The cell theory and the life of a cell

The cell theory was built step by step by three scientists:

• 1838, Matthias Schleiden: all plants are made of cells.
• 1839, Theodor Schwann: so are all animals.
• 1855, Rudolf Virchow: new cells come only from cells that already exist.

Put together, the cell theory says:

• every living thing is made of one or more cells;
• the cell is the basic unit of structure and function;
• every cell comes from another cell.

It ties all of biology together, from bacteria to humans.

Cells grow and divide in a controlled way, and die when they are no longer needed. Every cell has a definite life span, and dead cells are replaced.

Contact inhibition: knowing when to stop. Many animal cells stop dividing once they touch their neighbours, like people stopping in a full queue. Cancer cells lose this control. They keep dividing and pile up into a tumour. A benign tumour stays where it is; a malignant (cancerous) one can invade nearby tissue and spread. Plant cells, with their rigid walls, do not show contact inhibition.

Cells that die on purpose. Programmed Cell Death (PCD) is the body removing chosen cells in a planned way, controlled by genes. It is needed for growing, for quality control and for fighting disease. In an unborn baby, the hand first forms like a paddle; PCD removes the cells between the fingers. Without it we would have webbed hands.

In 1902, Gottlieb Haberlandt suggested that any living plant cell, given food and the right conditions, could grow into a whole plant. This ability is called totipotency. His idea became the basis of plant tissue culture: growing many new plants from tiny pieces in a laboratory.