TEXTBOOK MICRO CH3

Volutin granules

Also called metachromatic granules because of their staining characteristics, are inclusions that store polymerized inorganic phosphate that can be used in metabolism and assist in the formation of biofilms. Microbes known to contain volutin granules include the archaea Methanosarcina, the bacterium Coryne bacteriumdiphtheriae, and the unicellular eukaryotic alga Chlamydomonas.

Group translocation

Also transports substances into bacterial cells. In this case, as a molecule moves into a cell against its concentration gradient, it is chemically modified so that it does not require transport against an unfavorable concentration gradient. A common example of this is the bacterial phosphotransferase system, a series of carriers that phosphorylates (i.e., adds phosphate ions to) glucose or other sugars upon entry into cells. Since the phosphorylation of sugars is required during the early stages ofsugar metabolism, the phosphotransferase system is considered to be an energyneutral system

The Origins of Cell Theory

Although cells were first observed in the 1660s by Robert Hooke, cell theory was not well accepted for another 200 years. The work of scientists such as Schleiden, Schwann, Remak, and Virchow contributed to its acceptance.

Capsule

An organized layer located outside of the cell wall and usually composed of polysaccharides or proteins. The ability to produce a capsule can contribute to a microbe's pathogenicity (ability to cause disease) because the capsule can make it more difficult for phagocytic cells (such as white blood cells) to engulf and kill the microorganism. Streptococcus pneumoniae, for example, produces a capsule that iswell known to aid in this bacterium's pathogenicity.

Unique Characteristics of Prokaryotic Cells

Cell theory states that the cell is the fundamental unit of life. Cells can vary greatly between organisms, and even within the same multicellular organism. The two largest categories of cells—prokaryotic cells and eukaryotic cells—are defined by major differences in several cell structures. Prokaryotic microorganisms are classified within the domains Archaea and Bacteria, whereas eukaryotic organisms are classified within the domain Eukarya. Prokaryotic cells of the same species typically share a similar cell morphology and cellular arrangement. Most prokaryotic cells have a cell wall that helps the organism maintain cellular morphology and protects it against changes in osmotic pressure.

Extracellular Matrix

Cells of animals and some protozoans do not have cell walls to help maintain shapeand provide structural stability. Instead, these types of eukaryotic cells producean extracellular matrix for this purpose. They secrete a sticky mass of carbohydratesand proteins into the spaces between adjacent cells. Some protein components assemble into a basement membrane to which the remaining extracellular matrix components adhere. Proteoglycans typically form the bulky mass of the extracellular matrix while fibrous proteins, like collagen, provide strength.Both proteoglycans and collagen are attached to fibronectin proteins, which, in turn, are attached to integrin proteins. These integrin proteins interact with transmembrane proteins in the plasma membranes of eukaryotic cells that lack cell walls.In animal cells, the extracellular matrix allows cells within tissues to withstand external stresses and transmits signals from the outside of the cell to the inside. The amount of extracellular matrix is quite extensive in various types of connective tissues, and variations in the extracellular matrix can give different types of tissues their distinct properties. In addition, a host cell's extracellular matrix is often the site where microbial pathogens attach themselves to establish infection.For example, Streptococcus pyogenes, the bacterium that causes strep throat andvarious other infections, binds to fibronectin in the extracellular matrix of the cells lining the oropharynx (upper region of the throat).

Facilitated diffusion

Charged molecules, as well as large molecules, need the help of carriers or channels in the membrane. These structures ferry molecules across the membrane, a process known as facilitated diffusion.

Ignaz Semmelweis in 1847

Observed that mothers who gave birth in hospital wards staffed by physicians and medical students were more likely to suffer and die from puerperal fever after childbirth (10%-20% mortality rate) than were mothers in wards staffed by midwives (1% mortality rate). Semmelweis observed medical students performing autopsies and then subsequently carrying out vaginal examinations on living patients without washing their hands in between. He suspected that the students carried disease from the autopsies to the patients they examined. He suggested that the number of puerperal fever cases could be reduced if physicians and medical students simply washed their hands with chlorinated lime water before and after examining every patient. When this practice was implemented, the maternal mortality rate in mothers cared for by physicians dropped to the same 1% mortality rate observed among mothers cared for by midwives. This demonstrated that hand washing was a very effective method for preventing disease transmission.

Osmotic pressure

Occurs because of differences in the concentration of solutes onopposing sides of a semipermeable membrane. Water is able to pass through as emipermeable membrane, but solutes (dissolved molecules like salts, sugars, and other compounds) cannot. When the concentration of solutes is greater on one side of the membrane, water diffuses across the membrane from the side with the lower concentration (more water) to the side with the higher concentration (less water) until the concentrations on both sides become equal. This diffusion of water is called osmosis, and it can cause extreme osmotic pressure on a cell when its external environment changes.

