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COMPARISON OF EUKARYOTIC AND PROCARYOTIC CELL STRUCTURE

COMPARISON OF EUKARYOTIC AND

PROCARYOTIC CELL STRUCTURE

 

Eukaryotic Cell Structure

The following structures are associated with eukaryotic cells (see Table 1-1 and Figure I-I). In the diagnostic microbiology laboratory, the eukaryotic cell type occurs in medically important fungi and in parasites.

 

Cytoplasmic Structures

The nucleus of the eukaryotic cell contains the DNA of the cell in the form of discrete chromosomes (structures in the nucleus that carry genetic information; the genes). They are covered with basic proteins

called histones. The number of chromosomes in the nucleus varies according to the particular organism. A rounded, refractile body called a nucleolus is also located within the nucleus. The nucleolus is the site of

ribosomal RNA synthesis. The nucleus is bounded by a bilayered lipoprotein nuclear membrane.

The ER is a system of membranes that occur throughout the cytoplasm. It is found in two forms. The “rough” ER is covered with ribosomes, the site of protein synthesis. It is the ribosomes that give the ER the rough appearance. The smooth ER does not have  ribosomes on the outer surface of its membrane, hence the smooth appearance. Smooth ER does not synthesize proteins, but it does synthesize phospholipids (like rough ER). The major function of the Golgi apparatus or complex is to modify and package proteins sent to it by the rough ER depending on the protein’s final destination. Eukaryotic ribosomes, where protein synthesis occurs, are 80S in size and dissociate into two subunits: 60S and 40S. They are attached to the rough ER.Eukaryotic cells contain several membrane-enclosed organelles. Mitochondria are the main sites of energy production. They contain their own DNA and the electron transport system that produces energy for cell functions. Lysosomes contain hydrolytic enzymes for degradation of macromolecules and microorganisms within the cell. Peroxisomes contain protective enzymes that break down hydrogen peroxide and other peroxides generated within the cell. Chloroplasts, found in plant cells, are the sites of photosynthesis. Chloroplasts are the sites of energy production. Photosynthesis produces glucose from carbon dioxide and water. Fungi are not plants and therefore

have no chloroplasts.

 

Cell Envelope Structures

Plasma Membrane

The plasma membrane (PM) (Figure 1-2) is a phospholipid bilayer with embedded proteins that envelops

the cytoplasm and regulates transport of macromolecules into and out of the cell. A substantial amount of cholesterol is found. Cholesterol has a stabilizing effect and helps keep the membrane fluid. The polar

heads of the phospholipids are hydrophilic (water loving) and lie on both the intracellular and extracellular

fluids; their nonpolar tails are hydrophobic (water hating) and avoid water by lining up in the center of the PM “tail to tail.” It is this type of hydrophobic makeup of the interior of the PM that makes it potentially impermeable to water-soluble molecules. Proteins perform several important functions of the membrane. They may act as enzymes, hormone receptors, pore channels, and carriers. The presence of sterols is also a trait of eukaryotic cell membranes.

 

Cell Wall

The function of a cell wall is to provide rigidity and strength to the exterior of the cell. Most eukaryotic cells do not have cell walls. Fungi, however, have cell walls principally made of polysaccharides such as chitin, mannan, and glucan. Chitin is a distinct component of fungal cell walls.

Motility Organelles

Cilia are short projections (3 to 10 11m),usually numerous, that extend from the cell surface and are used for

locomotion. They are found in certain protozoa and in ciliated epithelial cells of the respiratory tract. Flagella are longer projections (> 150 !Am) used for locomotion by cells such as spermatozoa. The basal body, or kinetosome, is a small structure located at the base of cilia or flagella, where microtubule proteins involved in movement originate.

 

Prokaryotic Cell Structure

The bacterial cell is smaller and less compartmentalized than the eukaryotic cell. A variety of structures are, however, unique to prokaryotic cells (see Figure 1-1).

Cytoplasmic Structures

Bacteria do not contain a membrane-bound nucleus. Their genome consists of a single circular chromosome.

