Functional Anatomy of the Prokaryotic Cell

Prokaryote means, “before the nucleus”. Prokaryotic cells are simpler cells than

eukaryotes, but they are still able to carry on life processes.

Basic Characteristics of prokaryotes

• DNA in a prokaryote is not membrane bound. In other words, there is no nucleus.

• Unlike eukaryotes, which generally have multiple linear chromosomes, prokaryotes have one circular chromosome.

• Prokaryotes do not have membrane-bound organelles.• The cell walls of prokaryotes almost always contain

structure called peptidoglycan.• Prokaryotes divide by binary fission.

Structures External to Cell Wall

• Prokaryotes have three different types of appendages that Prokaryotes can have.– 1. Flagella for motility– 2. Fimbriae for attachment– 3. Pili for DNA transfer

• Prokaryotes use only flagella for motility. However, keep in mind that not all prokaryotes are motile.

• Flagella can be arranged 4 different ways in Prokaryotes.

• 1. Monotrichous, meaning single polar flagellum.• 2. Amphitrichous, meaning tuft of flagella at each

end of cell• 3. Lophotrichous, meaning two or more flagella at

one pole (or end of the cell)• 4. Peritrichous, meaning the flagella are distributed

over entire cell, much like cilia would be on a eukaryotic cell.

– Flagella in prokaryotes move in a circular motion like a propellar instead of the whip like motion used by eukaryotic flagella.

• Motility in Prokaryotes consists of “runs” and “tumbles” (See Fig. 4.5 in your textbook for an explanation of this concept.)– Motility enables the microbe to move towards favorable

conditions or away from unfavorable conditions.– Don’t be fooled though. Microbes are not “thinking”

entities. They move as a result of chemical messages that the cell receives from the environment.

• Spirochetes, a particular shape of bacteria, us a periplasmic flagella for motility.– It is anchored at one end of the cell and rotates to produce

propelling motion similar to a cork screw or winding telephone cord. Imagine taking a telephone cord and winding it until it can’t wind any more. When you let it go, it would rapidly unwind and move in many directions. This is similar to how the periplasmic flagellum works.

• Fimbriae are hair-like appendages used for attachment. There are generally many of them that surround the cell.

• Again, not all prokaryotes have fimbriae.– Neisseria gonorrhoeae uses fimbriae to colonize mucous

membranes. It is the organism responsible for gonorrhoea.

• Pili: (sometimes fimbriae and pili used interchangeably in some textbooks) – Pili are usually longer than fimbriae and number only 1 or 2 per

cell.– Pili are long hollow tubes that can attach one cell to another for a

brief period of time.– Join bacterial cells in preparation to transfer DNA from one cell to

another (also referred to as sex pili)– The Pilus is the actual tube through which the DNA is transferred.

• Like in eukaryotes, prokaryotes can have a glycocalyx. Again the glycocalyx is a sticky substance that surrounds the cells and is used for attachment. – For example, streptococci that coat your teeth adhere

to the teeth using the glycocalyx.– There are two forms the glycocalyx can take.

• 1.Capsule• Slime layer

– A capsule is a sugar coat that is thick and gummy that surrounds the cell. A capsule is generally an indicator of virulence in bacteria and aids in attachment and colonization of the host.

– A slime layer is a loose shield around the bacteria that helps prevent water and nutrient loss. Slime layers also help form biofilms (layers of bacteria that are impenetrable by antibiotics and other chemicals).

Cell Wall

• Almost all organisms have cell walls– (Mycobacteria is an exception)

• The function of the cell wall in prokaryotes is to prevent the cell from rupturing when the pressure inside cell is greater than pressure outside cell.

• The cell wall is composed of 2 main molecules:– N-acetylmuramic acid (NAM)– N-acetylglucosamine (NAM)– Also known as peptidoglycan.

Cell Wall Cont.• Many disease causing bacteria can be grouped into one of two

categories based on their cell wall structure.• These two groups are called gram positive (G+) and gram negative

(G-).• G + bacteria have a very thick and rigid cell wall it is composed of a

very thick layer of peptidoglycan– Peptidoglycan is made up of repeating units of NAG and NAM bound

together and layered on top of each other. (See fig.4.13 b)• G- bacteria have a more complex cell wall structure. It is composed

of an outer memb. (lipid bylayer) that contains LPS (lipopolysaccharide) on the outside of the outermembrane and phospholipids on the inside of the outer membrane.– Next is a thin layer of peptidoglycan that is loosely attached to the inside

of the outermembrane.– Surrounding the peptidoglycan on the top and bottom is the periplasmic

space– See fig. 4.14 (We will revisit this concept over and over throughout this

class so make sure you understand it.)

Structures Internal to Cell Wall• Cell Membrane (cytoplasmic memb)

– Encloses the cytoplasm of cell. Like eukaryotes, the cytoplasm contains a lot of water, unlike eukaryotes it does not have microtubules or microfilaments.

– It is a typical phospholipid bilayer. Therefore, what is the charge on this membrane?

– It also contains proteins like those discussed with eukaryotes. Keep in mind though that the protein structure is different between eukaryotes and prokaryotes. That’s an important concept when talking about drug treatments for disease.

• Fluid Mosaic Model is what we use to describe the cell membrane. – It state that the membrane is vicous or fluid. It allows the proteins

within the membrane to move throughout the phospholipids freely.• The function of the cell membrane is to selectively allow

materials such as nutrients and wastes to enter and exit.– The cell membrane is also site of cellular respiration.

Structures Internal to Cell Wall

• Cytoplasm: substance w/in cell membrane. – It is 80% water.– It contains the nuclear area (remember there are

no membrane bound organelles inside a prokaryotic cell).

