Selling the cell, seeking its structure
It's as regular as clockwork -- as reliable as Sesame Street. Every time you have life, you have cells. They go together, in fact, like rock -- and roll.

Cells are the basic units of structure and function in all living organisms. They are the smallest structure that can perform all activities of life. They may be shrimpy, but they can divide and form new cells.

All cells share certain fundamental properties. Cells:

are enclosed by a membrane that regulates the passage of materials between the cell and its surroundings and maintains essential concentrations of chemicals within the cell.

contain DNA that directs cell activities through gene expression, the process of interpreting the DNA code to make proteins. Proteins do the work of the cell and give the organism characteristics such as shape

perform DNA replication, meaning they copy DNA to pass identical copies to their progeny ("daughter") cells. (For some reason -- could it be fear of testosterone? -- cells never have "son cells."

have mechanisms to assimilate nutrients from the outside world and produce chemical energy to run the essential functions of the cell. Some also transform physical energy, as in the way a plant converts light into sugar through photosynthesis.

Nuts to the nucleus
The two major cell types, prokaryotes and eukaryotes, are based on fundamental structural differences. BTW, "kary" is Greek for "nut," or "nucleus."

Prokaryotic cells are simple and primitive, lacking both the nucleus and tiny organs called organelles. Prokaryotes ("before nucleus") comprise two huge domains of microscopic life: Bacteria and Archaea.

Above: The components of a typical rod-shaped bacterium. In addition to the structures discussed in the text, all bacteria have a rigid cell wall for support and ribosomes for the synthesis of proteins. Some bacteria have flagella for locomotion, pili for attachment and some have both.

Eukaryotes ("real nucleus"), include protists, fungi, plants and animals. By definition, these cells have a nucleus (a membrane-bound compartment housing DNA) and compartments (organelles) that carry out metabolism and other important functions. For example, eukaryotic cells process energy in an organelle called the mitochondrion. Cells may have thousands of organelles, all suspended in the cytoplasm, a thick fluid medium between the nucleus and cell wall.

The components of a generalized plant and animal cell, which is more complicated than a prokaryote. Most notably, the DNA is organized within a nucleus, and mitochondria generate energy. The plant cell (below) differs slightly from the animal cell (above), since it has a cell wall for structural support, chloroplasts for photosynthesis and a large vacuole to store chemicals for plant growth.

Meet the midgets
Fed up with "pro" and "eu"? Then let's turn up the volume on the big categories of small life. We'll start with those primitivos that can't even spring for compartments within their cells.
Stained slide of Bacillus anthracis. Stains bring out structures you normally would not see, such as the endospore which is remarkably heat resistant and tolerant to other harmful agents. It can remain dormant for years and germinate into a new bacterium when conditions are appropriate.
Courtesy Ken Todar.

Bacteria and Archaea

The oldest organisms on Earth, the prokaryotes, are positively codgerly, having appeared more than 4 billion years ago. They rule Earth in terms of overall impact: they are responsible for most chemical cycles that sustain life on Earth, such as making elements and nutrients available to other organisms. Prokaryotes are the most pervasive organisms, living in the weirdest environments (like above boiling temperature or locked deep in the rocks a thousand meters below ground), and they rule in terms of sheer numbers.

Halosphaera is a genus of green flagellates living in marine habitats, usually in cold water. Halosphaera also appears as a large planktonic cyst similar to the sporangia of Phytophthora.
©1994-2000 by Charles J. O'Kelly and Tim Littlejohn.

Prokaryotes include two domains: Bacteria (meaning "Bacteria") and Archaea (meaning "the oldsters"). Although similar in some cell structure (lacking a nucleus and organelles), Bacteria and Archaea differ in many key biochemical and physiological characteristics. In fact, the Archaea have at least as much in common with eukaryotes as they do with Bacteria. In terms of numbers, Bacteria account for most prokaryotes, and they are the most diverse functionally, with every major mode of nutrition and metabolism being represented among thousands of known species. About 5,000 species of prokaryotes are known, but the estimates of total prokaryotic diversity range from 400,000 to 4 million species.

Bacteria and Archaea:

lack membrane-bound compartments, such as a nucleus enclosing their DNA.

are small, single-celled organisms, ranging in diameter from 0.5 to 5 microns.

reproduce by binary fission, meaning a parent cell divides into two daughter cells. In this " asexual reproduction," the genome of each daughter is identical to the parent.

almost always have cell walls outside their cell membranes to provide protection and maintain the cell's shape.

can live on a wide range of food sources.

can grow in just about every environment on Earth, although not all grow easily in culture.

some use photosynthesis, just as plants and some protists

Got nucleus?
Now that we've dispensed with most of the cells on Earth, let's look at some microbes that resemble us -- the protists and fungi, both of which stash their DNA in a nucleus.

Protists

Protists were the first eukaryotes to evolve, probably 1.2 billion years ago.

Moldy 'n mealy... must be a Florida orange! The Rhizopus stolonifer fungus on an orange.
Courtesy To Volkm.

They are the most diverse group of eukaryotes in terms of structure and function; few characteristics are common to all species. Protists can use an animal-like nutritional mode (the protozoa), plant-like (the algae), or fungus-like. The 60,000 known species of protists include Phytophthora infestans, a fungus-like organism that caused Irish potato blight.

Most protists:

have organelles and a nucleus enclosing their DNA.

are larger than Bacteria, ranging in diameter from 10 to 100 microns.

take many shapes and colors.

are unicellular (although there are many multicellular species).

are always aquatic and almost always motile (unlike fungi, the other microbial eukaryotes).

reproduce asexually (progeny identical to parent) and sexually (progeny vary genetically).

some are photosynthetic.

Fungi

Fungi are eukaryotes that evolved after the protists, probably around 900 million years ago. They inhabit diverse environments and often live symbiotically with other organisms. Fungi may look like gross mold (which is indeed a fungus), but they're responsible for rot and recycling: they decompose dead organisms and recycle important chemicals back into the environment.

The three domains of life, Archaea, Bacteria, and Prokarya, represent a radically new view of life, based on genetic relationships. Adapted from Carl Woese, University of Illinois.

Fungi also help plants take up minerals from the soil. Of an estimated 1.5 million species worldwide, more than 100,000 species are known. Most fungi:

have a nucleus enclosing their DNA and other organelles.

have cells that are larger than Bacteria (10 to 100 microns in diameter).

are multicellular.

often produce specialized cells for specialized functions, such as hyphae or mycelia for normal growth and spores as resting structures or for dispersal.

are not aquatic and usually not motile (unlike the protists, the other microbial eukaryotes).

reproduce asexually and sexually.

will grow in culture (some require a host plant for all or some of the life cycle).

none are photosynthetic.