## TL;DR

Cells are the basic unit of life. Prokaryotes (bacteria, archaea): no nucleus, simple. Eukaryotes (animals, plants, fungi): nucleus, membrane-bound organelles. Key organelles: nucleus (DNA), mitochondria (energy, ATP), endoplasmic reticulum (protein synthesis), Golgi apparatus (protein packaging), lysosomes (digestion).

## Core Explanation

Mitochondria: 'powerhouse of the cell' — oxidative phosphorylation produces ATP. Chloroplasts: photosynthesis in plants. Cell membrane: phospholipid bilayer, selectively permeable. Cytoskeleton: microtubules, actin filaments — cell shape, transport, division. Human body: ~37 trillion cells. Red blood cells lack nuclei (mammals only).

## Further Reading

- [Molecular Biology of the Cell (Alberts et al.)](https://wwnorton.com/books/9780393884821)