What is a Cell?

Every living thing you've ever seen — bacteria, mushrooms, oak trees, you — is made of cells. Some organisms are a single cell; you're roughly 37 trillion of them, all cooperating. Cell theory, one of biology's foundational ideas, states this plainly: the cell is the basic structural and functional unit of life, every living thing is made of one or more cells, and every cell comes from a pre-existing cell.

What a cell actually needs to do

Regardless of organism, every cell has to solve the same handful of problems: keep its contents separate from the outside world, get energy, build proteins from genetic instructions, and reproduce. What differs enormously between organisms is how a cell is organized to do this — and that split falls into two broad categories.

Prokaryotic vs. eukaryotic

Prokaryotic cells (bacteria and archaea) are simple and small: no nucleus, no internal membrane-bound compartments. Their DNA just floats in the cytoplasm, in a region called the nucleoid. Simple isn't the same as primitive, though — bacteria are extraordinarily successful and metabolically diverse.

Eukaryotic cells (everything else — plants, animals, fungi, protists) are larger and more organized: DNA is enclosed in a nucleus, and the cell contains other membrane-bound compartments (organelles) that each specialize in one job — mitochondria generate usable energy, ribosomes build proteins, and so on. Multicellular life, including you, is built from eukaryotic cells.

Why this distinction matters

This isn't just a naming exercise. The prokaryote/eukaryote split is the deepest divide in the tree of life, older than the split between plants and animals. Understanding which structures a cell does and doesn't have tells you immediately what it can and can't do — which is exactly the foundation the rest of this course builds on: how cells get energy, how they read DNA to build proteins, and how they divide.