Cells

What is the basic unit of life?

In 1665, a physicist named Robert Hooke used one of the first microscopes to look more closely at the living world. A slice of cork caught his eye. Looking at thousands of tiny chambers, Hooke termed these structures cells because they reminded him of the rooms in a monastery.

Subsequent observations built upon Hooke’s work and led to what is known as the cell theory. The cell theory states the following:

• All living things are made of cells.
• Cells are the basic units of structure and function in living things.
• All cells are produced from existing cells.

Hooke's Microscope.	Cork cells as seen in early microscope.

How is life organized around cells?

An organism may be characterized as having different levels of organization built around the cell. Unicellular organisms are made up of only one cell, while some multicellular organisms are comprised of trillions of cells. In multicellular organisms, groups of cells sharing similar structure form tissues that carry out a specialized function. Different tissue layers form organs. And separate organs often work together to carry out major bodily functions, such as digestion or circulation. These “teams” of organs are referred to as organ systems. Growth of an organism is the result of an increase in cell size and, in multicellular organisms, an increase in the number of cells.

Levels of organization.

How are cells alike?

At first it might seem that the cells of such dissimilar creatures as the tiniest bacterium and a great blue whale would be very different. The cells of any organism, however, demonstrate all the characteristics of life: a life span that includes a living beginning, growth, development, reproduction, and death; the constant use of matter and energy; response to changing internal and external conditions; and the presence of DNA.
Although multicellular organisms can have diverse cell types, all cells have certain parts in common (see the interactive cell picture below). All cells are bound by a cell membrane and contain cytoplasm, which is where the cell parts lie. The nucleus of this cell contains the hereditary molecule DNA, which is also found in all cells. And, all cells contain ribosomes, which are so tiny that they can’t be seen in this cell. Ribosomes are structures that are responsible for constructing proteins — the molecules that carry out the instructions contained in DNA.

Cytoplasm | Nucleus | Cell Membrane

Roll your mouse over the names of the parts to see them in the picture.

How are cells different?

One of the most basic distinctions among life forms involves the cell. In some cells, including those of plants, animals, fungi and protists, the DNA is contained in a nucleus, which is itself a membrane-enclosed body. Cells that contain a nucleus are called eukaryotic cells (“true nucleus”) and the organisms with cells of this type are called eukaryotes. Within the cytoplasm of a eukaryotic cell, there are numerous compartmentalized bodies called organelles. Organelles serve specific purposes within the cell, ranging from building proteins to making energy. Different types of organisms may have different sets of organelles. Organisms that photosynthesize, like plants, contain chloroplasts, for example.

There are cells that aren’t as structurally complex as those with a nucleus. Prokaryotic cells (“before the nucleus”) lack a nucleus and organelles. Most single-celled organisms are prokaryotes that are known as bacteria and archaea. Prokaryotes are still capable of performing many of the same types of functions as eukaryotes. These functions occur in the cytoplasm instead of in organelles. In fact, many bacterial and archaeal cells are capable of carrying out very complicated biochemical reactions that allow them to harvest food and/or energy from very unlikely sources, such as pollutants and poisons.

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