Every body cell keeps the whole genome; differentiation is choosing which genes to use. Stem cells divide endlessly and can take different paths; niches tell them when. Surface area against volume limits cell size, and specialised cells find ways around it.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
· How these pages are made
Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
Learn
B2.3.1 From one fertilised egg to many cell types
Early divisions of the zygote produce unspecialised cells.
Differentiation is expressing some genes and not others, making one cell type's proteins.
The genome is unchanged: a differentiated cell keeps every gene but uses part.
A gradient of a signalling chemical across the embryo gives each cell a concentration.
Different concentrations switch on different genes, so one gradient can specify several cell types.
Students often think a specialised cell throws away the genes it does not need. In fact it keeps them all; a neuron and a liver cell differ in expression only.
Students often think a signal is simply present or absent, so it gives two cell types. In fact different thresholds along the gradient give a series of types.
B2.3.2 What makes a cell a stem cell
A stem cell is undifferentiated and has two defining properties.
It can divide endlessly by mitosis without losing its capacity to divide.
It can differentiate along different pathways into specialised types.
Both properties are needed; a dividing fibroblast only makes fibroblasts, for a limited time.
Students often think any dividing cell is a stem cell. In fact it must also be able to differentiate into other types.
Students often think a muscle or skin cell can switch type when needed. In fact differentiated cells keep their identity; new types come from stem cells.
B2.3.3 Niches: where adult stem cells live and who tells them what to do
A stem cell niche is a tissue location where local signals control stem cells.
The niche can maintain stem cells undifferentiated, or promote their proliferation and differentiation.
Bone marrow niche: stem cells replace the blood cells continually lost.
Hair follicle niche: stem cells rest between growth cycles, then build new hair when signalled.
Students often think a niche is just storage and the stem cells decide for themselves. In fact signals from the niche decide whether they rest or divide.
Students often think stem cells divide constantly. In fact between hair cycles they are held resting until the niche activates them.
B2.3.4 Totipotent, pluripotent, multipotent
Totipotent: any cell type, including placenta and membranes; the zygote and first few divisions.
Pluripotent: any body cell type but not extra-embryonic tissue; early embryo cells soon become this.
Multipotent: a limited range, usually one tissue's types; adult stem cells such as bone marrow.
Bone marrow stem cells make blood cells, never neurons or muscle.
Students often use totipotent and pluripotent as synonyms. In fact only a totipotent cell can also form the extra-embryonic tissues.
Students often think adult stem cells are pluripotent. In fact they are multipotent, restricted to one tissue's cell types.
B2.3.5 Human cells come in very different sizes
A sperm head is about 5 µm; with its tail, about 50 µm.
The egg is about 100 µm across, the largest human cell by volume.
Red blood cells are 7–8 µm; most white blood cells are 10–20 µm.
A neuron's axon can exceed a metre; a striated muscle fibre can be centimetres long.
Students often think all cells are roughly 10–20 µm. In fact human cells span from 5 µm sperm heads to metre-long axons.
Students often think sperm and egg are similar in size because each gives half the DNA. In fact the egg is vastly larger; it carries the cytoplasm.
B2.3.6 Surface area against volume limits cell size
Surface area-to-volume ratio is surface area divided by volume.
Double the side length: area × 4, volume × 8, so the ratio halves.
Exchange across the surface depends on area; the need for exchange depends on volume.
Past a certain size the surface cannot supply the volume; the cell divides or reshapes.
The cube model is simpler than a real cell, but the scale factors work for any shape: a model is a simplification.
Students often think a bigger cell has a bigger ratio because it has more surface. In fact volume grows faster than area, so the ratio falls.
Students often think a cube model says nothing about round cells. In fact area goes with length squared and volume with length cubed for any shape.
B2.3.7 Shapes that raise the ratio (HL) HL
Flattening: the erythrocyte is a biconcave disc, so no cytoplasm is far from the surface.
Microvilli: finger-like membrane projections on proximal convoluted tubule cells, facing the lumen, for reabsorption.
Invagination: infolding of the basal membrane in the same cells, with mitochondria between folds.
Each adds surface area without adding volume; shape, not just size, sets the ratio.
Students often think the biconcave shape is for holding more haemoglobin. In fact it gives more surface and shorter diffusion distances for gas exchange.
Students often think microvilli beat like cilia. In fact they do not move; they multiply the membrane area for transport proteins.
