EXTERNAL COMPOSITION OF A PROKARYOTIC CELL

PLASMA MEMBRANE

CELL WALL

GLYCOCALYX

CAPSULE

SLIME LAYER

FLAGELLUM

SEX PILUS

FIMBRAE

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  1. PLASMA MEMBRANE: All cells (Prokaryote and Eukaryote) have a plasma membrane; it is a baggie-like structure that holds the organelles inside of the cell. Any substance that can rupture the plasma membrane will kill the whole organism; therefore this structure is carefully studied. Alcohol, soaps, and other detergents easily rupture the plasma membrane.

The plasma membrane is semipermiable; it has pores in it that allow some substances to come and go (oxygen and water molecules), but does not allow other things to get inside or leave. Therefore it regulates the flow of nutrients in the cell. It allows low molecular weight (small sized) substances (such as water) to get in and out depending on their concentration within the cell and outside of it. This is called diffusion, and does not require the cell to expend any energy.

Inside all living cells, there is a certain amount of salt; the cytoplasm of bacteria contains 0.9% NaCl (salt). Water follows wherever salt is. If a cell is soaked in a salt solution (hypertonic solution), the concentration of salt outside of the cell is higher than the inside of the cell, so the water will follow salt out, and the cell will shrink. Conversely, if you soak a cell in pure water (hypotonic solution), there will be more salt inside of the cell, so water will diffuse into the cell, causing the cell to explode (osmotic shock). When a cell dies, it is called necrosis. The cell wall of bacteria is rigid and protects the organism from osmotic shock. Normally, there is equilibrium inside and outside of the cell (isotonic solution).

The plasma membrane is composed of a phospholipid bilayer. This means that there are two layers of a compound consisting of phosphates and lipids (fats). In the diagram below, the blue circle is the phosphate and the black line is the chain of lipids. The plasma membrane is bilayered, so there are two sets of these structures. They face each other at the lipid chain. Therefore, the outer and inner sides of the membrane are water soluble, and the area between is not water soluble. This gives the membrane semipermiablity, which allows it to take in certain substances and keep out other substances.

Embedded within the phospholipids bilayer are lipoproteins (LP), made of lipid (fat) and proteins. These special proteins can transport larger molecules (like sugars) directly into the cell, like allowing someone in through a revolving door. This is called active transport. It requires the cell to spend some energy in the form of ATP.

The plasma membrane is also the site of enzymes for energy production in the cell.

Gram negative organisms have an inner and an outer plasma membrane, whereas Gram positive organisms only have one plasma membrane. In Gram negative organisms, the outer plasma membrane contains a special structure called a lipopolysachharide (LPS), which means it is made of lipids (fats) and many sugars (polysaccharides). The LPS of the plasma membrane in bacteria is recognized as a foreign element by our immune system (white blood cells: WBC’s). This makes it an antigen (something our immune system sees as foreign and needs to be destroyed). This particular antigen (the LPS unit) is referred to as an O antigen. However, it is a weak antigen, and does not stimulate much of an immune response. Within the LPS membrane is a toxin called Lipid A. It is toxic when it is released from the LPS unit, but not while it is attached. This is important to keep in mind when designing antibiotics that attack the plasma membrane of bacteria, possibly releasing this toxin. The natural immune response can also release it. The inner from the outer plasma membrane in a Gram negative bacterium is separated by a cell wall.

  1. CELL WALL: Surrounds the plasma membrane, gives it structure and shape. It is more complex in Prokaryotes (bacteria) than in Eukaryotes (humans). It keeps the organism from exploding from osmotic shock (too much water entering into the cell). The cell wall is composed of peptidoglycan, which is a combination of peptide (protein) and glycan (sugar). Peptidoglycan is only found in bacteria, not in any other organism. Therefore, this structure is important to study because we can create antibiotics that attack peptidoglycan and it will not harm the cells of the patient. Peptidoglycan consists of a chain of two types of sugars (NAM and NAG) linked by proteins. The sugars are arranged in this order: NAG-NAM-NAG. This creates rigidity to help prevent osmotic lysis (rupture), and helps maintain the shape of the cell. Mycoplasma (causes TB or leprosy, depending on the species) is the only bacteria without a normal cell wall (its cell wall is 60% waxy). It is neither Gram-positive nor Gram-negative. It is called “Acid-fast” because it takes an acidic stain to color it.

