2.5Theme B — Form and function

Cell Specialisation — IB Biology SL Revision Notes & Topical Questions

5 min read 11 parts Lesson 12 of 13 1 quiz

0%read

Lesson summary

2.5 Cell Specialisation — IB BIOLOGY SL notes: Explain how differentiation arises from gene expression and morphogens, compare stem cells, and calculate SA:V ratios to explain cell size.After fertilisation the zygote divides by mitosis to form a ball of totipotent cells.A blastoc…

Unipotent cells
Stem cell niche
Therapeutic use
Division of labour
Red blood cell
Plasma cell

Part 1 of 9Development of specialised cells

After fertilisation the zygote divides by mitosis to form a ball of totipotent cells.

A blastocyst forms, and the inner cell mass cells are pluripotent.

Morphogen gradients give cells positional information.

Cells switch particular genes on and others off.

Cells differentiate into specialised types with specific structures and proteins.

Some multipotent stem cells stay in niches to replace cells throughout life.

Gene expression = the process by which a gene's information is used to make a product, usually a polypeptide
- In differentiation, different cell types express different genes.

Part 2 of 9Morphogens

Morphogen = a signalling chemical that diffuses through the early embryo and forms a concentration gradient
- Examples occur in Drosophila and in the vertebrate limb.
- The concentration a cell receives decides which genes it expresses.

Source cells secrete the morphogen.

The morphogen diffuses, forming a gradient from high to low concentration.

Cells bind the morphogen at different concentrations.

Different transcription factors are activated, switching on different genes.

Cells differentiate into different types according to their position.

Part 3 of 9Stem cell types and niches

Type	Potency	Source	Example use
Zygote and morula cells	Totipotent	1-4 day embryo	Can form the whole organism and placenta
Embryonic stem cells	Pluripotent	Inner cell mass of the blastocyst	Research and potential tissue replacement
Adult (somatic) stem cells	Multipotent	Bone marrow and skin	Bone marrow transplants for leukaemia

Stem cell types and niches

Unipotent cells

Unipotent cells they divide but produce only their own cell type, such as cardiomyocytes making new heart muscle cells
They have the lowest potency, below multipotent cells, and most cells in an animal body are unipotent.

Stem cell niche

Stem cell niche bone marrow makes blood cells and hair follicles make hair and skin
A niche keeps its stem cells inactive until they are needed.
It also stimulates them to divide and differentiate when replacements are required.

Therapeutic use

Therapeutic use a bone marrow transplant replaces blood cells after cancer treatment

Ethics

Ethics embryonic stem cells raise issues because embryos are destroyed

Part 4 of 9Division of labour and cell structure

Division of labour and cell structure

Division of labour

Division of labour differentiated cells take on particular roles so each works more efficiently
Specialisation changes cell shape and the organelles present.
Cells that secrete a lot of protein contain many ribosomes, extensive rough ER and Golgi.

Red blood cell

Red blood cell no nucleus or mitochondria, biconcave and flexible for capillaries

Neuron

Neuron a large cell body makes the proteins needed to maintain a very long axon

Sperm

Sperm a tail with mitochondria

Egg

Egg cortical granules and large stores

Plasma cell

Plasma cell an activated white blood cell enlarges to hold more rough ER and Golgi for antibody production

Muscle fibre

Muscle fibre large and long so it generates strong forces when it contracts

Part 5 of 9Sizes of specialised cells

Cell	Approximate size	Adaptation
Red blood cell	7-8 µm diameter	Biconcave with no nucleus, giving more haemoglobin and a large SA:V
Sperm	About 50 µm long	Flagellum, many mitochondria and acrosome
Egg (human)	About 110 µm	Large cytoplasm with nutrients
Motor neuron	Axon up to about 1 m	Long axon for rapid signalling
Type I pneumocyte	About 0.15 µm thick	Short diffusion distance

Typical range
- most cells are 10-100 µm, with limits set by SA:V and diffusion distance

Part 6 of 9Calculating SA:V

Cube

Cube surface area = 6 × L², volume = L³, so SA:V = 6 / L

Sphere

Sphere surface area = 4πr², volume = (4/3)πr³, so SA:V = 3 / r

Growth

Growth as a cell grows, volume increases faster than surface area, so SA:V decreases

