Cell Biology and Biomolecules
What the exam tests
This domain covers prokaryotic and eukaryotic cell organization, plant cell organelles, and the structure and function of carbohydrates, proteins, lipids, nucleic acids, and enzymes. Questions often ask you to identify a molecule from its properties or explain organelle function.
Core rules and concepts
- Plant cell wall is mainly cellulose; bacterial wall is peptidoglycan; fungal wall is chitin.
- Plastids: chloroplast for photosynthesis, chromoplast for pigments, leucoplast for storage.
- Vacuole maintains turgor and stores ions, pigments, and wastes.
- Carbohydrates: monosaccharides, disaccharides, polysaccharides. Starch is storage in plants; cellulose is structural.
- Proteins are polymers of amino acids joined by peptide bonds; the sequence is the primary structure.
- Lipids are esters of fatty acids and glycerol; saturated fatty acids have no double bonds, unsaturated ones have one or more.
- Enzymes are biological catalysts that lower activation energy without changing the equilibrium constant.
Worked example
A polysaccharide from a plant storage organ gives a blue-black colour with iodine. Identify it and explain. The iodine test indicates starch, the glucose storage polymer of plants. Cellulose is also a glucose polymer but is structural and does not give this starch-iodine colour. The reasoning uses both source and chemical test.
Traps that cost marks
Starch and cellulose differ in glycosidic linkage, not just in monomer. Do not say enzymes increase reaction energy; they lower activation energy. A saturated fatty acid is not the same as a trans fat. Remember that the plant cell wall is not a selective barrier - the plasma membrane is.