Cells — the building blocks of life

All living organisms are made of cells. CCEA Double Award Biology starts with cell biology because it underpins every other topic.

Animal cells — key organelles

Organelle	Function
Cell membrane	Controls what enters and leaves the cell; acts as a barrier
Cytoplasm	Jelly-like fluid where chemical reactions occur; contains organelles
Nucleus	Contains DNA; controls cell activities; contains genetic information
Mitochondria	Site of aerobic respiration; produces ATP energy
Ribosomes	Site of protein synthesis (translation)

Plant cells — additional organelles

Plant cells have everything animal cells have, PLUS:

Additional organelle	Function
Cell wall	Made of cellulose; provides rigid support and shape
Chloroplasts	Contains chlorophyll; site of photosynthesis; produces glucose
Vacuole	Large, permanent; contains cell sap; maintains turgor pressure

Memory tip: Plants have CWV extra — Cell Wall, Vacuole, Chloroplasts.

Bacterial cells — very different from animal and plant cells

Bacteria are prokaryotes — they do NOT have a membrane-bound nucleus. Their DNA floats freely in the cytoplasm. They also have:

• Cell wall (made of murein/peptidoglycan — not cellulose)
• Plasmids (small rings of extra DNA)
• Flagella (in some species — for movement)
• Ribosomes (smaller than in eukaryotic cells)

Bacteria do NOT have mitochondria or chloroplasts (their surface area-to-volume ratio is so small that diffusion is efficient enough).

Specialised cells (animal examples)

Red blood cells: biconcave disc shape (maximises surface area for oxygen absorption); no nucleus (more room for haemoglobin); flexible membrane (can squeeze through capillaries).

Sperm cells: long tail (flagellum) for swimming; many mitochondria (for ATP energy); acrosome at head (contains enzymes to penetrate the egg).

Nerve cells (neurons): very long axon (carries impulses over long distances); many dendrites (receive signals from other neurons); myelin sheath (speeds up nerve impulse transmission).

Specialised cells (plant examples)

Root hair cells: large surface area (long hair-like extension) to maximise water and mineral absorption; no chloroplasts (underground — no light); large vacuole.

Palisade mesophyll cells: packed with chloroplasts; elongated shape; near the top of the leaf (maximum light); thin walls allow CO₂ diffusion.

Levels of organisation

Cell → Tissue → Organ → Organ system → Organism.

Example: cardiac muscle cells → heart muscle tissue → heart (organ) → circulatory system → human.