B2.3.8 Two pneumocytes for one job (HL) HL
Type I pneumocytes are extremely thin, so gases diffuse the shortest possible distance.
They form most of the alveolar surface, where exchange happens.
Type II pneumocytes are thicker, with many lamellar bodies that discharge surfactant into the lumen.
One cell cannot be both thin and full of vesicles, so the tissue needs both.
Students often think a tissue is a group of identical cells. In fact many tissues hold several cell types because the overall function needs different adaptations.
Students often think type II cells do the gas exchange because they are bigger. In fact type I cells do; thinness is what matters.
B2.3.9 Cardiac muscle cells and striated muscle fibres (HL) HL
Both contain contractile myofibrils, giving a striated appearance.
Cardiac cells are short, branched, usually one nucleus; branches join neighbours into a network.
Hypothesis: branching lets contraction spread in all directions, so the wall contracts as one.
Striated fibres are very long, unbranched, with many nuclei; formed by fusion of many myoblasts.
Length lets a fibre contract as one unit; many nuclei supply mRNA for the huge cytoplasm. One membrane, one cytoplasm, many nuclei: whether a fibre is a cell is debated.
Students often think a fibre's nucleus divided repeatedly without the cell dividing. In fact many cells fused, each bringing its nucleus.
Students often think cardiac cells branch to raise their surface area-to-volume ratio. In fact branching is for coordination, letting the signal pass cell to cell.
B2.3.10 Sperm and egg are built for different jobs (HL) HL
Sperm: haploid nucleus, an acrosome with enzymes, a midpiece of mitochondria, a flagellum.
Very little cytoplasm, so it is light and streamlined for the journey.
Egg: about 100 µm, non-motile, haploid nucleus, large cytoplasm with stored nutrients for early divisions.
The zona pellucida (glycoproteins) surrounds it; cortical granules alter it to block further sperm.
Students often think a sperm swims on a food store in its head. In fact its midpiece mitochondria respire substrates from the surrounding fluid.
Students often think the egg has a cell wall. In fact the zona pellucida is a glycoprotein layer that acrosome enzymes digest through.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 In a human, a neuron and a liver cell are both descended by mitosis from the same zygote. What is the difference between them once they have differentiated?
Answer and reasoning
They express different subsets of the same complete set of genes. — All cells produced by mitosis from the zygote inherit the full genome. Differentiation is a change in which genes are expressed: the neuron switches on the genes for its proteins and the liver cell switches on a different set, while both keep every gene.
They have kept different sets of genes and discarded the rest. — A student who thinks differentiation discards unneeded genes picks this. Gene content is unchanged by differentiation; only gene expression differs, and the silent genes remain in the nucleus of every specialized cell.
They were set on different paths inside from fertilization. — A student who believes cell fate is fixed internally in each cell from the start picks this. The early cells are unspecialized and identical; their fates are set later by their positions in gradients of signalling chemicals.
They differ only until the body needs to swap them over. — A student who thinks specialized cells can switch type on demand picks this. In the human body differentiation is effectively one-way; new cells of another type come from stem cells, not from conversion of differentiated cells.
Ability to divide whenever a tissue is damaged and to produce more cells of its own type. — A student who thinks any cell that divides is a stem cell picks this. Differentiated cells such as fibroblasts divide during repair and produce their own type, but they have limited divisions and cannot differentiate along other pathways.
Ability to switch from one specialized type to another whenever a tissue requires this. — A student who thinks specialized cells are interchangeable picks this. A stem cell is undifferentiated; it produces other cell types by differentiating, not by converting one specialized type into another.
Capacity to divide endlessly and ability to differentiate along different pathways. — These are the two defining properties in the guide: a stem cell can divide endlessly, and it can differentiate along different pathways to give specialized cell types. Both are required.
Continuous division at all times, giving new cells whether or not they are needed. — A student who reads 'divide endlessly' as 'divide continuously' picks this. Stem cells have an unlimited capacity to divide but can be maintained in a resting state by their niche; the defining property is the capacity, not constant activity.
3 In a hair follicle, stem cells in a region below the skin surface stay inactive for months between hair growth cycles. At the start of a new cycle they begin to divide and produce the cells of a new hair. What does this show about the stem cell niche?
Answer and reasoning
It stores the stem cells, which decide on their own when to begin dividing. — A student who pictures the niche as a storage shelf picks this. The stem cells respond to signals from the niche; the timing of activation at the start of a growth cycle is set by the niche environment, not by the cells acting independently.