The cell wall of a Gram positive bacterium is different than a Gram negative bacterium, and the antibiotics to attack each type of cell wall are different. Gram positive organisms have much more peptidoglycan than Gram negatives. The peptidoglycan is what takes up the Crystal Violet stain. The mechanism of the Gram stain is based on differences in the structure of the cell walls of gram-positive and gram-negative bacteria. Crystal Violet, the primary stain, stains both gram-positive and gram-negative cells purple because the dye enters the cytoplasm of both types of cells. When the iodine is applied, it forms large crystals with the dye that are too large to escape through the cell wall. The application of alcohol dehydrates the peptidoglycan of gram-positive cells to make it more impermeable to the Crystal Violet-iodine. The effect on gram-negative cells is quite different; alcohol dissolves the outer membrane of the gram-negative cells and even leaves small holes in the thin peptidoglycan layer through which the Crystal Violet-iodine complex diffuses. Because gram-negative bacteria are colorless after the alcohol wash, the addition of safranin turns these cells pink. Although gram-positive and gram-negative cells both absorb safranin, the pink color of safranin is masked by the darker purple dye previously absorbed by gram-positive cells.

Not only do Gram negative bacteria have less peptidoglycan than Gram positives, they also have an inner and outer plasma membrane. The outer plasma membrane is external to the cell wall, and the inner plasma membrane is internal to the cell wall.

GRAM POSITIVE CELL WALL / GRAM NEGATIVE CELL WALL
No outer plasma membrane / Inner and outer plasma membrane
Thick peptidoglycan / Thin peptidoglycan
  1. GLYCOCALYX: many prokaryotes secrete on-air surface a substance called the glycocalyx (sugar coat). It is made inside the cell and secreted to the cell surface. If the substances organized and firmly attached to the cell wall, the glycocalyx is described as a capsule. If the substance is unorganized and loosely attached to the cell wall, the glycocalyx is described as a slime layer.
  2. CAPSULE: non-slimy protein (made of polypeptides) or sugars (polysaccharides) covering the bacterium. It is neatly organized. Not every bacterium has a capsule. You can do a capsule stain to see if it’s there. Its purpose is to store nutrients and also to protect it from phagocytosis (ingestion) by our protective white blood cells which are trying to eat it and kill it. Phagocytosis is inhibited by capsules. There are several types of white blood cells. One type is called a monocyte. Its job is to circulate in the blood stream, looking for bacteria and other debris to eat. Once the bacteria is phagocytized (ingested), the WBC releases a sac of enzymes to dissolve it. A monocyte has special receptor cells called chemoreceptors, which sense chemicals emitted by bacteria. If a monocyte senses a chemical made by bacteria in the tissues, it squeezes out of the blood vessel and travels in the tissues to the bacteria and phagocytizes it there. When a monocyte leaves the vessel to enter the tissues, it is called a macrophage. Sometimes, a macrophage is able to phagocytize a bacterium with a capsule (for example a tuberculosis organism in the lungs). The bacterium can then live inside the macrophage because the capsule also protects it from the destructive enzymes. The WBC now has become an infected host to the bacteria. The body will try to surround the infected WBC with calcium deposits to kill the WBC, but although the WBC dies, the bacteria go on living in this calcified nodule. An x-ray of a TB patient will show these nodules in the lungs. Another example is Streptococcus pneumoniae: some strains have a capsule, and so a virulent (cause disease), and other strains do not have a capsule, so they are avirulent (do not cause disease). A strain is a subtype or a variation of the typical organism. There can be thousands of strains of one bacterium. Some may cause disease and some do not. The capsule itself is an antigen, called the K antigen. It stimulates an immune response.
  1. SLIME LAYER: slimy protein covering the entire bacterium. Not neatly organized. Not every bacterium has a slime layer. The function of the slime layer is to attach to some structure in the host. An example is the bacteria in the mouth. Oral bacteria frequently have a slime layer. That’s why your teeth feel slimy. They break down sugars that we eat which are left behind on our teeth. For instance, they break down sucrose into its components: glucose and fructose. This process is called fermentation. The products of fermentation are acidic, so they break down the protective enamel on the tooth and cause a cavity.
  1. FLAGELLUM: whip-like tail used for motility. This structure is difficult to see under the microscope in live cells, but you can see the bacteria swimming around. To see flagella, you need a special stain, and that kills the bacterium. It is made of a protein called flagellin. The entire structure consists of a filament, hook, and turning disk within a basil body. It uses ATP for energy to turn the disk, which turns the flagella. The bacterium “decides” which way to move depending on the chemicals it senses in the environment. This is called chemotaxis (chemo = chemical; taxis = movement). It can also move in response to a physical stimulus. Bacteria flagella contain a protein called an H antigen (Flagellar antigen), which our white blood cells recognize as a bad foreign element. When a WBC comes across this antigen it stimulates an immune response to produce antibodies against the bacteria. There is one strain of E. coli called 0157.H7 (weird name!). The letter “O” followed by a number indicates the type of cell wall lipopolysaccharide (LPS) and the H7 indicates the type of flagellar antigen. This strain of E. coli is the main pathogen that you hear about on the news. It produces intestinal bleeding, especially in babies. It is found in cattle feces. If there is an outbreak of bleeding diarrhea that is traced to having eaten spinach at one restaurant, the Center for Disease Control (CDC) has to find out where that spinach came from and pull that product from the grocery store shelves because it probably did not have the fertilizer cleaned off it properly.

