MICROBIOLOGY 101/102 INTERNET TEXT

CHAPTER II: EUKARYOTIC vs. PROKARYOTIC CELLS

Updated: 12/16/99

GLOSSARIES

The Basic Principle of Life
- Eukaryotic vs Prokaryotic Cells

THE UNIVERSAL PRINCIPLE OF LIFE

Life is based on the universal principle of the SPECIFIC INTERACTION OF MOLECULES WITH EACH OTHER. Interacting molecules interact as pairs, one of which is called the LIGAND and the other the RECEPTOR. These ligand/receptor interactions involve associations of atoms that attract one another so as to stick or bind the molecules together through a variety of #weak bonds. The specificity is determined by the SHAPE of the interactive regions on the ligand/receptor pairs that are similar to interactions between a LOCK and its KEY or the pieces of a puzzle. Imagine a lock/key system that when the key is inserted into the lock it strongly sticks in place so it is difficult to remove. Imagine further, that the interaction between the lock/key cause one or both items to actually change their shape ("morph") and that in doing so some event is triggered to occur, like a bomb going off or a light going on or a door opening etc.

The cartoon above illustrates a number of basic ligand/receptor interactions that will be described and discussed throughout the remaining chapters. If you apply this principle to every biological phenomenon described in this course, you will find that understanding them becomes relatively easy. Further, you will find that your future understanding of biological phenomenon will present little problem for you. That is, you will be able to constantly amaze yourself and your friends with your brilliance.

EUKARYOTIC vs. PROKARYOTIC CELLS

All cellular life has the following characteristics in common.

All cells have a CELL MEMBRANE that separates the CHAOS outside a cell from the high degree of organization within the cell. A cell without a cell membrane is #NOT A CELL.
All cellular life CONTAINS DNA as its genetic material. All cells contain several varieties of RNA molecules and PROTEINS, most of the latter are enzymes.
All cells are composed of the same BASIC CHEMICALS: carbohydrates, proteins, nucleic acids, minerals, fats and vitamins.
All cells REGULATE the flow of nutrients and wastes that enter and leave the cell.
All cells REPRODUCE and are the result of reproduction.
All cells require a SUPPLY OF ENERGY.
All cells are HIGHLY REGULATED by ELABORATE SENSING SYSTEMS (chemical "noses") that allow them to be aware of every reaction that is occurring within them and many of the environmental conditions around them; this information is continually PROCESSED to make metabolic decisions.
- For example: Many flowers close up at night and open during the day. How do they know when the sun is up?
- When you were developing in your mother's uterus how did the cell that eventually became your eyes know to turn into an eye and in the right place at the correct moment in fetal development and relative to the other cells around it that will be turning into something else?

The above criteria are the MINIMAL REQUIREMENTS of life. TWO GENERAL CELL TYPES have evolved. These are called PROKARYOTIC and EUKARYOTIC cells. Current data supports the theory that PROKARYOTIC cells represent the initial or PRIMITIVE cell type on earth and that EUKARYOTIC cell types evolved from them.

Prokaryotic cells are LESS COMPLEX than eukaryotic cells at several levels. Eukaryotic cells are structurally & biochemically more COMPLEX and are considered to represent a later stage of evolution. There is strong data to support the idea that Eukaryotic cells EVOLVED FROM aggregates of Prokaryotic cells that became interdependent upon one another and eventually MERGED or FUSED into a single larger cell. Eukaryotic cells have a HIGHER DEGREE OF ORGANIZATION than do Prokaryotic cells, in that they contain many ORGANELLES or structures separated from the other cytoplasm components by a membrane, whereas prokaryotic cells contain no organelles. The major eukaryotic organelles are:

The NUCLEUS, which contains the DNA and the enzymes for DNA synthesis and the process of #transcription.

Figure 1A. Eukaryotic cell structure. The large structure on the bottom right is the eukaryotic nucleus, containing the cell's DNA. Mitochondria, which are approximately the SIZE OF BACTERIA and which are surrounded by their OWN MEMBRANE, lie outside of the nuclear membrane in the cytoplasm. The tiny dark bodies in the cytoplasm are ribosomes, and starch granules.

