IB Biology 2025 · Theme B · Form and function

B2.3 Cell specialization. Practice questions with markscheme.

71 original IB-style questions on B2.3, written from the 2025 guide: 31 multiple-choice, 23 short-answer, 9 data-based, 5 extended-response parts, 2 drawing, 1 labelling. Below are two ready-made practice papers built from them, one at standard level (20 marks) and one at higher level (21 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

6 statements at SL and HL, 4 additional higher level.

B2.3.1SL / HL Production of unspecialized cells following fertilization and their development into specialized cells by differentiation
B2.3.2SL / HL Properties of stem cells
B2.3.3SL / HL Location and function of stem cell niches in adult humans
B2.3.4SL / HL Differences between totipotent, pluripotent and multipotent stem cells
B2.3.5SL / HL Cell size as an aspect of specialization
B2.3.6SL / HL Surface area-to-volume ratios and constraints on cell size
B2.3.7HL Adaptations to increase surface area-to-volume ratios of cells
B2.3.8HL Adaptations of type I and type II pneumocytes in alveoli
B2.3.9HL Adaptations of cardiac muscle cells and striated muscle fibres
B2.3.10HL Adaptations of sperm and egg cells

In the bank for B2.3

31 multiple-choice
23 short-answer
9 data-based
5 extended-response parts
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

20 marks at SL · 21 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 B2.3 Cell specialization · SL

Name:

A motor neuron running from the spinal cord to a muscle in the foot can be more than 1 m long, and a striated muscle fibre in the thigh can be several centimetres long. Explain how the sizes of these two cells are related to their functions.

[3]

Researchers isolated three populations of unspecialized cells, P, Q and R, from adult mouse tissues. Single cells were placed one per well in culture. When a cell had grown into a colony, the colony was broken up and single cells from it were again placed one per well. This was repeated until the cells stopped dividing or 30 rounds had been completed. Colonies were then given signals that normally cause these cells to differentiate.

Population	Rounds of single-cell transfer completed	Cell types formed after the signals to differentiate
P	3 (division then stopped)	bone, cartilage and fat cells
Q	30 (still dividing)	none: the cells stayed unspecialized
R	30 (still dividing)	bone, cartilage and fat cells

(a)Deduce, with reasons, which population shows both properties of stem cells.[3]

(b)Explain why each colony was started from a single cell rather than from a small group of cells.[2]

Identical twins develop when a single early embryo separates into two parts. If the separation happens at the two-cell stage, each twin develops its own placenta. If it happens about five days after fertilization, when the embryo is a blastocyst with an inner cell mass, only the inner cell mass divides in two, and the twins share one placenta.

(a)Deduce what the twins that each have their own placenta show about the cells of the two-cell embryo.[2]

(b)Explain why twins formed by division of the inner cell mass share a placenta.[3]

The table shows the body mass and the mass-specific metabolic rate (oxygen consumed per gram of body per hour) for five mammals. Mass-specific metabolic rate reflects how fast each gram of tissue must be supplied with oxygen.

Mammal	Body mass / g	Mass-specific metabolic rate / cm³ O₂ g⁻¹ h⁻¹
shrew	4	7.4
mouse	25	2.5
rabbit	2000	0.75
human	65000	0.21
elephant	4000000	0.07

(a)Describe the relationship between body mass and mass-specific metabolic rate.[1]

(b)Analyse how this relationship is linked to the surface area-to-volume ratio of the animals.[2]

(c)Explain why a shrew must eat far more food per gram of body mass each day than an elephant.[2]

(d)Evaluate whether surface area-to-volume ratio alone fully accounts for the pattern in the data.[2]

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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Markscheme BbB-EAAAIAAAABQAOq8_

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. B2.3.5 [3]

the neuron has a very long (thin) extension / axon so that a single cell carries the impulse the whole distance from the spinal cord to the muscle, OWTTE;
the muscle fibre is long so that its contraction acts along a large part of the length of the muscle / a single fibre can shorten by a useful distance, OWTTE;
the muscle fibre is wide / has a large volume to contain many contractile myofibrils, giving a large force;
very large cells need adaptations, e.g. many nuclei in a muscle fibre / a narrow axon, to keep the cell supplied and controlled despite its size;

Beyond this subtopic. Award if given; the marks above can be earned without it.

C2.2 (so) no synapses are needed along the route, which would slow / could interrupt transmission of the impulse;

Accept 'nerve fibre' for axon. Credit is for linking size to function; a description of size alone does not score.

