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NDA Biology · Biochemistry

Biomolecules and Protein Structure

Living things are built from four kinds of biomolecule — carbohydrates, proteins, lipids and nucleic acids; proteins are chains of amino acids joined by peptide bonds, folded through four structural levels.

All Biochemistry notesNDA Biology strategy

Why this matters

The NDA tests this as a single recall fact — most often the four levels of protein structure, where a 'linear sequence of amino acids joined by peptide bonds' is the primary structure. Learn the four biomolecules and the primary → secondary → tertiary → quaternary ladder, and the marks are yours. EASY recall.

Concept 1 of 2

The four biomolecules of life

Intuition

Every living cell is built from four families of large molecules. Each is a polymer — a long chain assembled from small repeating units (monomers). Knowing which monomer builds which biomolecule is the foundation for everything else in this chapter.

Definition

The four classes of biomolecule and their building blocks:

  • Carbohydrates — monomer is a monosaccharide (e.g. glucose); store and supply energy (starch, glycogen, cellulose).
  • Proteins — monomer is an amino acid; build tissues, act as enzymes and hormones.
  • Lipids (fats and oils) — built from fatty acids + glycerol; long-term energy store and cell membranes.
  • Nucleic acids (DNA, RNA) — monomer is a nucleotide; store and carry genetic information.

Enzymes — biological catalysts that speed up reactions — are themselves proteins.

Worked example

Glycogen, an energy store in the liver, is a long chain of glucose units. Which class of biomolecule is it, and what is its monomer?
  1. Glucose is a monosaccharide (a simple sugar).
  2. A polymer of monosaccharides is a carbohydrate.
Answer:Glycogen is a carbohydrate; its monomer is glucose (a monosaccharide).

Concept 2 of 2

The four levels of protein structure

Intuition

A protein is a chain of amino acids that folds up into a precise 3-D shape — and that shape decides what the protein does. Biologists describe the folding in four levels, from the bare sequence (primary) up to an assembly of several chains (quaternary). The NDA's favourite is the primary level: a linear sequence of amino acids joined by peptide bonds.

Definition

The four levels, in order. Amino acids are linked by peptide bonds; the bonds that hold the higher folds are mostly hydrogen bonds and other weak interactions.

Primaryamino-acid sequence(peptide bonds)Secondaryα-helix / β-sheet(hydrogen bonds)Tertiary3-D fold ofone chainQuaternaryseveral chains(e.g. haemoglobin)
LevelWhat it is
PrimaryThe linear sequence of amino acids joined by peptide bonds
'A linear sequence of amino acids linked by peptide bonds' = PRIMARY structure — the bank's exact phrasing.
SecondaryLocal coiling/folding into an α-helix or β-pleated sheet, held by hydrogen bonds
TertiaryThe overall 3-D folded shape of a single polypeptide chain
QuaternaryTwo or more folded chains (subunits) assembled together (e.g. haemoglobin = 4 subunits)
Primary = sequence; Secondary = helix/sheet; Tertiary = 3-D fold of one chain; Quaternary = several chains together.
Practice this conceptself-check · 4 quick reps

Try it yourself

Haemoglobin is made of four separate folded polypeptide chains held together. Which level of protein structure does this describe?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Which level is a linear sequence of amino acids joined by peptide bonds?
  2. 2.
    Which level is the α-helix or β-pleated sheet?
  3. 3.
    Which bond links amino acids in a protein chain?
  4. 4.
    Haemoglobin's four-subunit assembly is which level?

From the bank · past-year question

Example 2BiochemistryEASY
A chain of peptide containing linear sequences of amino acid linked by peptide bonds best represent the

[Q107 · Apr · 2025]

Linear sequence + peptide bonds = PRIMARY, not secondary

The stem 'linear sequence of amino acids linked by peptide bonds' is the primary structure. Secondary (helix/sheet), tertiary (3-D fold) and quaternary (multiple chains) are the distractors. Peptide bonds define the primary chain; hydrogen bonds shape the higher levels.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (1)

The four levels of protein structure4 rows
LevelWhat it is
PrimaryThe linear sequence of amino acids joined by peptide bonds
'A linear sequence of amino acids linked by peptide bonds' = PRIMARY structure — the bank's exact phrasing.
SecondaryLocal coiling/folding into an α-helix or β-pleated sheet, held by hydrogen bonds
TertiaryThe overall 3-D folded shape of a single polypeptide chain
QuaternaryTwo or more folded chains (subunits) assembled together (e.g. haemoglobin = 4 subunits)
Primary = sequence; Secondary = helix/sheet; Tertiary = 3-D fold of one chain; Quaternary = several chains together.

Watch out for (1)

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