EM of Muscle Cell
Figure 1B. Eukaryotic muscle cell. This EM photograph shows muscle fibers surrounded by mitochondria and glycogen granules. The latter are converted to ATP by the mitochondria to supply the muscle fibers with energy.

Generalized Eukaryotic cell
Figure 3. Composite eukaryotic cell structure. This represents a composite eukaryotic cell. For example, chloroplasts are ABSENT in animal cells. In lab exercises #1 & 2 you will see a number of different microbial eukaryotes. You should be able to see some of their organelles such as the nucleus and chloroplasts.

MITOCHONDRIA are the ENERGY PRODUCING structures of Eukaryotic cells and supply the cells with #ATP. The mitochondria contain some DNA and some protein synthesizing machinery that resembles similar Prokaryotic machinery (Fig. 1A & B). The human mitochondria are inherited entirely from the mother (the egg) and thus changes (mutations) in mitochondrial DNA are traced through the mother only.

CHLOROPLASTS which allow plants to TRAP SUNLIGHT AS ENERGY and to carry out PHOTOSYNTHESIS. It is PHOTOSYNTHESIS, the process of using the trapped sunlight to fix carbon dioxide in the form of sugar, that makes human life possible on this planet since plants ultimately provided the food for most Eukaryotes.
GOLGI APPARATUS is a structure in eukaryotic cells that SECRETES substances.

Other cellular components: Click here to see excellent pictures of Eukaryotic cells and their components; unstained and stained with fluorescent dyes.

#RIBOSOMES which are the structures upon which proteins are made. All cells contain these, but they are different in prokaryotes and eukaryotes.
Photosynthetic cells contain forms of the light-trapping pigment, CHLOROPHYLL. Almost all life on earth is dependent upon chlorophyll. There are many photosynthetic bacteria (prokaryotes), but many of them have a mechanism of photosynthesis that is different from that in eukaryotic photosynthetic cells.
Motility structures. Not all cells are motile, but both groups have cells that are. Some cells have #FLAGELLA that are long thin structures that rotate so as to propel the cell; others crawl across solid surfaces and others have stiff fiber-like structures that move like oars. The bacterial flagella is composed of repeating protein subunits that form a stiff organ. The base of the bacterial flagella acts like a motor and contains a flexible "joint". The motor can change direction when necessary, but it only moves forward when the flagella turns in a counterclockwise direction for most bacteria. The eukaryotic flagella is very complex and is composed of several filaments. Click here to see how bacteria swim (requires Quicktime).

protozo1.gif (36394 bytes) .
Figure 4. Protozoan cells. The fine stiff hairs on the outside of these protozoa beat in rhythm to move these cells through liquid. In the first two laboratory exercises you will see a number of such protozoa

Budding Yeast
Figure 5. Actively reproducing yeast cells showing buds and bud-scars. These are actively growing yeast with lots of daughter yeast budding off from the parental cell. Note that every daughter cell leaves a "Bud Scar" behind on the parent yeast cell. What do you think happens when these scars build up to a high number? Click here for a colored version.

BinaryFissionAnimationC2.gif (10808 bytes)
Binary Fission Animation

Most cells divide by a process called BINARY FISSION where a cell divides in half forming two DAUGHTER cells which are smaller duplicates of each other. A few cells reproduce by BUDDING, where the daughter cell grows out of the parent and gradually increases in size. Click here to see bacteria reproducing by binary fission (you will need Quicktime loaded as a helper application).

All cells have their genes arranged in linear chains called CHROMOSOMES. Eukaryotic cells contain 2 (or more) copies of each gene (with some exceptions) carried in duplicate chromosomes. During cell division the chromosomes of eukaryotic cells undergo an organized process of chromosome replication that is visible under the light microscope. This process, called MITOSIS, insures that each daughter cell receives a complete copy of the parental genome. Prokaryotic cells usually contain only a single chromosome and, while its process of replication etc. is also highly organized, it is not visible under the light microscopes.

Figure 6. White blood cell attacking bacterial cells (choose "MEDICAL" and then "MACROPHAGE"). This is a picture of bacteria (the short, plump, curved rods) associated with a eukaryotic cell (upper right). The line on the bottom left represents 1 micrometer. Note the size comparison.

