board.

Self-check: can you do these?

• Which bond joins two amino acids, and what is released? (Answer: peptide bond, water)
• Name the monomer of cellulose. (Answer: beta-glucose)
• Why is a triglyceride a good energy store? (Answer: high ratio of C-H bonds, insoluble, compact)
• Which bonds hold the secondary structure of a protein? (Answer: hydrogen bonds)
• What colour shows a positive biuret test? (Answer: lilac or purple)