Cell Theory

The microscopes we use today are far more complex than those used in the 1600s by Antony van Leeuwenhoek, a Dutch shopkeeper who had great skill in crafting lenses. Despite the limitations of his now-ancient lenses, van Leeuwenhoek observed the movements of protista (a type of single-celled organism) and sperm, which he collectively termed “animalcules. ”

In a 1665 publication called Micrographia, experimental scientist Robert Hooke coined the term “cell” for the box-like structures he observed when viewing cork tissue through a lens. In the 1670s, van Leeuwenhoek discovered bacteria and protozoa. Later advances in lenses, microscope construction, and staining techniques enabled other scientists to see some components inside cells.

By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann were studying tissues and proposed the unified cell theory. The unified cell theory states that: all living things are composed of one or more cells; the cell is the basic unit of life; and new cells arise from existing cells. Rudolf Virchow later made important contributions to this theory.

Schleiden and Schwann proposed spontaneous generation as the method for cell origination, but spontaneous generation (also called abiogenesis) was later disproven. Rudolf Virchow famously stated “Omnis cellula e cellula”… “All cells only arise from pre-existing cells. “The parts of the theory that did not have to do with the origin of cells, however, held up to scientific scrutiny and are widely agreed upon by the scientific community today. The generally accepted portions of the modern Cell Theory are as follows:

The cell is the fundamental unit of structure and function in living things.
All organisms are made up of one or more cells.
Cells arise from other cells through cellular division.

The expanded version of the cell theory can also include:

Cells carry genetic material passed to daughter cells during cellular division
All cells are essentially the same in chemical composition
Energy flow (metabolism and biochemistry) occurs within cells

In the late 1600’s, an English scientist named Robert Hook was the first to observe plant cells with a crude microscope. Then, almost a century and a half later, in the 1830’s two German scientists proposed that all living things are composed of cells (Their names were Mathias Schleiden and Theodor Schwann). A German pathologist named Rudolph Virchow extended this idea by contending that cells arise only from other cells.

Since the late 1800’s, cell research has seen astounding gains and provided us with four concepts collectively known as cell theory.

Key Points

The cell theory describes the basic properties of all cells.

The three scientists that contributed to the development of cell theory are Matthias Schleiden, Theodor Schwann, and Rudolf Virchow.

A component of the cell theory is that all living things are composed of one or more cells.

A component of the cell theory is that the cell is the basic unit of life.

A component of the cell theory is that all new cells arise from existing cells.

Key Terms

cell theory: The scientific theory that all living organisms are made of cells as the smallest functional unit.

What is cell theory?

A cell is the basic structural and functional unit of living organisms. When you define cell properties, you define the properties of life.
The activity of an organism depends on both the individual and the collective activities of its cells.
According to the principle of complementarity of structure and function, the biochemical activities of cells are dictated by their shapes or forms, and by the relative number of their specific sub-cellular structures.
Continuity of life from one generation to another has a cellular basis.

Cells are the basis of life. Some connect body parts and store nutrients, others fight disease and transport gases. Some cells gather information and control certain body functions, while specialized cells are used for reproduction.

These concepts will be expanded on as we progress and links will be posted to new material as it’s available. For now, lets begin with the idea that the cell is the smallest living unit. No matter its form, or how it behaves, the cell is a microscopic package that contains all the necessary parts to survive in a changing world. This is why the loss of cellular homeostasis underlies virtually every disease known to man.

There are trillions of cells in the human body. These include over 200 different cell types that vary greatly in size, shape, and function. Red blood cells are disc-shaped, nerve cells branch, and kidney tubule cells are cubed. These are just a few examples of the shape cells take. Cells vary in length as well – ranging from 2 micrometers in the smallest cells to over a meter in the nerve cells you wiggle your toes with. Generally, a cell’s shape reflects its function. For example, the epithelial cells that line the inside of your cheek are flat and fit closely together like floor tile, forming a living barrier that protects underlying tissues from bacterial invasion.

Regardless of the type, all cells are mainly composed of carbon, nitrogen, hydrogen, oxygen, and trace amounts of a few other elements. In addition, all cells have the same basic parts and some common functions. Because of this, it is possible to speak of a generalized, or composite, cell.

Three basic parts of a cell

The plasma membrane: the outer boundary of the cell.
The cytoplasm: the intracellular fluid packed with organelles, small structures that perform specific cell functions.
The nucleus: an organelle that controls cellular activities. Typically the nucleus resides near the cell’s center.

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