The cell and cell theory

Some living things, like Amoeba or bacteria, are just one cell, and that one cell does everything needed for life: feeding, breathing, getting rid of waste, reproducing. Nothing smaller than a complete cell can live on its own. So the cell is the basic structural and functional unit of life: every living thing is built of cells, and life works through them.

How we learned this:

• Antonie von Leeuwenhoek first saw and described a live cell.
• Robert Brown discovered the nucleus.
• Matthias Schleiden (1838) found that plants are made of cells; Theodor Schwann (1839) found the same for animals. Schwann also noted the thin outer layer of cells (the plasma membrane), and that the cell wall is found only in plant cells. Together they said that bodies are made of cells and the products of cells.
• But they could not explain where new cells come from. Rudolf Virchow (1855) showed that cells divide, and that new cells come only from cells that already exist: "Omnis cellula-e cellula".

So the cell theory today says: (i) all living organisms are made of cells and products of cells; (ii) all cells arise from pre-existing cells. A cell cannot form from non-living material.

Cells are of two kinds. Eukaryotic cells have a proper nucleus wrapped in a membrane, and many membrane-bound organelles (like mitochondria, the Golgi apparatus and lysosomes). Prokaryotic cells, like bacteria, have neither.

Cells vary hugely in size. Mycoplasma, the smallest, is only about 0.3 µm (micrometre) across (the book prints "0.3 mm" by mistake). Bacteria are 3 to 5 µm, and a human red blood cell about 7 µm. The largest single cell is the egg of an ostrich. Nerve cells are among the longest.