Biomolecules: Carbohydrates, Lipids, Amino Acids, Proteins and Nucleic Acids
What this domain tests
Biomolecules questions ask you to classify carbohydrates, lipids, amino acids, proteins, and nucleic acids, then link structure to function and to simple chemical tests.
Core rules
- Monosaccharides are polyhydroxy aldehydes (aldoses) or ketones (ketoses). Glucose is an aldohexose; fructose is a ketohexose. Free aldehyde or ketone groups make sugars reducing; sucrose has no free reducing group because both anomeric carbons are in the glycosidic bond.
- Glycosidic bonds join monosaccharides. Starch and glycogen are storage homopolysaccharides of glucose; cellulose is a structural beta-1,4-glucan.
- Fatty acids are saturated or unsaturated. Triacylglycerols are storage lipids; phospholipids are amphipathic membrane lipids; cholesterol is a steroid precursor.
- Amino acids contain an alpha-carbon bearing amino and carboxyl groups; at physiological pH they exist as zwitterions. Peptide bonds join the alpha-carboxyl of one amino acid to the alpha-amino of the next.
- Protein structure runs primary, secondary, tertiary, quaternary. Denaturation disrupts non-covalent structure, not peptide bonds.
- Nucleic acids are polymers of nucleotides: phosphate, pentose, nitrogenous base. DNA has deoxyribose and thymine; RNA has ribose and uracil. Bases pair A-T/U and G-C.
Worked example
A pentapeptide has five amino acid residues, so it contains four peptide bonds. If it is made of Ala-Gly-Ser-Lys-Val, the N-terminal residue is alanine and the C-terminal residue is valine. Hydrolysis with acid breaks the four peptide bonds and gives five free amino acids; it does not change the side chains.
Traps
Fructose is a ketose but still reducing. Sucrose is nonreducing. Cellulose and starch differ in glycosidic linkage, not in monosaccharide units. DNA and RNA differ in pentose and one pyrimidine, not in the purine set.