It receives fresh stem cells sent from the bone marrow when a new hair is needed. — A student who thinks adult stem cells exist only in bone marrow picks this. The hair follicle has its own resident stem cells in a niche below the skin surface; bone marrow stem cells supply blood cells, not hair.
It converts ordinary dividing skin cells into stem cells at the start of each cycle. — A student who equates any dividing cell with a stem cell picks this. The follicle's stem cells are a maintained population of undifferentiated cells; differentiated skin cells that divide do not become stem cells.
It can maintain the stem cells or activate them, according to need. — A niche does two things: it can maintain stem cells in an undifferentiated resting state, and it can promote their proliferation and differentiation. The months of inactivity show maintenance; the start of a new cycle shows activation.
4 Which statement correctly distinguishes pluripotent stem cells from totipotent stem cells?
Answer and reasoning
Pluripotent cells can form any cell type of the body but not extra-embryonic tissues such as the placenta. — Totipotent cells of the very early embryo can form every cell, including the placenta and membranes. Within a few divisions the cells become pluripotent: they can still form any body cell type but no longer the extra-embryonic tissues.
Pluripotent and totipotent cells can both form every cell type, so the two terms are interchangeable. — A student who translates both prefixes as 'all' picks this. The terms differ precisely over the extra-embryonic tissues, which totipotent cells can form and pluripotent cells cannot.
Pluripotent cells are the stem cells found in adult tissues such as bone marrow and hair follicles. — A student who thinks adult stem cells can form any cell type picks this. Adult stem cells are multipotent, limited to the cell types of their own tissue; pluripotent cells are found in the early embryo.
Pluripotent cells appear only after birth, since all the cells of an embryo remain totipotent. — A student who thinks embryonic cells stay totipotent throughout development picks this. Embryonic cells are totipotent only for the first few divisions and soon become pluripotent, well before birth.
5 Which statement about the range of cell size in humans is correct?
Answer and reasoning
Red and white blood cells and gametes are all about 10 µm across. — A student who assumes all cells are one generic size picks this. Red blood cells are 7–8 µm, white blood cells mostly about 10–20 µm, a sperm head about 5 µm and an egg about 100 µm across.
The egg and the sperm are similar in size, as each supplies half the DNA. — A student who carries the equal genetic contribution over to cell size picks this. The egg, about 100 µm across, is one of the largest human cells; the sperm is one of the smallest, because it carries almost no cytoplasm.
A motor neuron can be over a metre long, yet only micrometres wide. — Cell size is an aspect of specialization. A single motor neuron running from the spinal cord to a muscle in the foot has an axon over a metre long, while its width remains microscopic; extreme length suits its function of carrying signals over a distance.
A muscle fibre several centimetres long must be a chain of short cells. — A student who thinks no single cell can be visible picks this. A striated muscle fibre is a single continuous unit with one plasma membrane that can run for centimetres; its length is part of its specialization.
6 A cube of side 1 mm has a surface area of 6 mm², a volume of 1 mm³ and a surface area-to-volume ratio of 6. The side length is increased to 4 mm. What is the new surface area-to-volume ratio?
Answer and reasoning
6, because the surface area and the volume are both multiplied by four — A student who scales area and volume in direct proportion to length picks this. Area scales with the square of length (× 16) and volume with the cube (× 64), so the ratio does change: it falls to 1.5.
24, because a larger cube has more surface, so the ratio rises fourfold — A student who confuses a larger surface area with a larger ratio picks this. The surface area does rise, to 96 mm², but the volume rises even faster, so the ratio falls from 6 to 1.5.
0.67, because it compares the 64 mm³ volume with the 96 mm² surface area — A student who divides the ratio the wrong way round picks this. The surface area-to-volume ratio is surface area divided by volume, 96 / 64 = 1.5; 64 / 96 is volume per unit area, which rises with size and hides the constraint.
1.5, because the area becomes 96 mm² and the volume becomes 64 mm³ — Surface area = 6 × 4² = 96 mm² and volume = 4³ = 64 mm³, so the ratio is 96 / 64 = 1.5. Multiplying the side by 4 multiplied the area by 16 and the volume by 64, so the ratio fell to a quarter of its original value.
