Cell Differentiation and Specialised Cells

When a fertilised egg divides by mitosis it first makes a ball of cells that all look much the same. Yet a finished organism contains hundreds of different cell types — nerve cells metres long, flat red blood cells, root hair cells that reach into the soil. How does one type of cell turn into so many? The answer is differentiation. This lesson, part of Topic B2 of OCR Gateway Science A, explains how cells become specialised for particular jobs, and looks closely at the key specialised cells of animals and plants. The recurring exam skill here is linking a cell's structure to its function — saying not just what a feature is but why it helps.

By the end of this lesson you should be able to explain what differentiation is, describe how named specialised animal and plant cells are adapted to their functions, and link each adaptation clearly to the job the cell does.

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What Is Differentiation?

Differentiation is the process by which a cell becomes specialised for a particular job, developing a specific shape and the right set of sub-cellular structures. The crucial point — and a favourite of examiners — is how this happens:

Every cell in an organism contains the same DNA (the same genes), but a specialised cell switches on only the genes it needs. Switching different genes on or off makes different proteins, which gives each cell type its particular structure and function. That is why a nerve cell and a red blood cell can look completely different even though they carry an identical genome.

flowchart TD
  A["Unspecialised cell<br/>(same genome in every cell)"] --> B["Differentiation<br/>(specific genes switched on or off)"]
  B --> C["Nerve cell<br/>long, with branches"]
  B --> D["Red blood cell<br/>biconcave, no nucleus"]
  B --> E["Sperm cell<br/>tail and many mitochondria"]
  B --> F["Root hair cell<br/>long extension into soil"]
  B --> G["Palisade cell<br/>packed with chloroplasts"]

In animals, most cells differentiate at an early stage and then stay specialised — once a cell has become, say, a muscle cell, it cannot turn into a nerve cell. In plants, by contrast, many cells keep the ability to differentiate throughout the plant's life. The unspecialised cells that retain the ability to divide and differentiate are called stem cells, which are the subject of the next lesson.

Exam Tip: The most common differentiation question asks how cells become different even though they have the same DNA. The mark-worthy idea is that different genes are switched on (and others off) in different cells, so different proteins are made. Avoid vague answers like "the cells just change".

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Specialised Animal Cells

A specialised cell has features that suit its function. Learn at least two or three adaptations for each cell below, and — this is where the marks are — be ready to say how each feature helps.

Cell	Adaptation	How it helps the function
Sperm cell	Long tail (flagellum)	Allows it to swim to the egg
	Many mitochondria in the mid-piece	Release energy (aerobic respiration) for swimming
	Acrosome (enzyme cap on the head)	Enzymes digest the egg membrane so the sperm can enter
	Streamlined head	Reduces resistance while swimming
Egg cell (ovum)	Large store of nutrients in the cytoplasm	Feeds the developing embryo after fertilisation
	Membrane changes after fertilisation	Stops more than one sperm entering
Ciliated epithelial cell	Tiny hair-like cilia on the surface	Beat to sweep mucus (with trapped dust/microbes) along, e.g. out of the airways
Red blood cell	Biconcave disc shape	Large surface area for absorbing oxygen
	No nucleus	More room for haemoglobin to carry oxygen
	Packed with haemoglobin	Binds and transports oxygen
Nerve cell (neurone)	Very long	Carries electrical impulses over large distances
	Branched ends (dendrites)	Connect to many other cells
	Insulating (myelin) sheath	Speeds up the impulse
Muscle cell	Contains protein fibres that can shorten	The cell contracts to produce movement
	Many mitochondria	Release the energy needed for contraction

A closer look at the red blood cell

The red blood cell is the highest-frequency exam example, so it is worth dwelling on. It is one of the few animal cells with no nucleus — losing the nucleus frees up space, so the cell can be packed with haemoglobin, the red pigment that binds oxygen. Its biconcave disc shape (like a doughnut without a hole right through) gives it a large surface area compared with its volume, so oxygen can diffuse in and out quickly. It is also small and flexible, so it can squeeze through the narrowest blood vessels (capillaries).

Exam Tip: For the red blood cell, two adaptations score most reliably: the biconcave disc shape (large surface area for gas exchange) and the absence of a nucleus (more space for haemoglobin). Always pair the feature with the reason.

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Specialised Plant Cells

Plant cells specialise too, and several of them connect directly to later parts of B2 on transport and exchange.

Cell	Adaptation	How it helps the function
Root hair cell	Long hair-like extension	Large surface area to absorb water and mineral ions from the soil
	Thin cell wall	Short distance for water to cross
	Many mitochondria	Release energy for active transport of mineral ions
Xylem vessel	Hollow tubes with no end walls and no living contents (dead)	Form a continuous pipe so water and minerals can flow up the plant
	Walls strengthened with lignin	Provide support and keep the tube open
Phloem cell	Sieve plates with pores between cells; living	Allow dissolved sugars to flow up and down the plant (translocation)
Palisade mesophyll cell	Packed with chloroplasts	Absorb the most light for photosynthesis
	Tall, column shape near the upper leaf surface	Catches as much light as possible

A closer look at the root hair cell

The root hair cell is a perfect example of structure matching function, and it links forward to osmosis and active transport. Its long, thin extension dramatically increases the surface area in contact with the soil water, so it can absorb water (by osmosis) and dissolved mineral ions far faster than a flat cell could. Its thin wall gives a short distance for water to cross. Crucially, it contains many mitochondria: minerals are often more concentrated inside the cell than in the soil, so they must be taken in by active transport, which needs energy released by respiration.