Because of their small size and deceptively simple cell organization, bacteria were long dismissed as primitive organisms, distinguished mostly by the eukaryotic features they lack: They have no membrane-bounded nuclei, no energy-producing organelles, no sex. This point of view turns out to be more than a little misleading. Bacteria are the diverse and remarkably successful products of nearly 4 billion years of evolution. Today, bacterial cells outnumber eukaryotic cells by several orders of magnitude. Even in your own body, bacteria outnumber human cells 10 to 1.

Fig. 26.1 illustrates the bacterial cell, which was briefly introduced in Chapter 5. The cell’s DNA is present in a single circular chromosome, in contrast to the multiple linear chromosomes characteristic of eukaryotic cells. Many bacteria carry additional DNA in the form of plasmids, small circles of DNA that replicate independently of the cell’s circular chromosome. In general, plasmid DNA is not essential for the cell’s survival, but it may contain genes that have adaptive value under specific environmental conditions. No nuclear membrane separates DNA from the surrounding cytoplasm, and so transcribed mRNA is immediately translated into proteins by ribosomes.

Bacteria lack the membrane-bounded organelles found in eukaryotic cells. Instead, cell processes such as metabolism are carried out by proteins that float freely in the cytoplasm or are embedded in the plasma membrane. A few bacteria, notably the photosynthetic bacteria, contain internal membranes similar to those found in chloroplasts and mitochondria. The light reactions of photosynthetic bacteria take place in association with membranes distributed within the cytoplasm.

Structural support is provided by a cell wall made of peptidoglycan, a complex polymer of sugars and amino acids. Some bacteria have thick walls made up of multiple peptidoglycan layers, while others have thin walls surrounded by an outer layer of lipids. For many years, it was believed that bacteria lacked the cytoskeletal framework that organizes cytoplasm in eukaryotic cells. However, careful studies now show that bacteria do have an internal scaffolding of proteins that plays an important role in determining the shape, polarity, and other spatial properties of bacterial cells (Chapter 10).