IB Biology 2025 · Theme B · Form and function

B1.2 Proteins. Practice questions with markscheme.

70 original IB-style questions on B1.2, written from the 2025 guide: 31 multiple-choice, 23 short-answer, 9 data-based, 3 extended-response parts, 3 drawing, 1 labelling. Below are two ready-made practice papers built from them, one at standard level (20 marks) and one at higher level (20 marks); switch between them above the paper. Hand one out as a class quiz or homework, or sit it yourself and mark against the scheme. Print it, project it, or build a fresh one on the same topic.

What the guide asks for

5 statements at SL and HL, 7 additional higher level.

B1.2.1SL / HL Generalized structure of an amino acid
B1.2.2SL / HL Condensation reactions forming dipeptides and longer chains of amino acids
B1.2.3SL / HL Dietary requirements for amino acids
B1.2.4SL / HL Infinite variety of possible peptide chains
B1.2.5SL / HL Effect of pH and temperature on protein structure
B1.2.6HL Chemical diversity in the R-groups of amino acids as a basis for the immense diversity in protein form and function
B1.2.7HL Impact of primary structure on the conformation of proteins
B1.2.8HL Pleating and coiling of secondary structure of proteins
B1.2.9HL Dependence of tertiary structure on hydrogen bonds, ionic bonds, disulfide covalent bonds and hydrophobic interactions
B1.2.10HL Effect of polar and non-polar amino acids on tertiary structure of proteins
B1.2.11HL Quaternary structure of non-conjugated and conjugated proteins
B1.2.12HL Relationship of form and function in globular and fibrous proteins

In the bank for B1.2

31 multiple-choice
23 short-answer
9 data-based
3 extended-response parts
3 drawing
1 labelling
41 higher level only

Every question is original and tagged to a guide statement. See the whole bank →

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The practice paper

20 marks at SL · 20 at HL · each has its own code

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Biology · topic quiz

Standard level · topic practice, not an exam format

30 minutes20 marks

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Covers B1.2 Proteins · SL

Name:

Which groups are bonded to the central (alpha) carbon atom in the generalized structure of an amino acid?

[1]

An amine group, a phosphate group, a hydrogen atom and an R group
Two amine groups, a carboxyl group and a hydrogen atom
A carboxyl group, a hydroxyl group and two R groups
An amine group, a carboxyl group, a hydrogen atom and an R group

A tripeptide contains one glycine, one alanine and one serine. How many different tripeptides could be made that each contain exactly these three amino acids, one of each?

[1]

Human insulin is made of 51 amino acids in two polypeptide chains, while the muscle protein titin contains more than 30 000 amino acids. Which statement about these two proteins is correct?

[1]

Titin must be built from more than 20 different amino acids, because 20 kinds could not make such a long chain
Both are built from the same 20 amino acids, differing in the number and the sequence in which they are joined
Insulin has a shorter chain because it uses fewer of the 20 kinds of amino acid than titin does
Both proteins contain each of the 20 amino acids in equal numbers, arranged in different orders

A polypeptide is 100 amino acids long. Which expression gives the number of different possible amino acid sequences of this length?

[1]

20¹⁰⁰
100!
20 × 100
100²⁰

Young rats were fed diets that were complete except that each lacked one amino acid. Rats fed a diet lacking threonine lost body mass, while rats fed a diet lacking glycine grew as well as rats on the complete diet. Deduce what these results show about threonine and glycine in rats.

[2]

The table shows the relative molecular masses of three amino acids and of a tripeptide made from one molecule of each of them.

Molecule	Relative molecular mass
alanine	89
cysteine	121
valine	117
tripeptide made from alanine, cysteine and valine	291

(a)Calculate the difference between the total relative molecular mass of the three amino acids and the relative molecular mass of the tripeptide.[1]

(b)Explain the difference that you calculated in (a).[2]

(c)Predict the relative molecular mass of the polypeptide formed when a second alanine is joined to the tripeptide.[1]

(d)Suggest why the relative molecular mass of a polypeptide cannot be used to work out the sequence of its amino acids.[1]

The diversity of possible polypeptides is effectively infinite. Outline why.

[2]

Egg-white solutions were heated for 10 minutes at a range of temperatures and cooled to 20 °C. The cloudiness (turbidity) of each sample, a measure of denatured, aggregated protein, was then measured in arbitrary units.

Heating temperature / °C	Turbidity after cooling / a.u.
20	0
40	1
50	2
60	18
70	64
80	95
90	98

(a)Describe the effect of heating temperature on turbidity.[2]

(b)Explain, in terms of protein structure, why turbidity increases at high temperatures.[3]

(c)Explain why the turbidity did not disappear when the samples were cooled.[2]

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One mark per point; / separates alternative wording within a point, OR separates alternative answers, words in brackets are not required, underlined words are essential. OWTTE = or words to that effect.

