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Chapter 16: 1-11

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Single Cells and Microbial Life, including Viruses

WebLecture Topics

The Origin of Life

Evolutionary biologists accept the idea that life arose from non-living materials through a sequence of chemical events that eventually produced a cell: an enclosed, self-perpetuating organism capable of growth by processing nutrition, waste production, and response to its environment. The definition of life as cell-based leads to some interesting issues, since there are certain molecular forms that share growth and reproduction characteristics but not cellular structure with the forms we recognize as "living". These forms — viruses and prions, primarily — cannot be ignored, because their similarities enable them to interact with living cells in ways both advantageous and disastrous.

A note on Viruses

We have already discussed viruses somewhat, but it is useful to review them here. They are not considered living organisms, since they have no true membrane, no nucleus, no cell organelles, and while they may have either DNA or RNA, they do not have both. For some time, biologists thought that viruses might be one of the links between protosymbionts (non-living membrane-encases spheres of RNA) and living cells. However, viruses which are pathogens (disease-causing) are usually species-specific. Now most biologists think viruses had their origin in cells that broke down or bits of cells that escaped from the parent cell.

There are two kinds of viruses. Lytic viruses destroy the host cell by injecting their own DNA into the cell, using the cell's own RNA and ribosomes, and replicating so many times that they use up the cell's own materials. The cell eventually ruptures, the new virus cells escape and attack other cells. Temporate viruses also invade the host cell, but do not necessarily then kill the cell. They inject their own DNA into the host DNA, which continues to help the host cell function as well as perform the new DNA's instructions. Often these include making viruses (in which case, the host cell may lyse or break apart as with the lytic viruses).

A virus which has RNA instead of DNA uses the host's DNA polymerase, reverse transcriptase, to create new DNA. These viruses are called retroviruses, and are the leading cause of viral diseases in humans.

Bacteria or prokaryotes

Prokaryotes differ from all other kinds of living organisms in several ways. Although they have ribosomes (the protein factories usually found embedded in endoplasmic reticulum), prokaryotic cells do not have nuclei, mitochondria, chloroplasts, Golgi bodies, or lysosomes. Their DNA is a single long loop strand which duplicates itself just before a bacterial cell reproduces by binary fission. They often have flagella, cilia, and fimbriae that allow them to move or to stick to surfaces of cells and materials. These structures, while similar in function, do not have the same structure as the microtubule-based flagella and cilia of eukaryotic cells.

Prokaryotes do exchange some DNA materials. In the process of conjugation, both archaea and bacteria make pili or extensions from one individual to another, which the primary cell (the one extending the pili) uses to pump a piece of DNA into the target cell. In transformation, a cell will take up DNA found in the environment (perhaps released by another bacteria). In transduction, viruses inject their own or absorbed DNA or RNA materials into the target cell.

Prokaryotes have membranes similar to those of eukaryotic cells. Bacteria have a cell membrane that is enclosed by a layer of proteins and peptidoglycan, a molecule of sugars linked by peptides (amino acid chains). The amount of peptidoglycan shows up when bacteria are stained with Gram stain; those bacteria with high amounts of peptidoglycan absorb the stain well--they are considered gram-positive cells. Those with a thin peptidoglycan layer don't absorb the stain as readily and are gram-negative. This characteristic is often used to identify a sample of bacteria, but obviously can't be used to identify archaea.

Archaea are found in more extreme habitats — methanogens are anaerobes (can't live in oxygen), halogens live in salt bonds, thermoacidophiles live in hot (boiling) acidic water like the pools at Yellowstone Park. Bacteria live in less exotic places, but do live in MORE environments than archaea. One idea is that archaea are older, but were crowded out of most environments by the more successful (more places, more species) bacteria.

Modern Considerations for Classification

During a transition period over the last few decades, when biologists were moving from a single class (Monera) for prokaryotes to a two-group system, "bacteria" was still used as the general term for both groups, with "eubacteria" referring to those protista with peptidoglycan in their cell walls, and "archaea" bacteria, considered older because it was simpler, identifying protista cells with no peptidoglycan. As more differences were discovered, the names simplified to bacteria and archaea, which are now considered fundamentally different domains.

Modern biology studies often focus on energy and nutrition mechanisms as a way of grouping organisms. For prokaryotes, we consider whether their energy comes from light or chemical bonds, and whether their carbon comes from inorganic or organic sources.


Types of bacteria

Archaebacteria (no peptidoglycan) Found in extreme environments (methane atmosphere, salt concentrations, and high heat) not habitable by other organisms.


Extreme salt environments such as salt lakes


Found in hot springs and near deep-sea vents at temperatures above boiling.


Anaerobic, methane producing; some forms are parasitic.

Eubacteria with gram-negative walls (low peptidoglycan)

This list is intended to help you see the variety; don't worry about memorizing it! Pay attention to the four main groups in the left hand column.


Spiral-shaped, both free-living and symbiotic; includes Lyme disease

Aerobic Rods and cocci

Oxygen-using heterotrophs which use chemical reactions to derive energy directly from simple sugars. Important to nitrogen cycle; one species causes Legionnaire's disease.

Facultative anaerobes

Decomposers that live in no-oxygen atmospheres (compost heaps). Includes E. coli, salmonella, and typhoid fever bacteria.

Rickettsias and chlamydias.

Rickettsias are rod-shaped parasites like Rocky Mountain spotted fever; they live in insects. Chlamydias are spherical, have no peptidoglycan, and are parasites that depend on their host for ATP; they include trachoma (causes blindness).


Saprobes--organisms that feed on decaying organic matter. Similar to slime molds.


Blue-green algae-like organisms which use photosynthesis and fix nitrogen; very important for balanced environment.

Eubacteria with gram-positive walls (high peptidoglycan)



Spherical bacteria found in mucus membranes and digestive tracts of their hosts; can cause sinusitis, ear infections, pneumonia.


Anaerobic bacteria which often produce toxins; includes the bacteria which cause tetanus and botulism.

Lactic acid bacteria

Long filaments. Our old friends from the lactic acid operon; very important because they ferment sugars (used to make yogurt, pickles, black olives).


Rod-shaped; responsible for tuberculosis and leprosy.


Closely resemble fungi (molds) in form and life cycles; but since they have peptidoglycan, they are classed with the bacteria.

Bacteria with no cell walls

Mycoplasmas live in sewage and break down organic matter; cause few diseases.