Cell Structure & Organisation

• Cell theory says all living things are made of cells, cells are the basic unit of life, and new cells only come from existing cells.
• Cells organise into a hierarchy: organelles → cells → tissues → organs → organ systems.
• There are two broad types of cell: eukaryotic (complex, membrane-bound organelles — animals, plants, fungi) and prokaryotic (simple, no membrane-bound organelles — bacteria).
• Eukaryotic cells share a "protein production line": nucleus → ribosomes → rough ER → Golgi apparatus → cell membrane — this is how proteins are made, processed, and exported.
• Prokaryotic cells are smaller, have circular DNA free in the cytoplasm, smaller (70S) ribosomes, and a peptidoglycan cell wall.
• Light microscopes use light and glass lenses (lower resolution, can view living specimens). Electron microscopes use electron beams (much higher resolution, specimen must be dead).
• You need to be able to calculate magnification and use an eyepiece graticule + stage micrometer to measure real specimen sizes.
• Practical skills: preparing slides (dry mount, wet mount, squash, smear), staining, and making correctly-formatted biological drawings.

Where the idea came from

Before microscopes were powerful enough, nobody knew what living things were actually made of. Robert Hooke was the first person to view cells (in cork) and gave them the name "cell" because the boxy compartments reminded him of the small rooms (cells) monks lived in. Later, Matthias Schleiden (studying plants) and Theodor Schwann (studying animals) put the pieces together in 1837 into what we now call cell theory — a "unifying concept," meaning it applies to every living thing on Earth and is universally accepted by biologists.

The three core ideas

1. All living organisms are made up of one or more cells.
2. Cells are the basic functional unit of living organisms (the smallest unit that can be said to be "alive").
3. New cells are only produced from pre-existing cells (cells don't just spontaneously appear).

Features every cell shares

Regardless of whether a cell is a bacterium or part of your brain, it will always have these four things:

• Cell surface membrane — the boundary between inside and outside
• Cytoplasm — the jelly-like substance everything sits in
• DNA — the genetic instructions
• Ribosomes — the protein-making machinery

Beyond these four "universal" features, different cell types have very different extra structures — this is exactly what separates prokaryotes from eukaryotes (covered below). When you look at a cell under a microscope and study its internal structures, you're studying its ultrastructure.