IB Biology 2025 · Theme B · Form and function

B2.2 Organelles and compartmentalization. Practice questions with markscheme.

45 original IB-style questions on B2.2, written from the 2025 guide: 19 multiple-choice, 15 short-answer, 5 data-based, 3 extended-response part, 2 drawing, 1 labelling. Below is a 22-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

3 statements at SL and HL, 6 additional higher level.

B2.2.1SL / HL Organelles as discrete subunits of cells that are adapted to perform specific functions
B2.2.2SL / HL Advantage of the separation of the nucleus and cytoplasm into separate compartments
B2.2.3SL / HL Advantages of compartmentalization in the cytoplasm of cells
B2.2.4HL Adaptations of the mitochondrion for production of ATP by aerobic cell respiration
B2.2.5HL Adaptations of the chloroplast for photosynthesis
B2.2.6HL Functional benefits of the double membrane of the nucleus
B2.2.7HL Structure and function of free ribosomes and of the rough endoplasmic reticulum
B2.2.8HL Structure and function of the Golgi apparatus
B2.2.9HL Structure and function of vesicles in cells

In the bank for B2.2

19 multiple-choice
15 short-answer
5 data-based
3 extended-response part
2 drawing
1 labelling
28 higher level only

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

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The practice paper

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Biology · topic quiz

Standard level · topic practice, not an exam format

35 minutes22 marks

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Covers B2.2 Organelles and compartmentalization

Name:

A gene from a eukaryote was inserted into a bacterium. The gene was transcribed, but the protein produced was faulty. Which statement is the most likely explanation, based on the way the two types of cell are organized?

[1]

The bacterium has no ribosomes of its own, so the mRNA was translated by ribosomes that had entered with the gene
The bacterial cell wall prevented the mRNA from leaving the cell to reach ribosomes in the surrounding medium
The bacterium lacks a nucleus, so the DNA could not be transcribed into mRNA in the correct sequence
The bacterium has no nuclear compartment, so ribosomes translated the mRNA before it had been modified

What is an advantage of keeping hydrolytic enzymes inside lysosomes?

[1]

The enzymes are isolated from structures they could damage
The enzymes work best at the neutral pH of the cytoplasm
The enzymes are broken down as soon as they have acted
The enzymes can pass freely into the cytoplasm when needed

Which of the following is NOT considered an organelle?

[1]

A mitochondrion
The cytoplasm
A ribosome
A lysosome

In eukaryotes, transcription occurs in the nucleus and translation in the cytoplasm. What does this separation make possible?

[1]

Export of DNA to the ribosomes in the cytoplasm
Modification of mRNA before it is translated
Faster translation because the mRNA is unmodified
Simultaneous transcription and translation of the same RNA

Which is an advantage of compartmentalization in cells?

[1]

Enzymes are spread evenly through the whole cytoplasm
Enzymes and their substrates become more dilute in each compartment
Different compartments can maintain different optimal conditions
All reactions can occur at the same rate throughout the cell

Progress in science often follows the development of a new technique. Before ultracentrifuges were invented, the function of individual organelles could not be studied. What did the ultracentrifuge make possible?

[1]

Magnifying organelles with enough resolution for the internal membranes of each type to be seen and their structure worked out
Keeping organelles alive outside the cell for long periods so that their growth and division could be observed directly
Breaking every organelle in a cell down into its component molecules so that the chemical composition of the whole cell could be analysed
Separating the organelles of broken cells into fractions by size and density so that the activity of each fraction could be tested

In a bacterium, ribosomes begin to translate an mRNA molecule while it is still being transcribed. Explain why this does not happen in a eukaryotic cell and what advantage the eukaryotic arrangement provides.

[3]

Outline the advantages of compartmentalization in the cytoplasm of cells.

[3]

The drawing shows a cell as it appears in an electron micrograph, with four structures labelled I to IV. Identify structures I, II, III and IV.

[4]

II.

III.

IV.

10.

Cell biologists can separate organelles by breaking cells open gently and spinning the mixture in a centrifuge at increasing speeds; larger, denser components sediment first. A liver-cell homogenate was centrifuged in stages, and the main organelle in each collected fraction was identified. The relative abundance of one enzyme, catalase, and of DNA was measured in each fraction.

