Microbiology Chapter 3: Functional Anatomy of Prokaryotic and Eukaryotic Cells PDF

Microbiology Chapter 3 Functional Anatomy of Prokaryotic and Eukaryotic Cells Karla Cristine C. Doysabas, DVM, MSc., Ph.D.  Prokaryotic vs. Eukaryotic PROKARYOTES EUKARYOTES DNA is not enclosed within a membrane DNA is found in nucleu...

Microbiology Chapter 3 Functional Anatomy of Prokaryotic and Eukaryotic Cells Karla Cristine C. Doysabas, DVM, MSc., Ph.D.  Prokaryotic vs. Eukaryotic PROKARYOTES EUKARYOTES DNA is not enclosed within a membrane DNA is found in nucleus, and is found in and is one circular chromosome multiple chromosomes DNA is consistently associated with DNA is not associated with histones; other chromosomal proteins called histones and proteins are associated with the DNA with nonhistones have a number of membrane-enclosed lack membrane-enclosed organelles organelles cell walls almost always contain the cell walls, if present, are chemically simple complex polysaccharide peptidoglycan divide by binary fission. during this process, divide by mitosis. this process is guided by the DNA is copied and cell splits into two mitotic spindle cells  The Prokaryotic Cell bacteria and archaea 0.2-2.0 µm in diameter, 2-8 µm in length basic shapes coccus/ cocci- round or oval, elongated or flattened on one side diplococci- remain in pairs after dividing streptococci- divide and remain attached in chainlike patterns tetrads- divide in 2 planes and remain in groups of four sarcinae- divide in 3 planes and remain attached in tubelike groups of 8 staphylococci- divide in multiple planes and form grape like clusters bacillus/ bacilli diplobacilli- appear in pairs after division streptobacilli- occur in chains coccobacilli- oval and look so much like cocci spiral vibrios- look like curved rods spirilla- helical shape, like a corkscrew, and fairly rigid bodies spirochetes- helical and flexible, move by means of axial filaments Arrangements of cocci. (a) Division in one plane produces diplococci and streptococci. (b) Division in two planes produces tetrads. (c) Division in three planes produces sarcinae, and (d) division in multiple planes produces staphylococci. Bacilli. (a) Single bacilli. (b) Diplobacilli. In the top micrograph, a few joined pairs of bacilli could serve as examples of diplobacilli. (c) Streptobacilli. (d) Coccobacilli. Spiral bacteria Star-shaped and rectangular prokaryotes. (a) Stella (star-shaped (b) Holoarcula, a genus of halophilic archaea (rectangular cells) monomorphic: maintain a single shape pleomorphic: can have many shapes, not just one  Structures External to the Cell Wall Glycocalyx- substances that surround cells viscous, gelatinous polymer that is external to the cell wall and composed of polysaccharide, polypeptide, or both made inside the cell and secreted to the cell surface if the substance is organised and is firmly attached to the cell wall, it is described as a capsule if the substance is unorganised and only loosely attached to the cell wall, it is called slime layer extracellular polymeric substance: a glycocalyx made of sugar; protects the cells, facilitates communication between them, and enables the cells to survive by attaching to various surfaces. A glycocalyx protects against desiccation, attaches cells to surfaces, and helps pathogens evade the immune system Flagella- (sing. flagellum) long filamentous appendages that propel bacteria range in length from 10 μm to 20 μm and are many times longer than the diameter of the cell atrichous- without projections monotrichous- single polar flagellum amphitrichous- a tuft of flagella at each end of the cell lophotrichous- 2 or more flagella at one pole of the cell peritrichous- flagella distributed over the entire cell Flagella has 3 basic parts filament- constant in diameter and contains the globular protein flagellin arranged in several chains that intertwine and form a helix around a hollow core hook- slightly wider, where filament is attached basal body- anchors the flagellum to the cell wall and plasma membrane an assembly of more than 20 different proteins that form a central rod and set of enclosing rings Gram-positive bacteria have a pair of rings embedded in the cell membrane and one in the cell wall, while gram-negative bacteria have a pair of rings embedded in the cell membrane and another pair in the cell wall represents a powerful biological motor or rotary engine that generates a propeller-type rotation of the flagellum The structure of a bacterial flagellum. The parts and attachment of a flagellum of a gram-negative bacterium and gram-positive bacterium are shown in these highly schematic diagrams. Flagella Motility Each bacterial flagellum is a semirigid, helical structure that moves the cell by rotating from the basal body rotation of a flagellum is either clockwise or counterclockwise around its long axis motility: ability of an organism to move by itself run or swim: bacterium moves in one direction for a length of time “Runs” are interrupted by periodic, abrupt, random changes in direction called “tumbles.”