12 Chapter 1 unity of the microbial world

Some microorganisms are prokaryotic and others are eukaryotic (Table 1-2). All eubacteria (commonly referred to simply as bacteria) and archaebacteria (often called Archaea) are prokaryotic cells. (The differences between eubacteria and archaebacteria are discussed in Chapter 3.) All other living organisms are composed of eukaryotic cells. Three of the major groups of microorganisms— fungi, algae, and protozoa— are composed of eukaryotic cells. Plant cells and animal cells, including human cells, are eukaryotic. The difference between prokaryotic and eukaryotic cells sets the eubacteria and archaebacteria apart from all other organisms in a fundamental way.

All eubacteria and archaebacteria are prokaryotic cells that lack membrane-bound organelles; all other living organisms are composed of eukaryotic cells that contain membrane-bound organelles.

Acellular Nonliving Viruses

So far we have considered organisms that have cells and clearly meet all the criteria of living systems. These criteria include a high degree of organization, ability to exchange materials with their surroundings, ability to transform energy, ability to grow, and ability to reproduce independent progeny. Some microorganisms, however, do not have cells and do not meet all these criteria. Viruses are acellular (noncellular) microorganisms, meaning that they have neither prokaryotic nor eukaryotic cells. By some criteria, these organisms give the appearance of being alive, but by many other criteria they clearly are nonliving entities.

Viruses lack the fundamental structure of living systems. Viruses have a genetic molecule — which

may be either DNA or RNA (ribonucleic acid)—surounded by a protein coat (FIG. 1-5). Although the viral genetic molecule is capable of directing viral reproduction, viruses do not have the cellular support structures and metabolic machinery necessary to perform other life functions. They depend on living host cells to produce the materials necessary for ther replication. Although some viruses are surrounded by an envelope composed of a membrane that the obtain from a host cell in which they replicate viruses do not have functional plasma membrans separating them from their surroundings. They have no means of carrying out independent metabolism or replication.

Viruses rely entirely on the metabolic activities of living cells to provide energy and materials for their replication. On their own, viruses are inanimate оbjects. They passively interact with their environment and are unable to reproduce themselves. They do not transform energy, carry out metabolism, or active respond to their environment. All of these are essential characteristics of living systems. Thereto viruses can be considered as nonliving. However when viruses are able to enter (infect) living cells, the viral nucleic acid molecule has the capability of directing the replication of the complete virus. With the confines of a living cell, the genetic information the viral nucleic acid takes over control of the metabolic activities of that cell. Many microbiologis, therefore, view viruses as genetic extensions of the host cells in which they replicate.