7 Which features of a proximal convoluted tubule cell increase its surface area-to-volume ratio, and how? HL
Answer and reasoning
Microvilli, which beat to sweep the filtrate across the cell so that more of it can be absorbed. — A student who confuses microvilli with cilia picks this. Microvilli do not move; they are static folds of membrane whose only role here is to multiply the surface area for reabsorption.
A small cell size, because shape cannot alter the ratio of a cell that has a given volume. — A student who thinks only size affects the ratio picks this. Microvilli and invaginations show that shape does alter the ratio: they multiply the membrane area of a cell whose volume is unchanged.
Its large size, because a bigger cell has more surface and so a higher surface area-to-volume ratio. — A student who confuses a larger absolute surface area with a larger ratio picks this. A larger cell has a smaller ratio, because volume grows faster than surface area; proximal convoluted tubule cells raise their ratio by adding membrane (microvilli and invaginations), not by being large.
Microvilli and basal invaginations, which add membrane area without adding volume. — Microvilli on the surface facing the filtrate and invaginations of the membrane on the basal side both extend the plasma membrane while the cell's volume stays the same, so the ratio rises and more membrane is available for reabsorption and transport to the blood.
8 How are type I and type II pneumocytes adapted to their functions in the alveolar epithelium? HL
Answer and reasoning
Type I cells are extremely thin for a short diffusion distance; type II cells hold many lamellar bodies that discharge surfactant. — Type I pneumocytes are stretched extremely thin so that gases diffuse only a short distance between alveolar air and blood. Type II pneumocytes contain many secretory vesicles, the lamellar bodies, which discharge surfactant into the alveolar lumen.
Type II cells are the thicker gas-exchange cells; type I cells are thin cells that line the gaps between them. — A student who thinks the thicker, organelle-rich cell must do the main work picks this. Gas exchange depends on a short diffusion distance, so it is the extremely thin type I cells that form the exchange surface.
Type I cells secrete surfactant from their lamellar bodies; type II cells are thin to allow rapid diffusion. — A student who has attached gas exchange to type II cells and swapped the roles picks this. It is type I cells that are extremely thin for diffusion and type II cells that contain the lamellar bodies that discharge surfactant.
Type I and type II cells are one cell type at two stages, with the thin type I cells maturing into type II cells. — A student who expects a tissue to contain a single cell type picks this. The alveolar epithelium contains two distinct cell types because thinness for diffusion and a cytoplasm full of secretory vesicles cannot both be built into one cell.
9 A heart drug is found to block the passage of signals across the junctions where the branches of neighbouring cardiac muscle cells meet, without affecting the myofibrils inside each cell. Using the hypothesis for why cardiac muscle cells are branched, what effect on the heart wall would be predicted? HL
Answer and reasoning
Contraction would still spread normally, because cardiac cells are fused into long multinucleate fibres, as in skeletal muscle. — A student who assumes cardiac muscle is built like skeletal muscle picks this. Cardiac cells are short, separate and usually uninucleate; they are joined where their branches meet rather than fused, so a signal can only cross from one cell to the next through those junctions.
Contraction would still be coordinated, because each cell is triggered by its own nerve ending, not its neighbours. — A student who thinks every muscle cell is stimulated separately by a nerve, as skeletal muscle fibres are, picks this. In the heart the signal to contract passes from cell to cell across the junctions between branches, so blocking those junctions would stop it spreading.
Contraction would no longer spread through the wall in all directions, so it would not contract as a coordinated unit. — The hypothesis for branching is that it lets each cell connect with several neighbours, so the signal to contract, and the contraction itself, can pass through the network in every direction. If signals could not cross the junctions, each cell would contract in isolation and the wall would lose its coordinated contraction.
Oxygen uptake would fall, because the junctions where branches meet are the extra surface through which the cells exchange materials. — A student who explains every unusual cell shape as a way of increasing surface area-to-volume ratio picks this. The junctions are sites where the signal to contract passes between cells, not exchange surfaces; cardiac cells obtain oxygen from the capillaries around them, and the drug would affect coordination, not oxygen uptake.
10 Which pairing correctly matches an adaptation of a human gamete to its function? HL
Answer and reasoning
Sperm acrosome: stores the fuel that the mitochondria respire while the sperm swims. — A student who assumes the sperm carries a food store in its head picks this. The acrosome contains enzymes for digesting through the layers around the egg; the midpiece mitochondria respire substrates absorbed from the surrounding fluid.
