A-Level Biology Practice: Biological Molecules

18 MCQ practice problems on biological molecules. Select an answer, submit, and review the explanation. Questions follow A-Level examination style (AQA/OCR/Edexcel) and cover carbohydrates, lipids, proteins, nucleic acids, enzymes, and biochemical tests.

What These Questions Test

These problems test your understanding of the four major biological macromolecules, their structures, functions, and the biochemical tests used to identify them.

Typical question types:

Carbohydrates: Monosaccharides (glucose, fructose, galactose), disaccharides (maltose, sucrose, lactose), and polysaccharides (starch, glycogen, cellulose). Know the glycosidic bond and how it forms by condensation. Reducing and non-reducing sugar tests (Benedict’s and hydrochloric acid + Benedict’s).
Lipids: Triglycerides (three fatty acids + glycerol) and phospholipids (two fatty acids + glycerol + phosphate). The emulsion test (ethanol emulsion) and lipid test (grease spot on paper). Know the difference between saturated and unsaturated fatty acids.
Proteins: Primary, secondary ( $\alpha$ -helix, $\beta$ -pleated sheet), tertiary, and quaternary structure. Peptide bonds form by condensation. The biuret test for proteins. Enzymes are globular proteins that act as biological catalysts.
Nucleic Acids: DNA (double-stranded, deoxyribose, thymine) vs RNA (single-stranded, ribose, uracil). Nucleotides: phosphate + pentose sugar + nitrogenous base. DNA replication is semi-conservative. Transcription and translation.
Enzymes: Lock-and-key model and induced-fit model. Enzyme inhibition: competitive (binds to active site) and non-competitive (binds elsewhere). Factors affecting enzyme activity: temperature, pH, substrate concentration.

Approach Strategy

Draw the molecule. For structural questions, sketch the molecule from memory. This helps you identify functional groups and bonds.
Use the test results. Biochemical tests are specific. Benedict’s tests for reducing sugars (brick-red precipitate). Iodine tests for starch (blue-black). Biuret tests for proteins (purple). Ethanol emulsion tests for lipids (white emulsion).
Connect structure to function. The shape of a protein determines its function. Denaturation destroys the tertiary structure, making the protein non-functional.
Know the condensation and hydrolysis distinction. Condensation joins monomers (releases water). Hydrolysis breaks polymers (uses water).

Intuition

Biological molecules are like building blocks. Monomers are the individual bricks; polymers are the walls built from them. The type of brick determines the type of wall: glucose bricks make starch or cellulose; amino acid bricks make proteins with wildly different shapes and functions.

Enzymes are molecular machines. The active site is like a mould that fits a specific substrate. If you change the shape of the mould (denature it), the substrate no longer fits and the reaction cannot proceed.

Common Mistakes

Confusing DNA and RNA. DNA has deoxyribose, thymine, and is double-stranded. RNA has ribose, uracil, and is in most cases single-stranded. A common error is mixing up which base goes with which nucleic acid.
Forgetting that condensation releases water. When a glycosidic or peptide bond forms, one water molecule is released. The reverse (hydrolysis) uses one water molecule. This is a frequent exam question.
Mixing up competitive and non-competitive inhibition. Competitive inhibitors bind to the active site and can be overcome by increasing substrate concentration. Non-competitive inhibitors bind elsewhere and cannot be overcome by adding more substrate.
Assuming all proteins are enzymes. All enzymes are proteins, but not all proteins are enzymes. Other proteins include structural proteins (collagen), transport proteins (haemoglobin), and hormones (insulin).

Cross-References

Cell Biology: Biological molecules form the structural basis of cells
Ecology: Biological molecules are the foundation of all living organisms

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