2.5Theme B — Form and function

Cell Specialisation — IB Biology HL Revision Notes & Topical Questions

5 min read 12 parts Lesson 13 of 14 1 quiz

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2.5 Cell Specialisation — HL Notes — IB BIOLOGY HL notes: Explain how cell differentiation arises from gene expression and morphogens and how the size and shape of specialised cells relate to function.Stem cell = a cell that can divide by mitosis an unlimited number of times (sel…

Stem cell niche
Niche conditions
Division of labour
Structural adaptations
Size effect
Small cells

Part 1 of 10Stem cells and niches

Stem cell

Stem cell a cell that can divide by mitosis an unlimited number of times (self-renewal) and can differentiate.

Each new cell may remain a stem cell or differentiate into a specialised cell such as a blood or muscle cell.
After differentiation the cell is no longer a stem cell.

Stem cell niche

Stem cell niche a location where stem cells remain so that a tissue can regenerate and repair.

Bone marrow supplies stem cells that replace red blood cells, white blood cells and platelets continually.
The hair follicle root supplies stem cells for continual hair growth.

Niche conditions

Niche conditions the environment must keep stem cells inactive and also be able to stimulate their division and differentiation.

Part 2 of 10Potency of stem cells

Potency	Differentiation ability	Example
Totipotent	Any embryonic cell type plus extra-embryonic (placental) cells	Zygote and embryo cells up to the 16-cell stage
Pluripotent	Any embryonic cell type but not extra-embryonic cells	Embryonic stem cells of the blastocyst inner cell mass
Multipotent	Closely related cell types	Bone marrow stem cells forming blood cells
Unipotent	Only their own lineage	Cardiomyocytes replacing heart muscle; most animal cells

Therapy
- bone marrow transplants replace blood cells after cancer treatment.
- Ethical issues arise with embryonic stem cells because embryos are destroyed.

Part 3 of 10Differentiation and gene expression

Division of labour

Division of labour after fertilisation the zygote divides by mitosis and cells specialise to form tissues, which makes them more efficient.

Gene expression

Gene expression every cell has the same genes, and differentiation depends on which genes are expressed.

Morphogens

Source cells secrete a morphogen.

It diffuses to form a concentration gradient.

Cells bind the morphogen at different concentrations, which gives positional information.

Different transcription factors switch on different genes.

Cells differentiate into different types according to position.

Structural adaptations
- specialisation changes cell shape and organelle content.
- Protein-making cells have many ribosomes.

Part 4 of 10Cell size and specialisation

Cell	Size feature	Reason
Red blood cell	Small, about 7–8 µm	Passes narrow capillaries
Active white blood cell	Larger than inactive	Room for rER and Golgi for antibody synthesis
Sperm	Long, narrow streamlined head	Movement to the egg with low resistance
Egg	Largest volume	Stored food reserves
Nerve cell	Large cell body, long axon	Protein synthesis to maintain a long axon for rapid impulses
Muscle fibre	Large in length and diameter	Exerts force in contraction

Part 5 of 10Surface area to volume ratio

Metabolic rate depends on cell volume, while exchange rate depends on surface area.

Size effect

Size effect as a cell grows its volume rises faster than its surface area, so SA:V falls.
For a cube, SA = 6 × L², V = L³ and SA:V = 6 ÷ L.
For a sphere, SA = 4πr², V = (4/3)πr³ and SA:V = 3 ÷ r.

Small cells

Small cells single-celled organisms have a high SA:V, low needs and a short diffusion distance, so diffusion alone suffices.

Large cells

Large cells the SA:V is too low and the diffusion distance to the centre is long, so growth must stop and cells divide.

Multicellular organisms

Multicellular organisms they evolved gas exchange systems, digestive systems and circulatory systems.
Heat loss also depends on SA:V, so small animals lose heat faster.

Typical cell size is 10–100 µm.

Part 6 of 10Agar cube model of diffusion

Nature of science

Nature of science models simplify complex systems so that predictions can be tested.

Method

Make agar coloured with dilute sodium hydroxide and Universal Indicator, or with indicator only.

Cut cubes of different sizes, such as 0.5, 1 and 2 cm, and calculate SA, V and SA:V.

Place each in the same volume of dilute hydrochloric acid, which has a higher molarity than the sodium hydroxide.

