The Discovery of Cells and Cell Theory

Cell theory is the scientific principle that all living things are made of one or more cells, the cell is the basic unit of life, and all cells come from existing cells. In brief: the cell was first named by Robert Hooke in 1665 after he looked at thin slices of cork under a microscope, and cell theory was formulated by Matthias Schleiden, Theodor Schwann and Rudolf Virchow in the nineteenth century. A bacterium is a single cell, and an adult human body contains tens of trillions of them.

Cell theory is one of the foundations of biology, in the same way that atomic theory is a foundation of chemistry. This guide traces the discovery of cells step by step, states the three parts of the theory, explains modern additions and exceptions, and works through the key experiments with a timeline.

How were cells discovered?

Cells are too small to see with the naked eye, so their discovery had to wait for the microscope. Early compound microscopes appeared in Europe around 1600, and by the middle of the seventeenth century, scientists were using them to examine nature.

In 1665 the English scientist Robert Hooke examined a thin slice of cork. He saw many small box-like compartments that reminded him of the cells of a monastery, so he called them “cells”. What he saw were the empty cell walls of dead plant tissue.

Around the same time, the Dutch tradesman Antonie van Leeuwenhoek made simple microscopes with a single lens that magnified much more strongly. In the 1670s he described living single-celled organisms in pond water, and he later saw bacteria, sperm cells and red blood cells. He called the moving creatures “animalcules”.

What does cell theory state?

It took about 170 years after Hooke for the idea to take shape as a general theory. In the 1830s, improved microscopes made it possible to see the contents of cells clearly.

• In 1838 the German botanist Matthias Schleiden concluded that all plants are made of cells.
• In 1839 the German zoologist Theodor Schwann extended this to animals, and proposed that cells are the basic unit of structure for all living things.
• In 1855 the physician Rudolf Virchow stated “omnis cellula e cellula”, meaning that every cell comes from a cell. He is often credited with this idea, though the Polish-German scientist Robert Remak had published similar observations earlier, and the credit is debated.

The classical cell theory is summarized in three statements:

1. All living organisms are composed of one or more cells.
2. The cell is the basic unit of structure and organization in organisms.
3. All cells arise from pre-existing cells.

Why are cells called the units of life?

A cell is the smallest unit that can carry out all the functions of life on its own. It has a boundary, the cell membrane, that separates it from its surroundings. It contains genetic material, takes in energy and materials, carries out chemical reactions and can reproduce.

Parts of a cell, such as a nucleus or a mitochondrion, cannot live independently. A single-celled organism like an amoeba carries out all life processes in one cell. In a multicellular organism, cells specialize, and tissues, organs and organ systems are built from them.

Discovery timeline

Worked example: testing a sample against cell theory

A student looks at a drop of pond water under a microscope and sees a moving green object, a thin strand with several small boxes in a row, and a tiny irregular particle that does not move. Decide which of these fit cell theory.

1. Moving green object: It has a visible boundary and contains green structures, which suggests chloroplasts. It moves and responds to light. It is likely a single-celled alga, which is a cell and a whole organism.
2. Strand with boxes in a row: Each box has a wall and contents, and together they form a filament. This fits a multicellular alga made of cells.
3. Tiny particle: Without a membrane, contents or signs of activity, it cannot be classed as a cell. It could be dust or a grain of sand.

The three-part test for the first two objects is: Is it made of cells? Does it behave as a unit of life? Could it have come from existing cells? A reliable conclusion needs more than one observation, and a stain or higher magnification can help confirm the structures.

What are the modern additions and exceptions?

Modern biology has added statements to the original theory. Cells contain hereditary information in the form of DNA, which is passed from cell to cell during division. Metabolism and energy flow occur within cells, and the chemical composition of cells is similar in all species.

Some cases do not fit the theory neatly:

• Viruses are not cells and cannot reproduce on their own. Whether they are alive is debated.
• Some tissues are unusual. Skeletal muscle fibers are long cells with many nuclei, and some fungi have threads with many nuclei and no cross-walls.
• The first cells must have arisen from nonliving matter. The theory describes life today, while the origin of life is a separate question still under study.

Why does cell theory matter today?

Cell theory is the basis for medicine and biotechnology. Diseases such as cancer are understood as problems of cell growth and division. Infections are diagnosed by finding cells of bacteria in samples, and vaccines are tested on cell cultures.

Stem cell research, tissue engineering and the production of medicines in cell cultures depend on treating the cell as the unit of life. Forensic scientists and food inspectors also use cell observation, for example to identify bacteria in contaminated food.

Which mistakes are common?

Mistake 1: Crediting Hooke with cell theory

Hooke named and described cells in cork, but the theory that all organisms are made of cells came 170 years later.

Mistake 2: Assuming Hooke saw living cells

Cork is dead tissue. He saw the walls that were left behind.

Mistake 3: Saying that all organisms have many cells

Many organisms, such as bacteria and amoebae, have just one. Cell theory says one or more.

Mistake 4: Treating cell theory as a final, unchanging statement

The three classical statements remain valid, but scientists have added to them as knowledge of DNA, viruses and cell structure grew.