Cells — the units of life
Biology · Key Stage 3 · The School
Everything alive is made of cells
Every living thing — from bacteria to blue whales — is built from cells, most far too small to see. An ANIMAL cell has a nucleus (the control centre holding DNA), cytoplasm (where reactions happen), a cell membrane (the gatekeeper controlling what enters and leaves), and mitochondria (where respiration releases energy).
What plant cells add
A PLANT cell has all of that plus three extras: a rigid cell wall of cellulose (support), chloroplasts holding chlorophyll (where photosynthesis makes food from light), and a large vacuole of sap (storage and firmness). The extras exist because plants make their own food and cannot move to find it.
From cells to systems
Cells with the same job group into TISSUES (muscle tissue); tissues build ORGANS (the heart); organs cooperate in SYSTEMS (the circulatory system); systems make the ORGANISM. Cell → tissue → organ → system → organism: the ladder of life's organisation.
How anyone knows this, and doing the magnification sum
None of this was guessed. Robert Hooke named the "cell" in 1665 after seeing box-like compartments in a thin slice of cork, and Anton van Leeuwenhoek, grinding better lenses, was the first to see living single-celled organisms. You can repeat the observation: a slice of onion epidermis under a school microscope shows cell walls as a clear brick-like grid. The calculation that goes with it is magnification = image size ÷ actual size. If a cell measures 30 mm across your drawing and is really 0.06 mm across, the magnification is 30 ÷ 0.06 = 500, written ×500. Get both measurements into the SAME unit before dividing — millimetres against micrometres is the error that produces answers a thousand times out.