– It contains ribosomes.– It contains inclusions

Structures Internal to Cell Wall• Nuclear area

– The nuclear area contains the single circular chromosome found in prokaryotes.

– Sometimes plasmids are also found in the nuclear area.• Plasmids are small, circular pieces of DNA that contain extra genes

that the cell can use.• Sometimes they carry genes for antibiotic resistance or for toxins• They are also used very frequently for genetic engineering because

they are easily manipulated and can be easily moved into a cell.

• Ribosomes– Ribosomes in prokaryotes are free. – Like ribosomes in eukaryotes, they are the site for protein

synthesis– There is a difference in the structure and mass of the ribosomes in

eukaryotes and prokaryotes. This is important to know because it helps to treat bacterial infections.

Structures Internal to Cell Wall

• Inclusions– Storage for reserve nutrients

• Polysaccharide granules: store glycogen and starch• Lipid inclusions• Sulfur granules• Carboxysomes: store enzymes for carbon fixation from

carbon dioxide• Gas vacuoles: control buoyancy to receive sufficient oxygen,

light and nutrients• Magnetosomes: store iron oxide, act like magnet to reach

attachment sites

Endospores

• Some bacteria, such as Bacillus and Clostridium, form resting cells called endospores upon depletion of nutrients in the environment.

• Endospores are essentially highly durable dehydrated cells, similar to a plant seed. It contains all the genetic information of the original cell and will form a living cell again once the environmental conditions are right.

• Endospores are formed inside the cell and then released into the environment when the cell dies.

• Upon release, endospores can survive extreme heat, lack of water, exposure to many toxic chemicals, and radiation.

• 25 million year old endospores trapped in amber germinated when placed in nutrient media.

Endospores

• Sporulation is the process of endospore formation.• Endospores return to a vegetative state (living cell) by a process called

germination.• Sporulation does not increase the number of cells but rather preserves

the genetic information of the parent cell until conditions are right for it to grow again.

• Endospores are important to the food industry. They are can be responsible for food contamination.– Endospores can survive boiling water for several hours– Endospores can germinate and produce toxins when conditions are right.– For example, botulism is caused by an endspore forming bacteria,

Clostridium botulinum. This organism grows in environments without oxygen. So during the canning process if endospores are introduced to the food and correct cooking temperatures and times are not observed the endospores can survive. Once the cans or bottles are sealed an oxygenless, or anaerobic environment is created. The endospore can now germinate and form toxins. The toxins are responsible for the symptoms and illness caused in botulism.

Shape and Arrangement of Bacterial Cells

• Bacterial cells come in all kinds of shapes and sizes and arrangements.

• Coccus, single ball shaped cell (cocci, many ball shaped cells)– Diplococci, two ball shaped cells connected together.– Streptococci, chains of ball shaped cells.– Tetrads, four ball shaped cells bound together.– Sarcinae, groups of eight ball shaped cells connected

together– Staphylococci, balled shaped bacteria connected

together in what look like grape clusters

• Bacillus (bacilli) is rod shaped bacteria– Diplobacilli, two rods connected together.

– Streptobacilli, chains of rods connected end to end.

– Coccobacilli, short fat rods that look similar to cocci.

– (Bacillus has to meanings. Bacillus, the genus name of bacteria and bacillus, the cellular shape of a bacterium.)

• Other shapes– Vibrio, comma shape.

– Spirilla, spirochetes and others. See figure 4.22 for great diagrams of each shape.

Taxonomy• A new classification system now divides all

life into three domains instead of two. • This classification system is based on the phylogeny of

each group of organisms. Phylogeny is the evolutionary history of a group of organisms. In other words, how are they related?

• New Classification system consists of the 3 Domains:– 1. Eukarya: animals, plants, fungi, protists– 2. Bacteria: pathogenic, nonpathogenic, photoautrophic– 3. Archaea: prokaryotic type single celled organism without

peptidoglycan in it’s cell wall. Interestingly, archaea are more closely related to eukaryotes than to prokaryotes.

• Archaea generally live in extreme environments– Methanogens (produce methane from carbon dioxide and

hydrogen) – Extreme halophiles (live in high salt concentrations)– Hyperthermophiles (hot, acidic environments)– These are all examples of archaea.

• The current scientific theory is that the 3 domains arose from one single organism called the universal ancestor. (See fig. 1.15)

• Classification of each organism is important because it provides us with valuable information, such as cellular shape, metabolic functions, growth characteristics, etc. about each one.

• This is a brief review of the classification system used for all organisms. (I will never ask you to reproduce this list but it will help you to understand how prokaryotes are classified and named.)– Domain– Kingdom– Phylum– Class– Order– Family– Genus– Species

• A Eukaryotic Species is a group of closely related org. that breed among themselves.– Dogs are a great example of this.

• A Prokaryotic Species is a population of cells with similar characteristics (notice, they don’t breed with each other like the eukaryotic species.) – Staphylococcus epidermidis and Staphylococcus aureus are cells

that belong to the same genus, Staphylococcus but the species are different. That means that they might look different from each other when grown on a plate or they might make different enzymes or one might cause disease more frequently than the other.

• A Prokaryotic Strain is a member of same species that is not exactly identical to the rest of the species.– Let’s say that our Staphylococcus aureus develops resistance to

Penicillin. It still has all of the same characteristics of Staph. Aureus but it now has an additional characteristic that sets it apart from the rest of the species.

• Bacterial organisms are named using both Genus and Species names.– Sometimes organisms are named using the name of

scientist who discovered the organism.• For examaple, Eschericia coli discovered by a scientist who

had the last name Eschericia.

• When writing the name of an organism it is proper to capitalize the genus name and italicize both the genus and species names. If the names are hand written then it is proper to underline both name.

• Now your ready to complete Homework 3. Homework 3 is due Monday, Sept. 11 by midnight.