Fraction (order of sedimentation)	Main component	Relative DNA content	Relative catalase activity
1 (first / lowest speed)	nuclei	+++	low
2	mitochondria	+	low
3	peroxisomes	none	high
4 (last / highest speed)	ribosomes	none	low

(a)Explain why nuclei are found in the first fraction.[1]

(b)Deduce, with a reason, which organelle is the main site of catalase activity in these cells.[2]

(c)Suggest why fraction 2 contains a small amount of DNA.[1]

(d)Outline how this separation technique illustrates the principle of division of labour among organelles.[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. B2.2.2 [1]

D: in a eukaryote the nuclear envelope separates transcription from translation, so mRNA can be modified before it meets ribosomes; in a prokaryote ribosomes can bind the mRNA immediately, so the unmodified message is translated; A fails because bacteria have their own ribosomes; B fails because translation occurs inside the cell, not in the medium; C fails because the stem states that transcription did occur;

2. B2.2.3 [1]

A: compartmentalization isolates dangerous (hydrolytic) enzymes from the rest of the cell and lets the lysosome interior stay acidic (optimal for hydrolases) while the cytoplasm does not;

3. B2.2.1 [1]

B: organelles are discrete subcellular structures with specific functions; the cytoplasm (and the cell wall) are not organelles;

4. B2.2.2 [1]

B: the nuclear envelope keeps ribosomes away from primary transcripts, so mRNA can be processed/modified (capping, poly-A tail, splicing) before translation, impossible in prokaryotes, where the two are simultaneous;

5. B2.2.3 [1]

C: compartments concentrate enzymes with their substrates, maintain distinct internal conditions (e.g. acidic lysosomes), and isolate hazardous contents;

6. B2.2.1 [1]

D: cell fractionation uses the ultracentrifuge to separate organelles of different size and density into fractions, and the function of each organelle can then be tested in isolation; A describes the electron microscope, which reveals structure rather than function; B is wrong because organelles are not cultured as living units; C fails because fractionation separates intact organelles rather than breaking them all down to molecules;

7. B2.2.2 [3]

in a eukaryotic cell transcription takes place in the nucleus, whereas ribosomes / translation are in the cytoplasm;
the nuclear envelope / membrane separates the mRNA from ribosomes (until the mRNA has left through the nuclear pores), OWTTE;
(so) the mRNA can be modified / processed after transcription and before it meets a ribosome;
a bacterium has no nucleus, so mRNA may meet ribosomes immediately and cannot be modified first;
modification allows the mRNA to be altered / different versions of the protein to be made from one gene / faulty transcripts to be removed before translation, OWTTE;

Accept 'post-transcriptional modification', 'splicing' or 'adding a cap and tail' as examples of modification, but examples are not required.

8. B2.2.3 [3 max]

enzymes and substrates are concentrated together (in small volumes), speeding reactions;
harmful substances (e.g. hydrolytic enzymes in lysosomes) are isolated from the rest of the cell;
conditions (e.g. pH) can be maintained at different optima in different compartments;
incompatible processes can occur simultaneously (in separate compartments), OWTTE;

9. B2.2.1 [4]

I, cell wall;
II, chloroplast;
III, (large central) vacuole;
IV, nucleus;

Accept nuclear envelope/nuclear membrane for IV. Do not accept plasma membrane for I: the leader ends on the thick outer layer, not the membrane inside it.

10. B2.2.1-B2.2.2

(a) [1]
- nuclei are the largest/densest organelles, so they sediment first / at the lowest speed;
(b) [2]
- peroxisomes;
- because catalase activity is high(est) in fraction 3 (and low elsewhere);
(c) [1]
- mitochondria contain their own (small, circular) DNA;
(d) [2 max]
- different organelles contain different enzymes/components (catalase in peroxisomes, DNA in nuclei/mitochondria);
- (so) each organelle carries out a distinct function for the cell;
- compartmentalization keeps each activity localized/concentrated, OWTTE;
Content pivot.