Biological Molecules is a core part of A-Level Biology. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
TriglycerideGlycerol + 3 fatty acids joined by ester bonds
Ester bondBond between -OH (glycerol) and -COOH (fatty acid), releasing water
PhospholipidGlycerol + 2 fatty acids + phosphate group — forms bilayers
Peptide bondBond between amino group and carboxyl group of two amino acids
R groupVariable side chain that defines each amino acid's properties
Quaternary structureMultiple polypeptide chains joined into a functional protein (e.g. haemoglobin)
Active siteRegion of enzyme where substrate binds — shape is complementary
Induced fitActive site flexes around substrate, straining its bonds
KmSubstrate concentration at ½ Vmax — low Km = high affinity
VmaxMaximum reaction rate when enzyme is saturated with substrate
Competitive inhibitorSimilar to substrate — blocks active site, raises Km
Non-competitive inhibitorBinds allosteric site, changes enzyme shape — lowers Vmax
MonomerSmall repeating unit (e.g. glucose, amino acid, nucleotide)
PolymerLong chain of monomers joined by condensation
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
What reads the mRNA codons during translation?✗ DNA directly reads the codons on mRNA and adds amino acids. ✓ tRNA molecules carrying specific amino acids — their anticodons pair with mRNA codons at the ribosome.
What makes the antigen-binding site of an antibody specific to one antigen?✗ All antibodies have the same binding site; specificity comes from where they go in the body. ✓ The variable region — its amino acid sequence (and thus shape) differs between antibodies, complementing a specific antigen.
What happens to an enzyme when it is denatured?✗ The enzyme has died because enzymes are alive. ✓ Its tertiary structure (3D shape) changes, particularly the active site, so it can no longer bind substrate effectively.
Why are enzymes specific to certain substrates?✗ Each enzyme can react with any substrate, but works fastest with one type. ✓ Only substrates with complementary shape and chemistry to the active site can bind effectively.
How does a competitive inhibitor affect enzyme kinetics?✗ Lowers Vmax, Km unchanged ✓ Raises Km, Vmax unchanged
Practise Biological Molecules — free games
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