Biomolecules and Enzymes
Biomolecules and Enzymes
This domain tests the structure and function of water, carbohydrates, lipids, proteins and nucleic acids, the standard food tests, and enzyme kinetics. Questions often require you to match a monomer to its polymer, interpret a test result, or explain why reaction rate changes.
Core rules
- Monosaccharides join by glycosidic bonds to form polysaccharides. Amino acids join by peptide bonds; the sequence of amino acids determines protein shape. Nucleotides form polynucleotides linked by phosphodiester bonds.
- Benedict's test detects reducing sugars: heat with Benedict's solution and a brick-red precipitate forms. Iodine turns blue-black with starch. Biuret reagent turns purple with protein. An emulsion test gives a white emulsion with lipid.
- Enzymes are biological catalysts, usually proteins, that lower activation energy. They have an active site complementary to the substrate. Rate rises with temperature or substrate concentration until denaturation or active-site saturation limits it.
Worked example
At fixed enzyme concentration, increasing substrate concentration raises reaction rate at first, then the curve plateaus. Reasoning: more substrate molecules collide with active sites initially, but once every active site is occupied, added substrate cannot increase rate further. The plateau is saturation, not denaturation.
Traps
Enzymes are not used up in the reaction. Boiling denatures most enzymes by changing active-site shape; it does not simply 'kill' a chemical. Benedict's test needs heat. Sucrose is not a reducing sugar until hydrolysed. A cofactor such as a mineral ion may be required for activity.