All multicellular organisms begin life as a single fertilised egg cell (zygote). This one cell divides repeatedly by mitosis to produce all the billions of cells in the organism.
As cells divide, they become progressively more specialised — this process is called DIFFERENTIATION.
Differentiation means a cell switches on certain genes and switches off others, causing it to develop a specific structure suited to a specific function.
The result: specialised cells that are experts at one job, making the whole organism far more efficient.
In ANIMALS: most differentiation occurs early during embryonic development. Once differentiated, most animal cells lose the ability to become a different type — they are committed.
In PLANTS: differentiation can continue throughout the plant's life. Meristem cells (at root and shoot tips) remain undifferentiated and can keep producing new specialised cells.
Specialised Animal Cells
SPERM CELL — function: swim to and fertilise an egg.
Adaptations:
• Streamlined head — reduces drag when swimming.
• Flagellum (tail) — rotates to propel the sperm through fluid.
• Many mitochondria in the midpiece — aerobic respiration provides ATP energy for swimming.
• Acrosome (cap on the head) — contains enzymes that digest through the egg's outer membrane.
• Haploid nucleus — contains only 23 chromosomes (half the normal number) so that after fertilisation the normal 46 is restored.
RED BLOOD CELL (erythrocyte) — function: carry oxygen from lungs to tissues.
Adaptations:
• Biconcave disc shape — large surface area for oxygen absorption; thin centre = short diffusion distance for O₂.
• No nucleus — creates more space for HAEMOGLOBIN, the oxygen-carrying protein.
• Flexible — can squeeze through narrow capillaries without tearing.
• Packed with haemoglobin — each cell contains ~270 million haemoglobin molecules.
NEURONE (nerve cell) — function: transmit electrical impulses rapidly over long distances.
Adaptations:
• Very long axon — can be over a metre long (e.g. sciatic nerve), allowing signals to travel from brain to toe without interruption.
• Myelin sheath — fatty insulating layer that speeds up impulse conduction (signals jump between gaps called nodes of Ranvier).
• Dendrites — short branching fibres that receive signals from other neurones.
• Synaptic terminals — the axon ends in bulb-like structures that release neurotransmitters to communicate with the next cell.
MUSCLE CELL — function: contract to produce movement.
Adaptations:
• Contains contractile proteins (actin and myosin) that slide past each other to shorten the cell.
• Many mitochondria — large energy demand for constant contraction.
• Can store glycogen as an energy reserve.
Specialised Plant Cells
ROOT HAIR CELL — function: absorb water and mineral ions from soil.
Adaptations:
• Long, thin hair-like projection extending from the cell — massively increases the surface area in contact with soil water.
• No chloroplasts — underground, no light available for photosynthesis.
• Thin cell wall — short diffusion distance for water and minerals.
• Large permanent vacuole — maintains a low water potential inside the cell to draw water in by osmosis.
XYLEM CELL — function: transport water and dissolved minerals from roots up to leaves.
Adaptations:
• Dead cells — no cytoplasm or nucleus (no living contents obstructing flow).
• Hollow lumen — forms a continuous open tube for water to flow through.
• Walls thickened with LIGNIN — a hard, waterproof substance that prevents collapse under pressure and makes xylem very strong.
• No end walls — cells are stacked end-to-end to form an uninterrupted column.
PHLOEM CELL — function: transport dissolved sugars (sucrose) from leaves to the rest of the plant.
Adaptations:
• Living cells with little cytoplasm.
• Sieve plates — porous plates at the end walls that allow sugar solution to flow between cells.
• Companion cells alongside each sieve tube cell — provide energy (ATP) for active loading of sugars.
• Found at the top of the leaf — closest to sunlight.
• Tall, column shape — allows maximum chloroplasts to be stacked near the leaf surface.
⚠️ Common Mistake
Students often say 'root hair cells absorb water by active transport' — this is WRONG. Water enters by OSMOSIS (passive). Mineral IONS are absorbed by active transport. These are two different processes happening in the same cell. Don't mix them up.
📌 Key Note
Differentiation = cells becoming specialised by switching genes on/off. Animals: mostly at embryo stage. Plants: meristems continue throughout life. Every adaptation has a specific reason — always link structure to function.
🎯 Matching Activity — Match the Specialised Cell to its Key Adaptation
Match each cell type to the adaptation that makes it suited to its function. — drag the symbols on the right to match the component names on the left.
Sperm cell
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Red blood cell
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Neurone
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Root hair cell
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Xylem
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Palisade cell
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Hollow, dead, lignified walls — forms a tube for water transport
Long axon with myelin sheath — fast impulse transmission
Packed with chloroplasts near leaf surface — photosynthesis
Biconcave disc, no nucleus — maximises haemoglobin space
Flagellum and many mitochondria — to swim to the egg
Long projection, no chloroplasts — absorbs water and minerals from soil
🎯 Test Yourself
Question 1 of 4
1. Why does a sperm cell contain many mitochondria?
2. A red blood cell has no nucleus. How does this benefit its function?
3. Why does a root hair cell have no chloroplasts?
4. What structural feature of xylem cells makes them well-suited for water transport?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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