Cells: Basic Units of Life

Cell Discovery and Cell Theory

The invention of the microscope in the seventeenth century was a great help in the study of living organisms. For the first time, biologists could see beyond the limits of the human eye. The discovery of cells, the smallest living unit that carries on the activities of an organism, would not have been possible without the work of many scientists and the development of the right scientific tools, such as the microscope.

In 1665, an English scientist named Robert Hooke made a simple microscope and looked at a slice of cork, the dead cells of oak bark. He was amazed at the tiny boxlike cavities the cork was made of. Because these cavities reminded him of the cells in which monks live at a monastery, he gave the name “cellulae” (the Latin word meaning “small rooms”) to these tiny units of cork. It is from Hooke’s work that we have the term cell.

Spontaneous Generation

By the mid 1800s, scientists had learned a great deal about cells. For example, they knew that cells were the basic units of plants and animals, and they knew about some structures inside cells. What they did not know was where cells came from. Many people believed that living things arose through spontaneous generation, which is the belief that living things come from nonliving matter. As early as the fifth century BC, philosophers had concluded that living things developed from nonliving elements in nature. Today this idea may seem unrealistic based on what you know. Yet these conclusions were based on people’s observations. Some of the important research involving spontaneous generation is summarized below.

Francesco Redi

Even before the invention of the microscope, some critics had questioned the idea of spontaneous generation. In 1668, the Italian physician Francesco Redi proposed that maggots did not suddenly appear out of nowhere on rotting meat. Instead he believed that maggots came from eggs laid by flies that land on the meat. To test his hypothesis, Redi set out jars: Some of the jars were open to air, some were completely sealed, and some were covered with cheesecloth. Air can pass through cheesecloth, but flies cannot. Redi observed that maggots appeared only in the jars open to air that could be reached by flies.

John Needham

In 1745, an English clergyman named John Needham designed an experiment to test spontaneous generation. He boiled chicken broth to kill the microorganisms in it, and then he put the broth into a flask, sealed it, and waited. In time, microorganisms appeared in the broth. Needham concluded that spontaneous generation did indeed occur. Others thought that organisms in the air had grown in Needham’s broth and that Needham was wrong.

Louis Pasteur

In 1859, a French chemist named Louis Pasteur designed an experiment to test spontaneous generation. He put broth in special S-shaped flasks and boiled it. This made it possible for air to reach the broth inside, but any microorganisms in the air would get stuck in the neck of the flask and be unable to reach the broth. Because no microorganisms grew in this broth, Pasteur knew that spontaneous generation did not occur. To confirm that microorganisms came from particles in the air, Pasteur then tilted the flasks so that air reached the broth without traveling through the bend in the tube. Microorganisms were then found in the broth. In this way, Pasteur showed that microorganisms exist in the air; they do not arise from nonliving matter.

The Cell Theory

Around the same time that Pasteur was conducting his experiments, a German physician named Rudolf Virchow was studying cells. Like Pasteur, Virchow did not support the idea of spontaneous generation. In 1855, Virchow presented a hypothesis that cells are produced only by other cells. This last piece of information about cells helped complete what is now one of the fundamental ideas of modem biology-the cell theory. The cell theory includes the following three principles:

  1. All living organisms are made up of one or more cells.
  2. Cells are the basic units of structure and function in all living organisms.
  3. New cells are produced from existing cells.

Specialized Cells and Cellular Organization

Your community could not function if many different people did not perform specific jobs to make life in the community possible. In thinking about your community, you can subdivide, or organize into smaller units, different groups of people. Groups of people with similar jobs often work together, for example, in a hospital or restaurant.

Just as people perform specialized jobs within a community, cells perform specialized tasks within an organism. Some cells are equipped to do a particular job. These cells are called specialized cells. For example, pancreatic cells produce insulin, and white blood cells fight infection. In most multicellular organisms, cells are organized into tissues, organs, and body systems as shown in the image on page 101.


In most multicellular organisms, groups of similar cells are organized into tissues. There are four types of tissue, and each type has specific functions. Muscle tissues, for example, make the heart beat, the stomach digest food, and the body move. Nerve tissues form long fibers to carry impulses to the brain. Epithelial tissues form linings and coverings for body. Connective tissues form cartilage and ligaments.


A group of tissues can work together to form an organ, which performs a more complex function than individual cells and tissues perform. The heart, brain, and stomach are organs. The heart pumps blood throughout the body. It is made up of muscle tissue, blood tissue, epithelial tissue, and nerve tissue.

Body Systems

Organs that work together to perform a set of related tasks form a body system. Recall that the stomach is one of several organs of the digestive system. Other organs of the digestive system include the esophagus and the intestines. Together, all the body systems in a multicellular organism carry out the processes needed to keep the entire organism alive.

Limits on Cell Size

You may be wondering why many small cells group together in an organism instead of simply growing larger. Most cells are small for a reason. As a cell grows larger, both its volume and its surface area increase. Volume is the amount of space an object takes up. The surface area is a measure of the size of the outer surface of an object. For example, when you wrap a gift box, you cover the surface area of the box. The surface area of a cell determines the amount of material that can enter and leave the cell.

As a cell grows, the volume increases at a faster rate than the surface area. If the cell grows too large, it will require more materials than can pass through its surface area. Therefore there is a limit to how large a cell can grow. Instead of continuing to grow larger, cells form new cells that enable an organism to grow and develop tissues, organs, and body systems. The ratio of a cell’s surface area to its volume is a factor that limits cell size. For example, when the sides of a cube double in length from 1 mm to 2 mm, the surface-area-to-volume ratio decreases from 6:1 to 3:1.