Chapter 4

Eukaryotic Cell Structure1515.1 A typical eukaryotic cellCompare and contrast eukaryotic, bacterial, and archaeal cells in terms of their use of membranes, size, morphological diversity, and organelles.2Eukaryotic MicroorganismsProminent members of ecosystemsUseful as model systems and industrySome are major human pathogensTwo groups of eukaryotes commonly possess microbial membersprotists fungi34

Common Features of Eukaryotic CellsMembrane-delimited nucleiMembrane-bound organelles that perform specific functionsIntracytoplasmic membrane complex serves as transport systemMore structurally complex and generally larger than bacterial or archaeal cells556

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5.2 Eukaryotic cell envelopesIdentify the types of eukaryotic microbes that have cell walls and distinguish them from plant cell walls.Compare and contrast the cell envelopes of members of Bacteria, Archaea, and Eukarya in terms of their component layers, molecular make-up, and function.8Eukaryotic Cell EnvelopesConsists of the plasma membrane and all coverings external to itPlasma membrane is a lipid bilayermajor membrane lipids include phosphoglycerides, sphingolipids, and cholesterol, all of which contribute to strength of membranemicrodomains participate in variety of cellular processes9910

Eukaryotic Cell Envelopes - 2Unlike the peptidoglycan in the cell wall of Bacteria and Archaea, many eukaryotes lack or have a chemically distinct cell wall Cell walls of photosynthetic algae have cellulose, pectin, and silicaCell walls of fungi consist of cellulose, chitin, or glucan 115.3 Cytoplasm of eukaryotesDescribe the functions of the cytoplasm.Identify the three filaments that make up the cytoskeleton of eukaryotic cells and describe their functions.12The Cytoplasm of EukaryotesConsists of liquid, the cytosol, and many organellesCytoskeletonvast network of interconnected filaments within the cytoplasmic matrixfilaments that form the cytoskeleton: microfilaments (actin), microtubules, intermediate filaments, and motor proteinsplays role in both cell shape and cell movement131314

MicrofilamentsSmall protein filaments, 4 to 7 nm in diameterScattered within cytoplasmic matrix or organized into networks and parallel arraysComposed of actin proteinInvolved in cell motion and shape changes15

15Intermediate FilamentsHeterogeneous elements of the cytoskeleton, ~10 nm in diameterKeratin and vimentin classesRole in cell is unclearplay structural role some shown to form nuclear laminaothers help link cells together to form tissues1616MicrotubulesShaped like thin cylinders ~25 nm in diameter of - and -tubulinHelp maintain cell shapeInvolved with microfilaments in cell movements Participate in intracellular transport processes 17

175.4 Organelles of the secretory and endocytic pathwaysDifferentiate the two types of endoplasmic reticulum in terms of structure and function.Outline the pathway of molecules through the secretory pathway, noting the structures involved and their role in the process.List the endocytic pathways observed in mammalian cells, noting the structures involved and their role in the process, and noting those pathways that have been observed in eukaryotic microbes.18Secretory Endocytic Pathway Intricate complex of membranous organelles and vesicles that move materials into the cell from outside, from inside to outside, and within the cellEndoplasmic reticulum (ER)Golgi apparatusLysosomes1919Endoplasmic Reticulum (ER)Irregular network of branching and fusing membranous tubules and flattened sacs (cisternae s., cisterna)Rough ERribosomes attachedsynthesis of secreted proteins by ER-associated ribosomesSmooth ERdevoid of ribosomessynthesis of lipids by ER-associated enzymes2020Functions of ERTransports proteins, lipids, and other materials within cellMajor site of cell membrane synthesis2121The Golgi ApparatusMembranous organelle made of cisternae stacked on each otherCis and trans facesDictyosomes=stacks of cisternaeInvolved in modification, packaging, and secretion of materials22

22LysosomesMembrane-bound vesicles found in most eukaryotesInvolved in intracellular digestion Contain hydrolases, enzymes which hydrolyze molecules and function best under slightly acidic conditionsMaintain an acidic environment by pumping protons into their interior2323The Secretory PathwayUsed to move materials to various sites within the cell, as well as to either the plasma membrane or cell exteriorProteins destined for the cell membrane, endosomes, and lysosomes or secretion are synthesized by ribosomes on rough endoplasmic reticulum (RER)Targeted to RER lumen and are released in small budding vesicles from RER2424Secretory Pathway - 2Released in small vesicles cis face of Golgi apparatus trans face of Golgi apparatusmodification of proteins occurs in Golgi; targets protein for final destinationTransport vesicles released from trans face of Golgi

25The Secretory Pathway - 3After release some vesicles deliver their contents to endosomes and lysosomesTwo types of vesicles deliver proteins to cell membraneconstitutive delivery to membranesecretory vesicles in multicellular eukaryotes store proteins until signal to release2626The Secretory Pathway - 4Quality assurance mechanismunfolded or misfolded proteins are secreted into cytosol, targeted for destruction by ubiquitin polypeptidesproteasomes destroy targeted proteins

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The Endocytic PathwayEndocytosis used by all eukaryotic cellsused to bring materials into the cellsolutes or particles taken up and enclosed in vesicles pinched from plasma membranein most cases materials are then delivered to lysosome and destroyed 2828Types of EndocytosisPhagocytosis: use of cell surface protrusions to surround and engulf particlesClathrin-dependent: clathrin protein-coated pits have external receptors that specifically bind macromoleculesCaveolae-dependent endocytosis: may play role in signal transduction, transport of small as well as macromolecules