Egg zona pellucida: a cell wall that gives the egg mechanical protection from damage. — A student who calls any layer outside the membrane a cell wall picks this. Animal cells have no cell wall; the zona pellucida is a glycoprotein layer that sperm bind to and digest through, then modified to block further sperm.
Egg and sperm of matching size: each gamete carries half of the chromosomes needed. — A student who links equal genetic contribution to equal size picks this. The gametes are extremely different in size: the egg is about 100 µm across and full of cytoplasm, while the sperm is tiny and streamlined for swimming.
Egg cytoplasm with nutrient stores: supplies the embryo's first divisions. — The egg is large because its cytoplasm contains stored nutrients (yolk droplets) that supply the embryo during its first divisions, before it can obtain nutrients from the mother. This is why the egg is one of the largest human cells.
Read the ones marked not yet in Learn, then Verify.
Verify confirm before you go
6 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 In an early-stage embryo a signalling protein is made by cells at one end and spreads towards the other end. Cells at three different distances from the source develop into three different cell types. Which explanation is best supported?
Answer and reasoning
The protein is an on/off switch, so the cells that received it differ from the cells that did not. — A student who treats a signal as present or absent picks this. An on/off switch could divide the embryo into two cell types only; three types at three distances show that cells respond to the level of the protein, not merely to its presence.
Different genes are switched on above different concentrations, so each position expresses its own set. — The protein forms a concentration gradient. Cells respond to the concentration they experience, with different genes switched on at different thresholds, so cells at high, medium and low concentration express different genes and differentiate into three types.
Each cell's fate was fixed internally at fertilization; the protein only marks the region afterwards. — A student who thinks fate is programmed inside each cell from the start picks this. The cells are genetically identical and unspecialized; the gradient is what makes their gene expression differ, so the protein is a cause, not a label.
The protein removes different genes from cells at different distances, so each type keeps only the genes it needs. — A student who thinks differentiation removes genes picks this. All the cells keep the whole genome; the gradient changes which genes are expressed at each position, not which genes are present.
2 A cell taken from a 4-cell human embryo can develop into a complete individual with its own placenta. A cell from the inner cell mass of a blastocyst can form any tissue of the body but not a placenta. A stem cell from adult bone marrow forms only blood cells. How should the three cells be classified?
Answer and reasoning
Totipotent for both embryo cells, then multipotent. — A student who thinks all embryonic cells stay totipotent picks this. The inner cell mass cell cannot form the placenta, so it has already become pluripotent; totipotency is lost within the first few divisions.
Totipotent, then pluripotent, then finally multipotent. — Forming a whole individual including the placenta is totipotency. Forming any body tissue but not the placenta is pluripotency. Forming only the cell types of one tissue, here blood, is multipotency.
Totipotent, then pluripotent for both of the others. — A student who believes adult stem cells can form any cell type picks this. The bone marrow cell forms only blood cells, which is the limited range that defines a multipotent stem cell.
Pluripotent for both embryo cells, then finally multipotent. — A student who uses pluripotent and totipotent interchangeably picks this. A cell that can form the placenta as well as the embryo is totipotent; pluripotent cells cannot form extra-embryonic tissue.
3 A single-celled organism is growing. Why can it not keep growing indefinitely without dividing?
Answer and reasoning
Its volume, which sets its need for exchange, grows faster than the surface through which exchange occurs. — The need for exchange of materials depends on the volume of cytoplasm, while the rate of exchange depends on the area of the surface. Volume increases with the cube of length and surface with the square, so demand outgrows supply and sets a limit on size.
Its surface area-to-volume ratio rises as it grows, until the membrane cannot hold the cell together. — A student who thinks the ratio increases with size picks this. The ratio falls as the cell grows, and the constraint is about supplying the cytoplasm with materials, not about the membrane failing mechanically.
Its larger surface area increases its need for materials until supply cannot keep up with demand. — A student who attaches the need for exchange to the surface picks this. Need is set by the volume of metabolizing cytoplasm; the surface is what supplies that need, and it is the surface that fails to keep up.
Its surface area and volume increase by the same factor, so the limit must come from the nucleus instead. — A student who scales area and volume in step picks this. They do not increase by the same factor: volume grows with the cube of length and surface area with the square, and it is this mismatch that constrains cell size.
4 A student argues: 'Real cells are not cubes, so the cube model of surface area-to-volume ratio proves nothing about real cells.' Which response best evaluates this claim?
