Living cells exist on this planet in two basic patterns: procaryotes and eucaryotes. As we saw in Chapter 23, procaryotes are older and simpler in design, and are represented today only by bacteria and blue-green algae. The eucaryotic pattern is newer and more complex and is found in every other type of living cell: green, red, and other algae, fungi, protozoa, and higher plants and animals.

Both kinds of cells carry out the essential functions outlined at the beginning of Chapter 22: They propagate, grow, metabolize at the expense of their surroundings, use and protect themselves from their environment, and evolve in response to slow changes in the environment. They may go about things in different ways, but they all face similar challenges and have similar goals: to meet those challenges well enough to survive.

Right: Electron micrograph of a bacterium (staphylococcus aureus). Notice the cell wall outside the bacterial membrane.