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Endocytosis Clathrin-coated vesicles and some caveolin-coated vesicles deliver contents to endosomes (organelles with hydrolytic enzymes)Early endosomes develop into late endosomes which fuse with lysosomesCaveosomes fuse with early endosomes30AutophagyDelivery of materials to be digested by route that does not involve endocytosisMacroautophagy involves digestion and recycling of cytoplasmic components Double membrane surrounds cell component forming an autophagosomeAutophagosome fuses with a lysosome31

31Once Lysosome Is FormedDigestion occurs without release of lysosome enzymes into cytoplasmic matrixAs contents are digested, products leave lysosome and can be used as nutrients Resulting lysosome called a residual body which can release contents to cell exterior by process called lysosome secretion

32325.5 Organelles involved in genetic control of the cellDescribe the structure of the nucleus, chromosomes, nucleolus, and eukaryotic ribosomes.Compare and contrast the chromosomes and ribosomes of bacterial, archaeal, and eukaryotic cells.3334Organelles Involved in Genetic Control of the Cell NucleusRibosomesThe Nucleus - 1Membrane-bound spherical structure that houses genetic material of eukaryotic cellContains dense fibrous material called chromatincomplex of DNA, histones, and other proteinsfive types of histones form nucleosomesH1, H2A, H2B, H3, and H4chromatin condenses into chromosomes during division

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35The Nucleus - 2Nuclear envelopedouble membrane structure that delimits nucleuscontinuous with ERpenetrated by nuclear poresassociated proteins make up the nuclear pore complex pores allow materials to be transported into or out of nucleus36

36The Nucleolus 1 nucleolus/nucleusOrganelle but not membrane enclosedImportant in ribosome synthesisdirects synthesis and processing of rRNAdirects assembly of rRNA to form partial ribosomal subunitsribosomes mature in cytoplasm37

37Eukaryotic Ribosomes - 1Larger (more mass) than the 70S bacterial and archaeal ribosomes80S in size60S + 40S subunitsMay be attached to ER or free in cytoplasmic matrix60S is bound subunit to ER38

38Eukaryotic Ribosomes - 2Proteins made on ribosomes of RER are often secreted or inserted into ER membrane as integral membrane proteinsFree ribosomes synthesize nonsecretory and nonmembrane proteins some proteins are inserted into organelles

39395.6 Organelles involved in energy conservationDraw a mitochondrion and identify its component parts.Compare and contrast mitochondria and hydrogenosomes in terms of their structure and the chemical processes they carry out.Draw a chloroplast and identify its component parts.40Organelles Involved in Energy ConservationMitochondriaHydrogenosomesChloroplasts41Endosymbiotic HypothesisMitochondria, hydrogenosomes, and chloroplasts are all thought to have evolved from bacterial cells that invaded or were ingested by early ancestors of eukaryotic cellsmitochondria and chloroplasts are very similar to extant bacteria and cyanobacteria, respectively42MitochondriaThe power houses of the cell are found in most eukaryotic cellsSite of tricarboxylic acid cycle activitySite where ATP is generated by electron transport and oxidative phosphorylationAbout the same size as bacterial cellsReproduce by binary fission as do bacterial cells4343Mitochondrial StructureOuter membranecontains porins similar to the outer membrane of Gram-negative bacteriaInner membranehighly folded to form cristae (s., crista)location of enzymes and electron carriers for electron transport and oxidative phosphorylation4444Mitochondrial StructureMatrix enclosed by inner membranecontains ribosomes (same size as bacterial), mitochondrial DNA (may be closed circular like bacterial DNA)contains enzymes of the tricarboxylic acid cycle and enzymes involved in catabolism of fatty acids45

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HydrogenosomesSmall energy conservation organelles in some anaerobic protistsDescended from common mitochondrial ancestordouble membrane, no cristae, usually lack DNAATP is generated by fermentation process rather than respirationCO2, H2, and acetate are products46ChloroplastsType of plastidpigment-containing organelles observed in plants and algaeSite of photosynthetic reactionsSurrounded by double membrane4747Chloroplast StructureThe stroma (a matrix) is within inner membrane contains DNA, ribosomes, lipid droplets, starch granules, and thylakoids thylakoids flattened, membrane-delimited sacsgrana (s., granum) stacks of thylakoidssite of light reactions (trapping of light energy to generate ATP, NADPH, and oxygen)4848

Chloroplast StructureStroma is site of dark reactions of photosynthesis (formation of carbohydrates from water and carbon dioxide)Algal chloroplasts many contain a pyrenoidparticipates in polysaccharide synthesis49495.7 External structuresDescribe the structures of eukaryotic flagella and cilia.Compare and contrast bacterial, archaeal, and eukaryotic flagella.List the types of motility observed in eukaryotic microbes.50External Cell CoveringCiliaFlagella5151

Cilia and FlagellaFlagella (s., flagellum)100-200 m longmove in undulating fashionCilia (s., cilium)5-20 m longbeat with two phases, working like oars

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Ultrastructure of Flagella and CiliaMembrane-bound cylinders ~2 m in diameterAxoneme: set of microtubules in a 9+2 arrangementBasal bodyat base of flagellum or ciliumdirects synthesis of flagella and cilia54

545.8 Comparison of bacterial, archaeal, and eukaryotic cellsCreate a Venn diagram or concept map that clearly distinguishes bacterial, archaeal, and eukaryotic cells in terms of their genome organization, organelles, cell envelopes, ribosome size and component molecules, and cytoskeleton.Determine the type of microbe when given a description of a newly discovered microbe.55Comparison of Bacterial, Archaeal, and Eukaryotic CellsDifferences in eukaryotic cellseukaryotic nucleuslarger, more complexmeiosis, mitosiscomplex processesMolecular unity basic to all three cellsbiochemical processes, metabolic pathwaysgenetic code5657

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