“Tumbles” are caused by a reversal of flagellar rotation Taxis- the movement of a bacterium toward or away from a particular stimulus. Such stimuli include chemicals (chemotaxis) and light (phototaxis) If the chemotactic signal is positive, called an attractant, the bacteria move toward the stimulus with many runs and few tumbles. If the chemotactic signal is negative, called a repellent, the frequency of tumbles increases as the bacteria move away from the stimulus H antigen: useful for distinguishing among serovars, or variations within a species, of gram- negative bacteria (e.g., 50 different H antigens for E. coli) Flagella and bacterial motility Archaella Motile archaeal cells have archaella (singular: archaellum) A knoblike structure anchors archaella to the cell rotate like flagella, an action that pushes the cell through water, use ATP for energy lack a cytoplasmic core. consist of glycoproteins called archaellins. Axial Filaments Spirochetes move by means of axial filaments, or endoflagella, bundles of fibrils that arise at the ends of the cell beneath an outer sheath and spiral around the cell have a structure similar to that of flagella. The rotation of the filaments produces a movement of the outer sheath that propels the spirochetes in a spiral motion Fimbriae and Pili Many gram-negative bacteria contain hairlike appendages that are shorter, straighter, and thinner than flagella consist of a protein called pilin arranged helically around a central core Fimbriae (singular: fimbria) can occur at the poles of the bacterial cell or can be evenly distributed over the entire surface of the cell few to several hundred per cell can adhere to each other and to surfaces. As a result, they are involved in forming biofilms and other aggregations on the surfaces of liquids, glass, and rocks. help bacteria adhere to epithelial surfaces in the body. e.g. E. coli O157 can adhere to the lining of the small intestine Pili (singular: pilus) are usually longer than fimbriae and number only one or two per cell involved in motility and DNA transfer twitching motility, a pilus extends by the addition of subunits of pilin, makes contact with a surface or another cell, and then retracts (powerstroke) as the pilin subunits are disassembled (Pseudomonas aeruginosa, Neisseria gonorrhoeae) gliding motility: the smooth gliding movement of myxobacteria provides a means for microbes to travel in environments with a low water content, such as biofilms and soil Conjugation (sex) pili: used to bring bacteria together, allowing the transfer of DNA from one cell to another, a process called conjugation. The exchanged DNA can add a new function to the recipient cell, such as antibi- otic resistance or the ability to digest its medium more efficiently Fimbriae. The fimbriae seem to bristle from this E. coli cell, which is beginning to divide.  The Cell Wall a complex, semirigid structure responsible for the shape of the cell. Almost all prokaryotes have a cell wall that surrounds the underlying, fragile plasma (cytoplasmic) membrane and protects it and the interior of the cell from adverse changes in the outside environment prevent bacterial cells from rupturing when the water pressure inside the cell is greater than that outside the cell helps maintain the shape of a bacterium and serves as a point of anchorage for flagella contributes to the ability of some species to cause disease and is the site of action of some antibiotics  Cell Wall Composition and Characteristics composed of a macromolecular network called peptidoglycan (also known as murein), which is present either alone or in combination with other substances Peptidoglycan- consists of a repeating disaccharide connected by polypeptides to form a lattice that surrounds and protects the entire cell The disaccharide portion is made up of monosaccharides called N- acetylglucosamine (NAG) and N-acetyl- muramic acid (NAM) (from murus, meaning wall), which are related to glucose  Cell Wall Composition and Characteristics The various components of peptidoglycan are assembled in the cell wall Alternating NAM and NAG molecules are linked in rows of 10 to 65 sugars to form a carbohydrate “backbone” (the