Chapter 4: Tour Of The Cell

Chapter 4: Tour of the CellBIO100Fall 2007THE MICROSCOPIC WORLD OFCELLS Cells must be tiny for materials to move inand out of them and fast enough to meet thecell’s metabolic needs. Organisms are eitherSingle-celled, such as most bacteria andprotistsMulticelled, such as plants, animals, and mostfungi.1

Microscopes as Windowsto CellsThe light microscopeis used by manyscientists Light passesthrough thespecimenLensesenlarge, ormagnify, theimage.(a) Light micrograph (LM) of a white blood cell(stained purple) surrounded by red blood cellsFigure 4.2AHow We Study Cells Light Microscope: Simple vs.Compound?Magnification First cells observed by Robert Hooke in 1665using a light microscope.An increase in the specimen’s apparent sizeResolving power The ability of an optical instrument to show twoobjects as separate.2

Cells were first discovered in 1665 byRobert Hooke The accumulation of scientific evidence ledto the cell theory, p. 57 The electron microscope (EM) uses abeam of electrons It has a higher resolving power than thelight microscope.The electronmicroscopecan magnifyup to100,000X Such powerreveals thediverse partswithin a cell.Electron microscope All living things are composed of cellsAll cells form from previously existing cellsCells are the smallest units capable of carryingout the processes of life: ex. respiration,digestion, reproduction, growth, ingestion, etc.Unaided eye Light microscope Figure 4.33

Metric Prefixes:Kilo 1000so 10 Kcalories 10 000 calories 10 CHecto 100Deka 10Unit 1 ex. meter, liter, gramdeci 0.1centi 0.01so 1 cm 10 mmmilli 0.001 The scanningelectronmicroscope(SEM) is usedto study thedetailedarchitecture ofthe surface ofa cell.(b) Scanning electron micrograph (SEM) of a whiteblood cellFigure 4.2B The transmission electron microscope(TEM) is useful for exploring theinternal structure of a cell.(c) Transmission electron micrograph (TEM) of a white blood cellFigure 4.2C4

The Two Major Categories of Cells The countless cells on earth fall intotwo categories Prokaryotic cellsEukaryotic cellsProkaryotic and eukaryotic cells differin several respects.Prokaryotic cellEukaryotic cell Nucleoid regionNucleusOrganellesFigure 4.4Prokaryotic cells Are smaller than eukaryotic cellsLack internal structures surrounded bymembranesLack a nucleus.5

ProkaryoticflagellaNucleoid region (DNA)RibosomesPlasmamembraneCell wallCapsulePiliFigure 4.5A Panoramic View of Eukaryotic umNot in mostplant asmicreticulum (ER)Smoothendoplasmicreticulum (ER)GolgiapparatusFigure 4.6A, p. 59 An idealized plant cell.Not in animal sCell wallRough endoplamsicreticulum asmicreticulum (ER)PlasmodesmataGolgi apparatusFigure 4.6B, p. 596

The nucleus is an organelle which contains longfibers made of DNA molecules and associatedproteins. Each fiber, known as chromatin, becomesa chromosomeHumans have 46 chromosomes in the nucleus ofeach and every cellAlso within the nucleus is the nucleolus whichis a ball-like mass of fibers and granules whichproduces the component parts of ribosomes.Ribosomes move through the pores of thenucleus then are responsible for proteinsynthesis. Some are associated with “rough”ER others remain suspended in the cytosol.MEMBRANE STRUCTURE ANDFUNCTION The plasma membrane separates the livingcell from its nonliving surroundingsThe entire region of cell between thenucleus and plasma membrane is thecytoplasmCytoplasm consists of organellessurrounded by a liquid known as cytosol.7

A Fluid Mosaic of Lipids and Proteins The membranes of cells are composedof LipidsProteins. Phospholipids form a two-layeredmembrane, the phospholipid bilayer. The lipids belong to a special categorycalled phospholipidsOutside cellHydrophilicheadHydrophobictailCytoplasm(inside cell)(a) Phospholipid bilayer of membrane Figure 4.7AMost membranes have specificproteins embedded in the phospholipidbilayer.Hydrophilicregion ofproteinPhospholipidbilayerHydrophobicregion of protein(b) Fluid mosaic model of membraneFigure 4.7B8

CytoplasmFibers ofextracellularmatrixc Enzymatic activitySome functions of membrane proteins. b Cell signalinga Attachment tocytoskeleton andextracellularmatrixe Intercellularjoiningd TransportCytoskeletonf Cell-cellrecognitionCytoplasmFigure 4.8 Membrane phospholipids and proteinscan drift about in the plane of themembraneThis behavior leads to the description of amembrane as a fluid mosaic Molecules can move freely within themembraneA diversity of proteins exists within themembrane.Selective Permeability Membranes of the cell are selectivelypermeable They allow some substances to cross moreeasily than othersThey block passage of some substancesaltogether.9

