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By the end of this section, you will be able to:
  • Describe the basic structure of a typical prokaryote
  • Describe important differences in structure between Archaea and Bacteria

There are many differences between prokaryotic and eukaryotic cells. However, all cells have four common structures: the plasma membrane, which functions as a barrier for the cell and separates the cell from its environment; the cytoplasm, a jelly-like substance inside the cell; nucleic acids, the genetic material of the cell; and ribosomes, where protein synthesis takes place. Prokaryotes come in various shapes, but many fall into three categories: cocci (spherical), bacilli (rod-shaped), and spirilli (spiral-shaped) ( [link] ).

Part a: The micrograph shows ball-shaped cocci about 0.9 microns long. Part b: The micrograph shows hotdog-shaped bacilli about 2 microns long. Part c: The micrograph shows corkscrew-shaped spirilli that are quite long and 2 microns in diameter.
Prokaryotes fall into three basic categories based on their shape, visualized here using scanning electron microscopy: (a) cocci, or spherical (a pair is shown); (b) bacilli, or rod-shaped; and (c) spirilli, or spiral-shaped. (credit a: modification of work by Janice Haney Carr, Dr. Richard Facklam, CDC; credit c: modification of work by Dr. David Cox; scale-bar data from Matt Russell)

The prokaryotic cell

Recall that prokaryotes ( [link] ) are unicellular organisms that lack organelles or other internal membrane-bound structures. Therefore, they do not have a nucleus but instead generally have a single chromosome—a piece of circular, double-stranded DNA located in an area of the cell called the nucleoid. Most prokaryotes have a cell wall outside the plasma membrane.

In this illustration, the prokaryotic cell is rod shaped. The circular chromosome is concentrated in a region called the nucleoid. The fluid inside the cell is called the cytoplasm. Ribosomes, depicted as small circles, float in the cytoplasm. The cytoplasm is encased by a plasma membrane, which in turn is encased by a cell wall. A capsule surrounds the cell wall. The bacterium depicted has a flagellum protruding from one narrow end. Pili are small protrusions that project from the capsule all over the bacterium, like hair.
The features of a typical prokaryotic cell are shown.

Recall that prokaryotes are divided into two different domains, Bacteria and Archaea, which together with Eukarya, comprise the three domains of life ( [link] ).

 The trunk of the phylogenetic tree is a universal ancestor. The tree forms two branches. One branch leads to the domain bacteria, which includes the phyla proteobacteria, chlamydias, spirochetes, cyanobacteria, and Gram-positive bacteria. The other branch branches again, into the eukarya and archaea domains. Domain archaea includes the phyla euryarchaeotes, crenarchaeotes, nanoarchaeotes, and korarchaeotea.
Bacteria and Archaea are both prokaryotes but differ enough to be placed in separate domains. An ancestor of modern Archaea is believed to have given rise to Eukarya, the third domain of life. Archaeal and bacterial phyla are shown; the evolutionary relationship between these phyla is still open to debate.

The composition of the cell wall differs significantly between the domains Bacteria and Archaea. The composition of their cell walls also differs from the eukaryotic cell walls found in plants (cellulose) or fungi and insects (chitin). The cell wall functions as a protective layer, and it is responsible for the organism’s shape. Some bacteria have an outer capsule    outside the cell wall. Other structures are present in some prokaryotic species, but not in others ( [link] ). For example, the capsule found in some species enables the organism to attach to surfaces, protects it from dehydration and attack by phagocytic cells, and makes pathogens more resistant to our immune responses. Some species also have flagella (singular, flagellum) used for locomotion, and pili (singular, pilus) used for attachment to surfaces. Plasmids, which consist of extra-chromosomal DNA, are also present in many species of bacteria and archaea.

Characteristics of phyla of Bacteria are described in [link] and [link] ; Archaea are described in [link] .

