Yeast are living organisms that belong to the Kingdom Fungi. Species of the yeast genus Saccharomyces are used in baking and brewing. These yeasts metabolize sugars and produce carbon dioxide as a waste product. In fresh bread dough, the accumulated carbon dioxide makes the bread rise and become spongy. When yeast break down sugar in an environment without oxygen, ethyl alcohol also is produced as a waste product.

This activity begins by asking students to predict what might happen when yeast are “fed” table sugar (sucrose, C₁₂H₂₂O₁₁). Sucrose consists of two smaller sugar molecules (monosaccharides): glucose and fructose.

The complete Food and Fitness Activities Guide for Teachers may be downloaded as a PDF file from the Teacher Resources menu on BioEd Online: http://www.bioedonline.org/resources/nsbri.cfm

Classification systems attempt to solve the problem of providing meaningful groupings of organisms.  The Swedish scientist, Carolus von Linnaeus, is credited with introducing binomial nomenclature and hierarchical classification as an organized way of naming and describing organisms and their relationships to one another.  Binomial nomenclature refers to the use of a two-part name for each species (one name designating genus and one designating species).

Linnaeus described a hierarchical classification system using seven taxonomic categories, or taxa (Kingdom, Phylum, Class, Order, Family, Genus, Species).  Beginning with species, each category becomes progressively more comprehensive.  For example, while the leopard, tiger and domestic cat all belong to different genera, they are grouped together in the same family.

Taxonomy is the science of classification.  When taxonomic systems include hypothesized evolutionary relationships among groups, the field generally is referred to as Phylogenetics.  Systematics is a larger field involving classifying organisms based on their phylogenetic relationships.  Systematics can be thought of as the study of biological diversity and how that diversity evolved.  In a sense, Charles Darwin introduced systematics in his revolutionary work, The Origin of Species.  He wrote, "The natural system is founded on descent with modification; that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, and, in so far, all true classification is genealogical" (Darwin, 1859).

Carolus von Linnaeus created a hierarchical classification system using seven taxonomic categories, or taxa (Kingdom, Phylum, Class, Order, Family, Genus, Species).  These categories are based on shared physical characteristics, or phenotypes, within each group.  Beginning with kingdom, each successive level of classification becomes more and more specific.  Organisms within the same order have more in common with one another than organisms within the same class. For example, all species of bears are mammals, but not all mammals are bears.  A useful pneumonic tool to help students remember the hierarchical classification system is: "King Phillip Came Over For Green Soup," with the first letter of each word representing each category, beginning with kingdom and ending with species.

In the 18th Century, organisms were considered to belong to one of two kingdoms, Animalia or Plantae. As biologists gathered more information about the diverse forms of life on Earth, it became evident that the two-kingdom system did not accurately reflect relationships among different groups of organisms, and the number of kingdoms increased. In 1969, Robert Whittaker proposed a five-kingdom system consisting of monerans, protists, fungi, plants and animals. In the last few years, comparative studies of nucleotide sequences of genes coding for ribosomal RNA and other proteins have allowed biologists to recognize important distinctions between bacteria and archaebacteria. The graphic on this slide illustrates the phylogenetic relationships drawn from this information using a three-domain and a six-kingdom arrangement, compared to the traditional five kingdom system.

Most disease-causing organisms, or pathogens, are too small to be seen without a microscope. Some (e.g., most viruses) are even too small to be visible under a light microscope and must be viewed with the more powerful electron microscope. Because of their microscopic size, these minute organisms often are referred to as microbes or microorganisms. The study of these organisms is called microbiology, and scientists who study these organisms are microbiologists. Not all microbes cause disease; many are beneficial and even essential. Bacteria, in the digestive system, for example are important partners in digestion. Microbes that cause disease are sometimes informally referred to as “germs” or “bugs”.

The five main groups of pathogens are bacteria, viruses, protozoa, fungi, and helminths. Bacteria are simple, single-celled organisms that lack an organized nucleus or membrane enclosed organelles. They often have a cell wall (prokaryotes), and their cells usually are rod-shaped or spherical. Commonly known diseases caused by bacteria are diarrheal diseases, pneumonia, strep throat, tuberculosis, and anthrax. 

Viruses are particles of nucleic acid (DNA or RNA) surrounded by a protective coat that replicate within specific host cells and can spread from cell to cell. Infectious diseases caused by viruses include the flu, the common cold, AIDS, chickenpox, and hepatitis. 

Protozoa are single-celled, motile, eukaryotic organisms, found in the Kingdom Protista, that can be human parasites. A protozoan known as Plasmodium (over 170 species), causes malaria, an infectious disease that is one of the world’s top killers.

Fungi are made of eukaryotic cells (organized nucleus and membrane enclosed organelles). All fungi, with the exception of the yeast group, are multi-cellular organisms that absorb nutrients from the environment. Fungi can cause athlete’s foot, sinusitis, skin diseases, and vaginal infections.

Helminths (worms and flukes) are invertebrate animals, some of which are parasitic. Wuchereia bancrofti is transmitted to humans by way of the mosquito. The mature adults pass into lymphatic glands, obstructing lymphatic drainage and resulting in a disfiguring condition, known as elephantiasis.

In our previous presentation about phylogenetic classification, we introduced classifying organisms under a broad three-domain system versus classifying organisms using a five, six, or more kingdom approach. For the purpose of this discussion, we will refer to the traditional five-kingdom system.  Organisms are divided into each of five kingdoms based on defining characteristics, such as: cell type; cell structures; whether the organism is unicellular, multicellular, or has both forms; and nutrition.  As new information is gathered, classifying approaches are constantly being refined.