The traffic of some substances canonly occur through transport proteins Glucose, for example, requires a transportprotein to move it into the cell.THE NUCLEUS AND RIBOSOMES:GENETIC CONTROL OF THE CELL The nucleus is the manager of the cell Genes found on the chromosomes within thenucleus store information necessary toproduce proteins.Structure and Function of the Nucleus The nucleus is bordered by a doublemembrane called the nuclear envelope It contains chromatinIt contains a nucleolus.10

re 4.9Ribosomes Ribosomes build all the cell’s proteins.How DNA Controls the CellDNA DNA controls thecell by transferringits codedinformation intoRNA The information inthe RNA is usedto make proteins.Figure 4.101 Synthesis ofmRNA in thenucleusmRNANucleusCytoplasm2 Movement ofmRNA intocytoplasm vianuclear pore3 Synthesis ofprotein in thecytoplasmmRNARibosomeProtein11

THE ENDOMEMBRANE SYSTEM:MANUFACTURING AND DISTRIBUTINGCELLULAR PRODUCTS Many of the membranous organelles in thecell belong to the endomembrane system.The Endoplasmic Reticulum Theendoplasmicreticulum(ER) Produces anenormousvariety ofmoleculesIs composed ofsmooth andrough ER.NuclearenvelopeRibosomesRough ERSmooth ERFigure 4.11Rough ER Again, the “roughness” of the roughER is due to ribosomes that stud theoutside of the ER membrane.12

The functions of the rough ER include Producing proteinsProducing new membrane.After the rough ER synthesizes amolecule it packages the molecule intotransport vesicles4Transport vesiclebuds offRibosome3Secretoryprotein insidetransportvesicleProtein12Rough ERPolypeptideFigure 4.12Smooth ER The smooth ER lacks the surfaceribosomes of ER and produces lipids,including steroids.13

The Golgi ApparatusWorks in partnership with the ERRefines, stores, and distributes the products ofcells. Transportvesiclefrom ER“Receiving” side ofGolgi apparatusGolgi apparatusNew vesicle formingTransport vesiclefrom the Golgi“Shipping” side ofGolgi apparatusPlasma membraneFigure 4.13Lysosomes A lysosome is a membrane-enclosedsac It contains digestive enzymesThe enzymes break down macromolecules. Solysosomes are responsible for intracellulardigestion.If its membrane were to break its contentswould digest the cell Lysosomes have several types ofdigestive functions They fuse with food vacuoles to digest thefood.LysosomeDigestive enzymesPlasmamembraneDigestionFoodFood vacuole(a) Lysosome digesting foodFigure 4.14a14

They break down damaged organellesThey carry out the intracellular digestion.LysosomeDigestionDamagedorganelle(b) Lysosome breaking down damaged organelleFigure 4.14bVacuoles Vacuoles are membranous sacs Two types are the contractile vacuoles ofprotists and the central vacuoles of plants.CentralvacuoleContractilevacuoles(a) Contractile vacuoles in a protist(b) Central vacuole in a plant cellFigure 4.15Rough ERTransportvesicle from ER A review of le from GolgiSecretoryproteinVacuoleLysosomePlasma membraneFigure 4.1615

CHLOROPLASTS AND MITOCHONDRIA:ENERGY CONVERSION Cells require a constant energy supply to doall the work of life.Nuclei, chloroplasts, and mitochondria areorganelles having double membranes.and outerCHLOROPLASTS Innermembranes ofenvelopeChloroplastsare the sites ofphotosynthesis,the conversionof light energyto chemicalenergy. GranumSpace betweenmembranesStroma (fluid inchloroplast)Figure 4.17Mitochondria Mitochondria are the sites of cellular respiration,which involves the production of ATP from rixSpace betweenmembranesFigure 4.1816

ATPTHE CYTOSKELETON:CELL SHAPE AND MOVEMENT The cytoskeleton is an infrastructure of thecell consisting of a network of fibers.Maintaining Cell Shape One function ofthe cytoskeleton Providemechanicalsupport tothe cell andmaintain itsshape.Figure 4.19A17

The cytoskeletoncan change theshape of a cell This allows cells likeamoebae to move.Figure 4.19BCilia and Flagella Cilia and flagella are motileappendages. Flagella propelthe cell in a whiplike motion Cilia move in acoordinated backand-forth motion.Figure 4.20A, B18

Some cilia orflagella extendfrom nonmovingcells The humanwindpipe is linedwith cilia.Figure 4.20CCELL SURFACES:PROTECTION, SUPPORT, AND CELL-CELLINTERACTIONSMost cells secrete materials that areexternal to the plasma membrane. Plant Cell Walls and Cell Junctions Plant cells are encased by cell walls These provide support for the plant cells.Walls of two adjacentplant cellsVacuolePlasmodesmata(channels between cells)Figure 4.2119

Animal Cell Surfaces and Cell Junctions Animal cells lack cell walls They secrete a sticky covering called theextracellular matrixThis layer helps hold cells together. THE ORIGIN OF MEMBRANES Phospholipids were probably among the firstorganic molecules on the early Earth.SUMMARY OF KEY CONCEPTS The Two Major Categories of Cells.Visual Summary 4.120

Membrane Structure and Function A Fluid Mosaic of Lipids and Proteins.Outside philicCytoplasm (inside cell)Visual Summary 4.221

The nucleus is the manager of the cell THE NUCLEUS AND RIBOSOMES: GENETIC CONTROL OF THE CELL Genes found on the chromosomes within the nucleus store information necessary to produce proteins. The nucleus is bordered by a double membrane called the nuclear envelope Structure and Function of the Nucleus It contains chromatin It