CCEA A Level Biology 1.1 Molecules Exam Questions

About CCEA A Level Biology 1.1 Molecules

Section 1.1 opens Unit AS 1: Molecules and Cells and gives you the biochemistry that the rest of the CCEA course builds on. You need the structure and function of carbohydrates, lipids, proteins and nucleic acids, how condensation builds polymers and hydrolysis breaks them, and how DNA copies itself semi-conservatively. CCEA also includes prions and the Meselson-Stahl experiment, which many other boards leave out. The content links directly to enzymes (1.2), membranes (1.4) and protein synthesis in A2 2. Examiners like questions on bonds (glycosidic, ester, peptide), interpreting Meselson-Stahl results, and the practical tests for biological molecules.

What you need to know

• Carbohydrates Monosaccharides join by condensation to form glycosidic bonds; maltose is glucose + glucose and sucrose is glucose + fructose. Starch and glycogen are storage polymers of alpha-glucose, while cellulose is a structural polymer of beta-glucose. Ribose and deoxyribose are pentose sugars used in nucleic acids and ATP.
• Lipids A triglyceride forms from glycerol and three fatty acids by condensation, making ester bonds. Unsaturated fatty acids have C=C double bonds. Phospholipids have a hydrophilic phosphate head and hydrophobic tails, so they form membrane bilayers.
• Protein structure Amino acids join by peptide bonds. Primary structure is the amino acid sequence; secondary structure (alpha-helix, beta-pleated sheet) is held by hydrogen bonds; tertiary structure involves hydrogen, ionic and disulfide bonds plus hydrophobic interactions; quaternary structure has more than one polypeptide, as in haemoglobin.
• Fibrous, globular and conjugated proteins Collagen is fibrous and structural; enzymes are globular. Conjugated proteins such as haemoglobin and glycoproteins have a prosthetic group. Prions are abnormally folded proteins rich in beta-sheet that cause scrapie, BSE and CJD.
• Nucleic acids A nucleotide is a pentose sugar, a phosphate and a nitrogenous base. DNA is an antiparallel double helix with complementary pairing (A with T, C with G); RNA is single stranded, contains ribose and uses uracil instead of thymine.
• DNA replication Replication is semi-conservative: DNA helicase separates the strands and DNA polymerase builds new strands from free nucleotides. The Meselson-Stahl experiment with 15N and 14N DNA provided the evidence.
• Practical work Iodine for starch, Benedict's for reducing sugars, a glucose-specific test strip for glucose and biuret for protein. Amino acids are separated by paper chromatography and identified using Rf = distance moved by spot / distance moved by solvent front.

Common mistakes in 1.1 Molecules

• Saying cellulose is made of alpha-glucose. Cellulose is a polymer of beta-glucose, with alternate monomers inverted, which allows straight chains linked by hydrogen bonds.
• Describing hydrolysis as adding water to break a molecule without naming the bond. Always name it, for example hydrolysis of the glycosidic bond in maltose.
• Calling disulfide bonds disulphide bridges between any amino acids. They form only between cysteine residues containing sulfur.
• Misreading Meselson-Stahl results after two generations. After one generation all DNA is intermediate; after two generations half is intermediate and half is light.
• Measuring Rf to the edge of the spot instead of its centre, or from the wrong origin. Measure from the origin line to the centre of the spot and to the solvent front.

Try three 1.1 Molecules exam-style questions