Biology  by Bradford
IB Biology 2025 · Theme B · Form and function

B1.2 Proteins. Practice questions with markscheme.

47 original IB-style questions on B1.2, written from the 2025 guide: 21 multiple-choice, 15 short-answer, 6 data-based, 3 drawing, 2 extended-response part. Below is a 20-mark standard-level practice paper built from them, ready to hand out as a class quiz or homework, or to sit 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.

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

In the bank for B1.2

  • 21 multiple-choice
  • 15 short-answer
  • 6 data-based
  • 3 drawing
  • 2 extended-response part
  • 26 higher level only

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

Make your own

The practice paper

Take it on screen → Build a fresh paper Paper code BbB-EAAABAAAABQAOqr9
Biology · topic quiz
Standard level · topic practice, not an exam format
30 minutes20 marks

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Covers B1.2 Proteins
Name:
1.

An enzyme loses all activity when heated to 70 °C and does not regain it on cooling. Which statement best explains this?

[1]
  1. Heat broke the hydrogen bonds and other interactions holding the tertiary structure, and the polypeptide cannot refold correctly
  2. Heat hydrolysed the peptide bonds, breaking the polypeptide into free amino acids that cannot rejoin when the mixture cools
  3. Heat altered the primary structure by changing the sequence of amino acids, so a different protein forms on cooling
  4. Heat removed the hydrophobic amino acids from the surface of the protein, so it can no longer dissolve in the cytoplasm
2.

Students use a molecular model kit to join two amino acids into a dipeptide. Which feature of the finished model represents a real property of the dipeptide, rather than an artefact of the kit?

[1]
  1. The bright colours distinguishing the carbon, oxygen and nitrogen atoms
  2. The rigid plastic rods holding the two amino acids in a fixed shape
  3. The empty space around the model, standing for the cytoplasm of the cell
  4. The removal of an −OH and an −H, leaving one water molecule unbuilt
3.

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

[1]
  1. Titin must be built from more than 20 different amino acids, because 20 kinds could not make such a long chain
  2. Insulin has a shorter chain because it uses fewer of the 20 kinds of amino acid than titin does
  3. Both proteins contain each of the 20 amino acids in equal numbers, arranged in different orders
  4. Both are built from the same 20 amino acids, differing in the number and the sequence in which they are joined
4.

Nine of the twenty amino acids are described as "essential" in the human diet. What does essential mean here?

[1]
  1. They are required only during infancy and childhood growth
  2. They cannot be synthesized by the body and must come from food
  3. They yield more energy per gram than the other amino acids
  4. They are the only amino acids incorporated into human proteins
5.

A dietician is planning meals for a patient who eats no animal products. The protein of cereals such as wheat contains little lysine, while the protein of pulses such as lentils contains little methionine. Both lysine and methionine are essential amino acids.

(a)Suggest why eating cereals and pulses in the same meal helps this patient.[2]
(b)Suggest why the body cannot make lysine from other amino acids that are present in excess.[2]
6.

All twenty amino acids used to make polypeptides share a generalized structure, yet each is chemically distinct. Outline the generalized structure and the source of the differences between amino acids.

[2]
7.

Amino acids are joined together by condensation reactions to form dipeptides and longer chains.

(a)State the word equation for the formation of a dipeptide.[1]
(b)Calculate the number of water molecules released when a polypeptide of 150 amino acids is formed from free amino acids.[1]
(c)State the name of the bond formed between two amino acids and the type of bond it is.[1]
8.

Draw a diagram to show the condensation reaction between two amino acids to form a dipeptide. Show the structures of the reactants and the product, identify the bond formed and state what is released.

[4]
9.

Draw the generalized structure of an amino acid, labelling the functional groups.

[3]

Original practice questions © Biology by Bradford · CC BY-NC-SA 4.0 · Not affiliated with or endorsed by the International Baccalaureate Organization.
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Show the markscheme

Markscheme BbB-EAAABAAAABQAOqr9

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. B1.2.5 [1]
  • A: denaturation is loss of 3D structure through disruption of weak bonds; the primary structure and peptide bonds are unaffected but the shape is usually not recovered;
2. B1.2.2 [1]
  • D: condensation removes the components of one water molecule as the peptide bond forms, so those atoms are genuinely absent from the dipeptide; the colours, the rigidity and the surrounding space are all conventions of the kit;

Design move: criteria rather than instances — every option is a true statement about the model, but only one is a true statement about the molecule.

3. B1.2.4 [1]
  • D: 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. B1.2.3 [1]
  • B: non-essential amino acids can be made (by transamination etc.); essential ones must come from dietary protein, a reason for balanced/varied diets (especially vegan diets);
5. B1.2.3
  • (a) [2 max]
    • the cereal supplies the methionine the pulse lacks, and the pulse supplies the lysine the cereal lacks;
    • so all essential amino acids are available at the same time, in the proportions needed for protein synthesis;
    • a shortage of any one essential amino acid would halt translation of proteins containing it, so the combination prevents this, OWTTE;
  • (b) [2]
    • humans lack the genes for (and therefore the enzymes of) the pathway that builds the lysine side chain;
    • so the R group cannot be synthesized from another amino acid and lysine must be supplied in the diet, OWTTE;
6. B1.2.1 [2 max]
  • a central/α-carbon is bonded to an amine (–NH₂) group and a carboxyl (–COOH) group;
  • a hydrogen atom and an R group (side chain) complete the four bonds of the α-carbon;
  • the R group is different in each of the twenty amino acids (giving each its distinct chemical properties), OWTTE;
7. B1.2.2
  • (a) [1]
    • amino acid + amino acid → dipeptide + water;

    Accept 'two amino acids → dipeptide + water'. Do not accept an equation that omits water as a product.

  • (b) [1]
    • 149;

    Do not accept 150.

  • (c) [1]
    • peptide bond, (which is a) covalent bond;

    Both the name and the type are needed for the mark.

8. B1.2.2 [4]
  • two generalized amino acids drawn correctly (central carbon with –NH2, –COOH, –H and –R), with the R groups distinguished (e.g. R1 and R2);
  • the carboxyl group of one amino acid reacts with the amine group of the other, shown by the –OH from –COOH and the –H from –NH2 being removed;
  • dipeptide product drawn with the peptide bond as –CO–NH– (the carbonyl carbon bonded to the nitrogen) and labelled peptide bond;
  • one molecule of water (H2O) released/produced per peptide bond formed (condensation);

The free –NH2 and –COOH must remain at the two ends of the dipeptide. Do not accept the bond drawn between the two carboxyl groups or the two amine groups.

9. B1.2.1 [3]
  • central (alpha) carbon atom drawn with four groups attached by single bonds;
  • amine group (–NH2) and carboxyl group (–COOH) drawn with the correct atoms and bonds and labelled;
  • hydrogen atom and R group (variable side chain) attached to the central carbon and labelled;

Accept the zwitterion form (–NH3+ and –COO−). Do not accept –COH or –NH for the functional groups, or the R group attached to the nitrogen.

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