What is Cell Theory? - Definition & Parts

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  • 0:00 Cell Theory
  • 1:00 Living Things are Made…
  • 1:26 Cells are the Basic…
  • 2:03 Cells Come from Other Cells
  • 2:38 Lesson Summary
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Lesson Transcript
Instructor: Lisa Roundy

Lisa has taught at all levels from kindergarten to college and has a master's degree in human relations.

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'. 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.

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