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.

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.

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).

Protists form a broad base across the bottom of the food chain, and they supply approximately one-half of the world's oxygen (unicellular algae compose a large portion of the world's phytoplankton).  Protists, along with bacteria and fungi, are responsible for decomposing and recycling nutrients.

Many protist are helpful.  Euglena are used to help treat sewage because of their unique ability to switch from an autotrophic to a heterotrophic nutritional mode, helping to maintain oxygen levels in the balance.  Another helpful protist is Trichonympha which lives in the digestive system of termites and produces cellulase, an enzyme that enables termites to digest wood.

Animal-like protists are responsible for diseases such as malaria, amoebic dysentery, toxoplasmosis, African Sleeping Sickness and Giardiasis in humans.  Some protists dramatically have affected human history.  Phytopthana infestans, a water mold, destroyed potato crops throughout Ireland in the 1840s, leading to the Great Potato Famine and the eventual migration of large numbers of people into the United States. 

Some protists have medicinal and industrial uses.  Carrageenan, from algae, is used to produce a thickening agent in ice cream, pudding, and candy.  Chemicals from algae are used to manufacture waxes, plastics, paints and lubricants.  Other chemicals made from Protists are used in treatment of ulcers, high blood pressure, and arthritis.

Animals, fungi and some protists, and bacteria are dependent on plants for food and oxygen.  Humans have used plants for medicinal purposes since the emergence of the human mind, from about 1 million to 100,000 years ago.  Today, medicines from plants include heart medications, pain relievers, decongestants, stimulants, and drugs for treatment of cancer.

Plants provide raw materials for construction and many kinds of manufacturing.  Industrial uses for plants are numerous; for example, agriculture is a vital industry throughout the world, and wood is the next most valuable resource.  Just stop for a minute and think of all the products made from wood: paper, rayon, cabinets, guitars, toys, framework for houses, just to name a few.  Wood still is used as fuel for heating and cooking in many parts of the world.  Cotton is one of the world's most important fibers.

It is important to think about plant diversity and the problem of extinction.  Many plants are lost every day to exploding human populations and to the destruction of natural habitats to make room for human settlements.  According to E.O. Wilson, a leading voice for the preservation of biodiversity, ninety-nine percent of all species that ever lived are now extinct.  "An awful symmetry of another kind binds the rise of humanity to the fall of biodiversity: the richest nations preside over the smallest and least interesting biotas, while the poorest nations, burdened by exploding populations and little scientific knowledge, are stewards of the largest," he said. Wilson wrote that biological diversity is the key to the maintenance of the world as we know it.

A light microscope is so named because it uses visible light to produce a magnified image. Compound light microscopes are indispensable to almost any teaching laboratory in biological science, yet many of us have a difficult time using them. Part of the problem is that with any light microscope, a user must select the right magnification, contrast and resolution, position, and focal plane, all at the same time. A second complication stems from the fact that most teaching lab microscopes are designed for bright field viewing only. A good bright field microscope can produce excellent high resolution images. However, many light microscopes are equipped with specialized optics that enhance contrast so that any specimen, living or preserved, can be imaged. 

For satisfactory contrast and resolution, some specimens are best examined using phase contrast or dark field optics. Polarized light provides the basis for differential interference contrast (D.I.C.), which produces three dimensional images. Specialized optics are usually necessary for imaging very small unstained living organisms, such as bacteria or the smallest protists. To maximize their capabilities, most research microscopes are equipped with some combination bright field and specialized optics.

Here, we will explore the features of compound microscopes, principles of imaging, magnification, contrast, and resolution. We also will look at the components of compound light microscopes and their functions.

To measure a vertical dimension using the fine focus, one must be able to focus separately on the top, and on the bottom of a specimen. For example, to measure the depth of liquid under a cover slip, it is necessary to measure the distance from the top of the slide to the bottom of the cover slip. To do so while lowering the focus requires one to focus on the bottom of the cover slip (or on a small object attached to it). While keeping track of the distance traveled, one then focuses on the top of the slide or on a small object on top of the slide.

It is easiest to measure the depth of an object that provides distinctly different cross sectional views from top to bottom. For example, the amoeboid protists known as Difflugia develop a shell, called a test, that encloses the cell. The test is usually textured and curved, and it is semi-transparent. When the organism is active, pseudopodia protrude from an aperture in the base of the test.

Depending on the type of microscope you are using, you will either raise the stage or lower the nosepiece until the object comes into view. When the central part of the test comes into focus, you are looking at the very top. When the aperture appears in focus, you are viewing the bottom of the test. The tips of the pseudopodia are attached to the surface of the slide. When you measure the distance traveled in the vertical direction from the top of the test to the tip of the pseudopodia, you have measured the height of the organism.

I would not rely too heavily on the precision of measurements using a fine focus control. For one thing, depth of focus limits the accuracy with which one can estimate a position. There also is the matter of hysteresis. We have hysteresis when the direction of change has a significant effect on a result. If I focus precisely on a specimen, then de-focus by moving the stage down a precise number of turns, the specimen is not in focus when I move the stage back up the exact number of turns. No machinery is perfect. Hysteresis results from slippage, for example, as a knob is turned. To minimize the effect, one should approach calibrated positions by moving a control in the same direction each time.