glycan portion of peptidoglycan). Adjacent rows are linked by polypeptides (the peptide portion of peptidoglycan). Although the structure of the polypeptide link varies, it always includes tetrapeptide side chains, which consist of four amino acids attached to NAMs in the backbone. Parallel tetrapeptide side chains may be directly bonded to each other or linked by a peptide cross-bridge, consisting of a short chain of amino acids. The structure of peptidoglycan in gram-positive bacteria. Together the carbohydrate backbone (glycan portion) and tetrapeptide side chains (peptide portion) make up peptidoglycan. The frequency of peptide cross-bridges and the number of amino acids in these bridges vary with species of bacteria. The small arrows indicate where penicillin interferes with the linkage of peptidoglycan rows by peptide cross- bridges. Cell Walls and the Gram Stain Mechanism  Atypical Cell Walls Mycoplasma- have no walls or have very little wall material Mycoplasmas are the smallest known bacteria that can grow and reproduce outside living host cells. Because of their size and because they have no cell walls, they pass through most bacterial filters and were first mistaken for viruses plasma membranes are unique among bacteria in having lipids called sterols, which are thought to help protect them from lysis Archaea may lack walls or may have unusual walls composed of polysaccharides and proteins but not peptidoglycan walls do, however, contain a substance similar to peptidoglycan called pseudomurein Pseudomurein contains N-acetyltalosaminuronic acid instead of NAM and lacks the D-amino acids found in bacterial cell walls Acid-Fast Cell Walls genus Mycobacterium and pathogenic species of Nocardia contain high concentrations (60%) of a hydrophobic waxy lipid (mycolic acid) in their cell wall that prevents the uptake of dyes, including those used in the Gram stain mycolic acid forms a layer outside of a thin layer of peptidoglycan. The mycolic acid and peptidoglycan are held together by a polysaccharide  Damage to the Cell Wall antimicrobial drugs target cell wall synthesis exposure to the digestive enzyme lysozyme particularly active on the major cell wall components of most gram-positive bacteria, making them vulnerable to lysis. catalyzes hydrolysis of the bonds between the sugars in the repeating disaccharide “backbone” of peptidoglycan gram-positive cell wall is almost completely destroyed by lysozyme protoplast: wall-less cell, spherical and is still capable of carrying on metabolism L forms: some members of the genus Proteus, as well as other genera, can lose their cell walls and swell into irregularly shaped cells may form spontaneously or develop in response to penicillin (which inhibits cell wall formation) or lysozyme (which removes the cell wall). L forms can live and divide repeatedly or return to the walled state spheroplast: When lysozyme is applied to gram-negative cells, usually the wall is not destroyed to the same extent as in gram-positive cells; some of the outer membrane also remains; spherical  Structures Internal to the Cell Wall plasma (cytoplasmic) membrane (or inner membrane) is a thin structure lying inside the cell wall and enclosing the cytoplasm of the cell consists primarily of phospholipids, which are the most abundant chemicals in the membrane, and proteins lack sterols, thus are less rigid than eukaryotic membranes  Plasma membrane structure The phospholipid molecules are arranged in two parallel rows, called a lipid bilayer Each phospholipid molecule contains a polar head, composed of a phosphate group and glycerol that is hydrophilic (water-loving) and soluble in water, and nonpolar tails, composed of fatty acids that are hydrophobic (water-fearing) and insoluble in water polar heads are on the two surfaces of the lipid bilayer, and the nonpolar tails are in the interior of the bilayer peripheral proteins: easily removed from the membrane by mild treatments and lie at the inner or outer surface of the membrane. function as enzymes that catalyze chemical reactions, as a “scaffold” for support, and as mediators of changes in membrane shape during movement integral proteins: can be removed from the membrane only after disrupting the lipid bilayer (by using detergents, for example). Most integral proteins penetrate the membrane completely and are called transmembrane proteins. Some integral proteins are channels that have a pore, or hole, through which substances enter and exit the cell glycoproteins: proteins attached to carbohydrates glycolipids: lipids attached to carbohydrates Plasma membrane. (a) A diagram and micrograph showing the lipid bilayer forming the inner plasma membrane of the gram- negative bacterium Vibrio cholerae. Layers of the cell wall, including the outer membrane, can be seen outside the inner membrane. (b) A portion of the inner membrane showing the lipid bilayer and proteins. The outer membrane of gram- negative bacteria is also a lipid bilayer. (c) Space-filling models of several phospholipid molecules as they are arranged in the lipid bilayer.  