Cells are the fundamental units of life, and as such are the smallest distinct elements of living things that retain all of the key properties associated with living things, including metabolism, the ability to reproduce and a means of maintaining chemical balance. Cells are either prokaryotic, a term referring to bacteria and a smattering of single-celled organisms, or eukaryotic, which refers to plants, fungi and animals.
Bacterial and other prokaryotic cells are far simpler in almost every way than their eukaryotic counterparts. All cells at a minimum include a plasma membrane, cytoplasm and genetic material in the form of DNA. While eukaryotic cells feature a wide variety of elements beyond these essentials, these three things account for almost the entirety of bacterial cells. Bacterial cells, however, do include a few features that eukaryotic cells no not, most notably a cell wall.
A single eukaryotic organism can have trillions of cells, although yeast are unicellular; bacterial cells, on the other hand, have only one cell. Whereas eukaryotic cells include a variety of membrane-bound organelles, such as the nucleus, mitochondria (in animals), chloroplasts (plants' answer to mitochondria), Golgi bodies, the endoplasmic reticulum and lysosomes, bacterial cells have no organelles. Both eukaryotes and prokaryotes include ribosomes, the tiny structures responsible for protein synthesis, but these are typically more easily visualized in eukaryotes because so many of them cluster along the linear, ribbon-like endoplasmic reticulum.
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It is easy to regard bacterial cells, and bacteria themselves, as "primitive," owing to both their greater evolutionary age (about 3.5 billion years, vs. about 1.5 billion for prokaryotes) and their simplicity. This, however, is misleading for a number of reasons. One is that, from the sheer standpoint of species survival, more complex does not necessarily mean more robust; in all likelihood, bacteria as a group will outlast humans and other "higher" organisms once conditions on Earth change sufficiently. A second reason is that bacterial cells, though simple, have evolved a variety of potent survival mechanisms that eukaryotes have not.
A Bacterial Cell Primer
Bacterial cells come in three basic shapes: rod-like (the bacilli), round (cocci), and spiral-shaped (spirilli). These morphological bacterial cell characteristics can be handy in diagnosing infectious diseases caused by known bacteria. For example, "strep throat" is causes by species of Streptococci, which, as the name implies, are round, as are Staphylococci. Anthrax is caused by a large bacillus, and Lyme disease is caused by a spirochete, which is spiral-shaped. In addition to the varying shapes of individual cells, bacterial cells tend to be found in clusters, the structure of which varies depending on the species in question. Some rods and cocci grow in long chains, while certain other cocci are found in clusters somewhat reminiscent of the shape of individual cells.
Most bacterial cells can, unlike viruses, live independently of other organisms, and are not reliant on other living things for metabolic or reproductive needs. Exceptions, however, do exist; some species of Rickettsiae and Chlamydiae are obligately intracellular, meaning that they have no option but to inhabit the cells of living things to survive.
Bacterial cells' lack of a nucleus is the reason prokaryotic cells were originally distinguished from eukaryotic cells, as this difference is evident even under microscopes of comparatively low magnification power. Bacterial DNA, while not surrounded by a nuclear membrane like that of eukaryotes, nevertheless tends to cluster closely, and the resultant rough formation is called a nucleoid. There is considerably less DNA overall in bacterial cells than in eukaryotic cells; if stretched end to end, a single copy of the typical eukaryrote's genetic material, or chromatin, would stretch to about 1 millimeter, whereas that of a bacteria would span about 1 to 2 micrometers – a 500- to 1,000-fold difference. The genetic material of eukaryotes includes both DNA itself and proteins called histones, whereas prokaryotic DNA has a few polyamines (nitrogen compounds) and magnesium ions associated with it.
The Bacterial Cell Wall
Perhaps the most obvious structural difference between bacterial cells and other cells is the fact that bacteria possess cell walls. These walls, made out of peptidoglycan molecules, lie just outside the cell membrane, which cells of all types feature. Peptidoglycans consist of a combination of polysaccharide sugars and protein components; their main job is to add protection and rigidity to the bacteria and offer an anchoring point for structures such as pili and flagella, which originate in the cell membrane and extend through the cell wall to the external environment.
If you were a microbiologist operating in a bygone century and wanted to create a drug that would be dangerous to bacterial cells while mostly harmless to human cells, and had knowledge of the respective structures of these organisms' cellular composition, you might go about this by designing or finding substances that are toxic to cell walls while sparing other cell components. In fact, this is precisely how a lot of antibiotics operate: They target and destroy the bacteria cell walls, killing the bacteria as a result. Penicillins, which emerged in the early 1940s as the first class of antibiotics, act by inhibiting the synthesis of the peptidoglycans that make up the cell walls of some, but not all, bacteria. They do this by inactivating an enzyme that catalyzes a process called cross-linking in susceptible bacteria. Over the years, antibiotic administration has selected for bacteria that happen to produce substances called beta-lactamases, which target the "invading" penicillins. Thus a longstanding and never-ending "arms race" remains in effect between antibiotics and their tiny, disease-causing targets.