1. B1.2.1 [1]

D: every amino acid has the same core: α-carbon bonded to –NH₂, –COOH, –H and a variable R group (side chain);

2. B1.2.4 [1]

C: amino acids can be joined in any order and each order (read from the amine end) is a different tripeptide; 3 choices for the first position × 2 for the second × 1 for the third = 6;

3. B1.2.4 [1]

B: all polypeptides are made from the 20 genetically coded amino acids, joined in any number and any order; A fails because chain length is not limited by the number of kinds of amino acid; B fails because length depends on how many amino acids are joined, not on how many kinds are used; C fails because amino acids can occur in any proportion, not equal numbers;

4. B1.2.4 [1]

A: each of the 100 positions can be any of the 20 amino acids, so 20 multiplied by itself 100 times;

5. B1.2.3 [2 max]

threonine is an essential amino acid (for rats), whereas glycine is non-essential;
rats cannot synthesize threonine, so without it in the diet protein synthesis is limited and growth stops / body mass falls;
rats can make glycine from other amino acids, so a diet lacking it does not limit protein synthesis, OWTTE;

Award converse. Do not accept 'threonine is more important' without reference to synthesis or diet.

6. B1.2.2

(a) [1]
- 89 + 121 + 117 = 327, and 327 − 291 = 36;
(b) [2 max]
- two peptide bonds form to join three amino acids;
- each peptide bond forms by a condensation reaction that releases one molecule of water;
- the water is made from the –OH of a carboxyl group and an –H of an amine group;
- each water molecule has a relative molecular mass of 18, so 2 × 18 = 36 is lost, OWTTE;
(c) [1]
- 362 (291 + 89 − 18);
Accept ECF from (a): 291 + 89 − half the value calculated in (a).
(d) [1]
- the mass depends only on which amino acids (and how many of each) are present, not on the order in which they are joined;
- the same amino acids joined in a different order give the same mass / different combinations of amino acids can give the same total mass, OWTTE;

7. B1.2.4 [2 max]

20 different amino acids can occur at each position (in the chain);
chains can be any length / hundreds–thousands of amino acids long;
(so) the number of possible sequences is 20ⁿ / astronomically large, OWTTE;

8. B1.2.5

(a) [2]
- little/no change up to ~50 °C;
- (then) a steep rise between 50 and 80 °C, levelling off by 90 °C, OWTTE;
(b) [3 max]
- heat increases (molecular) vibration, breaking the (weak) bonds/interactions that hold the three-dimensional shape (hydrogen/ionic bonds, hydrophobic interactions);
- the proteins unfold / lose their three-dimensional shape (denaturation);
- the unfolded chains stick to one another / aggregate / clump, forming insoluble (light-scattering) particles;
- (because) hydrophobic R groups that were buried in the core become exposed and cluster together, OWTTE;
(c) [2]
- denaturation (of this kind) is (effectively) irreversible;
- the aggregated/tangled chains cannot refold to the original conformation (the primary structure alone cannot re-establish it once aggregated), OWTTE;

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Biology · topic quiz

Higher level · topic practice, not an exam format

30 minutes20 marks

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Covers B1.2 Proteins · HL

Name:

Which distribution of amino acids would be expected in a protein that spans the plasma membrane?

[1]

Hydrophobic R groups on the surface of the region within the bilayer; hydrophilic R groups on the surfaces exposed to water
Hydrophilic R groups throughout, because the protein must interact with water on both sides of the membrane
Hydrophobic R groups buried in the centre and hydrophilic R groups on all outer surfaces, as in a soluble enzyme
Hydrophilic R groups facing the bilayer core, so that the protein is anchored by hydrogen bonds to the phospholipid tails

The table shows the R groups of four amino acids, W to Z, as they are at pH 7. Which R group is hydrophilic but uncharged?

[1]

Amino acid	R group at pH 7
W	–CH₂–OH
X	–CH(CH₃)₂
Y	–CH₂–COO⁻
Z	–CH₂–CH(CH₃)₂

The table shows four pairs of R groups that lie close to each other in a folded polypeptide. Identify the type of bond or interaction most likely to form between the two R groups of each pair.

[4]

Pair	R group 1	R group 2
1	–CH₂–SH (cysteine)	–CH₂–SH (cysteine)
2	–CH₂–COO⁻	–(CH₂)₄–NH₃⁺
3	–CH₂–OH	–CH₂–CONH₂
4	–CH(CH₃)₂	–CH₂–C₆H₅

The proportion of the amino acids of a protein that lies in alpha-helix and in beta-pleated sheet can be estimated from the way the protein absorbs polarized light. The table shows results for four proteins.