Plasma membrane functions serve as a selective barrier through which materials enter and exit the cell selective permeability (sometimes called semipermeability): certain molecules and ions are allowed to pass through the membrane but others are stopped important to the breakdown of nutrients and the production of energy contain enzymes capable of catalyzing the chemical reactions that break down nutrients and produce ATP. In some bacteria, pigments and enzymes involved in photosynthesis are found in infoldings of the plasma membrane that extend into the cytoplasm (chromatophores) antimicrobial agents specifically damage plasma membranes. These compounds include certain alcohols and quaternary ammonium compounds, which are used as disinfectants. By disrupting the membrane’s phospholipids, a group of antibiotics known as the polymyxins cause leakage of intracellular contents and subsequent cell death  The Movement of Materials across Membranes Passive Processes: substances cross the membrane from an area of high concentration to an area of low concentration Active Process: the cell must use energy to move substances from areas of low concentration to areas of high concentration (against the concentration gradient)  Passive process Simple Diffusion is the net (overall) movement of molecules or ions from an area of high concentration to an area of low concentration The movement continues until the molecules or ions are evenly distributed (equilibrium) transport certain small molecules, such as oxygen and carbon dioxide, across their cell membranes The principle of simple diffusion. (a) After a dye pellet is put into a beaker of water, the molecules of dye in the pellet diffuse into the water from an area of high dye concentration to areas of low dye concentration. (b) The dye potassium permanganate in the process of diffusing Facilitated diffusion integral membrane proteins (transporter proteins or permeases) function as channels or carriers that facilitate the movement of ions or large molecules across the plasma membrane Some transporters permit the passage of mostly small, inorganic ions that are too hydrophilic to penetrate the nonpolar interior of the lipid bilayer (nonspecific) Other transporters, which are common in eukaryotes, are specific and transport only specific, usually larger, molecules, such as simple sugars (glucose, fructose, and galactose) and vitamins extracellular enzymes: can break down large molecules into simpler ones (such as proteins into amino acids, or polysaccharides into simple sugars) Osmosis net movement of water molecules across a selectively permeable membrane from an area with a high concentration of water molecules (low concentration of solute molecules) to an area of low concentration of water molecules (high concentration of solute molecules) aquaporins: water channels Osmotic pressure is the pressure required to prevent the movement of pure water (water with no solutes) into a solution containing some solutes isotonic solution is a medium in which the overall concentration of solutes equals that found inside a cell (iso means equal) hypotonic solution outside the cell is a medium whose concentration of solutes is lower than that inside the cell (hypo means under or less) hypertonic solution is a medium having a higher concentration of solutes than that inside the cell (hyper means above or more) The principle of osmosis. (a) Setup at the beginning of an osmotic pressure experiment. Water molecules start to move from the beaker into the sack along the concentration gradient. (b) Setup at equilibrium. The osmotic pressure exerted by the solution in the sack pushes water molecules from the sack back into the beaker to balance the rate of water entry into the sack. The height of the solution in the glass tube at equilibrium is a measure of the osmotic pressure.  Active process In active transport, the cell uses energy in the form of ATP to move substances across the plasma membrane substances actively transported are ions (for exam- ple Na+, K+, H +, Ca2+, and Cl-), amino acids, and simple sugars There appears to be a different transporter for each substance or group of closely related substances. Active transport enables microbes to move substances across the plasma membrane at a constant rate, even if they are in short supply group translocation: a special form of active transport that occurs exclusively in prokaryotes, the substance is chemically altered during transport across the membrane enables a cell to accumulate various substances even though they may be in low concentrations outside the cell  Cytoplasm refers to the substance of the cell inside the plasma membrane about 80% water and contains primarily proteins (enzymes), carbohydrates, lipids, inorganic ions, and many low-molecular- mass compounds thick, aqueous, semitransparent, and elastic nucleoid (containing DNA), particles called ribosomes, and reserve deposits called inclusions cytoskeleton is a collective term for a series of fibers (small rods and cylinders) in the cytoplasm Components include MreB and ParM, cresetin, and FtsZ, which correspond to the micro- filaments, intermediate filaments, and microtubules of the eukaryotic cytoskeleton, respectively assumes roles in cell division, maintaining cell shape, growth, DNA movement, protein targeting, and alignment of organelles  The Nucleoid usually contains a single long, continuous, and frequently circularly arranged thread of double-stranded DNA called the bacterial chromosome Unlike the chromosomes of eukaryotic cells, bacterial chromosomes are not surrounded by a nuclear envelope (membrane) and do not include histones spherical, elongated, or dumbbell-shaped plasmids: small usually circular, double-stranded DNA molecules extrachromosomal genetic elements; that is, they are not connected to the main bacterial chromosome, and they replicate independently of chromosomal DNA may carry genes for such activities as antibiotic resistance, tolerance to toxic metals, the production of toxins, and the synthesis of enzymes. Plasmids can be transferred from one bacterium to another  Ribosomes are composed of two subunits, each of which consists of protein and a type of RNA called ribosomal RNA (rRNA) Accordingly, prokaryotic ribosomes are called 70S ribosomes, and those of eukaryotic cells are known as 80S ribosomes S refers to Svedberg units, which indicate the relative rate of sedimentation during ultra-high-speed centrifugation Sedimentation rate is a function of the size, weight, and shape of a particle Antibiotics such as streptomycin and gentamicin attach to the 30S subunit and interfere with protein synthesis. Other antibiotics, such as erythromycin and chloramphenicol, interfere with protein synthesis by attaching to the 50S subunit The prokaryotic ribosome. (a) A small 30S subunit and (b) a large 50S subunit make up (c) the complete 70S prokaryotic ribosome.  Inclusions reserve deposits Cells may accumulate certain nutrients when they are plentiful and use them when the environment is deficient Some inclusions, such as magnetosomes, are membrane- enclosed organelles, while other inclusions, such as carboxysomes, are enclosed in protein complexes Metachromatic granules are large inclusions that take their name from the fact that they sometimes stain red with certain blue dyes such as methylene blue Collectively known as volutin (represents a reserve of inorganic phosphate (polyphosphate) that can be used in the synthesis of ATP) found in algae, fungi, and protozoa, as well as in bacteria characteristic of Corynebacterium diphtheriae Polysaccharide Granules consist of glycogen and starch, their presence can be demonstrated when iodine is applied to the cells; glycogen granules appear reddish brown and starch granules appear blue Lipid inclusions- appear in various species of Mycobacterium, Bacillus, Azotobacter, Spirillum, and other genera. A common lipid-storage material, one unique to bacteria, is the polymer poly-β-hydroxybutyric acid Sulfur Granules Certain bacteria—for example, the “sulfur bacteria” that belong to the genus Acidithiobacillus—derive energy by oxidizing sulfur and sulfur-containing compounds Carboxysomes inclusions that contain the enzyme ribulose 1,5-bisphosphate carboxylase. Photosynthetic bacteria use carbon dioxide as their sole source of carbon and require this enzyme for carbon dioxide fixation. nitrifying bacteria, cyanobacteria, and acidithiobacilli. Gas vacuoles Hollow cavities found in many aquatic prokaryotes, includ- ing cyanobacteria, anoxygenic photosynthetic bacteria, and halobacteria maintain buoyancy so that the cells can remain at the depth in the water appropriate for them to receive sufficient amounts of oxygen, light, and nutrients Magnetosomes inclusions of iron oxide (Fe3O4) surrounded