Biological molecules
Biological molecules tests the structure and function of water, carbohydrates, lipids, proteins and nucleic acids, plus food tests and condensation/hydrolysis. Expect questions linking molecular shape to properties and interpreting qualitative tests.
Key molecules
- Water is polar, forms hydrogen bonds, has high specific heat capacity and is a solvent; these properties support transport and temperature stability.
- Monosaccharides are reducing sugars; sucrose is non-reducing. Starch is a storage polysaccharide of alpha-glucose; cellulose is structural with beta-glucose and cross-linking hydrogen bonds.
- Triglycerides have glycerol and three fatty acids; phospholipids have a phosphate group and are amphipathic.
- Proteins are polymers of amino acids joined by peptide bonds; primary to quaternary structure determines function.
Food tests
Benedict's test detects reducing sugars with a brick-red precipitate on heating; sucrose needs hydrolysis first. Iodine tests starch (blue-black). Biuret tests peptide bonds (purple). Emulsion test detects lipids (milky white).
Worked example
A tripeptide is made from three amino acids. Each peptide bond forms by condensation and releases one water molecule, so three amino acids form two peptide bonds and release two water molecules. The molecular mass falls by 2 x 18 = 36. Hydrolysis reverses this.
Traps
Benedict's is semi-quantitative, not a precise concentration measure. Do not call sucrose a reducing sugar. Lipids are not polymers. A positive emulsion test needs ethanol and water layers.