Cell — Structure and Functions

In 1665, he observed cork (tree bark) under a microscope and saw small hollow compartments — he named them 'cells' (from Latin, meaning small room). He had not actually seen living cells, but the name stuck.

Unicellular — all life functions are carried out within a single cell: amoeba (feeding, breathing, movement — everything), paramecium, chlamydomonas. Multicellular — many cells, each with a specific role: humans, plants — cells → tissue (same function) → organ (heart) → system (digestive). In multicellular organisms, cells are specialised.

The ostrich egg (yolk) — more than 15 cm — is the largest single cell in the world. The smallest is the bacterium (0.1–0.5 micrometre). The size of a cell is related to its function.

The nucleus contains chromosomes — threads of DNA — and segments of DNA called genes, which carry hereditary information (traits passed from parents — colour, height, eye shape). The nucleus sets the rules for the cell's growth, repair, and reproduction — just like a boss. If the nucleus is removed, the cell dies.

(1) Cell wall — a rigid layer (cellulose) outside the membrane — provides protection and keeps the plant upright; (2) Chloroplast — a plastid containing chlorophyll — carries out photosynthesis; (3) Large vacuole — stores water and food (when water is low, the leaf wilts). Animal cells — have only a membrane and no large vacuole.

Prokaryotic — no well-defined nucleus (no surrounding membrane — DNA floats in the cytoplasm): bacteria. Eukaryotic — nucleus enclosed by a membrane (a proper control room): plants, animals, humans. Prokaryotes are the oldest cells (the first inhabitants of Earth).