Record the time for the indicator to change colour throughout, or the depth of change in a set time such as 5 minutes.

Analysis
- convert times to rates and plot rate of diffusion against SA:V.
- Smaller cubes with larger SA:V change colour fully faster.

Part 7 of 10Cells with increased surface area

1Red blood cell

Red blood cell flattened and biconcave to maximise surface area and minimise volume, so oxygen diffuses in and out quickly.

2Proximal convoluted tubule cell

Proximal convoluted tubule cell reabsorbs glucose and mineral ions from filtrate.
Microvilli on the apical membrane increase area.
Invaginations (infoldings) of the basal membrane increase area.

Part 8 of 10Pneumocytes

Pneumocytes

Alveolar epithelium

Alveolar epithelium millions of alveoli give a large area, and the wall is one cell thick for a short diffusion distance.

Type I pneumocytes

Type I pneumocytes extremely thin cells making up most of the epithelium.
Capillary walls are also one cell thick, so less than 0.5 µm separates air from blood.

Type II pneumocytes

Type II pneumocytes rounded cells making about 5% of the epithelium, with many secretory vesicles.
- (lamellar bodies)
They secrete pulmonary surfactant, which forms a monolayer with hydrophobic tails facing the air.
Surfactant lowers surface tension, keeps alveolar shape and stops alveoli sticking together and collapsing.
The moist film lets oxygen dissolve before diffusing into blood, and carbon dioxide diffuses out from it.

Part 9 of 10Muscle fibres

Organisation from skeletal muscle to muscle fibres and myofibrils

Skeletal muscle is striated, and cardiac muscle is in the heart, and smooth muscle is in vessels and organs.

Striated muscle fibre

Striated muscle fibre it is cell-like and surrounded by the sarcolemma.
It has many nuclei, so it challenges the idea that a cell has one nucleus.
Contractile proteins are organised in myofibrils of actin and myosin that slide past each other.
The sarcoplasmic reticulum stores calcium and sends signals to all parts of the fibre at once.
T-tubules fold in from the sarcolemma close to the SR.
Many mitochondria supply ATP by aerobic respiration.

Cardiac muscle

Cardiac muscle it is striated and has one nucleus per cell.
It is myogenic and contracts without nerve or hormone stimulation, although nerves and hormones adjust the rate.
It does not fatigue and has many mitochondria.
Branched fibres joined by intercalated discs let contraction spread quickly across the chambers.
Myofibrils are present as in skeletal fibres.

Part 10 of 10Sperm and egg cells

Sperm and egg: contrasting structures and adaptations

Cell	Adaptation	Function
Sperm	Haploid nucleus in a streamlined head	Fuses with the egg nucleus to form a diploid zygote
Sperm	Acrosome with hydrolytic enzymes	Digests a path through the zona pellucida
Sperm	Many mitochondria in the middle piece	Supplies energy for movement
Sperm	Flagellum of microtubules	Propels the sperm
Egg	Haploid nucleus	Fuses with the sperm nucleus
Egg	Zona pellucida	Hardens to prevent polyspermy
Egg	Cortical granules	Release enzymes into the zona pellucida to prevent polyspermy
Egg	Follicle cells	Nourish and protect the egg
Egg	Nutrient-rich cytoplasm	Feeds the early embryo

Meiosis in the egg
- the final stage is completed only after fertilisation.

Key terms

Term	Meaning
Stem cell niche	the location where stem cells are maintained in the body.
Potency	the range of cell types a stem cell can form.
Morphogen	a signalling chemical that forms a gradient and gives positional information.
Surface area to volume ratio	surface area divided by volume, which falls as size increases.
Pulmonary surfactant	a secretion of type II pneumocytes that lowers surface tension in alveoli.
Sarcolemma	the plasma membrane of a muscle fibre.
Intercalated disc	a branched connection between cardiac muscle cells that speeds contraction.
Acrosome	the sperm organelle containing hydrolytic enzymes.

Avoid these mistakes

Specialised cells have lost genes — they keep the whole genome and express only some genes.

Totipotent and pluripotent are the same — only totipotent cells can form extra-embryonic cells.

Large cells are better at exchange — SA:V falls as cells grow, which limits diffusion.

Cardiac muscle needs nerves to contract — it is myogenic, and nerves only adjust the rate.

Last updated: 25 October 2026

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A ⇔ A = I / M ⇔ I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.