A Tour through the Kingdoms of Life...

In olden days, there were only two Kindgoms, Plantae and Animalia.

As humans learned more about organisms, the Kingdoms were divided to reflect true evolutionary relationships.

The Five Kingdom System is still the one you'll most commonly see these days:

• Kingdom Monera (PROKARYOTIC: The Bacteria)
• Kingdom Protista (EUKARYOTIC: The Unicellular & Colonial Eukaryotes)
• Kingdom Fungi (EUKARYOTIC: The Fungi)
• Kingdom Animalia (EUKARYOTIC: The Animals)
• Kingdom Plantae (EUKARYOTIC: The Plants)

More recently, with the advent of electron microscopy and sophisticated DNA analysis techniques, we can hone in more closely on which organisms are truly descended from a common ancestor (i.e., which taxa are MONOPHYLETIC).

The most recent classification system adds a taxonomic level ABOVE THE KINGDOM, the DOMAIN. In this system, there are three domains:

• DOMAIN ARCHAEA - Containing the archaebacteria, formerly in Kingdom Monera
• DOMAIN BACTERIA - Containing the true bacteria, formerly in Kingdom Monera
• DOMAIN EUKARYA - Containing all eukaryotes

Of these, Domain Archaea is believed to be the oldest, and is probably ancestral to the eukaryotes.

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Today, we'll have a quick look at some of the "simplest" organisms...and a few that may not be true "organisms" in the usual sense of the word, the VIRUSES.

VIRUSES

The first virus ever discovered was the tobacco mosaic virus (1883)

A virus is little more than an ornate coat of protein surrounding a nucleic acid genome (set of genes encoding the information needed to manufacture a virus).

A virus with DNA as its nucleic acid is called a virus.

A virus with RNA as its nucleic acid is called a retrovirus.

Viruses cannot reproduce themselves. They can reproduce ONLY by invading a living cell and commandeering the cell's protein machinery.

Animal viruses come in a huge host of types, and have many different ways to infect a cell and replicate within a cell.

Plant viruses are a serious, costly agricultural problem.

Viruses may have evolved from mobile genetic elements that were once part of a living cell.

Other strange, pathogenic (i.e., disease-causing) particles such as VIROIDS (naked fragments of RNA that act as plant pathogens) and PRIONS (proteinaceous infectious particles that cause normal proteins to convert to a pathogenic form that destroys the central nervous system of animals) are even simpler than viruses, and not yet fully understood.

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BACTERIA

There are now two Domains of organisms which were once considered to be of the same lineage, but are now known to be of separate evolutionary origin.

Domain Archaea - the archaebacteria

• methanogens (methane-producing bacteria; strict anaerobes)
• extreme halophiles (salt-loving archaebacteria; residents of very salty habitats)
• extreme thermophiles (heat-loving dwellers of hotsprings)

Domain Bacteria - the "true bacteria"

• oodles of different types, classified on the basis of shape, cell wall structure and other characteristics.

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prokaryotes exist just about everywhere!

almost all true bacteria have a cell wall outside the plasma membrane, and it's made at least partly of peptidoglycan

many bacteria are capable of independent movement

recall: prokaryotic cell structure and DNA form are different from eukaryotic!

prokaryotic populations have the capacity to increase incredibly rapidly, if conditions are conducive to their growth!

prokaryotes are often grouped into four categories, depending on how they obtain their energy and their carbon (for building their organic macromolecules):

• photoautotrophs (photosynthetic, like plants)
• chemoautotrophs (use CO2 to get carbon, but oxidize inorganic substances (e.g., ammonia, iron ions, sulfur compounds) to obtain energy.
• photoheterotrophs (get carbon from organic molecules, but use light energy to generate ATP)
• chemoheterotrophs (get both energy and carbon from organic sources, similar to the way fungi and animals do).

prokaryotes are vital decomposers in all ecosystems

many prokaryotes are symbiotic

many symbiotic prokaryotes cause disease (are pathogens), but most are free-living and harmless.

Koch's Postulates are used to determine whether a bacterium (or anything) is pathogenic:

the same pathogen must be found in each diseased organism

must be able to isolate & grow organism in pure culture from diseased organism sample

disease is caused in a healthy organism by introducing the cultured bacteria into the organism

must be able to isolate the same organism from the newly infected organism

some bacteria are "opportunistic" pathogens--they can cause disease under the right circumstances (e.g., in an immunosuppressed patient)

both harmful and beneficial prokaryotes are economically important!

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PROTISTS

Once subsumed under "Kingdom Protista," these organisms are so numerous and diverse that they almost defy description!

At last count, there were FIVE different main groups that used to be considered "Protista" (they're named Archaezoa, Euglenozoa, Alveolata, Stramenopila and Rhodophyta--and sometimes CHLOROPHYTA is included).

These taxa have been separated on the basis of extremely subtle cellular and sub-cellular structures that are beyond the scope of what we're doing here.

Eukaryotic cells such as those represented by the Protists are believed to have evolved from prokaryotic cells. Currently, there are two main models as to how this could have occurred...

The Autogenous Model (membrane inpocketings gave rise to organelles)

The Endosymbiont Model (smaller prokaryotes took up residents in larger ones, and became inseparably symbiotic)

Let's have a quick tour of some of the protists that used to be considered most closely related to plants....

[And here's the slide show!]