Characteristics of the five phyla of bacteria are described. The first phylum described is proteobacteria, which includes five classes, alpha, beta, gamma, delta and epsilon. Most species of Alpha Proteobacteria are photoautotrophic but some are symbionts of plants and animals, and others are pathogens. Eukaryotic mitochondria are thought be derived from bacteria in this group. Representative species include Rhizobium, a nitrogen-fixing endosymbiont associated with the roots of legumes, and Rickettsia, obligate intracellular parasite that causes typhus and Rocky Mountain Spotted Fever (but not rickets, which is caused by Vitamin C deficiency). A micrograph shows rod-shaped Rickettsia rickettsii inside a much larger eukaryotic cell. Beta Proteobacteria is a diverse group of bacteria. Some species play an important role in the nitrogen cycle. Representative species include Nitrosomas, which oxidize ammonia into nitrate, and Spirillum minus, which causes rat bite fever. A micrograph of spiral-shaped Spirillum minus is shown. Gamma Proteobacteria include many are beneficial symbionts that populate the human gut, as well as familiar human pathogens. Some species from this subgroup oxidize sulfur compounds. Representative species include Escherichia coli, normally beneficial microbe of the human gut, but some strains cause disease; Salmonella, certain strains of which cause food poisoning, and typhoid fever; Yersinia pestis–the causative agent of  Bubonic plague; Psuedomonas aeruganosa– causes lung infections; Vibrio cholera, the causative agent of cholera, and Chromatium–sulfur producing bacteria bacteria that oxidize sulfur, producing H2S. Micrograph shows rod-shaped Vibrio cholera, which are about 1 micron long. Some species of delta Proteobacteria generate a spore-forming fruiting body in adverse conditions. Others reduce sulfate and sulfur. Representative species include Myxobacteria, which generate spore-forming fruiting bodies in adverse conditions and Desulfovibrio vulgaris, an aneorobic, sulfur-reducing bacterium. Micrograph shows a bent rod-shaped Desulfovibrio vulgaris bacterium with a long flagellum. Epsilon Proteobacteria includes many species that inhabit the digestive tract of animals as symbionts or pathogens. Bacteria from this group have been found in deep-sea hydrothermal vents and cold seep habitats. The next phylum described is chlamydias. All members of this group are obligate intracellular parasites of animal cells. Cells walls lack peptidoglycan. Micrograph shows a pap smear of cells infected with Chlamydia trachomatis. Chlamydia infection is the most common sexually transmitted disease and can lead to blindness. All members of the phylum Spirochetes have spiral-shaped cells. Most are free-living anaerobes, but some are pathogenic. Flagella run lengthwise in the periplasmic space between the inner and outer membrane. Representative species include Treponema pallidum, the causative agent of syphilis and Borrelia burgdorferi, the causative agent of Lyme disease Micrograph shows corkscrew-shaped Trepanema pallidum, about 1 micron across. Bacteria in the phylum Cyanobacteria, also known as blue-green algae, obtain their energy through photosynthesis. They are ubiquitous, found in terrestrial, marine, and freshwater environments. Eukaryotic chloroplasts are thought be derived from bacteria in this group.  The cyanobacterium Prochlorococcus is believed to be the most abundant photosynthetic organism on earth, responsible for generating half the world’s oxygen. Micrograph shows a long, thin rod-shaped species called Phormidium. Gram-positive Bacteria have a thick cell wall and lack an outer membrane. Soil-dwelling members of this subgroup decompose organic matter. Some species cause disease. Representative species include Bacillus anthracis, which causes anthrax; Clostridium botulinum, which causes botulism; Clostridium difficile, which causes diarrhea during antibiotic therapy; Streptomyces, from which many antibiotics, including streptomyocin, are derived; and Mycoplasmas, the smallest known bacteria, which lack a cell wall. Some are free-living, and some are pathogenic. Micrograph shows Clostridium difficile, which are rod-shaped and about 3 microns long.
Phylum Proteobacteria is one of up to 52 bacteria phyla. Proteobacteria is further subdivided into five classes, Alpha through Epsilon. (credit “Rickettsia rickettsia”: modification of work by CDC; credit “Spirillum minus”: modification of work by Wolframm Adlassnig; credit “Vibrio cholera”: modification of work by Janice Haney Carr, CDC; credit “Desulfovibrio vulgaris”: modification of work by Graham Bradley; credit “Campylobacter”: modification of work by De Wood, Pooley, USDA, ARS, EMU; scale-bar data from Matt Russell)

