Since we are already familiar with plants and animals, it may be helpful to think about microorganisms in comparison. Like plants and animals, microorganisms (with the exception of viruses) are composed of cells, but in contrast with more complex organisms, bacteria and protists are single-cell organisms. Take a minute to think about the implications of this. Complex organisms organize the processes of life among many cells. Microorganisms must be able to carry out all functions within the confines of a single cell. Bacteria (Prokaryotes) must accomplish this in cells that are ten- to one hundred times smaller than eukaryotic cells.

We noted in previous lectures that the differences between plants and animals stem from a fundamental difference in metabolism. This is also a useful way to differentiate the prokaryotes. Bacteria are characterized by metabolic diversity. Whereas plants and animals are metabolic specialists, bacteria are generalists. They can use a variety of energy sources, including those used by plants and animals (sunlight and organic compounds) as well as chemical energy sources like reduced sulfur compounds. It is not unusual for a single kind of bacterium to utilize two or more alternative energy sources.

As prokaryotes, bacteria are haploid organisms. This means that they have only one copy of each chromosome. Think about chromosomes as the cookbooks that contain recipes for each part of an organism. Diploid organisms have two copies of each recipe (one from each parent) where haploids have only one copy of each recipe. This means that diploids have a backup copy in case anything goes wrong (mutation) but haploids (including the prokaryotes) have a single copy. This is not necessarily a handicap. Haploids can reproduce more efficiently since they only have to replicate a single copy of their genome, and, since any mutation will be visible in a haploid cell, prokaryotes can evolve quite rapidly in the face of environmental change. Most bacteria are also able to exchange genetic material by conjugation or transformation.

Prokaryotes are also characterized by cell structure without internal compartments. Their physiology, although broadly similar to that of eukaryotes, is different enough that some chemicals (antibiotics) disrupt the metabolism of prokaryotes but not eukaryotes. Remember, though, that these organisms are not simple in the sense that they are completely understood, or even that they are uninteresting. Some of the most thought-provoking living things are bacteria. Many show complex behaviors and developmental patterns. Some live in extreme conditions like very high salt concentrations or boiling sulfuric acid. Bacteria are an indispensable part of life for more complex organisms. In addition to playing an indirect role in our lives through decomposition and other biogeochemical cycles, bacteria are important symbionts, inhabiting the skin, digestive and reproductive tracts of humans and other complex organisms. The bacteria inhabiting your colon outnumber the cells in the rest of your body, for example. Some people think of bacteria mainly as disease agents (or pathogens), but in reality, the vast majority of bacteria are beneficial to other living things

More simple yet than bacteria are the viruses. Viruses are not alive - they have no metabolic or reproductive capacity of their own. Instead, they are basically a box of genes (see Figure 18.10), specialized in taking over the metabolism of a host cell. Bacteriophages (viruses that infect bacteria) are useful models for the behavior of viruses. The life cycle of a typical bacteriophage is outlined in Figure 18.12. The virus must attach, insert its genetic material, replicate and assemble viral progeny, and release newly constructed viruses. Note the implications of recognition and attachment for host-specificity.

Protists are single-cell eukaryotic organisms. As eukaryotes, these cells have internal compartmentalization. Although all have single-cell stages of the life-cycle, many also have complex and interesting aggregate or multicellular forms (see the slime mold in Figure 18.16). The internal compartmentalization of eukaryotes allows protists to echo the tissues and organs of animals with organelles devoted to locomotion, digestion, excretion, etc. The animal-like protozoans are the most complex single cells known. Many protists are similar to both plants and animals in their mode of nutrition and metabolism.

Some protists are pathogenic (disease causing). Examples are the parasites that cause malaria and giardia. These eukaryotic parasites are much more difficult to eradicate from the body because, as eukaryotes, their physiology is much closer to that of the host. Antibiotics that exploit the differences between prokaryotes and eukaryotes are not useful against protists. The malarial parasite employs other strategies to avoid the immune system - it spends a great deal of time inside the hosts cells, and therefore out of the reach of the immune system, and it has a number of developmental stages, making it difficult for the immune system if the host to recognize it as any one invader. (See section 18.11 in your text.)