2. B2.3.2

(a) [3 max]
- R;
- R was still dividing after 30 rounds, showing the capacity to divide (apparently) endlessly / self-renew;
- R formed several / three types of specialized cell, showing it can differentiate along different pathways;
- P can differentiate but its capacity to divide is limited (division stopped after 3 rounds), whereas Q divides (apparently) endlessly but cannot differentiate, OWTTE;
Do not award the first point for R without a reason.
(b) [2 max]
- all cells in a colony started from one cell are descended from it / form a clone;
- (so) the division and differentiation seen in the colony show what a single cell can do / show that both properties belong to the same cell;
- a group could contain a mixture, e.g. cells that only divide (like Q) with cells that only differentiate (like P), which together would falsely appear to have both properties, OWTTE;

3. B2.3.4

(a) [2 max]
- each cell (of the two-cell embryo) can develop into a complete organism / every cell type of the body;
- including the placenta / extra-embryonic tissues;
- (so) the cells are totipotent;
Do not accept pluripotent.
(b) [3 max]
- by the blastocyst stage the cells are no longer all totipotent / some specialization has begun;
- cells of the inner cell mass are pluripotent;
- they can form all the cell types of the body, so each half can form a complete twin;
- but they cannot form the placenta, so neither half forms a placenta of its own;
- the placenta forms from the other (outer) cells of the blastocyst, which were not divided / are shared by both twins, OWTTE;

4. B2.3.6

(a) [1]
- as body mass increases, mass-specific metabolic rate decreases (a negative relationship), OWTTE;
(b) [2]
- smaller animals have a larger surface area-to-volume ratio, so they lose heat faster across their surface;
- (so) each gram of tissue must respire faster to replace the heat lost / maintain body temperature, giving a higher mass-specific metabolic rate, OWTTE;
(c) [2]
- the shrew's mass-specific metabolic rate is much higher (7.4 vs 0.07), so each gram of its body uses far more oxygen/energy;
- (so) it must take in proportionally much more food to fuel this high rate of respiration, OWTTE;
(d) [2]
- SA:V explains much of the trend (heat loss scales with surface, demand with volume);
- but other factors also vary between these species (e.g. fur/insulation thickness, activity level, diet, body shape such as ear size), and only five species are shown, so SA:V is a major but not the sole explanation, OWTTE;
NOS: reward recognition of confounds/limited sample. All five species are mammals, so do not credit endothermy as a factor that varies.

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Biology · topic quiz

Higher level · topic practice, not an exam format

30 minutes21 marks

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Covers B2.3 Cell specialization · HL

Name:

What is a stem cell niche?

[1]

A set of genes that is expressed only in embryonic tissues
A microenvironment in the body that maintains stem cells
A laboratory culture medium used to grow stem cells
A stage of the cell cycle in which differentiation occurs

A sample of alveolar epithelium was treated with a drug that stops vesicles fusing with the plasma membrane, so no exocytosis can take place. Which effect on the pneumocytes would be expected?

[1]

Type II pneumocytes would lose their lamellar bodies, since these form by fusion with the membrane
Oxygen could no longer cross type I pneumocytes, since gases are carried through cells in vesicles
Lamellar bodies would build up in type II pneumocytes, and less surfactant would reach the alveolus
Type I pneumocytes would become thicker, increasing the diffusion distance between air and blood

Cells in an embryo become different from one another even though they contain the same genome. What causes this differentiation?

[1]

Different cells express different genes
Mutations occur in each cell type
The cytoplasm digests unused genes
Different cells lose different genes

Which feature is shared by cardiac muscle cells and striated muscle fibres?

[1]

Branching to form an interconnected network
A length that can extend over several centimetres
Many nuclei in each cell or fibre
Contractile myofibrils in the cytoplasm

Agar cubes of different side lengths are used as a model of cells. Using the data, deduce how the surface area, the volume and the surface area-to-volume ratio each depend on the side length of the cube.

[3]

Side length / mm	Surface area / mm²	Volume / mm³	Surface area-to-volume ratio / mm⁻¹
1	6	1	6.0
2	24	8	3.0
4	96	64	1.5
8	384	512	0.75

A zygote (fertilized egg) is unspecialized, and every cell type in the body is descended from it. Discuss whether the zygote should be described as a stem cell.