Questions & Answers

there are 3 trimester in human pregnancy
ROHIN Reply
I don't know answer of this question can u help me
ROHIN
yes
Bisa
what is a cell
Fatima Reply
what is genetic
Janet Reply
I join
Janet
what are the branchas of biology
Prisca Reply
zoology, ecology
Millicent
biochemistry,cytology,herpetology...etc
R0se
genetics, microbiology,botany and embryology
Muhammad
what is a cell
Kulunbawi Reply
cell is smallest unit of life. cells are often cell the building blocks of life...
Muhammad
the first twenty element
Orapinega Reply
what are the characteristics of living things?
R0se
growth,respiration,nutrition,sensitivity, movement,irritability, excretion,death.
Obinna
What is the difference between adaptation and competition in animals
Adeyemi Reply
What is biology
Adeyemi
it is a natural science stadey about living things
Zamiil
Biology is the bronch of science which deals with the study of life is called biology
Aziz
what is the x in 300 stands for?
Ogbudu Reply
the properties of life
Clarinda Reply
response to the environment, reproduction, homeostasis, growth,energy processing etc.....
Pushpam
hello.
Daniela
hi
MacPeter
Good
Thomas
what is reproduction
Tims
Reproduction is a fundamental feature of all known life,each individual organism exist as a result of re production.....or else Multiplying...
R0se
a complete virus particle known as
Darlington Reply
These are formed from identical protein subunitscalled capsomeres.
Pushpam
fabace family plant name
Pushpam Reply
in eukaryotes ...protein channel name which transport protein ...
Pushpam Reply
in bacteria ...chromosomal dna duplicate structure called
Pushpam
what is a prokaryotic cell and a eukaryotic cell
Matilda Reply
There are two types of cells. Eukaryotic and Prokaryotic cells. Prokaryotic cells don't have a nucleus or membrane enclosed organelles (little organs within that cell). They do however carry genetic material but it's not maintained in the nucleus. Prokaryotic cells are also one celled.
juanita
Prokaryotic cells are one celled (single celled).
juanita
Prokaryotic cells are Bacteria and Archea
juanita
Prokaryotic cells are smaller than Eukaryotic cells.
juanita
Eukaryotic cells are more complex. They are much bigger than Prokaryotic cells.
juanita
Eukaryotic cells have a nucleus and membrane bound organelles.
juanita
Eukaryotic cells are animals cells which also includes us.
juanita
Eukaryotic cells are also multicellular.
juanita
nice explaination
Amna
eukaryotic cells are individual cells .. but eukaryotes are multicellular organisms which consist of many different types of eukaryotic cells
Will
also eukaryotic cells have mitochondria. prokaryotic cells do not
Will
Good
John
in prokaryotes only ribosomes are present... in eukaryotes mitochondria ...glogi bodies ..epidermis .....prokaryotes one envelop but eukaryotes compartment envelop....envelop mean membrane bound organelles......
Pushpam
prokaryotic cell are cells dat have no true nuclei i.e no cell membrane while eukaryotic cell are cell dat have true nuclei i.e have cell membrane
Divine
grt
Thomas
we have 46 pair of somatic cell and 23 pair of chromosomes in our body, pls can someone explain it to me. pls
Matilda Reply
we have 22 pairs of somatic chromosomes and one pair of sex chromosome
Amna
thanks
Matilda
we have 23 pairs of chromosomes,22 pairs of somatic and one pair of sex chromosomes
Amna
23 chromosomes from dad & 23 chromosomes from mom 23 +23=46 total chromosomes
juanita
X & Y chromosomes are called sex cells, the very presence of a Y chromosome means the person is Male.
juanita
XX Female XY Male
juanita
If a Karyotype has more than 46 Chromosomes then nondisjunction occured. For example, having an extra chromosome 21 will cause Down Syndrome.
juanita
in mammal state the different vertebrae and their location in the body
Igbinigie Reply

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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