Cell Specialisation and Differentiation

GCSE Biology · Cell Biology

Differentiation

Differentiation is the process by which a cell becomes specialised for a particular function. As a cell differentiates, it develops specific features (structural adaptations) that allow it to carry out its role efficiently.

In animals, most cells lose the ability to differentiate early in development (except stem cells)
In plants, many cells retain the ability to differentiate throughout the plant's life (at meristems)

Specialised Animal Cells

Sperm cell — function: to reach and fertilise the egg

Streamlined shape (head and tail) for swimming
Long tail (flagellum) for movement
Acrosome (tip of head) contains enzymes to digest the egg membrane
Many mitochondria in the middle section to provide energy for swimming
Haploid nucleus (23 chromosomes) so that when it fuses with the egg, the resulting zygote has the correct diploid number (46)

Nerve cell (neurone) — function: to carry electrical impulses around the body

Long axon to carry impulses over long distances
Branched endings (dendrites) to connect with other neurones and form a network
Myelin sheath (insulating fatty layer) to speed up impulse transmission
Many mitochondria to provide energy for transmitting impulses
Synaptic knobs at the end to pass impulses to the next neurone using neurotransmitters

Red blood cell — function: to carry oxygen around the body

Biconcave disc shape gives a large surface area to volume ratio for efficient oxygen absorption
Contains haemoglobin — a protein that binds to oxygen to form oxyhaemoglobin
No nucleus — more room for haemoglobin, so it can carry more oxygen
Flexible — can squeeze through narrow capillaries

White blood cell — function: to defend the body against pathogens

Can change shape to engulf (phagocytose) microorganisms
Some produce antibodies (lymphocytes) that are specific to particular antigens on pathogens
Some produce antitoxins to neutralise toxins
Has a nucleus (unlike red blood cells)

Muscle cell — function: to contract and produce movement

Contains protein fibres (actin and myosin) that can slide past each other to shorten the cell
Many mitochondria to provide energy for contraction
Stores glycogen which can be converted to glucose for respiration

Specialised Plant Cells

Root hair cell — function: to absorb water and mineral ions from the soil

Long hair-like projection extending into the soil increases the surface area for absorption
Large permanent vacuole to speed up osmosis by maintaining a low water potential inside the cell
Many mitochondria to provide energy for active transport of mineral ions
Thin cell wall for easier absorption

Xylem cells — function: to transport water and dissolved minerals up the plant

Cells are dead and hollow — no cytoplasm or organelles, so water flows freely
Walls are thickened with lignin (waterproof and strong) for support
Cells are arranged end to end with no end walls, forming a continuous tube
Also provide structural support to the plant

Phloem cells — function: to transport dissolved sugars (sucrose) and amino acids around the plant (translocation)

Sieve tube elements are living cells arranged end to end
Sieve plates (perforated end walls) allow dissolved substances to flow through
Companion cells have many mitochondria and provide energy for the loading of sucrose (active process)
Sieve tube elements have very little cytoplasm and no nucleus, so they rely on companion cells

Palisade mesophyll cell — function: to carry out photosynthesis

Found near the upper surface of the leaf to absorb maximum light
Tall, column-shaped cells packed tightly together
Packed with chloroplasts containing chlorophyll to absorb light energy
Large vacuole pushes chloroplasts to the edges of the cell, closer to the light

Guard cells — function: to open and close stomata to control gas exchange and water loss

Kidney/bean-shaped in pairs around a stoma
Inner wall is thicker than the outer wall, so when the cell becomes turgid it bends, opening the pore
Contain chloroplasts for photosynthesis, providing energy
When the plant loses water, guard cells become flaccid and the stomata close to reduce water loss

Exchange Surfaces

Organisms need efficient exchange surfaces. These surfaces are adapted by having:

A large surface area (e.g. villi in the small intestine, alveoli in the lungs)
A thin membrane (short diffusion distance)
A good blood supply to maintain the concentration gradient
Ventilation (in lungs) to maintain the concentration gradient

Exam Tips

When describing a specialised cell, always link the adaptation to the function — for example, "has many mitochondria to provide energy for active transport"
Red blood cells having no nucleus is one of the most commonly examined points
Remember that xylem cells are dead but phloem cells are alive