Flagella, Pili and Endospores
Some bacteria feature external structures that assist the bacteria in their navigation of the physical world. For example, flagella (singular: flagellum) are whip-like appendages that provide a means of locomotion for bacteria that possess them, similar to that of tadpoles. Sometimes they are found at one end of a bacterial cell; some bacteria have them at both ends. The flagella "beat" much like a propeller does, allowing bacteria to "chase" nutrients, "escape" from toxic chemicals or move toward light (some bacteria, called cyanobacteria, rely on photosynthesis for energy like plants do and thus require regular exposure to light).
Pili (singular: pilus), are structurally similar to flagella, as they are hairlike projections extending outward from the bacterial cell surface. Their function, however, is different. Rather than aiding in locomotion, pili help bacteria attach themselves to other cells and surfaces of various compositions, including rocks, your intestines and even the enamel of your teeth. In other words, they offer "stickiness" to bacteria in the way the characteristic shells of barnacles allow these organisms to adhere to rocks. Without pili, many pathogenic (i.e., disease-causing) bacteria are not infectious, because they cannot adhere to host tissues. A specialized type of pili are used for a process called conjugation, in which two bacteria exchange portions of DNA.
A rather diabolical construct of certain bacteria are endospores. Bacillus and Clostridium species can produce these spores, which are highly heat-resistant, dehydrated and inactive versions of normal bacterial cells that are created inside the cells. They contain their own complete genome and all metabolic enzymes. The key feature of the endospore is its complex protective spore coat. The disease botulism is caused by a Clostridium botulinum endospore, which secretes a deadly substance called an endotoxin.
Bacteria produce by a process called binary fission, which simply means splitting in half and creating a pair of cells that are each genetically identical to the parent cell. This asexual form of reproduction is in sharp contrast to the reproduction of eukaryotes, which is sexual in that it involves two parent organisms contributing an equal amount of genetic material to create an offspring. While sexual reproduction on the surface may seem cumbersome – after all, why introduce this energetically costly step if cells can just split in half instead? – it is an absolute assurance of genetic diversity, and this kind of diversity is essential to species survival.
Think about it: If every human being were genetically identical or even close, especially at the level of enzymes and proteins you cannot see but that serve vital metabolic functions, then a single type of biological adversary would be sufficient to potentially wipe out all of humankind. You already know that humans differ in their genetic susceptibility to certain things, from the major (some people can die from exposure to small exposures to allergens, including peanuts and bee venom) to the relatively trivial (some people cannot digest the sugar lactase, making them unable to consume dairy products without serious disruptions to their gastrointestinal systems). A species that enjoys a great deal of genetic diversity is largely protected from extinction, because this diversity offers the raw material on which favorable natural selection pressures can act. If 10 percent of the population of a given species happens to be immune to a certain virus that the species has yet to experience, this is a mere quirk. If, on the other hand, the virus manifests itself in this population, it might not be long before this happenstance 10 percent represents 100 percent of surviving organisms in this species.
As a result, bacteria have evolved a number of methods for ensuring genetic diversity. These include transformation, conjugation and transduction. Not all bacterial cells can make use of all of these processes, but between them, they allow all bacterial species to survive to a far greater extent than they would otherwise.
Transformation is the process of taking up DNA from the environment, and it is divided into natural and artificial forms. In natural transformation, DNA from dead bacteria is internalized via the cell membrane, scavenger-style, and incorporated into the DNA of the surviving bacteria. In artificial transformation, scientists intentionally introduce DNA into a host bacterium, often E. coli (because this species has a small, simple genome that is easily manipulated) in order to study these organisms or create a desired bacterial product. Often, the introduced DNA is from a plasmid, a naturally occurring ring of bacterial DNA.
Conjugation is the process by which one bacterium uses a pilus or pili to "inject" DNA into a second bacterium via direct contact. The transmitted DNA may, as with artificial transformation, be a plasmid or it may be a different fragment. The newly introduced DNA may include a vital gene that codes for proteins allowing for antibiotic resistance.
Finally, transduction relies on the presence of an invading virus called a bacteriophage. Viruses rely on living cells to replicate because, although they possess genetic material, they lack the machinery to make copies of it. These bacteriophages place their own genetic material into the DNA of the bacteria they invade and direct the bacteria to make more phages, the genomes of which then contain a mix of the original bacterial DNA and the bacteriophage DNA. When these new bacteriophage leave the cell, they can invade other bacteria and transmit the DNA acquired from the previous host into the new bacterial cell.