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Cell theory is a fundamental theory in biology that makes generalizations about cells. This lesson will help you understand the three main principles of cell theory.

Cell Theory

Imagine what it must have felt like to be Robert Hooke when he first saw cork under a microscope. He noticed small compartmentalized areas that reminded him of the small rooms in a monastery. He named these areas ‘cells’.

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We now know much more about these fundamental units of life because of the interest their discovery sparked. Further investigation led to the development of cell theory, or basic generalizations that are accepted by modern scientists about cells. Three main assumptions of cell theory were developed in the mid-1800s and remain a foundation of modern biology. These three assumptions are:

  1. All living things are made up of cells.
  2. Cells are the basic units of structure and function in living things.
  3. Living cells come only from other living cells.

Let’s look at each of these statements in further detail.

Living Things are Made of Cells

The first part of cell theory is the idea that all living things are made of cells. This means that in order for something to be alive, it must either be a cell or made up of many cells.

All animals and plants, including trees, are made of cells. An amoeba is made up of just one cell, but it’s still enough for it to be living. A rock, however, is’nt made up of cells, so it’s not alive.

Cells are the Basic Units of Life

The second part of cell theory states that cells are the basic units of life. They are the fundamental units of a living organism’s structure. They are also the basic units of function. This means that cells carry out all the processes necessary for an organism to survive.

A cell is the smallest part of a living thing that can still be considered alive. One-celled organisms are alive, each individual plant cell is alive, and each individual animal cell is alive. A whole cell as a unit is alive, but any separate part of a cell is not alive by itself.

Cells Come From Other Cells

The third part of cell theory, that living cells come only from other living cells, seems obvious to us today, but this thought wasn’t universally accepted in the 1800’s. At the time, some people still believed that life could be created from spontaneous generation, or simply arising from nonliving matter. We now know that pre-existing cells must divide to make new cells. So for new plant cells to be created, a plant cell must exist to create them, and for a single-celled organism to reproduce, it must divide into two organisms.

Lesson Summary

Three basic generalizations are accepted by modern science about the basic units of life, or cells. These generalizations are called cell theory and are one of the foundations of modern biology. The first part of cell theory states that for something to be alive it must be made up of one or many cells. The second part of cell theory defines cells as the smallest unit that can be considered life. They are the fundamental units of structure in living things and perform the functions necessary for them to survive.

The third part of cell theory is the idea that all cells must be created from other living cells. Cells cannot be created through spontaneous generation from non-living matter.

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