by invaginations of the plasma membrane Bacteria may use magnetosomes to move down- ward until they reach a suitable attachment site can decompose hydrogen peroxide, which forms in cells in the presence of oxygen Magnetosomes. This micrograph of Magnetospirillum magnetotacticum shows a chain of magnetosomes. This bacterium is usually found in shallow freshwater mud  Endospores When essential nutrients are depleted, certain gram- positive bacteria, such as those of the genera Clostridium and Bacillus, form specialized “resting” cells called endospores highly durable dehydrated cells with thick walls and additional layers formed internal to the bacterial cell membrane can survive extreme heat, lack of water, and exposure to many toxic chemicals and radiation The process of endospore formation within a vegetative cell takes several hours and is known as sporulation or sporogenesis  Endospores Depending on the species, the endospore might be located terminally (at one end), subterminally (near one end), or centrally inside the vegetative cell contains a large amount of an organic acid called dipicolinic acid (DPA), which is accompanied by a large number of calcium ions DPA protects the endospore DNA against damage highly dehydrated endospore core contains only DNA, small amounts of RNA, ribosomes, enzymes, and a few important small molecules An endospore returns to its vegetative state by a process called germination. Germination is triggered by high heat, such as is used in canning, or small triggering molecules called germinants (alanine and inosine) endospore’s enzymes then break down the extra layers surrounding the endospore, water enters, and metabolism resumes  The Eukaryotic Cell algae, protozoa, fungi, plants, and animals larger and structurally more complex than the prokaryotic cell  Flagella and Cilia flagella: If the projections are few and are long in relation to the size of the cell cilia (singular: cilium):If the projections are numerous and short Both flagella and cilia are anchored to the plasma membrane by a basal body, and both consist of nine pairs of microtubules (doublets) arranged in a ring, plus another two microtubules in the center of the ring, an arrangement called a 9 + 2 array Microtubules are long, hollow tubes made up of a protein called tubulin Eukaryotic flagella and cilia. (a) A micrograph of Euglena, a chlorophyll-containing protozoan, with its flagellum. (b) A micrograph of Tetrahymena, a common freshwater protozoan, with cilia. (c) The internal structure of a flagellum (or cilium), showing the 9 + 2 arrangement of microtubules. (d) The pattern of movement of a eukaryotic flagellum The Cell Wall and Glycocalyx Most eukaryotic cells have cell walls, but generally much simpler than those of prokaryotic cells Many algae have cell walls consisting of the polysaccharide cellulose in most fungi the principal structural component of the cell wall is the polysaccharide chitin, a polymer of N-acetylglucosamine (NAG) units The cell walls of yeasts contain the polysaccharides glucan and mannan Protozoa do not have a typical cell wall; they have a flexible outer protein covering called a pellicle eukaryotic cells, including animal cells, the plasma membrane is covered by a glycocalyx, a layer of material containing substantial amounts of sticky carbohydrates  The Plasma (Cytoplasmic) Membrane Eukaryotic membranes also contain carbo- hydrates, which serve as attachment sites for bacteria and as receptor sites that assume a role in such functions as cell– cell recognition also contain sterols, complex lipids not found in prokaryotic plasma membranes Group translocation does not occur in eukaryotic cells endocytosis: occurs when a segment of the plasma membrane surrounds a particle or large molecule, encloses it, and brings it into the cell phagocytosis: cellular projections called pseudopods engulf particles and bring them into the cell pinocytosis: the plasma membrane folds inward, bringing extracellular fluid into the cell, along with whatever substances are dissolved in the fluid receptor-mediated endocytosis: substances (ligands) bind to receptors in the membrane. When binding occurs, the membrane folds inward.  Cytoplasm cytoplasm of eukaryotic cells encompasses the substance inside the plasma membrane and outside the nucleus cytoskeleton of eukaryotes consists of small rods (microfilaments and intermediate filaments) and cylinders (microtubules) provides support, shape, and assistance in transporting substances through the cell cytoplasmic streaming: movement of eukaryotic cytoplasm from one part of the cell to another, which helps distribute nutrients and move the cell over a surface many of the important enzymes found in the cytoplasmic fluid of prokaryotes are sequestered in the organelles of eukaryotes  Ribosomes sites of protein synthesis in the cell somewhat larger and denser than those of prokaryotic cells. 