Active transport

Occurs when cells move molecules across their membrane against concentration gradients. A major difference between passive and active transport is that active transport requires adenosine triphosphate (ATP) or other forms of energy to move molecules "uphill." Therefore, active transport structuresare often called "pumps."

Peroxisomes

Peroxisomes form independently in the cytoplasm from the synthesis of peroxin proteins by free ribosomes and the incorporation of these peroxin proteins into existing peroxisomes. Growing peroxisomes then divide by a process similar to binary fission. Peroxisomes were first named for their ability to produce hydrogen peroxide, a highly reactive molecule that helps to break down molecules such as uric acid, amino acids, and fatty acids. Peroxisomes also possess the enzyme catalase, which can degrade hydrogen peroxide. Along with the SER, peroxisomes also play a role in lipidbiosynthesis. Like lysosomes, the compartmentalization of these degradative molecules within an organelle helps protect the cytoplasmic contents from unwanted damage.The peroxisomes of certain organisms are specialized to meet their particular functional needs. For example, glyoxysomes are modified peroxisomes of yeasts and plant cells that perform several metabolic functions, including the production of sugar molecules. Similarly, glycosomes are modified peroxisomes made by certain trypanosomes, the pathogenic protozoans that cause Chagas disease and African sleeping sickness.

Membrane Transport Mechanisms

Some molecules can move across the bacterial membrane by simple diffusion, but most large molecules must be actively transported through membrane structures using cellular energy. The processes of simple diffusion, facilitated diffusion, and active transport are used in both eukaryotic and prokaryotic cells. However, eukaryotic cells also have the unique ability to perform various types of endocytosis, the uptake of matter through plasma membrane invagination and vacuole/vesicle formation. A type of endocytosis involving the engulfment of large particles through membrane invagination is called phagocytosis, which means "cell eating." In phagocytosis, particles (or other cells) are enclosed in a pocket within the membrane, which then pinches off from the membrane to form a vacuole that completely surrounds the particle. Another type of endocytosis is called pinocytosis, which means "cell drinking." In pinocytosis, small, dissolved materials and liquids are taken into the cell through small vesicles. Saprophytic fungi, for example, obtain their nutrients from dead and decaying matter largely through pinocytosis.

Glycocalyces and S-Layers

A glycocalyx is a sugar coat, of which there are two important types: capsules and slimelayers.

Slime layer

A less tightly organized layer that is only loosely attached to the cell wall and can be more easily washed off. Slime layers may be composed of polysaccharides, glycoproteins, or glycolipids. In bacteria, S-layers are found outside the cell wall, but in some archaea, the S-layer serves as the cell wall. The exact function of S-layers is not entirely understood, and they are difficult to study; but available evidence suggests that they may play a variety of functions in different prokaryotic cells, such as helping the cell withstand osmotic pressure and, for certain pathogens, interacting with the host immune system.

Mitotic phase

A multistep process during which the duplicated chromosomes are aligned, separated, move to opposite poles of the cell, and then are divided into two identical daughter cells. The first portion of the mitotic phase is called karyokinesis, ornuclear division.

Microtubules

A third type of cytoskeletal fiber composed of tubulin dimers (α tubulinand β tubulin). These form hollow tubes 23 nm in diameter that are used as girderswithin the cytoskeleton. Like microfilaments, microtubules are dynamic and have theability to rapidly assemble and disassemble. Microtubules also work with motor proteins (such as dynein and kinesin) to move organelles and vesicles around within the cytoplasm. Additionally, microtubules are the main components of eukaryotic flagellaand cilia, composing both the filament and the basal body components.

Receptor-mediated endocytosis

A type of endocytosis that is initiated by specific molecules called ligands when they bind to cell surface receptors on the membrane.Receptor-mediated endocytosis is the mechanism that peptide and amine-derived hormones use to enter cells and is also used by various viruses and bacteria for entry into host cells. The process by which secretory vesicles release their contents to the cell's exterior is called exocytosis. Vesicles move toward the plasma membrane and then meld with the membrane, ejecting their contents out of the cell. Exocytosis is used by cells to remove waste products and may also be used to release chemical signals that can be taken up by other cells.

Gas vacuoles

Accumulations of small, protein-lined vesicles of gas. These gas vacuoles allow the prokaryotic cells that synthesize them to alter their buoyancy so that they can adjust their location in the water column.

Sulfur granules

Another type of inclusion, are found in sulfur bacteria of the genus Thiobacillus; these granules store elemental sulfur, which the bacteria use for metabolism.

Intermediate filaments

Are a diverse group of cytoskeletal filaments that act as cables within the cell. Intermediate filaments tend to be more permanent in the cell and maintain the position of the nucleus. They also form the nuclear lamina (lining or layer) just inside the nuclear envelope. Additionally, intermediate filaments play a role inanchoring cells together in animal tissues. The intermediate filament protein desmin is found in desmosomes, the protein structures that join muscle cells together and help them resist external physical forces. The intermediate filament protein keratin is astructural protein found in hair, skin, and nails.

Cilia

Are a similar external structure found in some eukaryotic cells. Unique to eukaryotes, cilia are shorter than flagella and often cover the entire surface of a cell; however, they are structurally similar to flagella (a 9+2 array of microtubules) and use the same mechanism for movement.

Kinetoplasts

Are a variation of the mitochondria found in some eukaryotic pathogens. In these organisms, each cell has a single, long, branched mitochondrion in which kinetoplast DNA, organized as multiple circular pieces of DNA, is found concentrated atone pole of the cell.

Carboxysomes

Are composed of outer shells of thousands of protein subunits. Their interior is filled with ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) and carbonic anhydrase. Both of these compounds are used for carbon metabolism

Bacterial membranes

Are composed of phospholipids with integral or peripheral proteins. The fatty acid components of these phospholipids are ester-linked and areoften used to identify specific types of bacteria. The proteins serve a variety of functions, including transport, cell-to-cell communication, and sensing environmental conditions. Archaeal membranes are distinct in that they are composed of fatty acidsthat are ether-linked to phospholipids.

Microfilaments

Are composed of two intertwined strands of actin, each composed of actin monomers. The actin filaments work together with motor proteins, like myosin, to effect muscle contraction in animals or the amoeboid movement of some eukaryotic microbes.

Hydrogenosomes

Are found in some anaerobic eukaryotes and serve as the location ofanaerobic hydrogen production. Hydrogenosomes typically lack their own DNA andribosomes.

Inclusions

As single-celled organisms living in unstable environments, some prokaryotic cells have the ability to store excess nutrients within cytoplasmic structures called inclusions. Storing nutrients in a polymerized form is advantageous because it reduces the buildup of osmotic pressure that occurs as a cell accumulates solutes. Various types ofinclusions store glycogen and starches, which contain carbon that cells can access forenergy.

Pili

Commonly refers to longer, less numerous protein appendages that aid in attachment to surfaces. A specific type of pilus, called the F pilus or sex pilus, is important in the transfer of DNA between bacterial cells, which occurs between members of the same generation when two cells physically transfer or exchange parts of their respective genomes.

Fimbriae

Commonly refers to short bristle-like proteins projecting from the cellsurface by the hundreds. Fimbriae enable a cell to attach to surfaces and to other cells. For pathogenic bacteria, adherence to host cells is important for colonization, infectivity,and virulence. Adherence to surfaces is also important in biofilm formation.

John Snow in 1848

Conducted studies to track the source of cholera outbreaks in London. By tracing the outbreaks to two specific water sources, both of which were contaminated by sewage, Snow ultimately demonstrated that cholera bacteria were transmitted via drinking water. Snow's work is influential in that it represents the first known epidemiological study, and it resulted in the first known public health response to an epidemic. The work of both Semmelweis and Snow clearly refuted the prevailing miasma theory of the day, showing that disease is not only transmitted through the air but also through contaminated items.

Photosynthetic Membrane Structures

Cyanobacteria and photosynthetic bacteria, have membrane structures that enable them to perform photosynthesis.In cyanobacteria, these membrane structures are called thylakoids; in photosynthetic bacteria, they are called chromatophores, lamellae, or chlorosomes. These structures consist of an infolding of the plasma membrane that encloses photosynthetic pigments such as green chlorophylls and bacteriochlorophylls.

Louis Pasteur in 1856

Discovered properties of fermentation by microorganisms. He had demonstrated with his swan-neck flask experiments that airborne microbes, not spontaneous generation, were the cause of food spoilage, and he suggested that if microbes were responsible for food spoilage and fermentation, they could also be responsible for causing infection.

Lazzaro Spallanzani (1729-1799)

Did not agree with Needham's conclusions, however, and performed hundreds of carefully executed experiments using heated broth. As in Needham's experiment, broth in sealed jars and unsealed jars was infused with plant and animal matter. Spallanzani's results contradicted the findings of Needham: Heated but sealed flasks remained clear, without any signs of spontaneous growth, unless the flasks were subsequently opened to the air. This suggested that microbes were introduced into these flasks from the air. In response to Spallanzani's findings, Needham argued that life originates from a "lifeforce" that was destroyed during Spallanzani's extended boiling. Any subsequent sealing of the flasks then prevented new life force from entering and causing spontaneous generation.

Archaeal cell wall structure

Differs from that of bacteria in several significant ways. First, archaeal cell walls do not contain peptidoglycan; instead, they contain a similar polymer called pseudopeptidoglycan (pseudomurein) in which NAM is replaced with a different subunit.Other archaea may have a layer of glycoproteins or polysaccharides that serves as the cell wall instead of pseudopeptidoglycan. Last, as is the case with some bacterialspecies, there are a few archaea that appear to lack cell walls entirely.

Endosymbiotic Theory

Endosymbiotic theory states that mitochondria and chloroplasts, organelles found in many types of organisms, have their origins in bacteria. Again, this hypothesis was not initially popular, but mounting genetic evidence due to the advent of DNA sequencing supported the endosymbiotic theory, which is now defined as the theory that mitochondria and chloroplasts arose as a result of prokaryotic cells establishing a symbiotic relationship within a eukaryotic host.More recent genetic sequencing and phylogenetic analysis show that mitochondrial DNA and chloroplast DNA are highly related to their bacterial counterparts, both in DNA sequence and chromosome structure. Additionally, mitochondrial and chloroplast ribosomes are structurally similar to bacterial ribosomes, rather than to the eukaryotic ribosomes of their hosts. Last, the binary fission of these organelles strongly resembles the binary fission of bacteria, as compared with mitosis performed by eukaryotic cells.

Cell Morphologies

Eukaryotic cell morphologies vary greatly and may be maintained by various structures, including the cytoskeleton, the cell membrane, and/or the cell wall

Unique Characteristics of Eukaryotic Cells

Eukaryotic cells are defined by the presence of a nucleus containing the DNA genome and bound by a nuclear membrane (or nuclear envelope) composed of two lipid bilayers that regulate transport of materials into and out of the nucleus through nuclear pores.

Ribosomes

Eukaryotic cells contain 80S ribosomes in the rough endoplasmic reticulum (membranebound-ribosomes) and cytoplasm (free ribosomes). They contain 70s ribosomes inmitochondria and chloroplasts.The differences between eukaryotic and prokaryotic ribosomes are clinically relevantbecause certain antibiotic drugs are designed to target one or the other. For example,cycloheximide targets eukaryotic action, whereas chloramphenicol targets prokaryoticribosomes. Since human cells are eukaryotic, they generally are not harmed byantibiotics that destroy the prokaryotic ribosomes in bacteria. However, sometimes

Endomembrane System

Eukaryotic cells have evolved an endomembrane system, containing membrane-bound organelles involved in transport. These include vesicles, the endoplasmic reticulum, and the Golgi apparatus.

Nucleus

Eukaryotic cells possess a nucleus, which is surrounded by a complex nuclear membrane that houses the DNA genome. By containing the cell's DNA, the nucleus ultimately controls all activities of the cell and also serves an essential role in reproduction and heredity. Eukaryotic cells typically have their DNA organized intomultiple linear chromosomes. The DNA within the nucleus is highly organized and condensed to fit inside the nucleus, which is accomplished by wrapping the DNA around proteins called histones. Although most eukaryotic cells have only one nucleus, exceptions exist. For example, protozoans of the genus Paramecium typically have two complete nuclei: a small nucleus that is used for reproduction (micronucleus) and a large nucleus that directs cellular metabolism (macronucleus). Additionally, some fungi transiently form cells withtwo nuclei, called heterokaryotic cells, during sexual reproduction. Cells whose nucleidivide, but whose cytoplasm does not, are called coenocytes.The nucleus is bound by a complex nuclear membrane, often called the nuclear envelope, that consists of two distinct lipid bilayers that are contiguous with each other. The nuclear envelope contains nuclear pores, which are large, rosette-shaped protein complexes that control the movement of materials into and out of the nucleus.The overall shape of the nucleus is determined by the nuclear lamina, a meshwork of intermediate filaments found just inside the nuclear envelope membranes. Outside the nucleus, additional intermediate filaments form a looser mesh and serve to anchor the nucleus in position within the cell.Eukaryotes are able to multiply through asexual reproduction, during which a single parent cell becomes two identical daughter cells. This process of clonal reproduction iscalled mitosis. Although mitosis may sound similar to asexual binary fission inprokaryotes, the processes are very different. In contrast to the single chromosome inmost prokaryotes, eukaryotic cells possess multiple chromosomes that must bereplicated and strategically divided between daughter cells. Therefore, mitosis is a much more complex cellular process than binary fission. The eukaryotic cell cycle is an ordered and carefully regulated series of events involving cell growth, DNA replication, and cell division to produce two clonal daughter cells. One "turn" or cycle of the cell cycle consist of two general phases: interphase and the mitotic phase. During interphase, the cell is not dividing, but rather is undergoing normal growthprocesses and DNA is replicated preparing for cell division. The three stages ofinterphase are called G1, S, and G2.

Fimbriae and Pili

Fimbriae and pili are structurally similar and, because differentiation between the two is problematic, these terms are often used interchangeably.

Flagella

Flagella are structures used by cells to move in aqueous environments. Bacterial flagella act like propellers. They are stiff spiral filaments composed of flagella in protein subunits that extend outward from the cell and spin in solution. The basal body is themotor for the flagellum and is embedded in the plasma membrane. A hook region connects the basal body to the filament. Gram-positive and gram-negative bacteria have different basal body configurations due to differences in cell wall structure. Different types of motile bacteria exhibit different arrangements of flagella.

Basal body

Found at the base of each cilium and flagellum. The basal body, which attaches the cilium or flagellum to the cell, is composed of an array of triplet microtubules similar to that of a centriole but embedded in the plasma membrane. Because of their shorter length, cilia use a rapid, flexible, waving motion. In addition to motility, cilia may have other functions such as sweeping particles past or into cells. For example, ciliated protozoans use the sweeping of cilia to move food particles into their mouth parts, and ciliated cells in the mammalian respiratory tract beat in synchrony to sweep mucus and debris up and out of the lungs.

Gram-positive cells

Have a cell wall consisting of many layers of peptidoglycan totaling 30-100 nm in thickness. These peptidoglycan layers are commonly embedded with teichoic acids (TAs), carbohydrate chains that extend through and beyond the peptidoglycan layer. TA is thought to stabilize peptidoglycan by increasing its rigidity. TA also plays a role in the ability of pathogenic gram-positive bacteria such as Streptococcus to bind to certain proteins on the surface of host cells, enhancing their ability to cause infection. In addition to peptidoglycan and TAs, bacteria of the family Mycobacteriaceae have an external layer of waxy mycolic acids in their cell wall, these bacteria are referred to as acid-fast, since acid-fast stains must be used to penetrate the mycolic acid layer for purposes of microscopy.

Amphitrichous flagella

Have a flagellum or tufts of flagella at each end. An example is Spirillum minor, the cause of spirillary (Asian) rat-bite fever or sodoku.

lophotrichous flagella

Have a tuft at one end of the cell. The Gram-negativebacillus Pseudomonas aeruginosa, an opportunistic pathogen known for causing many infections, including "swimmer's ear" and burn wound infections, has lophotrichousflagella. Flagella that cover the entire surface of a bacterial cell are called peritrichous flagella.The Gram-negative bacterium E. coli shows a peritrichous arrangement of flagella.

Magnetosomes

Inclusions of magnetic iron oxide or iron sulfide surrounded by a lipidlayer. Magnetotactic bacteria, such as Magnetospirillum magnetotacticum, contain magnetosomes. These allow cells to align along a magnetic field, aiding their movement.

Karyokinesis

Is divided into a series of phases—prophase, prometaphase, metaphase, anaphase, and telophase—that result in the division of the cell nucleus.

Disproving Spontaneous Generation

Louis Pasteur is credited with conclusively disproving the theory of spontaneousgeneration with his famous swan-neck flask experiment. He subsequently proposed that "life only comes from life." His swan-neck flasks design allowed air inside the flasks to be exchanged with air from the outside, but prevented the introduction of any airborne microorganisms, which would get caught in the twists and bends of the flasks' necks.While some scientists were arguing over the theory of spontaneous generation, other scientists were making discoveries leading to a better understanding of what we now call the cell theory. Modern cell theory has two basic tenets: -All cells only come from other cells (the principle of biogenesis). -Cells are the fundamental units of organisms. Today, these tenets are fundamental to our understanding of life on earth. However, modern cell theory grew out of the collective work of many scientists.

Lysosomes

Lysosomes contain digestive enzymes that break down small particles ingested by endocytosis, large particles or cells ingested by phagocytosis, and damaged intracellular components. Compartmentalization of the digestive enzymes within the lysosome allows the cell to efficiently digest matter without harming the cytoplasmic components of the cell.

Filamentous Appendages

Many bacterial cells have protein appendages embedded within their cell envelopes thatextend outward, allowing interaction with the environment. These appendages canattach to other surfaces, transfer DNA, or provide movement. Filamentous appendages include fimbriae, pili, and flagella.

Simple diffusion

Molecules moving from a higher concentration to a lower concentration with the concentration gradient are transported by simple diffusion, also known as passive transport. Some small molecules, like carbon dioxide, may cross themembrane bilayer directly by simple diffusion.

Chloroplasts

Plant cells and algal cells contain chloroplasts, the organelles in which photosynthesis occurs. All chloroplasts have at least three membrane systems: the outer membrane, the inner membrane, and the thylakoid membrane system. Inside the outer and inner membranes is the chloroplast stroma, a gel-like fluid that makes up much of a chloroplast's volume, and in which the thylakoid system floats. The thylakoid system is a highly dynamic collection of folded membrane sacs. It iswhere the green photosynthetic pigment chlorophyll is found and the light reactions ofphotosynthesis occur. In most plant chloroplasts, the thylakoids are arranged in stacks called grana (singular: granum), whereas in some algal chloroplasts, the thylakoids are free floating.Other organelles similar to mitochondria have arisen in other types of eukaryotes, but their roles differ.

Plasmids

Prokaryotic cells may also contain extrachromosomal DNA, or DNA that is not part ofthe chromosome. This extrachromosomal DNA is found in plasmids, which are small, circular, double-stranded DNA molecules. Plasmids are more commonly found in bacteria; however, plasmids have been found in archaea and eukaryotic organisms. Plasmids often carry genes that confer advantageous traits such as antibiotic resistance; thus, they are important to the survival of the organism.

The Nucleoid

Prokaryotic chromosomes are typically circular, haploid (unpaired), and not bound by acomplex nuclear membrane. Prokaryotic DNA and DNA-associated proteins areconcentrated within the nucleoid region of the cell. In general, prokaryotic DNA interacts with nucleoid-associated proteins (NAPs) that assist in the organization and packaging of the chromosome. In bacteria, NAPs function similar to histones, which are the DNA-organizing proteins found in eukaryotic cells. In archaea, the nucleoid is organized by either NAPs orhistone-like DNA organizing proteins.

Ribosomes

Prokaryotic ribosomes are found in the cytoplasm. They are called 70S ribosomes whereas eukaryotic cytoplasmic ribosomes have a size of 80S. The S stands for Svedberg unit, a measure of sedimentation in an ultracentrifuge, which is based on size, shape, and surface qualities of the structure being analyzed. Eukaryotic cells contain 80S ribosomes in the rough endoplasmic reticulum (membranebound-ribosomes) and cytoplasm (free ribosomes). They contain 70s ribosomes in mitochondria and chloroplasts. The differences between eukaryotic and prokaryotic ribosomes are clinically relevant because certain antibiotic drugs are designed to target one or the other. For example,cycloheximide targets eukaryotic action, whereas chloramphenicol targets prokaryoticribosomes. Since human cells are eukaryotic, they generally are not harmed byantibiotics that destroy the prokaryotic ribosomes in bacteria. However, sometimes negative side effects may occur because mitochondria in human cells containprokaryotic ribosomes.

Robert Koch

Proposed a series of postulates (Koch's postulates) based on the idea thatthe cause of a specific disease could be attributed to a specific microbe. Using these postulates, Koch and his colleagues were able to definitively identify the causative pathogens of specific diseases, including anthrax, tuberculosis, and cholera.Koch's "one microbe, one disease" concept was the culmination of the 19th century's paradigm shift away from miasma theory and toward the germ theory of disease

John Needham (1713-1781)

Published a report of his own experiments, inwhich he briefly boiled broth infused with plant or animal matter, hoping to kill allpreexisting microbes. He then sealed the flasks. After a few days, Needham observed that the broth had become cloudy and a single drop contained numerous microscopic creatures. He argued that the new microbes must have arisen spontaneously. In reality, however, he likely did not boil the broth enough to kill all preexisting microbes.

Endospores

Some genera of bacteria have the ability to form endospores, structures that essentially protect the bacterial genome in a dormant state when environmental conditions are unfavorable. When living conditions improve, endospores undergo germination, re entering a vegetative state. After germination, the cell becomes metabolically active again and is able to carry out all of its normal functions, including growth and cell division. Endospores of certain species have been shown to persist in a dormant state for extended periods of time, up to thousands of years.

Nucleolus

The nucleolus is a dense region within the nucleus where ribosomal RNA (rRNA) biosynthesis occurs. In addition, the nucleolus is also the site where assembly of ribosomes begins. Pre-ribosomal complexes are assembled from rRNA and proteinsin the nucleolus; they are then transported out to the cytoplasm, where ribosomeassembly is completed

Flagella and Cilia

Some eukaryotic cells use flagella for locomotion; however, eukaryotic flagella are structurally distinct from those found in prokaryotic cells. Whereas the prokaryotic flagellum is a stiff, rotating structure, a eukaryotic flagellum is more like a flexible whip composed of nine parallel pairs of microtubules surrounding a central pair of microtubules. This arrangement is referred to as a 9+2 array. The parallel microtubulesuse dynein motor proteins to move relative to each other, causing the flagellum to bend.

Plasma Membrane

Structures that enclose the cytoplasm and internal structures of the cell are known collectively as the cell envelope. All cells (prokaryotic and eukaryotic) have a plasma membrane (also called cytoplasmic membrane or cell membrane) that exhibits selective permeability, allowing some molecules to enter or leave the cell while restricting the passage of others. The plasma membrane of eukaryotic cells is structurally similar to that found in prokaryotic cells, and membrane components move according to the fluid mosaic model. However, eukaryotic membranes contain sterols, which alter membrane fluidity, as well as glycoproteins and glycolipids, which help the cell recognize other cells and infectious particles. Additionally, many eukaryotic cells contain some specialized lipids, includingsphingolipids, which are thought to play a role in maintaining membrane stability as wellas being involved in signal transduction pathways and cell-to-cell communication.

Cyanobacteria

Such as Anabaena cylindrica and bacteria such as Halothiobacillusneapolitanus produce carboxysome inclusions. These structures are considered proto-organelles because they compartmentalize important compounds or chemical reactions, much like many eukaryotic organelles.

Gram-negative bacteria

Tetrapeptide chains extending from each NAM unit are directly cross-linked, whereas in Gram-positive bacteria, these tetrapeptide chains are linked by pentaglycine cross-bridges. Peptidoglycan subunits are made inside of thebacterial cell and then exported and assembled in layers, giving the cell its shape. Have a much thinner layer of peptidoglycan (no more than about 4nm thick) than gram-positive cells, and the overall structure of their cell envelope ismore complex. In gram-negative cells, a gel-like matrix occupies the periplasmic space between the cell wall and the plasma membrane, and there is a second lipid bilayer called the outer membrane, which is external to the peptidoglycan layer. This outer membrane is attached to the peptidoglycan by murein lipoprotein. The outer leaflet of the outer membrane contains the molecule lipopolysaccharide (LPS), which functions as an endotoxin in infections involving gram-negative bacteria, contributing to symptoms such as fever, hemorrhaging, and septic shock. Each LPS molecule iscomposed of Lipid A, a core polysaccharide, and an O side chain that is composed ofsugar-like molecules that comprise the external face of the LPS. The composition of the O side chain varies between different species and strains of bacteria. Parts of the O side chain called antigens can be detected using serological or immunological tests to identify specific pathogenic strains like Escherichia coli O157:H7, a deadly strain of bacteria that causes bloody diarrhea and kidney failure.

Mitochondria

The mitochondrial genome was found to be bacterial, when it was sequenced in 1976. These findings ultimately supported the endosymbiotic theory proposed byLynn Margulis, which states that mitochondria originally arose through an endosymbiotic event in which a bacterium capable of aerobic cellular respiration was taken up byphagocytosis into a host cell and remained as a viable intracellular component.Each mitochondrion has two lipid membranes. The outer membrane is a remnant of the original host cell's membrane structures. The inner membrane was derived from the bacterial plasma membrane. The electron transport chain for aerobic respiration uses integral proteins embedded in the inner membrane.The mitochondrial matrix, corresponding to the location of the original bacterium's cytoplasm, is the current location of many metabolic enzymes. It alsocontains mitochondrial DNA and 70S ribosomes. Invaginations of the inner membrane, called cristae, evolved to increase surface area for the location of biochemical reactions.The folding patterns of the cristae differ among various types of eukaryotic cells and are used to distinguish different eukaryotic organisms from each other.

Golgi Apparatus

The Golgi apparatus is composed of a series of membranous disks called dictyosomes, each having a single lipid bilayer, that are stacked together. The Golgi apparatus processes proteins and lipids, typically through the addition of sugar molecules, producing glycoproteins or glycolipids, components of the plasma membrane that are used in cell-to-cell communication.Different types of cells can be distinguished from one another by the structure and arrangement of the glycolipids and glycoproteins contained in their plasma membranes.Transport vesicles leaving the ER fuse with a Golgi apparatus on its receiving, or cis, face. The proteins are processed within the Golgi apparatus, and then additional transport vesicles containing the modified proteins and lipids pinch off from the Golgi apparatus on its outgoing, or trans, face. These outgoing vesicles move to and fuse with the plasma membrane or the membrane of other organelles. Exocytosis is the process by which secretory vesicles (spherical membranous sacs)release their contents to the cell's exterior. All cells have constitutive secretory pathways in which secretory vesicles transport soluble proteins that are released from the cell continually (constitutively). Certain specialized cells also have regulated secretory pathways, which are used to store soluble proteins in secretory vesicles. Regulated secretion involves substances that are only released in response to certain events or signals. For example, certain cells of the human immune system (e.g., mast cells) secrete histamine in response to the presence of foreign objects or pathogens in the body. Histamine is a compound that triggers various mechanisms used by the immune system to eliminate pathogens.

The Theory of Spontaneous Generation

The Greek philosopher Aristotle (384-322 BC) was one of the earliest recorded scholars to articulate the theory of spontaneous generation, the notion that life can arise from nonliving matter.This theory persisted into the 17th century, when scientists undertook additional experimentation to support or disprove it. Experimentation by Francesco Redi (1626-1697) in the 17th century presented the first significant evidence refuting spontaneous generation by showing that flies must have access to meat for maggots to develop on the meat. He predicted that preventing flies from having direct contact with the meat would also prevent the appearance of maggots

Cytoskeleton

The cytoskeleton, composed of microfilaments, intermediate filaments, and microtubules, provides structural support in eukaryotic cells and serves as a network for transport of intracellular materials.

Tonicity

The degree to which a particular cell is able to with stand changes in osmotic pressure.

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is an interconnected array of tubules and cisternae (flattened sacs) with a single lipid bilayer. The spaces inside of the cisternae are called lumen of the ER. There are two types of ER, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).The smooth endoplasmic reticulum plays a role in lipid biosynthesis, carbohydrate metabolism, and detoxification of toxic compounds. The rough endoplasmic reticulum contains membrane-bound 80S ribosomes that synthesize proteins destined for the cell membrane. SER does not have ribosomes and, therefore, appears "smooth." It is involved in biosynthesis of lipids, carbohydrate metabolism, and detoxification of toxic compounds within the cell.

The Germ Theory of Disease

The miasma theory of disease was widely accepted until the 19th century, when it was replaced by the germ theory of disease thanks to the work of Semmelweis, Snow,Pasteur, Lister, and Koch, and others. Prior to the discovery of microbes during the 17th century, other theories circulated about the origins of disease. For example, the ancient Greeks proposed the miasma theory, which held that disease originated from particles emanating from decomposing matter, such as that in sewage or cesspits. Such particles infected humans in close proximity to the rotting material. Diseases including the Black Death, which ravaged Europe's population during the Middle Ages, were thought to have originated in this way.

Cell Wall

The primary function of the cell wall is to protect the cell from harsh conditions in the outside environment. The major component of bacterial cell walls is called peptidoglycan (or murein); it is only found in bacteria. Each layer is composed of long chains of alternating molecules of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). In addition to a plasma membrane, some eukaryotic cells have a cell wall. Cells of fungi, algae, plants, and even some protists have cell walls. Depending upon the type of eukaryotic cell, cell walls can be made of a wide range of materials, including cellulose (fungi and plants); biogenic silica, calcium carbonate, agar, and carrageenan (protists and algae); or chitin (fungi). In general, all cell walls providestructural stability for the cell and protection from environmental stresses such asdesiccation, changes in osmotic pressure, and traumatic injury.

Cytokinesis

The second portion of the mitotic phase is the physical separation of the cytoplasmic components into the two daughter cells.

Hypotonic medium

The solute concentration inside the cell exceeds that outside of the cell, so water will move by osmosis into the cell. This causes the cell to swell and potentially lyse, or burst. The cells that lack a cell wall are more prone to lysis.

Hypertonic medium

The solute concentration outside the cell exceeds that inside the cell, so water diffuses out of the cell and into the external medium. The cells that lack a cell wall can become dehydrated, causing crenation, or shriveling of the cell. By contrast, cells that possess a cell wall undergo plasmolysis.

Isotonic medium

The solute concentrations inside and outside the cell areapproximately equal, so there is no net movement of water across the cell membrane.

Fluid mosaic model

The structure of the plasma membrane is often described in terms of the fluid mosaic model, which refers to the ability of membrane components to move fluidly within the plane of the membrane, as well as the mosaic-like composition of the components, which include a diverse array of lipid and protein components.

Singular flagellum

Typically located at one end of the cell (polar), is said to have a monotrichous flagellum. An example of a monotrichously flagellated bacterial pathogen is Vibrio cholerae, the Gram-negative bacterium that causes cholera.

Joseph Lister

Was trying to determine the causes of post surgical infections. He insisted on hand washing and extreme cleanliness during surgery. In 1867, to further decrease the incidence of post surgical wound infections, Lister began using carbolic acid (phenol) spray disinfectant/antiseptic during surgery. His extremely successful efforts to reduce post surgical infection caused his techniques to become a standard medical practice.

Counterclockwise direction

When running, flagella rotate in a counterclockwise direction, allowing the bacterial cell to move forward. When tumbling, flagella are splayed out while rotating in a clockwise direction, creating a looping motion and preventing meaningful forward movement but reorienting the cell toward the direction of the attractant. When an attractant exists, runs and tumbles still occur; however, the length of runs is longer, while the length of the tumbles is reduced, allowing overall movement toward the higher concentration of the attractant. When no chemical gradient exists, the lengthsof runs and tumbles are more equal, and overall movement is more random.

Centrosomes

Which are essentially microtubule-organizing centers, at opposite ends of the cell. Each centrosome is composed of a pairof centrioles positioned at right angles to each other, and each centriole is an array of nine parallel microtubules arranged in triplets.

Polyhydroxybutyrate (PHB)

Which can be produced by species of Bacillus and Pseudomonas, is an example of an inclusion that displays phospholipid monolayer structure. Industrially, PHB has also been used as a source of biodegradable polymers for bioplastics.