[3]

Cells of the proximal convoluted tubule in the nephron reabsorb large quantities of useful substances from the filtrate. The membrane facing the filtrate carries many finger-like projections, and the membrane facing the tissue fluid and blood capillaries is deeply infolded. Outline how these two features adapt the cells for reabsorption.

[2]

(a)Deduce what the twins that each have their own placenta show about the cells of the two-cell embryo.[2]

(b)Explain why twins formed by division of the inner cell mass share a placenta.[3]

The diagram shows a section through a human egg cell shortly after ovulation, with the cells that surround it. Identify the structures labelled I to IV.

[4]

II.

III.

IV.

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Markscheme BbB-EAEAIAAAABQAE9st

1. B2.3.3 [1]

B: niches (e.g. in bone marrow and hair follicles) provide the signals that keep stem cells undifferentiated, quiescent or dividing as needed;

2. B2.3.8 [1]

C: type II pneumocytes store surfactant in many secretory vesicles (lamellar bodies) and discharge it to the alveolar lumen by exocytosis, so blocking exocytosis leaves the lamellar bodies in the cytoplasm and less surfactant reaches the lumen; A and D fail because type I pneumocytes do not rely on vesicles: gases diffuse straight through their extremely thin cytoplasm; C fails because lamellar bodies form inside the cell, so they would accumulate rather than disappear;

3. B2.3.1 [1]

A: differentiation results from differential gene expression (switched on/off by signalling gradients/position in the embryo), not gene loss;

4. B2.3.9 [1]

D: both muscle types contain contractile myofibrils; branching is a feature of cardiac cells only, whereas great length and many nuclei are features of striated fibres only;

5. B2.3.6 [3 max]

surface area is directly proportional to the square of the side length: doubling the side length multiplies surface area by four;
volume is directly proportional to the cube of the side length: doubling the side length multiplies volume by eight;
the surface area-to-volume ratio is inversely proportional to side length: doubling the side length halves the ratio;
the product of side length and ratio is constant (1 × 6.0 = 2 × 3.0 = 4 × 1.5 = 8 × 0.75 = 6), which is what inverse proportionality means;
so as a cell enlarges, the surface available for exchange grows more slowly than the volume that has to be supplied;

Volume grows fastest while the ratio that governs exchange falls, so the surface trend has the opposite sign to the size trend. Do not accept 'the ratio decreases' alone for the third point; proportionality must be stated.

6. B2.3.2 [3 max]

for: it is unspecialized and its descendants can differentiate along every pathway / it is totipotent;
for: it divides (by mitosis) to produce the cells of the embryo;
against: a stem cell divides to produce more stem cells like itself / can divide endlessly, whereas the zygote is a single cell present only at the start of development and is not maintained by its divisions;
against: after a few divisions its descendants are no longer totipotent / become pluripotent, so its full potential is not kept in a self-renewing population;
judgement: it has the differentiation potential of a stem cell but not self-renewal, so whether it counts depends on the definition used, OWTTE;

At least one point for and one point against for full marks.

7. B2.3.7 [2 max]

the projections (microvilli) and the infoldings (invaginations) both increase the area of plasma membrane with little increase in cell volume / increase the surface area-to-volume ratio of the cell;
microvilli increase the area across which substances are taken in from the filtrate;
invaginations increase the area across which substances pass out of the cell towards the blood;
(so) large quantities can be reabsorbed quickly / the rate of reabsorption per cell is high, OWTTE;

Accept the features described or named (microvilli, invaginations).

8. B2.3.4

(a) [2 max]
- each cell (of the two-cell embryo) can develop into a complete organism / every cell type of the body;
- including the placenta / extra-embryonic tissues;
- (so) the cells are totipotent;
Do not accept pluripotent.
(b) [3 max]
- by the blastocyst stage the cells are no longer all totipotent / some specialization has begun;
- cells of the inner cell mass are pluripotent;
- they can form all the cell types of the body, so each half can form a complete twin;
- but they cannot form the placenta, so neither half forms a placenta of its own;
- the placenta forms from the other (outer) cells of the blastocyst, which were not divided / are shared by both twins, OWTTE;

9. B2.3.10 [4]

I, follicle cell / corona radiata;
II, zona pellucida / (glycoprotein) jelly coat;
III, plasma membrane (of the egg);
IV, cortical granule(s) / vesicle(s) containing enzymes;

Do not accept cell wall for II or III. Do not accept lysosome, yolk or lipid droplet for IV.