Cell Specialisation

A human embryo starts as a single fertilised egg, but by adulthood you contain over 200 types of specialised cell — each with a distinct form fitted to a distinct function. Red blood cells carry oxygen, neurones conduct electrical impulses, muscle cells contract, epithelial cells line surfaces. This lesson covers OCR A-Level Biology A specification point 2.1.6 (d) — the production of specialised cells — and prepares for Lesson 11 on stem cells and Lesson 12 on tissue organisation.

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1. What Is Cell Specialisation?

Key Definition — Cell Specialisation (Differentiation): The process by which cells develop particular structural and functional characteristics that make them suited to a specific role. Differentiated cells express only a subset of the genes in their genome.

All specialised cells in your body contain the same genome. What differs is which genes are switched on. A muscle cell has the genes for making haemoglobin, but those genes are silent; a red blood cell has the genes for making actin and myosin, but those too are silent. Gene expression determines form and function.

Key points about specialisation:

• All cells arise from unspecialised stem cells (covered in Lesson 11).
• Differentiation is usually irreversible in adult cells, though laboratory techniques can reprogram cells.
• The structure of a specialised cell matches its function — "form follows function".

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2. How Cells Become Specialised

During embryonic development, cells receive different signals depending on where they lie in the developing body. These signals — including hormones, growth factors, and physical contact with neighbours — switch specific genes on or off. Over time, the cell's cytoplasm, shape and surface proteins all change to produce a mature, specialised phenotype.

Differentiation involves:

• Transcriptional control — transcription factors bind to specific promoters, activating some genes and silencing others.
• Epigenetic modification — DNA methylation and histone modification stably silence genes that are no longer needed.
• Changes in cell shape, size and organelle content — driven by the proteins the cell now makes.
• Changes in cell surface markers — determining what signals the cell responds to and which cells it sticks to.

Once a cell has differentiated, its gene expression pattern is usually stable throughout its life, because the chromatin modifications are passed on to daughter cells during mitosis.

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3. Specialised Animal Cells — Worked Examples

3.1 Erythrocytes (Red Blood Cells)

Function: Transport oxygen from lungs to tissues, and some carbon dioxide from tissues to lungs.

Adaptations:

• Biconcave disc shape — maximises the surface-area-to-volume ratio for gas exchange and allows flexibility to squeeze through capillaries.
• No nucleus, mitochondria, ribosomes or endoplasmic reticulum — the extra space is filled with haemoglobin.
• Very small (~7–8 μm diameter) — short diffusion distance.
• Packed with haemoglobin (~270 million molecules per cell) — carries oxygen.
• Flexible membrane — deforms to pass through narrow capillaries.
• Lifespan ~120 days (because they cannot repair themselves without a nucleus) — then broken down in the spleen and liver.

graph LR
  A[Erythrocyte structure] --> B[No nucleus / more Hb]
  A --> C[Biconcave disc / SA:V ratio]
  A --> D[Small ~7 um / short diffusion]
  A --> E[Flexible / squeezes through capillaries]

3.2 Neutrophils

Function: Phagocytosis of pathogens as part of the innate immune response. They are the most abundant white blood cell.

Adaptations:

• Multi-lobed nucleus — flexibility to squeeze through capillary walls into infected tissues.
• Many lysosomes — packed with hydrolytic enzymes for digesting engulfed pathogens.
• Many mitochondria — supply ATP for the energy-demanding process of phagocytosis.
• Receptors on surface — detect pathogen-associated molecules and chemotactic signals.
• Short lifespan (~5 days) — produced continuously in bone marrow.

3.3 Epithelial Cells — Squamous

Function: Form thin layers where rapid diffusion is needed (alveoli in the lungs, capillary walls).

Adaptations:

• Very flat and thin (flattened nuclei, minimal cytoplasm) — minimises diffusion distance.
• Single layer — short diffusion path for gases.
• Tight junctions between cells maintain integrity of the barrier.

3.4 Epithelial Cells — Ciliated

Function: Sweep mucus, trapped particles and microbes along a surface. Found lining the trachea, bronchi and oviducts.

Adaptations:

• Cilia on the apical (lumen-facing) surface — beat in a coordinated wave.
• Many mitochondria at the apical end — supply ATP for ciliary movement.
• Goblet cells (specialised for mucus secretion) are interspersed among ciliated cells.
• In the respiratory tract, the cilia-mucus system is the mucociliary escalator, vital for removing inhaled pathogens.

3.5 Sperm Cells

Function: Deliver paternal DNA to the ovum.

Adaptations:

• Streamlined head and long flagellum (tail) — for motility.
• Many mitochondria packed in the midpiece — supply ATP for flagellar movement.
• Acrosome at the tip — contains digestive enzymes (hyaluronidase, acrosin) to break through the zona pellucida of the egg.
• Haploid nucleus — contains 23 chromosomes (the product of meiosis).
• Reduced cytoplasm — saves weight, maximises motility.

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4. Specialised Plant Cells — Worked Examples

4.1 Palisade Mesophyll Cells

Function: The main site of photosynthesis in leaves.

Adaptations: