LEH MICRO Chapters 4; Functional anatomy of Prokaryotic and Eukaryotic cells

Eukaryotes

1. Their DNA is found in the cell's nucleus, which is separated from the cytoplasm by a nuclear membrane, and the DNA is found in multiple chromosomes. 2. Their DNA is consistently associated with chromosomal proteins called histones and with nonhistones. 3. They have a number of membrane-enclosed organelles, including-mitochondria, endoplasmic reticulum, Golgi complex, lysosomes, and sometimes chloroplasts. 4. Their cell walls, when present, are chemically simple. 5. Cell division usually involves mitosis, in which chromosomes replicate and an identical set is distributed into each of two nuclei. This process is guided by the mitotic spindle, a football-shaped assembly of micro-tubules. Division of the cytoplasm and other organelles follows so that the two cells produced are identical to each other.

Prokaryotes

1. Their DNA is not enclosed within a membrane and is usually a singular circularly arranged chromosome. 2. Their DNA is not associated with histones (special chromosomal proteins found in eukaryotes);other proteins are associated with the DNA. 3. They lack membrane-enclosed organelles. 4. Their cell walls almost always contain the complex polysaccharide peptidoglycan. 5. They usually divide by binary fission. During the process, the DNA is copied., and the cell splits into two cells. Binary fission involves fewer structures and processes than eukaryotic cell division.

Bacilli

Bacilli divide only across their short axis, so there are fewer groupings of bacilli than of cocci. Most bacilli appear in single rods, called single bacilli. Diplobacilli appear in pairs after division and streptobacilli occur in chains. Some bacilli look like straws. Others have tapered ends, like cigars. Still others have oval and look so much like cocci that they are called coccobacilli. "Bacillus" has two meanings in microbiology. As we have just used it, bacillus refers to a bacterial shape. When capitalized and italicized, it refers to a specific genus. Bacillus cells often form long, twisted chains of cells.

Compare the cell structure of prokaryotes and eukaryotes. (p.73)

Both are chemically similar in that they both contain nucleic acids, proteins, lipids, and carbohydrates. They also use the same kinds of chemical reactions to metabolize food, build proteins, and store energy. It is primarily the structure of cell walls and membranes, and the absence of organelles (specialized cellular structures that have specific functions), that distinguish prokaryotes from eukaryotes.

Cocci

Cocci are usually round but can be oval, elongated, or flattened on one side. When cocci divide to reproduce, the cells remain attached to one another. Cocci that remain in pairs after dividing are called diplococci; those that divide and remain attached in chain-like patterns are called streptococci.. Those that divide in two planes and remain in groups of four are known as tetrads. Those that divide in three planes and remain attached in cube-like groups of eight are called sarcinae. Those that divide in multiple planes and form grapelike clusters or broad sheets are called staphylococci.

Describe the structure and function of the glycocalyx. (pg. 67, 75 and 76)

Glycocalyx is a gelatinous polymer surrounding a cell. Many prokaryotes secrete this substance on their surface. It is not only gelatinous, but sticky. It is external to the cell wall and composed of polysaccharide, polypeptide, or both. It is mostly composed inside of the cell and then secreted to the surface. There are two types of glycocalyx: capsule and slime layer. The glycocalyx is described as a slime layer if the substance is unorganized and only loosely attached to the cell wall. The glycocalx is described as a capsule if the substance is organized and is firmly attached to the cell wall. Capsules often protect pathogenic bacteria from phagocytosis by the cells of the host (phagocytosis is the ingestion and digestion of microorganisms and other solid particles). Glucocalyx is a very important component of biofilms. It helps cells in a biofilm attach to their target environment and to each other. This is called extracellular polymeric substance (EPS). It protects the cells within it, facilitates communication among them , and enables the cells to survive by attaching various surfaces in their natural environment. Glycoclax also can protect a cell against dehydration, and its viscosity may inhibit the movement of nutrients out of the cell.

Compare and contrast the cell walls of gram-positive bacteria, gram-negative bacteria, acid-fast bacteria, archaea, and mycoplasmas. (pg. 65-67, 80-83)

Gram positive cell walls consist of many layers of peptidoglycan, forming a thick, rigid structure. By contrast, gram-negative cell walls contain only a thin layer of peptidoglycan. Acid-Fast Cell Walls: Acid-fast stain is used to identify all bacteria of the genus Mycobacterium and pathogenic species of Nocardia. These bacteria contain high concentrations (60%) of a hydrophobic waxy lipid (mycolic acid) in their cell wall prevents the uptake of dye, including those used in the Gram Stain. The mycolic acid forms a layer outside of a thin layer of peptidoglycan. The mycolic acid and peptidoglycan are held together by a polysccharide. The hydrophobic waxy cell wall causes both cultures of Mycobacterium to clump and to stick to the walls of the flask. Acid-fast bacteria can be stained with carbofuchsin; heating enhances penetration of the stain. The carbolfuchsin penetrates the cell wall, binds to cytoplasm, and resists removal by washing with acid-alcohol. Acid-fast bacteria retain the red color of carbolfuchsin because it is more soluble in the cell wall mycolic acid than in the acid-alcohol. If the mycolic acid layer is removed from the cell wall of acid fast bacteria, they will stain gram-positive with the Gram stain. Mycoplasma: Smallest known bacteria that can grow and reproduce outside living host cells. Because of their size, they have no cell walls, they pass through most bacterial filters and were first mistaken for viruses. Their plasma membranes are unique among bacteria in having lipids called sterols, which are thought to help protect them from lysis (rupture).

Fimbriae and Pili

Hairlike appendages that are shorter, straighter, and thinner than flagella and are used for attachment and transfer of DNA rather than for motility. These structure, which consist of a protein called pilin arranged helically around a central core, are divided into two types: fimbriae and pili, having two very different functions. Fimbriae can occur at the poles of the bacterial cell or can be evenly distributed over the entire surface of the cell. They can number anywhere from a few to several hundred per cell. They have a tendency to adhere to one another and to surfaces. Because of thism they are involved in forming biofilms and other aggregations of the surface of liquids, glass, and rocks. Fimbriae can also help bacteria adhere to epithelial surfaces in the body. The fimbriae of a causative agent of a disease helps the microbe colonize mucous membranes. Once colonization occurs, the bacteria will cause disease. When fimbriae are absent (because of genetic mutation), colonization cannot happen, and no disease ensues. Pili are usually longer than fimbriae and number only one or two per cell. Pili are involved in motility and DNA transfer. In one typ eof motility, called twitching motility, a pilus extends by the addition of subunits of pilin, makes contact with a surface or another cell, and then retracts (power-stroke) as the pilin sub-units are disassembled. This is called the grappling hook model of twitching motility and results in short, jerky, intermittent movements. The other type of motility associated with pili is gliding motility, the smooth gliding movement of myxobacteria. Although the exact mechanism is unknown for most myxobacteria, some utilize pilus retraction. Gliding motility provides a means for microbes to travel in environments with a low water content, such as biofilms and soil. Some pili are used to bring bacteria together allowing the transfer of DNA from one cell ro another, a process called conjugation. Such pili are called conjugation (sex) pili.

Flagella

Some prokaryotic cells have flagella, which are long filaments appendages that propel bacteria. Bacteria that lack flagella are referred to as atrichous (without projections). Flagella may be peritrichous (distributed over the entire cell). If polar, flagella may be monotrichous (a single flagellum at one pole), lophotrichous (a tuft of flagella coming from one pole), or amphitrichous (flagella at both poles of the cell). A flagellum has three basic parts. The long outer most region, the filament, is constant in diameter and contains the globular (roughly spherical) protein flagellin arranged in several chains that intertwine and form a helix around a hollow core. In most bacteria, filaments are not covered by a membrane or sheath, as in eukaryotic cells. The filament is attached to a slightly wider hook, consisting of a different protein. The third portion of a flagellum is the basal body, which anchors the flagellum to the cell wall and the plasma membrane. The basal body is composed of a small central rod inserted in to a series of rings. Gram-negative bacteria contain two pairs of rings; the outer paid of rings is anchored to various portions of the cell wall, and the inner pair of rings is anchored to the plasma membrane. In gram-positive bacteria, only the inner pair is present. Each prokaryotic flagellum is a semi-rigid, helical structure that moves the cell by rotating from the basal body. The rotation of a flagellum is either clockwise or counterclockwise around its long axis. The movement of a prokaryotic flagellum results from rotation from its basal body and is similar to the movement of the shaft of an electric motor. As the flagella rotate, the form a bundle that pushes against the surrounding liquid and propels the bacterium. Flagellar rotation depends on the cell's continuous generation of energy. Bacterial cells can alter the speed and direction of rotation of flagella and thus are capable of various patterns of motility, the ability of an organism to move by itself. When a bacterium moves in one direction for a length of time, the movement is called a "run" or "swim". "Runs" are interrupted by periodic, abrupt, random changes in direction called "tumbles". Then, a "run" resumes. "Tumbles" are caused by a reversal of flagellar rotation. One advantage of motility is that it enables a bacterium to move towards a favorable environment or away from an adverse one. The movement of a bacterium toward of away from a particular stimulus is called taxis. Such stimuli include chemicals ( chemotaxis) an light (phototaxis) Motile bacteria contain receptors in various locations, such as in or just under the cell wall.

Spiral

Spiral bacteria have one or more twists; they are never straight. Bacteria that look like curved rods are called vibrios like a corkscrew; and fairly rigid bodies. Yet another group of spirals are helical and flexible; they are called spirochetes. Unlike the spirilla, which use propeller-like external appendages called flagella to move, spirochetes move by means of axial-filaments, which resemble flagella but are contained within a flexible external sheath.

Axial filaments

Spirochetes are a group of bacteria that have unique structure and motility. Spirochetes move by means of axial filaments, or endoflagella, bundles of fibrils that arise at the end of the cell beneath an outer sheath and spiral around the cell. Axial filaments, which are anchored at one end of the spirochete, have a structure similar to that of flagella. The rotation of the filaments produces a movement of outer sheath that propels the spirochetes in a spiral motion. This movement is similar to the way a corkscrew moves through a cork.

What determines the name of bacterium?

The shape of bacterium is determined by heredity. Genetically, most bacteria are monomorphic; that is, they maintain a single shape. However a number of environmental conditions can alter that shape. If the shape is altered, identification becomes difficult. Genetically pleomorphic bacterium have many shapes, not just one.

Identify the three basic shapes of bacteria. (pg. 73-75)

The three basic shapes of bacteria are spherical coccus (plural: cocci, meaning berries), rod-shaped bacillus (plural: bacilli, meaning little staffs), and spiral. In addition to these common shapes, there are star-shaped cells (genus Stella); rectangular, flat cells (halophilic archaea) of the genus Haloarcula; and triangular cells.