One of the hallmark characteristics of living things is that they perform chemical reactions. These reactions are collectively known as metabolism. Cells, the basic units of life, can perform many of these metabolic reactions. In a multicelled organism, the cells group together to form tissues that perform the same functions. Tissues group together to form organs, and finally, several organs exist together in a system. In this lesson, we will see how and why this hierarchy is established.
The Basic Unit of Life
All organisms from the smallest single-celled protists to huge whales and giant redwood trees are based on tiny microscopic cells. The types and number of cells may vary, but the cell is the basic unit of life.
The cell is the minimum amount of organized living matter complex enough to carry out the functions of life as outlined in Lessons 1 and 2. In the most basic sense, a cell is made of a gelatinous living substance we call protoplasm, which contains many small structures, all surrounded by a membrane.
The cell membrane separates the living cell from the rest of the environment. However, this membrane is not just a static solid wall. It must allow food molecules and oxygen to enter and wastes to exit. Thus, the cell membrane is semipermeable because it allows some things to pass through, but not others. It must also communicate and associate with the membranes of other cells.
Inside the cell membrane is a substance called protoplasm in which many tiny structures called organelles (because they act like small versions of organs) are suspended. Some of the more important organelles and their functions are listed in the following table
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