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 polypeptides
- B1.2.1The structure of an amino acid
- B1.2.2Peptide bonds: condensation
- B1.2.3Essential amino acids
- B1.2.4An infinite variety of peptide chains
- B1.2.5Heat, pH and denaturation
From R-groups to whole proteins
- B1.2.6 · HLThe chemistry of R-groups
- B1.2.7 · HLPrimary structure decides the shape
- B1.2.8 · HLSecondary structure: the α-helix
- B1.2.8 · HLSecondary structure: the β-pleated sheet
- B1.2.9 · HLTertiary structure: hydrogen and ionic bonds
- B1.2.9 · HLTertiary structure: disulfide and hydrophobic
- B1.2.10 · HLPolar and non-polar amino acids
- B1.2.11 · HLQuaternary structure: non-conjugated
- B1.2.11 · HLQuaternary structure: conjugated
- B1.2.11 · HL · Nature of scienceImaging proteins: cryo-EM
- B1.2.12 · HLGlobular proteins: insulin
- B1.2.12 · HLFibrous proteins: collagen
B1.2 Proteins: summary
Amino acids
- α-carbon + amine group + carboxyl group + H + R-group.
- Condensation forms peptide bonds: amino acid + amino acid → dipeptide + water.
Diet and variety
- Essential amino acids must come from food; vegans combine legumes and cereals.
- 20 amino acids in any order and number: 20ⁿ possible sequences.
Denaturation
- Heat or pH breaks bonds between R-groups; the shape and function are lost.
- Peptide bonds stay intact.
HL · R-groups and folding
- Hydrophobic or hydrophilic (polar, or charged: acidic or basic).
- Sequence decides shape; secondary structure: α-helix, β-sheet (backbone H-bonds).
HL · Tertiary
- Hydrogen, ionic, disulfide bonds and hydrophobic interactions between R-groups.
- Non-polar core in soluble proteins; hydrophobic belts in membrane proteins.
HL · Quaternary and shape
- Insulin, collagen non-conjugated; haemoglobin conjugated (haem).
- Globular insulin vs fibrous collagen: form suits function.
Key terms
- Amino acid
- an α-carbon with an amine group, carboxyl group, H and R-group.
- Peptide bond
- the C–N bond formed when two amino acids condense.
- Polypeptide
- a chain of many amino acids joined by peptide bonds.
- Essential amino acid
- cannot be made by the body, so must come from food.
- Denaturation
- loss of a protein’s 3D shape, and so its function.
- Primary structure
- the sequence of amino acids in a polypeptide.
- Secondary structure
- α-helix or β-sheet, held by backbone hydrogen bonds.
- Tertiary structure
- the 3D fold of one polypeptide, held by R-group bonds.
- Quaternary structure
- two or more polypeptides linked in one protein.
- Conjugated protein
- includes a non-protein prosthetic group, e.g. haem.
- Globular / fibrous
- compact and soluble / long and insoluble.
Sample exam questions
Three of the 36 multiple-choice questions for B1.2. Try each one before opening the answer.
Question 1. Why must people on a vegan diet plan their sources of protein?
- Plants contain no amino acids.
- Plant proteins cannot be digested.
- Vegans need non-essential amino acids.
- Plant foods may lack an essential one.
Show the answer
Answer: D. Many plant foods are low in one or more essential amino acids, so foods must be combined.
Question 2. Insulin has two polypeptide chains of 21 and 30 amino acids. How many water molecules are released when the two chains are made?
- 51
- 49
- 50
- 102
Show the answer
Answer: B. A chain of n amino acids has n − 1 peptide bonds: 20 + 29 = 49.
Question 3. Which groups are bonded to the alpha carbon of every amino acid?
- An amine, a hydroxyl, a hydrogen and an R-group
- A carboxyl, a phosphate, a hydrogen and an R-group
- An amine, a carboxyl, a hydrogen and an R-group
- An amine, a carboxyl, a methyl and a hydroxyl
Show the answer
Answer: C. The alpha carbon carries an amine group, a carboxyl group, a hydrogen atom and an R-group.
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
The study notes for B1.2 are free to read, no account needed. Every question and full markschemes are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.