Two Basic Types Of Cells

Fossil evidence suggests that the earliest cells on Earth were simple cells similar to some present-day bacteria. As cells evolved, they differentiated into two major types: prokaryotes and eukaryotes.

Prokaryotes

Prokaryotes (proh-KAR-ee-OHTS) are organisms that lack a membrane-bound nucleus and membrane-bound organelles. Although prokaryotic cells lack a nucleus, their genetic information—in the form of DNA—is often concentrated in a part of the cell called the nucleoid. Figure 4-7 shows a typical prokaryotic cell. Prokaryotes are divided into two domains: Bacteria and Archaea (ahr-KEE-uh). The domain Bacteria includes organisms that are similar to the first cellular life-forms. The domain Archaea includes organisms that are thought to be more closely related to eukaryotic cells found in all other kingdoms of life.

Eukaryotes

Organisms made up of one or more cells that have a nucleus and membrane-bound organelles are called eukaryotes (yoo-KAR-ee-OHTS). Eukaryotic cells also have a variety of subcellular structures called organelles, well-defined, intracellular bodies that perform specific functions for the cell. Many organelles are surrounded by a membrane. The organelles carry out cellular processes just as a person's pancreas, heart, and other organs carry out a person's life processes. Eukaryotic cells are generally much larger than prokaryotic cells, as seen in Figure 4-8, which shows a white blood cell (eukaryote) destroying tiny bacterial cells (prokaryotes).

figure 4-8

A white blood cell (eukaryotic) changes shape as it attacks purple-stained bacterial cells that are much smaller (prokaryotic).

figure 4-8

A white blood cell (eukaryotic) changes shape as it attacks purple-stained bacterial cells that are much smaller (prokaryotic).

figure 4-9

In a multicellular eukaryotic organisms, cells organize into tissues. Tissues organize into organs. Organs are part of organ systems, in which organs work together to perform body functions.

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