Eukaryotic cells range in size between 2 and 100 micro meters (=10^-6 meters) and are usually much larger than prokaryotic cells which run between 0.5 and 2 micrometers. However, a large prokaryotic cell that is 600 micro meters long has recently been discovered. This bacterium is much larger than the PARAMECIA you observed in lab and can be seen by the naked eye. So far it can't be grown in culture.

Until recently, only eukaryotic cells were thought to exist in MULTICELLULAR coordinated groups such as organs and tissues, but recent evidence indicates that some prokaryotic cells contain DIFFERENTIATED cells in their colonial mass that do different activities important to the survival of the colony. This is not too unexpected when you consider that whatever a bacterium does is regulated by its IMMEDIATE ENVIRONMENT and that the environment in a colony is very different (pH, nutrients, wastes) in different parts of the colony (e.g. the colony center vs the outer edge of the colony).

xeflare1.gif (22133 bytes)
Figure 7. Mobile cells forming flares at the edge of a colony. Some highly motile cells produce aggressively spreading colonies. At the edges of these colonies special large motile cells are formed. These cells move in a coordinated fashion, forming long chains of unconnected giant cells which produce "flares" or streams of giant cells that rapidly move out from the colony edge in this characteristic fashion. Recent data indicate that these flare-cells are differentiated from the cells further back in the colony.

Below are some electron micrographs of various eukaryotic & prokaryotic cells.

Fungi Hyphae and Spores em11a.gif (45141 bytes)
Figure 8. Electron micrographs of fungi (left) and blood cells (right). The LEFT fig. is that of a fungi with reproductive spores on the ends of hyphae. The RIGHT figure is of blood. Most of the cells (hole in center) in the picture are red blood cells (RBC) that carry oxygen to the tissue cells. RBC cells are unusual in that they lack a nucleus, which is discarded after they are formed. Another type of cell is seen in lower left & upper right ; it is probably a white blood cell. Click here for a colored version of mold go to "FUNGI & SLIME MOLDS". Click here for a colored version of blood go to "MEDICAL".

E. coli Cells
Figure 9. Electron micrograph of E. coli cells. These cells are approximately 2 micrometers long and 0.5 to 1.0 micrometers wide. The number of cells in the process of binary fission indicates that this was a rapidly growing culture. Click here and choose "BACTERIA 1 & 2" for colored versions of many bacterial forms.

SUMMARY

In summary, prokaryotic and eukaryotic cells are more alike than different and all the current scientific evidence strongly indicates that all cells are related through evolution. The differences are only in the details, important as those details may be. What are your thoughts on yourself as an AGGREGATE of smaller, prokaryotic cells that have come together as a team?

THE TWO GROUPS OF PROKARYOTES

Prokaryotes are divided into two major taxonomic groups, the EUBACTERIA and the ARCHAEA or ARCHAEBACTERIA. The Eubacteria are the more commonly known form, but since the recognition of the Archaea they have been found in thousands of environments so we can't say which group is the most common on the planet. However, there are no reports of any pathogenic Archaea. The Archaea are generally found in environmental extremes (as far as we're concerned) in that they exist in hot springs at 100^oC, in the bottom of the ocean at ~110^oC & in water the density of gold, in saturated salt ponds and other environments that would seem to be inamicable to life. They can metabolize unusual substances for energy like methane, sulfur, and hydrogen gas. Their origin and relationship to eubacteria and eukaryotes is currently being hotly debated among microbiologists. However, some of their biochemical characteristics resemble eukaryotes more than they do eubacteria.

INTERNET ADDRESS OF INTEREST AND FUN

For another discussion on the similarities and differences between Eukaryotic and Prokaryotic cells click here (good drawings) and follow the directions.

To see dust mite that causes allergies, click here, choose "ARACHRIDS" then "DUST MITE".

To view a number of microbes click here and follow the many links.

To view Bluegreen bacteria which are thought to have been among the first life forms on the earth click here. This site has a lot of interesting URLs, feel free to explore them; a guarantee you'll find some interesting microbes.

Click here for a self assessment test of what you have learned.

Copyright © Dr. R. E. Hurlbert, 1999.
This material may be used for educational purposes only and may not be duplicated for commercial purposes.
SCIENCE HALL, ROOM 440CA
PHONE: 509-335-5108
FAX: 509-335-1907
E-mail address: hurlbert@wsu.edu or hurlbert@pullman.com

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