⚗️ Chemistry introductory Lesson 3 of 5 2 min read

Biochemistry Basics: Macromolecules

The four classes of biological macromolecules, how their monomers assemble, and how enzymes speed the reactions of life.

Reading level

What you'll learn

Name the four classes of biological macromolecules and their building-block monomers.
Describe the four levels of protein structure and the forces that stabilize each.
Compare the roles of carbohydrates, lipids, and nucleic acids in the cell.
Explain how enzymes lower activation energy and how inhibitors alter enzyme kinetics.

The Four Classes of Macromolecules

Class	Monomer	Key functions
Carbohydrates	Monosaccharides	Energy storage (glycogen), structural (cellulose), signaling
Lipids	Fatty acids / glycerol	Membrane bilayers, energy storage, hormones (steroids)
Proteins	Amino acids	Enzymes, structural (collagen), transport (hemoglobin), signaling
Nucleic acids	Nucleotides	Information storage (DNA), gene expression (RNA)

Proteins

Proteins are polymers of amino acids linked by peptide bonds (condensation reaction).

Protein structure has four levels:

Primary — linear sequence of amino acids (determined by gene)
Secondary — local folding into α-helices and β-sheets (H-bonds between backbone atoms)
Tertiary — overall 3D fold (hydrophobic core, disulfide bridges, salt bridges)
Quaternary — assembly of multiple polypeptide subunits (e.g., hemoglobin = 4 subunits)

Carbohydrates

Monosaccharides (glucose, fructose, galactose) link via glycosidic bonds to form:

Disaccharides — sucrose (glucose + fructose), lactose (glucose + galactose)
Polysaccharides — glycogen (animal energy storage), starch (plant), cellulose (structural; β-1,4 linkage humans cannot digest)

Lipids

Triacylglycerols (fats) store long-term energy. Phospholipids (two fatty acid tails + phosphate head) self-assemble into bilayers — the basis of all cellular membranes. Cholesterol modulates membrane fluidity. Steroid hormones (cortisol, estrogen, testosterone) are synthesized from cholesterol.

Enzymes

Enzymes are protein (or RNA) catalysts that lower activation energy without being consumed. Key concepts:

Active site — complementary to substrate (lock-and-key / induced-fit models)
Km (Michaelis constant) — substrate concentration at half-maximal velocity; reflects affinity
Vmax — maximum reaction velocity at enzyme saturation
Inhibition — competitive (↑ Km, same Vmax) vs. non-competitive (same Km, ↓ Vmax)

Medical Relevance

Many drugs are enzyme inhibitors (e.g., statins inhibit HMG-CoA reductase in cholesterol synthesis; ACE inhibitors lower blood pressure by blocking angiotensin-converting enzyme).

Going deeper advanced

Extra depth for when you're ready — expanded automatically in Advanced mode.

Chirality and biological stereospecificity

Nineteen of the twenty standard amino acids are chiral, and life uses almost exclusively the L-enantiomer, just as sugars are predominantly D; enzymes and receptors are themselves chiral, so they distinguish mirror-image molecules. This stereospecificity is why one enantiomer of a drug can be therapeutic while its mirror image is inactive or toxic, the classic cautionary example being thalidomide. Functional groups such as hydroxyl, carboxyl, amino, and phosphate confer the reactivity and charge that define each monomer's behavior.

The peptide bond as a resonance-stabilized amide

The peptide bond is an amide in which the nitrogen lone pair delocalizes into the carbonyl, giving the C-N linkage partial double-bond character. This resonance makes the six atoms of the peptide unit essentially planar and restricts rotation about the C-N bond, so backbone conformation is described mainly by the phi and psi dihedral angles around the neighboring bonds. That planarity and the resulting rotational constraints are what make regular secondary structures like the alpha-helix and beta-sheet geometrically possible.

Key terms

Macromolecule: A large biological polymer built from repeating monomer subunits.
Monomer: A small molecule that links with others to form a polymer, such as an amino acid or monosaccharide.
Peptide bond: The covalent bond linking amino acids in a protein, formed by a condensation (dehydration) reaction.
Phospholipid bilayer: A double layer of phospholipids that forms the basic structure of all cell membranes.
Enzyme: A protein or RNA catalyst that lowers a reaction's activation energy without being consumed.
Active site: The region of an enzyme that binds substrate and carries out catalysis.
Km (Michaelis constant): The substrate concentration at half-maximal reaction velocity, reflecting enzyme-substrate affinity.
Denaturation: The loss of a protein's functional three-dimensional shape due to heat, pH, or chemical stress.

Check your understanding

5 questions · answers reveal instantly.

1.Which macromolecule uses amino acids as its monomer?
2.The linear sequence of amino acids in a protein defines its:
3.What is the primary structural role of phospholipids in cells?
4.How does an enzyme speed up a reaction?
5.A competitive inhibitor typically affects enzyme kinetics by:

Citations & References

Links open publicly available educational and peer-reviewed sources.