2.18: Cell Theory

What are you made of?

Cells make up all living things, including your own body. This picture shows a typical group of cells, but not all cells look alike. Cells can differ in shape and sizes, and the different shapes usually indicate different functions.

Introduction to Cells

A cell is the smallest structural and functional unit of an organism. Some organisms, like bacteria, consist of only one cell. Big organisms, like humans, consist of trillions of cells. Compare a human to a banana. On the outside, they look very different, but if you look close enough you’ll see that their cells are actually very similar.

Observing Cells

Most cells are so small that you cannot see them without the help of a microscope. It was not until 1665 that English scientist Robert Hooke invented a basic light microscope and observed cells for the first time, by looking at a piece of cork. You may use light microscopes in the classroom. You can use a light microscope to see cells (Figure below). But many structures in the cell are too small to see with a light microscope. So, what do you do if you want to see the tiny structures inside of cells?

Figure \(\PageIndex{1}\): The outline of onion cells are visible under a light microscope.

In the 1950s, scientists developed more powerful microscopes. A light microscope sends a beam of light through a specimen, or the object you are studying. A more powerful microscope, called an electron microscope, passes a beam of electrons through the specimen. Sending electrons through a cell allows us to see its smallest parts, even the parts inside the cell (Figure below). Without electron microscopes, we would not know what the inside of a cell looked like.

Figure \(\PageIndex{2}\): An electron microscope allows scientists to see much more detail than a light microscope, as with this sample of pollen.

Cell Theory

In 1858, after using microscopes much better than Hooke’s first microscope, Rudolf Virchow developed the hypothesis that cells only come from other cells. For example, bacteria, which are single-celled organisms, divide in half (after they grow some) to make new bacteria. In the same way, your body makes new cells by dividing the cells you already have. In all cases, cells only come from cells that have existed before. This idea led to the development of one of the most important theories in biology, the cell theory.

Cell theory states that:

1. All organisms are composed of cells.
2. Cells are alive and the basic living units of organization in all organisms.
3. All cells come from other cells.

As with other scientific theories, many hundreds, if not thousands, of experiments support the cell theory. Since Virchow created the theory, no evidence has ever been identified to contradict it.

Specialized Cells

Although cells share many of the same features and structures, they also can be very different (Figure below). Each cell in your body is designed for a specific task. In other words, the cell's function is partly based on the cell's structure. For example:

• Red blood cells are shaped like flat discs to move easily and quickly through blood vessels and deliver oxygen all over the body.
• Nerve cells are long and stringy in order to form a line of communication with other nerve cells, like a wire. Because of this shape, they can quickly send signals, such as the feeling of touching a hot stove, to your brain.
• Skin cells are flat and fit tightly together to protect your body.

As you can see, cells are shaped in ways that help them do their jobs. Multicellular (many-celled) organisms have many types of specialized cells in their bodies.

Figure \(\PageIndex{3}\): Red blood cells (left) are specialized to carry oxygen quickly through the blood. Neurons (center) are shaped to conduct electrical impulses to many other nerve cells. These epidermal cells (right) make up the “skin” of plants. Note how the cells fit tightly together.

Levels of Organization

While cells are the basic units of an organism, groups of cells can perform a job together. These cells are called specialized because they have a special job. Specialized cells can be organized into tissues. For example, your liver cells are organized into liver tissue. Your liver tissue is further organized into an organ, your liver. Organs are formed from two or more specialized tissues working together to perform a job. All organs, from your heart to your liver, are made up of an organized group of tissues.

These organs are part of a larger system, the organ systems. For example, your brain works together with your spinal cord and other nerves to form the nervous system. This organ system must be organized with other organ systems, such as the circulatory system and the digestive system, for your body to work. Organ systems work together to form the entire organism. There are many levels of organization in living things (Figure below).

Figure \(\PageIndex{4}\): Levels of organization, from the atom (smallest) to the organism (largest). Notice that organelles are inside a cell, and organs are inside an organism.

Summary

• Cells were first observed under a light microscope, but today's electron microscopes allow scientists to take a closer look at the inside of cells.
• Cell theory says that:
  • All organisms are composed of cells.
  • Cells are alive and the basic living units of organization in all organisms.
  • All cells come from other cells.
• Cells are organized into tissues, which are organized into organs, which are organized into organ systems, which are organized to create the whole organism.

Review

1. What type of microscope would be best for studying the structures found inside of cells?
2. What are the three basic parts of the cell theory?
3. According the cell theory, can you create a cell by combining molecules in a laboratory? Why or why not?
4. Give an example of a specialized cell.
5. What is a tissue?
6. What is the relationship between tissues and organs?