1. What Is a Cell? Cell Theory & Discovery

Every living thing — a single bacterium, a blade of grass, you — is built from cells. The cell is the smallest unit that is truly alive. This big idea is summed up by the cell theory, which has three parts:

• All living things are made of one or more cells.
• The cell is the basic unit of structure and function in living things.
• All cells come from existing cells (cells don't appear from nothing).

This understanding depended on the microscope. In 1665 Robert Hooke looked at a thin slice of cork and named the little boxes he saw 'cells.' Soon after, Antonie van Leeuwenhoek used better lenses to see actual living microbes. In 1838–39, Schleiden and Schwann concluded that all plants and animals are made of cells, and in 1855 Virchow added that every cell comes from another cell.

Cells are usually tiny — most can only be seen with a microscope — which is exactly why they went unnoticed for so long.

⚠️ Common misconception: Hooke did not see living cells. He saw the empty walls of dead cork cells. It took stronger microscopes (Leeuwenhoek) to reveal living cells.

💡 Tip: memorize the three parts of cell theory — made of cells, basic unit of life, cells from cells.