Flagella come in various arrangements:

A.Peritricous: Many flagella all around the perimeter of the cell.

B.Lophotrichous: A group of flagella gathered at one end of the cell.

C.Amphitrichous: One flagellum coming out of each end of the cell.

D.Monotrichous: Only one flagellum, comes out of one end of the cell.

Flagella cause various types of motility:

  1. Run: move in a straight line from point A to point B.
  2. Tumble: roll around themselves like a rock tumbling down a slope.
  3. Run and Tumble: Doing both movements alternately.
  1. AXIAL FILAMENTS: These are special flagella found only in a type of bacteria called a spirochete (spiral shaped). The axial filament attaches from the “head” to the “tail end” of the bacterium. When it contracts, it allows the spirochete to move in a motion like a corkscrew. This allows it to penetrate tissue. An example of a spirochete is the bacterium that causes syphilis.
  1. SEX PILUS: longer than flagella. Helps cells connect to each other during conjugation.
  1. FIMBRAE: hair-like structures also made of protein. In Eukaryotes, they are called cilia. In bacteria, fimbrae allow them to attach to the host like roots of a plant in the soil. An example is Neisseria gonorrhoeae (causes gonorrhea). This bacterium has fimbrae that allow them to get into the urinary tract and anchor there. This creates pus and painful urination in the patient.

INTERNAL COMPOSITION OF PROKARYOTE CELLS

1. CYTOPLASM: in prokaryotes, the cytoplasm refers to the watery substance inside of the plasma membrane. It is made up of 80% water and contains proteins (enzymes), carbohydrates, and lipids. It also contains the following:

A. NUCEOID: a nuclear area (prokaryotes have no nucleus). There is only one chromosome, and the DNA is circular instead of linear. The chromosome contains the cell’s genetic information, which carries all of the information required for the cell’s structure and function. Since there is not much DNA, prokaryotes have no histones, which are structures eukaryotes use to organize their DNA by wrapping around it.

B. PLASMIDS: some bacteria contain these small pieces of DNA fragments which are separate from the chromosome. These little plasmids may carry genes for antibiotic resistance, production of toxins, etc. Plasmids can be transferred from one bacterium to another. In fact, plasmid DNA is used for gene manipulation and biotechnology.

C. RIBOSOMES: these are like little factories that make proteins. All eukaryotic and prokaryotic cells contain ribosomes. Cells that have high rates of protein synthesis have more ribosomes than others. The cytoplasm can contain tens of thousands of these ribosomes, which give the cytoplasm a granular appearance. Several antibiotics work by inhibiting the protein synthesis of ribosomes, such as streptomycin, gentamicin, erythromycin, and chloramphenicol.

D. INCLUSIONS are reserve deposits of nutrients within the cytoplasm. These nutrients can be in the form of phosphate, glycogen, starch, and lipids.

2. ENDOSPORES: when essential nutrients are depleted, certain gram-positive bacteria form specialized resting cells called endospores.

An example is Clostridium, which causes diseases such as gangrene, tetanus, botulism, and food poisoning. Another example is Bacillus, some species of which cause anthrax and food poisoning.

Only bacteria make endospores. They are highly durable, dehydrated cells with thick walls. They are formed inside the cell membrane; when released into the environment, they can survive extreme heat, lack of water, and exposure to toxic chemicals and radiation.

One endospore that was estimated to be 7500 years old germinated when it was placed in a nutrient medium. Other endospores which were found fossilized in tree resin have germinated even after 40 million years!

The process of endospore formation within a vegetative (parent) cell is known as sporulation. When a key nutrient becomes unavailable, the cytoplasm of the vegetative cell dries up, the cell wall ruptures, and the endospore is released into the environment. An endospore that is located at one end of the cell is called a terminal endospore. If it is near the end of the cell it is called a sub terminal endospore, and if it is in the center it is called a central endospore. These distinctions make it possible to identify the species of bacteria being viewed. Endospores require a special stain to be visualized.

An endospore returns to its vegetative state by a process called germination. It is triggered by a change in the environment. Water enters into the endospore, and metabolism resumes. Only one cell comes from one endospore, therefore sporulation is not reproduction. Endospores are important from a clinical viewpoint in the food industry because they are resistant to processes that normally kill vegetative cells. Such processes include heating, freezing, desiccation (drying), use of chemicals, and radiation. Endospores can survive in boiling water for several hours or more. Endospore-forming bacteria are a problem in the food industry because they aren't likely to survive under-processing, and if conditions for growth occur, some species produce toxins and disease.