Bacteria are the most numerous and ancient life forms found on Earth.  They can live in places normally found inhospitable to other organisms (too cold, too dark, too hot, etc.).  Bacteria are unicellular organisms that do not contain a nucleus or internal compartments, and their genome does not contain introns.  Most species of bacteria can be assigned to two groups, based on the amount of peptidoglycan found in their cell walls.  Bacteria with a thick layer of peptidoglycan in their cell walls are called "gram-positive" because they retain a blue color after staining (following a technique developed by Christian Gram.)  Bacteria with a thin layer of peptidoglycan sandwiched between other layers stain orange-red following the same procedure and are called "gram-negative."  The three most common shapes of bacteria are spherical (cocci), rod (bacilli), and helices (spirilla).

The number of ways that bacteria can obtain nutrition and respire contributes to their ability to inhabit so many diverse places on Earth.  To obtain energy and carbon, bacteria can be photoautotrophic- harness light energy to drive metabolic processes and use CO2 as a carbon source, while others are chemoautotrophic- oxidize inorganic substances for energy and use CO2 as a carbon source, photoheterotrophic- use light to generate energy but obtain carbon from other organic molecules, or chemoheterotrophic- consume organic molecules for both energy and carbon.   The chemoheterotrophs include saprobes, decomposers that absorb their nutrients from the body fluids of living hosts.  Bacteria also form many diverse symbiotic relationships with other organisms.

Bacteria exhibit wide variation in their use of oxygen and can be classified based on their dependence upon it.  Obligate aerobes must have oxygen for cellular respiration; facultative anaerobes use oxygen if it is present, but also can grow by fermentation in an anaerobic environment.  Obligate anaerobes can not tolerate oxygen at any level. 

Bacterial reproduction normally occurs asexually by binary fission.  Bacteria do have the ability to transfer genes or segments of genes, and they do so using three mechanisms: conjugation, transformation and transduction.  Conjugation involves the direct transfer of genetic material between prokaryotes.  In transformation, the cells absorb fragments of DNA from the surrounding environment (even from other species).  Transduction occurs when bacterial viruses play a role in transferring genetic material between prokaryotes. 

These abilities, along with a rapid reproductive rate, leaves little surprise as to why bacteria are "masters" of change and adaptation.

A research team led by Carl Woese at the University of Illinois, first recognized the distinction between bacteria and archaea, also known as archaebacteria.  By analyzing RNA in subunits of ribosomes, they defined the early branching of the prokaryotes into Archaea and Eubacteria.  In addition to their unique composition of ribosomal RNA, archaea also are distinguished by the lack of peptidoglycan in their cell walls and their unusual membrane lipids not found in other organisms.  Unlike traditional bacteria, archaebacterial genes contain introns similar to those found in eukaryotes.

Archaea live in the most extreme or harsh environments on Earth and are classified based on the environment in which they can be found.  Methanogens produce energy from organic compounds in the presence of carbon dioxide, nitrogen and water.  They produce methane and can not live in an oxygen-containing environment.  Thermophiles live in very hot water found in areas around hot springs and ocean hydrothermal vents, and Halophiles are found in water with a high saline content, like the Great Salt Lake in Utah.

Members of the microbial kingdom Protista originally were defined by structure (mainly unicellular eukaryotes) and by the difficulty to classify them as either plant, fungi or animal.  More recently, the concept of protists was expanded to include certain multicellular organisms such as kelp (Copeland, 1956). Thus defined, members of Protoctista range from microscopic one-celled organisms like dinoflagellates, to multicellular organisms, like seaweed. To untangle this confusing kingdom, biologists now are turning to molecular analysis. 

When following the traditional five- or six-kingdom classification, the Protist group contains all eukaryotes that are not fungi, plants or animals. There are unicellular, colonial, and multicellular forms, some of which show cell specialization.  Protists groups include both autotrophs and heterotrophs, some of which function as detrivores.

Animal-like groups are often referred to as Protozoans.  The term Protozoa dates back to when members of this group were considered "first animals."  Plant-like forms are generally called algae.

Traits such as method of motility, presence or absence of a shell, manner of obtaining nutrition, and reproducing, are used to categorize and discuss this diverse group, but it is important to remember that these traits do not necessarily reflect evolutionary history.  Recent work suggests that green and red algae are more closely allied with land plants, and that slime molds are more closely allied to animals (Baldauf, et al. 2000).

Members of the kingdom Fungi are eukaryotic, heterotrophic, multicelled organisms (except for yeasts).  Examples of fungi are "mushrooms," puffballs, bracket fungi, molds, and mildews.  Fungi cell walls contain chitin, which is the same material found in the exoskeletons of arthropods.  Fungi are important decomposers in ecosystems as they break down organic materials such as dead organisms, leaves, old wood, and feces.

The body of the multicellular fungi consists of long, slender hyphae, some of which can specialize to hold spores, to anchor its body, to secrete enzymes, and to absorb decomposing material.  Fungi break down potential food sources by excreting strong hydrolytic enzymes (exoenzymes).  Once the food is broken down into smaller molecules, the fungi then absorb them into their bodies. 
 
Fungi are generally described and grouped according to the way they reproduce.  Reproduction can be both sexual and asexual, producing spores that disperse by wind or water.