The Cell as an Organism

How do individual cells maintain homeostasis?

Cells are the basic living units of all organisms, but sometimes a single cell is the organism. In fact, in terms of their numbers, unicellular organisms dominate life on Earth. A single-celled organism does everything you would expect a living thing to do. Just like other living things, unicellular organisms must maintain homeostasis, relatively constant internal physical and chemical conditions. To maintain homeostasis, unicellular organisms grow, respond to the environment, transform energy, and reproduce.

Unicellular organisms include both prokaryotes and eukaryotes. Prokaryotes, especially bacteria, are remarkably adaptable. Bacteria live almost everywhere—in the soil, on leaves, in the ocean, in the air, even within the human body.

Many eukaryotes, like the protozoan in Figure 7–21, also spend their lives as single cells. Some types of algae, which contain chloroplasts and are found in oceans, lakes, and streams around the world, are single celled. Yeasts, or unicellular fungi, are also widespread. Yeasts play an important role in breaking down complex nutrients, making them available for other organisms. People use yeasts to make bread and other foods.

Don't make the mistake of thinking that single-celled organisms are always simple. Prokaryote or eukaryote, homeostasis is still an issue for each unicellular organism. That tiny cell in a pond or on the surface of your pencil still needs to find sources of energy or food, to keep concentrations of water and minerals within certain levels, and to respond quickly to changes in its environment. The microscopic world around us is filled with unicellular organisms that are successfully maintaining that homeostatic balance.

FIGURE 7–21 Unicellular Life Single-celled organisms, like this freshwater protozoan, must be able to carry out all of the functions necessary for life (SEM 600X).