80S ribosomes, each of which consists of a large 60S subunit containing three molecules of rRNA and a smaller 40S subunit with one molecule of rRNA free ribosomes: unattached to any structure in the cytoplasm,synthesize proteins inside the cell membrane-bound ribosomes: attach to the nuclear membrane and the endoplasmic reticulum, synthesize proteins destined for insertion in the plasma membrane or for export from the cell  Organelles structures with specific shapes and specialized functions and are characteristic of eukaryotic cells include the nucleus, endoplasmic reticulum, Golgi complex, lysosomes, vacuoles, mitochondria, chloroplasts, peroxisomes, and centrosomes The Nucleus spherical or oval, is frequently the largest structure in the cell, and contains almost all of the cell’s hereditary information (DNA) surrounded by a double membrane called the nuclear envelope nuclear pores allow the nucleus to communicate with the cytoplasm Nucleoli are actually condensed regions of chromosomes where ribosomal RNA is being synthesized contains most of the cell’s DNA, which is combined with several proteins, including some basic proteins called histones and nonhistone When the cell is not reproducing, the DNA and its associated proteins appear as a threadlike mass called chromatin. During nuclear division, the chromatin coils into shorter and thicker rodlike bodies called chromosomes mitosis and meiosis are absent in prokaryotes The eukaryotic nucleus. (a) A micrograph of a nucleus. (b) Drawing of details of a nucleus. Endoplasmic Reticulum extensive network of flattened membranous sacs or tubules called cisternae Golgi Complex consists of 3 to 20 cisternae that resemble a stack of pita bread The cisternae are often curved, giving the Golgi complex a cuplike shape Proteins synthesized by ribosomes on the rough ER are surrounded by a portion of the ER membrane, which eventually buds from the membrane surface to form a transport vesicle transport vesicle fuses with a cistern of the Golgi complex, releasing proteins into the cistern The proteins are modified and move from one cistern to another via transfer vesicles that bud from the edges of the cisternae Some of the processed proteins leave the cisternae in secretory vesicles, which detach from the cistern and deliver the proteins to the plasma membrane, where they are discharged by exocytosis Lysosomes formed from Golgi complexes and look like membrane- enclosed spheres contain as many as 40 different kinds of digestive enzymes capable of breaking down various molecules Vacuoles a space or cavity in the cytoplasm of a cell that is enclosed by a membrane called a tonoplast Some vacuoles serve as temporary storage organelles for substances such as proteins, sugars, organic acids, and inorganic ions. Other vacuoles form during endocytosis to help bring food into the cell may take up water, enabling plant cells to increase in size and also providing rigidity to leaves and stems Mitochondria powerhouse of the cell 70S ribosomes and some DNA of their own, as well as the machinery necessary to replicate, transcribe, and translate the information encoded by their DNA Chloroplasts Algae and green plants a double membrane-enclosed structure that contains both the pigment chlorophyll and the enzymes required for the light- gathering phases of photosynthesis The chlorophyll is contained in flattened membrane sacs called thylakoids; stacks of thylakoids are called grana (singular: granum) Peroxisomes Organelles similar in structure to lysosomes, but smaller contain one or more enzymes that can oxidize various organic substances enzymes in peroxisomes oxidize toxic substances, such as alcohol also contain the enzyme catalase, which decomposes H2O2 Centrosomes located near the nucleus, consists of two components: the pericentriolar area and centrioles pericentriolar material is a region of the cytosol composed of a dense network of small protein fibers centrioles, each of which is composed of nine clusters of three microtubules (triplets) arranged in a circular pattern, an arrangement called a 9 + 0 array The Evolution of Eukaryotes endosymbiotic theory larger bacterial cells lost their cell walls and engulfed smaller bacterial cells. This relationship, in which one organism lives within another, is called endosymbiosis (symbiosis = living together)