This appears as a diffuse nucleoid or chromatin body (nuclear body), which is attached to a mesosome, a saclike structure in the cell membrane. Bacterial ribosomes are found free in the cytoplasm and attached to the cytoplasmic membrane. They are 70S in size and dissociate into two subunits, 50S and 30S in size (see Table 1 1). The S stands for Svedberg units that refer to sedimentation rates during highspeed centrifugation. Stained bacteria sometimes reveal the presence of granules in the cytoplasm (cytoplasmic granules). These granules are storage deposits and may consist of polysaccharides such as glycogen, lipids such as poly-13-hydroxybutyrate, or polyphosphates. Certain genera, such as Bacillus and Clostridium, produce endospores in response to harsh environmental conditions. Endospores are small, dormant, asexual spores that develop inside the bacterial cell as a means of survival, although they do become vegetative when the harsh conditions are removed. Their thick protein coat makes them highly resistant to chemical agents, temperature change, starvation, dehydration, ultraviolet and gamma radiation, and desiccation. Spores appear as highly refractile bodies in the cell. Spores

are visualized microscopically as unstained areas in a cell with the use of traditional bacterial stains or by using specific spore stains. Schaeffer-Fulton is the most commonly used endospore stain. The size, shape, and interior

location of the spore can be used as identifying characteristics.

Cell Envelope Structures

The cell envelope consists of the membrane and structures surrounding the cytoplasm. In bacteria, these are the cell membrane and the cell wall. Some species also produce capsules and slime layers.

Plasma Membrane (Cell Membrane)

The plasma membrane is a phospholipid bilayer with embedded proteins that envelop the cytoplasm. The prokaryotic plasma membrane is made of phospholipids and proteins but does not contain sterols,

unlike eukaryotic plasma membranes (except for Mycoplasma). The plasma membrane acts as an osmotic barrier (prokaryotes have a high osmotic pressure inside the cell) and is the location of the electron transport chain, where energy is generated. The general functions of the prokaryotic plasma membrane are identical to those in eukaryotes (see Figure 1-2).

Cell Wall

The cell wall of prokaryotes is a rigid structure that maintains the shape of the cell and prevents bursting of the cell from the high osmotic pressure inside it. There are several different types of cell wall structures in bacteria, which have traditionally been categorized according to their staining characteristics. The two major types of cell walls are the gram-positive and the gram-negative types (Figure 1-3). [n addition, mycobacteria have a modified gram-positive cell wall called an acid-fast cell wall, although they do stain gram-positive and mycoplasmas have no cell wall. Gram-Positive Cell Wall The gram-positive cell wall is composed of a very thick protective peptidoglycan (murein) layer. Because the peptidoglycan layer is the principal component of the gram-positive cell wall, many antibiotics effective against gram-positive organisms (e.g., penicillin) act by preventing synthesis of peptidoglycan. Gram-negative bacteria, which have a thinner layer of peptidoglycan and a different cell wall structure, are less affected by these antibiotics. The peptidoglycan or murein layer consists of glycan (polysaccharide) chains of alternating N-acetyl-c1- glucosamine (NAG) and N-acetyl-d-muramic acid (NAM) (Figure 1-4). Short peptides, each consisting of four amino acid residues, are attached to a carboxyl group on each NAM residue, The chains are then cross-linked to form a thick network via a peptide bridge (varying in number of peptides) connected to the tetrapeptides on the NAM. Other components of the gram positive cell wall that penetrate to the exterior of the cell are teichoic acid (anchored to the peptidoglycan) and Iipoteichoic acid (anchored to the plasma membrane). These two components are unique to the gram-positive cell wall (see Figure 1-3). Other antigenic polysaccharides may be present on the surface of the peptidoglycan layer. Acid-Fast Cell Wall Certain genera (Mycobacterium and Nocardia) have a gram-positive cell wall structure but, in addition, contain a waxy layer of glycolipids and fatty acids (mycolic acid) bound to the exterior of the cell wall. More than 60′){,of the cell wall is lipid, and the major lipid component is mycolic acid. This is a strong “hydrophobic” molecule that forms a lipid shell around the organism and affects its permeability. This makes Mycobacterium spp. difficult to stain with the Gram  . Because of their gram-positive nature, they stain a faint blue (gram-positive) color. The mycobacteria and nocardiae are best stained with an acid-fast stain, in which the bacteria are stained with carbolfuchsin, followed by acid-alcohol as a decolorizer. Other bacteria are decolorized by acid-alcohol, whereas mycobacteria and nocardiae retain the stain. They have

