Info

Animals

Fungi

Plants combining mitochondria and chloroplasts

Animals

Fungi

Nucleus

FIGURE 2.18 Symbiosis with Respiring Bacteria Gives Rise to the Primitive Eukaryote

The ancestor to the eukaryote, or "urkaryote" engulfs a respiring bacterium by surrounding it with an infolding of the cell membrane. Consequently there is now a double membrane around the newly enveloped bacterium. The symbiont, now called a "mitochondrion", divides by fission like a bacterium and provides energy for the primitive eukaryote. The mitochondrion develops infoldings of the inner membrane that increase its energy producing capacity.

Entamoeba A very primitive single-celled eukaryote that lacks mitochondria fermentation A biochemical process that releases energy without oxygen or light Giardia A very primitive single-celled eukaryote that lacks mitochondria

FIGURE 2.19 Entamoeba: an Anaerobic Eukaryote

Some single-celled eukaryotes lack true respiratory mitochondria and must grow by fermentation. Shown here is a false-color transmission electron micrograph of Entamoeba histolytica, a parasitic amoeba, which is ingesting human red blood cells (green ovals). The white/green oval (at left) with a blue and pink central circular area is the nucleus. Entamoeba invades and destroys the tissues of the intestines, causing amoebic dysentery. It may spread to the liver causing abscesses to develop. The infection is acquired through contamination of food or water or through the agency of flies. Magnification: x830. Courtesy of: London School of Hygiene & Tropical Medicine, Science Photo Library.

Biological classification attempts to impose a convenient filing system upon organisms related by continuous evolutionary branching.

Organisms Are Classified

Living organisms have two names, both printed in italics; for example, Escherichia coli or Saccharomyces cerevisiae.The first name refers to the genus (plural, genera), a group of closely related species.After its first use in a publication,the genus name is often abbreviated to a single letter, as in "E. coli." Next comes the species, or individual, name. The genus and species are the smallest subdivision of the system of biological classification. Classification of living organisms facilitates the understanding of their origins and the relationships of their structure and function. The highest level of classification is the domain. There are considered to be three domains:

1. Eubacteria These are prokaryotic cells (traditional bacteria). Interestingly, this group includes the genomes of mitochondria and chloroplasts that have been symbiotically related to eukaryotes.

2. Archaebacteria: From a structural viewpoint, these are prokaryotes like eubacteria in that they lack a nucleus. However, their gene sequences and other biochemical features indicate they are, if anything, slightly more closely related genetically to eukaryotes than to eubacteria.

3. Eukaryotes: Higher organisms whose DNA is carried on several chromosomes which are found inside the nucleus. Their cells are divided into separate compartments and usually contain other organelles in addition to the nucleus. Eukaryotes are divided into four kingdoms:

Protoctista—An accumulation of primitive, mostly single-celled eukaryotes often referred to as protists that don't belong to the other three main kingdoms. There are several groups that are distinct enough that some scientists would elevate them in rank to miniature kingdoms.

domain (of life) Highest ranking group into which living creatures are divided, based on the most fundamental genetic properties genus A group of closely related species kingdom Major subdivision of eukaryotic organisms, in particular the plant, fungus and animal kingdoms

Organisms Are Classified 39

Sperm

Sperm

Fertilization

Nucleus

Fertilization

Developing embryo

Organisms Are Classified 39

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