Answer and reasoning
The claim is right: a model is only valid when it has the same shape as the thing that it represents. — A student who judges a model by its resemblance to the real thing picks this. The value of a model lies in the relationship it isolates, not in looking like the system; the cube model isolates the effect of size and does so correctly.
The model is simplified on purpose; area and volume scale in the same way for any shape, so its conclusion holds. — Models are simplified versions of complex systems. Cubes are simpler than real cells, but scaling a shape of any kind multiplies area by the square and volume by the cube of the scale factor, so the conclusion that larger cells have a smaller ratio applies to real cells.
The claim is right for another reason: in real cells the surface area and the volume grow in proportion. — A student who scales area and volume in step picks this. In any object, cube or cell, area grows with the square of length and volume with the cube, so they do not grow in proportion; the cube model shows exactly this.
The claim is wrong, because in real cells the ratio increases with size, which the simple cube model fails to show. — A student who believes the ratio rises with size picks this. In real cells, as in cubes, the ratio falls as size increases; the cube model shows this correctly, which is why the claim is wrong for a different reason.
5 A human erythrocyte has a volume of about 90 µm³ and a surface area of about 135 µm². A sphere with the same volume would have a surface area of about 97 µm². What do these figures show about the adaptation of the erythrocyte? HL
Answer and reasoning
The biconcave shape gives it about 40% more room for haemoglobin than a sphere would provide. — A student who thinks the shape is for holding more haemoglobin picks this. The comparison is between shapes of the same volume, 90 µm³, so there is no extra room; the flattened cell gains surface area, not internal space.
The extra surface can only come from the erythrocyte being smaller than a typical sphere-shaped cell would be. — A student who thinks size is the only route to a high ratio picks this. The sphere in the comparison has exactly the same volume, so the extra surface comes from shape alone: flattening the cell adds area without adding volume.
Flattening gives it about 40% more surface than a sphere of the same volume, which speeds gas exchange. — 135 / 97 ≈ 1.4, so the biconcave disc has about 40% more surface area than a sphere of equal volume, and its ratio (1.5) is higher than the sphere's (about 1.08). More surface and a shorter diffusion distance increase the rate at which oxygen and carbon dioxide are exchanged.
Its ratio of 1.5 is higher than the sphere's ratio of 1.08 because the erythrocyte is the larger of the two cells. — A student who thinks a larger cell has a larger ratio picks this. The two have the same volume, so neither is larger; and in any case a larger object has a smaller ratio, not a larger one.
6 A striated muscle fibre has one continuous plasma membrane and cytoplasm and contains hundreds of nuclei. Which is the best assessment of whether it is a cell? HL
Answer and reasoning
It is not a cell, because a cell by definition contains exactly one nucleus, whereas this muscle fibre contains many hundreds. — A student who has made 'one nucleus' part of the definition of a cell picks this. Cell theory does not fix the number of nuclei; the nucleus count is one consideration in the debate, not a rule that settles it.
It is debatable: one membrane and cytoplasm argue for a cell, but many nuclei and an origin by fusion argue for a syncytium. — The guide asks for a discussion, not a verdict. A single membrane enclosing a single cytoplasm that works as a unit fits the idea of a cell; hundreds of nuclei and formation from many separate cells fit the idea of a multinucleate structure. Both positions can be defended.
It is a cell, because its nuclei arose by repeated mitosis inside a single cell that did not undergo cytokinesis. — A student who explains the many nuclei by mitosis without cytokinesis picks this. The fibre actually forms by fusion of many myoblasts, and that origin is the main argument against calling it a single cell.
It is a cell, because cardiac muscle cells also contain many nuclei, and they are accepted as cells by biologists. — A student who generalizes from skeletal to cardiac muscle picks this. Cardiac muscle cells are short, branched and usually have a single nucleus, so they offer no precedent for accepting a multinucleate fibre as a cell.
That was your twenty minutes. Real practice on B2.3 is past-paper questions marked against the mark scheme.
What the exam asks of B2.3
Paper 1A asks you to classify a cell as totipotent, pluripotent or multipotent, or to pick the largest or smallest human cell. Paper 1B commonly gives cube data and asks you to calculate surface area-to-volume ratios, then explain what happens as size increases; show the working and the units. Paper 2 uses *outline* for differentiation and stem cell properties, and *explain* for why cell size is constrained. At HL, expect *explain* for how a named adaptation increases the ratio, and *discuss* for whether a striated muscle fibre is a cell: give both sides before a conclusion.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
DP Science Cafe