Metabolism and exchange

Metabolism and exchange the rate of metabolism depends on volume and the rate of exchange depends on surface area

Heat loss

Heat loss it also depends on SA:V, so small animals lose heat more rapidly

Part 7 of 9Limits on cell size and multicellular solutions

Limits on cell size and multicellular solutions

Small cells

Small cells high SA:V and a short diffusion distance to every organelle

Large cells

Large cells low SA:V, so exchange is too slow and wastes and heat build up

Division

Division growth stops and the cell divides once SA:V becomes too small

Multicellular exchange systems

Multicellular exchange systems large organisms have a low SA:V and long diffusion distances
They evolved gas exchange and digestive systems as exchange surfaces.
They evolved circulatory systems to carry substances to internal tissues.

Adaptations that raise SA:V

Adaptations that raise SA:V flattening in red blood cells, microvilli in the ileum epithelium, elongation and small size

Part 8 of 9Agar cube diffusion model

Cut agar containing indicator, for example with dilute NaOH, into cubes of different sizes.

Calculate the surface area, volume and SA:V of each cube.

Place the cubes in equal volumes of dilute acid.

Measure the time for complete colour change, or the depth of change in a fixed time.

Convert the times to rates and plot rate against SA:V.

Result

Result smaller cubes, with a higher SA:V, are fully penetrated faster

Limitation

Limitation the cubes model cells in a simplified form and do not match real shapes exactly

Part 9 of 9More from the specification

Differentiation of Eukaryotic Cells

In multicellular organisms, unspecialised embryonic cells undergo selective gene expression to allow development into specialised eukaryotic cells with distinct structures.

Division of Labour and Efficiency

Cellular specialisation means distinct cell types perform dedicated physiological tasks, enhancing the organism's ability to satisfy metabolic requirements and adapt to its environment.

Structural Adaptations to Function

Specialised eukaryotic cells (such as neurones, erythrocytes,
and xylem vessels) develop distinct morphological adaptations that optimize their primary biological function.

Metabolic Requirements in Multicellular Systems

As organisms increase in size, simple diffusion is insufficient to meet metabolic requirements, requiring specialized eukaryotic cells organized into organ systems for transport and exchange.

Differentiation of Eukaryotic Cells

In multicellular organisms, unspecialised embryonic cells undergo selective gene expression to allow development into specialised eukaryotic cells with distinct structures.

Division of Labour and Efficiency

Cellular specialisation means distinct cell types perform dedicated physiological tasks, enhancing the organism's ability to satisfy metabolic requirements and adapt to its environment.

Structural Adaptations to Function

Specialised eukaryotic cells (such as neurones, erythrocytes,
and xylem vessels) develop distinct morphological adaptations that optimize their primary biological function.

Metabolic Requirements in Multicellular Systems

As organisms increase in size, simple diffusion is insufficient to meet metabolic requirements, requiring specialized eukaryotic cells organized into organ systems for transport and exchange.

Development

The changes as an organism becomes more complex and mature.

Key terms

Term	Meaning
Zygote	The first diploid cell of a new organism, formed by fusion of gametes, which is totipotent.
Differentiation	The process by which unspecialised cells become specialised by expressing some genes and not others.
Morphogen	A signalling chemical that forms a concentration gradient and gives cells positional information.
Stem cell	An unspecialised cell that can self-renew and differentiate into specialised cell types.
Stem cell niche	The specific location where stem cells are found and maintained, such as bone marrow.
Totipotent	Able to differentiate into any cell type, including extra-embryonic cells.
Multipotent	Able to differentiate into a limited range of related cell types.
Surface area-to-volume ratio	The surface area of a cell divided by its volume, which decreases as the cell grows.

Avoid these mistakes

Using volume instead of surface area to explain exchange rate — exchange depends on surface area and metabolism on volume

Saying SA:V increases as a cell grows — it decreases

Saying all stem cells are embryonic — adult multipotent stem cells exist in niches such as bone marrow

Saying morphogens act by moving the cells — they form a gradient that switches genes on or off

Last updated: 26 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.