Biomolecules
Domain 13: Biomolecules
This domain tests the structure and function of carbohydrates, proteins, enzymes, vitamins, nucleic acids, and hormones, with emphasis on classifying tests and linkages.
Core rules
- Monosaccharides are reducing sugars; sucrose is non-reducing because its glycosidic bond involves both anomeric carbons. Hydrolysis of sucrose gives glucose and fructose (invert sugar).
- Glucose is an aldohexose; fructose is a ketohexose. They form osazones and reduce Tollens'/Fehling's reagents. Starch and cellulose are polysaccharides of glucose but differ in glycosidic linkage: starch has α-linkages, cellulose β-linkages.
- Proteins are polymers of α-amino acids linked by peptide (amide) bonds. Primary structure is sequence; secondary structure includes α-helix and β-pleated sheet; tertiary is 3D folding; quaternary is subunit assembly.
- Denaturation disrupts hydrogen bonds and hydrophobic interactions without breaking peptide bonds.
- Enzymes are biological catalysts, highly specific; they lower activation energy. Vitamins are classified fat-soluble (A, D, E, K) and water-soluble (B group, C).
- Nucleic acids: DNA has deoxyribose and thymine; RNA has ribose and uracil. DNA is a double helix with A–T and G–C base pairing.
Worked example
Sucrose gives a negative Tollens' test. Its glucose and fructose units are joined through the anomeric carbons, so the sugar cannot open to the free aldehyde/keto form under the test conditions. After acid hydrolysis, the glycosidic bond breaks, glucose and fructose are released, and the mixture reduces Tollens' reagent.
Traps
- Calling all carbohydrates reducing sugars.
- Assuming sucrose is a monosaccharide.
- Saying denaturation breaks peptide bonds.
- Mixing DNA and RNA sugars or bases.
- Forgetting that enzymes are proteins (except catalytic RNA).