Protein	Alpha-helix / %	Beta-pleated sheet / %	Role
haemoglobin	75	0	oxygen transport in blood
silk fibroin	0	90	fibre of a silk thread
lysozyme	30	6	enzyme that digests bacterial cell walls
porin	3	65	channel through a bacterial outer membrane

(a)Identify the type of bond that holds both the alpha-helix and the beta-pleated sheet in shape.[1]

(b)Deduce the relationship between the proportion of beta-pleated sheet and the role of these proteins.[2]

(c)Suggest why silk fibres are very strong but stretch only slightly.[2]

(d)Suggest why this method gives no information about the order of the amino acids in these proteins.[1]

(e)Comment on the use of these four proteins as a basis for generalizing about protein structure.[2]

In sickle-cell haemoglobin (HbS) a single mutation in the β-globin gene replaces glutamate (a charged, hydrophilic amino acid) with valine (a non-polar amino acid) at position 6 of the 146 amino acids in the β chain. This position lies on the outer surface of the folded chain. Discuss the extent to which a change of one amino acid in the primary structure of a protein can be expected to alter its conformation and function, using HbS as one example.

[6]

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1. B1.2.10 [1]

A: the bilayer core is hydrophobic, so the membrane-spanning surface is hydrophobic; cytoplasmic and extracellular surfaces are hydrophilic;

2. B1.2.6 [1]

A: the –OH group of W is polar, so it forms hydrogen bonds with water, but it carries no charge; X and Z are non-polar hydrocarbon groups (hydrophobic); Y has lost a hydrogen ion from its carboxyl group, so it is negatively charged (acidic);

3. B1.2.9 [4]

pair 1: disulfide bond / disulfide bridge (a covalent bond);
pair 2: ionic bond (between the negatively charged carboxyl and the positively charged amine group);
pair 3: hydrogen bond (between polar groups);
pair 4: hydrophobic interaction (between non-polar R groups);

Accept 'salt bridge' for pair 2. Do not accept 'covalent bond' alone for pair 1, or 'hydrogen bond' for pair 2.

4. B1.2.8

(a) [1]
- hydrogen bonds (between the C=O and N-H groups of the polypeptide backbone);
Do not accept 'hydrogen bonds between R groups'.
(b) [2 max]
- the two proteins with a structural or barrel-forming role (silk fibroin and porin) have much the highest proportion of beta-pleated sheet;
- the two soluble globular proteins (haemoglobin and lysozyme) have little or none and are richer in alpha-helix;
- so a high beta-sheet content is associated with proteins that form sheets, fibres or barrels rather than compact soluble structures, OWTTE;
(c) [2 max]
- in a beta-pleated sheet the polypeptide is already almost fully extended, so there is little length left to pull out;
- many hydrogen bonds link neighbouring chains along the whole length of the sheet, and the load is shared between them;
- the covalent backbone must be broken for the fibre to fail, so it resists a large force, OWTTE;
(d) [1]
- it detects only the overall shape/conformation of the polypeptide backbone, and many different sequences can adopt the same conformation, OWTTE;
(e) [2 max]
- the four were chosen as clear contrasting examples, so they illustrate the association between structure and role well;
- but four proteins out of many thousands is far too small a sample to establish a general rule, and many proteins contain substantial amounts of both forms;
- a larger survey across many proteins of known role would be needed before generalizing, OWTTE;

5. B1.2.7 [6 max]

primary structure (the sequence and position of each amino acid) determines where each R group lies and so how the chain folds / its three-dimensional conformation;
in HbS only one of 146 amino acids is changed, so the primary structure differs at a single position;
a charged, hydrophilic R group on the surface is replaced by a non-polar/hydrophobic one, creating a hydrophobic patch on the surface of the molecule;
hydrophobic patches on different haemoglobin molecules can cluster together (away from water), so HbS molecules stick to each other (when deoxygenated) and the protein becomes less soluble / forms fibres, OWTTE;
in HbS the folded shape of each molecule is (almost) unchanged, yet its behaviour changes, because the change is on the surface where molecules meet, OWTTE;
a substitution in the hydrophobic core or in a binding site (e.g. the haem pocket) can disrupt folding or binding, so function is lost;
but a substitution by an amino acid with a similar R group (e.g. one non-polar for another) may leave the conformation (almost) unchanged;
a substitution at a position that is not critical to folding or binding may have little or no effect on function;
(so) the effect depends on where the change is and how different the new R group is, not on the number of amino acids changed, OWTTE;

Discuss: at least one point showing a large effect and one showing little or no effect are needed for full marks. Points about the disease or about natural selection are not asked for here and earn no marks.