Once students have completed their observations, either have them discuss their observations in small groups or conduct a class discussion. Help students to understand the functions of each structure they have located in the cells. Students should have been able to observe the following structures in one or both samples.

• Nucleus - structure in cell that holds hereditary information and is bounded by a membrane. Nuclei are found in plant, animal and protist cells. Usually, they are not usually visible in Elodea samples.
• Cytoplasm - all of the contents that fill the cell between the cell membrane and the nucleus. Plant cells often have a large sac surrounded by a membrane, called the central vacuole, within the cytoplasm. A large central vacuole sometimes causes the nucleus to appear pressed against the cell wall.
• Cell wall - strong wall outside the cell membrane (or plasma membrane) of some cells, such as those of plants. Animal cells do not have cell walls.
• Chloroplasts - Green structures, located within the cytoplasm, in which photosynthesis takes place. Chloroplasts are a kind of organelle, a specialized subunit within a cell that has a specific function and is surrounded by a membrane. Chloroplasts will be present only in the Elodea leaves.

As an assessment, ask students to record the similarities and differences they observed between the two cell samples.

Viewing this presentation fulfills part of the requirements for completing the short course on The Science of Microbes, offered on BioEd Online for professional development contact hours. The Science of Microbes Teacher's Guide may be obtained in its entirety from the Center for Educational Outreach, Baylor College of Medicine (1-800-798-8244).

You can download a PDF of this lesson, including the pre-assessment, from BioEd Online or K8 Science.

The Science of Microbes and accompanying online professional development were supported, in part, by Science Education Partnership Award number 5R25RR018605 from the National Center for Research Resources of the National Institutes of Health (NIH) to Baylor College of Medicine. The unit was developed in partnership with the Baylor-UT Houston Center for AIDS Research, an NIH-funded program (AI036211). The opinions, findings, and conclusions expressed in this presentation are solely those of the authors and do not necessarily reflect the views of Baylor College of Medicine or the sponsoring agencies.

Help students to make connections between the plant cells they just observed and other kinds of cells. Ask for examples of other organisms that consist of many cells (multicellular organisms). Like plants, animals are multicellular and have specialized cell types for different functions. Members of other groups, such as fungi and protists, may be single-celled (unicellular) or multicellular. Bacteria and related groups are exclusively unicellular.

Most organisms on Earth are unicellular and microscopic. Each tiny cell is capable of independent life and exhibits the following properties of living things: specialized structure; hereditary information that is passed to the next generation; adaptation to the environment as a result of natural selection; responsiveness to the environment; ability to process energy; regulation of an internal environment that is different from external environment; growth and development; and reproduction.

Have student groups revisit their concept maps and add information.

Viewing this presentation fulfills part of the requirements for completing the short course on The Science of Microbes, offered on BioEd Online for professional development contact hours. The Science of Microbes Teacher's Guide may be obtained in its entirety from the Center for Educational Outreach, Baylor College of Medicine (1-800-798-8244).

You can download a PDF of this lesson, including the pre-assessment, from BioEd Online or K8 Science.

The Science of Microbes and accompanying online professional development were supported, in part, by Science Education Partnership Award number 5R25RR018605 from the National Center for Research Resources of the National Institutes of Health (NIH) to Baylor College of Medicine. The unit was developed in partnership with the Baylor-UT Houston Center for AIDS Research, an NIH-funded program (AI036211). The opinions, findings, and conclusions expressed in this presentation are solely those of the authors and do not necessarily reflect the views of Baylor College of Medicine or the sponsoring agencies.

Observing Different Microbes is the fourth lesson in the unit, The Science of Microbes. This lesson addresses National Science Education Content Standards related to Inquiry and Life Science. See the downloadable lesson PDF (link below) for a complete list of the standards addressed.

In this activity, students will use a microscope to examine representatives from three different microbe groups: bacteria, fungi and protists. Specifically, students will observe bacteria (in yogurt), baker's yeast cells and paramecia.

Viewing this presentation fulfills part of the requirements for completing the short course on The Science of Microbes, offered on BioEd Online for professional development contact hours. The Science of Microbes Teacher's Guide may be obtained in its entirety from the Center for Educational Outreach, Baylor College of Medicine (1-800-798-8244).

You can download a PDF of this lesson, including the pre-assessment, from BioEd Online or K8 Science.

The Science of Microbes and accompanying online professional development were supported, in part, by Science Education Partnership Award number 5R25RR018605 from the National Center for Research Resources of the National Institutes of Health (NIH) to Baylor College of Medicine. The unit was developed in partnership with the Baylor-UT Houston Center for AIDS Research, an NIH-funded program (AI036211). The opinions, findings, and conclusions expressed in this presentation are solely those of the authors and do not necessarily reflect the views of Baylor College of Medicine or the sponsoring agencies.