therefore been designated acid-fast bacteria. Gram-Negative Cell Wall The cell wall of gramnegative microorganisms is composed of two layers. The inner peptidoglycan layer is much thinner than in gram-positive cell walls. Outside the peptidoglycan layer is an additional outer membrane unique to the gram-negative cell wall. The outer membrane containsproteins, phospholipids, and lipopolysaccharide (LPS) (see Figure 1-3). LPS contains three regions: an antigenic a-specific polysaccharide, a core polysaccharide, and an inner lipid A (also called endotoxin). The lipid A moiety is responsible for producing fever and shock conditions in patients infected with gramnegative bacteria. The outer membrane functions in the following ways: · It acts as a barrier to hydrophobic compounds and harmful substances. · It acts as a sieve, allowing water-soluble molecules to enter through protein-lined channels called porins. · It provides attachment sites that enhance attachment to host cells. Between the outer membrane and the inner membrane, and encompassing the thin peptidoglycan layer, is an area referred to as the peri plasmic space. Within the peri plasmic space is a gel-like matrix containing nutrient-binding proteins and degradative and detoxifying enzymes. Absence of Cell Wall Prokaryotes that belong to the Mycoplasma and Ureaplasma genera are unique in that they lack a cell wall and contain sterols in their cell membranes. Because they lack the rigidity of the cell wall, they are seen in a variety of shapes microscopically. Gram-positive and gram-negative cells can lose their cell walls and grow as L-forms in media supplemented with serum or sugar to prevent osmotic rupture of the cell membrane.

Surface Polymers

A variety of pathogenic bacteria produce a discrete organized covering termed a capsule. Capsules are usually made of polysaccharide polymers, although they may also be made of polypeptides. Capsules act as virulence factors in helping the pathogen evadephagocytosis. During identification of certain bacteriaby serologic typing, capsules sometimes must beremoved in order to detect the somatic (cell wall) antigens present underneath them. Capsule removal is accomplished by boiling a suspension of the microorganism. Salmonella typhi must have its capsular (Vi) antigen removed in order for the technologist to observe agglutination with Salmonella somatic (0)

antisera. The capsule does not ordinarily stain withuse of common laboratory stains, such as Gram or India ink. Instead, it appears as a clear area (“halolike”) between or surrounding the stained organism and the stained amorphous background material in a direct smear from a clinical specimen. Slime layers are similar to capsules but are more diffuse layers surrounding the cell. They also aremade of polysaccharides and serve either to inhibitphagocytosis or, in some cases, to aid in adherence to host tissue or synthetic implants.

Cell Appendages The flagellum is the organ of locomotion. Flagella are exterior protein filaments that

rotate and cause bacteria to be motile. Bacterial species vary in their possession of flagella from none (nonmotile) to many (Figure 1-5). Flagella that extend from one end of the bacterium are polar. Polar flagella

may occur singly at one or both ends or multiply in tufts. Flagella that occur on all sides of the bacterium are peritrichous. The number and arrangement of flagella are sometimes used for identification purposes.Flagella can be visualized microscopically with special flagellum stains. Pili (plural of pilus), also known as conjugation

 

pili,

are nonmotile, long, hollow protein tubes that connect two bacterial cells and mediate DNA exchange.

Fimbriae (plural of fimbria) are nonflagellar, sticky, proteinaceous, hairlike appendages that adhere somebacterial cells to one another, and to environmental surfaces.

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