IB Biology · Theme B: Form and function · SL and HL
B1.2 Proteins
A one-page summary of B1.2 Proteins, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
What is the relationship between amino acid sequence and the diversity in form and function of proteins?
How are protein molecules affected by their chemical and physical environments?
What B1.2 covers
Amino acids and the polypeptide chain
- B1.2.1Twenty amino acids, one shared backbone
- B1.2.2Peptide bonds: built by condensation
- B1.2.3Some amino acids must come from the diet
- B1.2.4Twenty amino acids, near-infinite chains
- B1.2.5Denaturation: shape lost, sequence intact
Structure and function: from R-group to whole protein
- B1.2.6 · HLR-groups set the chemistry
- B1.2.7 · HLPrimary structure sets the fold
- B1.2.8 · HLThe α-helix coils the backbone
- B1.2.8 · HLThe β-pleated sheet bonds between strands
- B1.2.9 · HLHydrogen and ionic bonds fold the chain
- B1.2.9 · HLDisulfide bridges and hydrophobic interactions lock it in
- B1.2.10 · HLPolarity decides what faces water
- B1.2.11 · HLNon-conjugated proteins: chains only
- B1.2.11 · HLHaemoglobin: subunits plus a prosthetic group
- B1.2.12 · HLGlobular proteins: compact and dynamic
- B1.2.12 · HLFibrous proteins: elongated and strong
B1.2 Proteins: summary
Amino acids & peptide bonds
- Backbone: amino + carboxyl + H + R-group on one alpha carbon.
- Condensation joins amino acids (releases water); hydrolysis breaks them apart.
Diet & sequence variety
- Essential amino acids must come from food; vegan diets need mixed plant sources.
- 20 amino acids → 20ⁿ possible sequences; order and length set the protein.
Denaturation (SL)
- Heat or pH unfolds the protein; weak interactions break.
- Peptide bonds stay intact — denaturation is not hydrolysis.
Primary & secondary HL
- Primary sequence determines the fold (sickle cell: Glu→Val).
- Secondary: α-helix and β-sheet held by backbone C=O···N–H hydrogen bonds.
Tertiary structure HL
- R-group interactions: hydrogen, ionic, disulfide bridges, hydrophobic core.
- Polarity places hydrophobic groups inside, hydrophilic groups outside.
Quaternary & form HL
- Subunits: insulin (non-conjugated), haemoglobin (conjugated, haem).
- Globular (soluble, dynamic) vs fibrous (insoluble, structural).
Key terms
- Amino acid
- Molecule with an alpha carbon bonded to an amino group, a carboxyl group, an H and an R-group.
- Peptide bond
- Covalent C–N bond between amino acids, formed by condensation.
- Polypeptide
- A chain of many amino acids linked by peptide bonds.
- Essential amino acid
- An amino acid the body cannot synthesise; it must come from the diet.
- Denaturation
- Loss of 3D shape without breaking peptide bonds; caused by heat or pH.
- Primary structure HL
- The linear amino acid sequence of a polypeptide.
- Secondary structure HL
- α-helix and β-pleated sheet, held by backbone hydrogen bonds.
- Tertiary structure HL
- Overall 3D fold of one chain, held by R-group interactions.
- Quaternary structure HL
- Assembly of two or more polypeptide chains into one protein.
- Conjugated protein HL
- Protein with a non-protein prosthetic group, e.g. haem in haemoglobin.
Sample exam questions
Three of the 36 multiple-choice questions for B1.2. Try each one before opening the answer.
Question 1. Valine and lysine are essential amino acids. Alanine and glycine are non-essential amino acids. What is a valid conclusion for one of these amino acids?
- Lysine is synthesised by the human body
- Valine must be supplied in the diet
- Alanine can only be obtained from food
- Glycine is not used in the human body
Show the answer
Answer: B. Valine is an essential amino acid: human cells cannot synthesise it, so it must be obtained from the diet. Lysine is also essential (not synthesised by the body), while alanine and glycine are non-essential — the body can synthesise them from other molecules, though it can still use dietary sources too.
Question 2. Insulin is a protein made of two polypeptide chains with a total of 51 amino acids. How many peptide bonds are there in a molecule of human insulin?
- 48
- 49
- 50
- 51
Show the answer
Answer: B. Each polypeptide chain has one fewer peptide bond than it has amino acids (a chain of n amino acids has n−1 peptide bonds, since a peptide bond only forms between adjacent residues). Insulin's two chains are 21 and 30 amino acids long, giving 20 + 29 = 49 peptide bonds — NOT 51−1=50, because the count must be done separately for each chain, not for the combined total.
Question 3. The R groups (side chains) of amino acids determine their chemical properties. Which category of amino acids would you expect to find buried in the interior of a soluble globular protein?
- Hydrophobic (nonpolar) amino acids, because they avoid contact with the aqueous environment
- Positively charged amino acids, because they are attracted to the negatively charged DNA
- All amino acids are distributed randomly throughout the protein
- Hydrophilic (polar) amino acids, because they interact favourably with water
Show the answer
Answer: A. In an aqueous environment, protein folding buries hydrophobic (nonpolar) R groups in the interior, away from water, and exposes hydrophilic (polar and charged) R groups on the surface where they can interact with water. This hydrophobic core is a major driving force for protein folding.
Linking questions
Questions that connect B1.2 to other parts of the course, the kind that come up in Paper 2.
- Ribosomes synthesise proteins from amino acids using the genetic code. How does the sequence of bases in DNA (A1.2) ultimately determine the primary structure of a protein? (see A1.2, D1.2)
- Mutations in DNA (D1.3) can alter a single amino acid in a protein. Using sickle cell disease as an example, explain how a change in primary structure can alter quaternary structure and reduce the protein's ability to perform its function. (see D1.3, D3.2)
Practise B1.2
Study notes, every question and full markschemes for B1.2 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.