Biology_Unit_5 (1110837), страница 2
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For example,human intestinal cells have closely packed, fingerlike extensionsthat increase their surface area, which greatly enhances theirability to absorb digested food molecules.Cells Have a DNA-Containing Central RegionThat Is Surrounded by CytoplasmAll cells are bounded by the plasma membrane, a bilayer made oflipids with embedded protein molecules (Figure 5.6). The lipid bilayer is a hydrophobic barrier to the passage of water-soluble substances, but selected water-soluble substances can penetrate cellmembranes through transport protein channels. The selectivemovement of ions and water-soluble molecules through the transport proteins maintains the specialized internal ionic and molecular environments required for cellular life.
(Membrane structureand functions are discussed further in Chapter 6.)FIGURE 5.4Research MethodLight and Electron MicroscopyPurpose: In biology, microscopy is used to view organisms, cells, and structures within cellsin their natural state or after being treated (stained) so that specific structures can be seenmore clearly. All of the photographs of cells and cell structures in this book were made usingmicroscopy.Protocol: A light microscope uses a beam of light to illuminate the specimen and forms amagnified image of the specimen with glass lenses. An electron microscope uses a beam ofelectrons to illuminate the specimen and forms an image with magnetic fields.
Electron microscopy provides higher resolution and higher magnification than light microscopy.Differences in refraction (theway light is bent) caused byvariations in the density of thespecimen are visualized asdifferences in contrast.Otherwise invisible structuresare revealed with thistechnique, and living cells inaction can be photographedor filmed.Nomarski (differentialinterference contrast):Similar to phase-contrastmicroscopy, special lensesenhance differences in density,giving a cell a 3D appearance.Fluorescence microscopy:Different structures ormolecules in cells are stainedwith specific fluorescent dyes.The stained structures ormolecules fluoresce when themicroscope illuminates themwith ultraviolet light, and theirlocations are seen by viewingthe emitted visible light.Transmission electronmicroscopy (TEM): A beamof electrons is focused on athin section of a specimen in avacuum.
Electrons that passthrough form the image;structures that scatter electronsappear dark. TEM is usedprimarily to examine structureswithin cells. Various stainingand fixing methods are used tohighlight structures of interest.Jeremy Pickett-Heaps,University of ColoradoDennis Kunkel Microscopy, Inc.Phase-contrast microscopy:Light illuminates the specimenat an angle, and only lightscattered by the specimenreaches the viewing lens of themicroscope. This gives a brightimage of the cell against ablack background.Dennis Kunkel Microscopy, Inc.Dark field microscopy:Dennis Kunkel Microscopy, Inc.Bright field microscopy:Light passes directly throughthe specimen. Many cellstructures have insufficientcontrast to be discerned.Staining with a dye is used toenhance contrast in aspecimen, as shown here, butthis treatment usually fixes andkills the cells.Jeremy Pickett-Heaps,University of ColoradoDennis Kunkel Microscopy, Inc.Micrographs are of the greenalga Scenedesmus.Dennis Kunkel Microscopy, Inc.Electron microscopyMicrographs are of theprotist Paramecium.Dennis Kunkel Microscopy, Inc.Light microscopyConfocal laser scanningmicroscopy: Lasers scanScanning electronmicroscopy (SEM): A beamacross a fluorescently stainedspecimen, and a computerfocuses the light to show asingle plane through the cell.This provides a sharper 3Dimage than other lightmicroscopy techniques.of electrons is scanned acrossa whole cell or organism, andthe electrons excited on thespecimen surface areconverted to a 3D-appearingimage.Interpreting the Results: Different techniques of light and electron microscopy produce images that reveal different structures or functions of the specimen.A micrograph is a photograph of an image formed by a microscope.CHAPTER 5THE CELL: AN OVERVIEW91tains the organelles, the cytosol, and the cytoskeleton.The organelles (“little organs”) are small, organizedstructures important for cell function.
The cytosol isan aqueous (water) solution containing ions and various organic molecules. The cytoskeleton is a proteinbased framework of filamentous structures that,among other things, helps maintain proper cell shapeand plays key roles in cell division and chromosomesegregation from cell generation to cell generation. Thecytoskeleton was once thought to be specific to eukaryotes, but recent research has shown that all major eukaryotic cytoskeletal proteins have functional equivalents in prokaryotes.Many of the cell’s vital activities occur in the cytoplasm, including the synthesis and assembly of most ofthe molecules required for growth and reproduction (except those made in the central region) and the conversion of chemical and light energy into forms that can beused by cells. The cytoplasm also conducts stimulatorysignals from the outside into the cell interior and carriesout chemical reactions that respond to these signals.4x3x2xxTotal surfacearea6x 26(2x)2 = 24x 26(3x)2 = 54x 26(4x)2 = 96x 2Totalvolumex3(2x)3 = 8x 3(3x)3 = 27x 3(4x)3 = 64x 3Surface area/volume ratio6:13:12:11.5:1FIGURE 5.5Relationship between surface area and volume.
The surface area of an object increases as asquare of the linear dimension, whereas the volume increases as a cube of that dimension.A central region of all cells contains DNA molecules, whichstore hereditary information. The hereditary information is organized in the form of genes—segments of DNA that code for individual proteins. The central region also contains proteins thathelp maintain the DNA structure and enzymes that duplicateDNA and copy its information into RNA.All the parts of the cell between the plasma membrane andthe central region comprise the cytoplasm. The cytoplasm con-HydrophobictailPhospholipidmoleculeTransport proteinchannelsFIGURE 5.6Organisms fall into two fundamental groups, prokaryotes andeukaryotes, based on the organization of their cells.
Prokaryotes(pro before; karyon nucleus) make up two domains of organisms, the Bacteria and the Archaea. TheDNA-containing central region of prokaryotic cells,the nucleoid, has no boundary membrane separatingit from the cytoplasm. Many species of bacteria contain few if any internal membranes, but a number ofother bacterial species contain extensive internalmembranes.The eukaryotes (eu true) make up the domainEukarya, which includes all the remaining organisms. The DNA-containing central region of eukary100 nmotic cells, a true nucleus, is separated by membranesfrom the surrounding cytoplasm. The cytoplasm ofeukaryotic cells ty pically contains extensivePhospholipidmembrane systems that form organelles with theirbilayerown distinct environments and specialized functions.
As in prokaryotes, a plasma membrane surrounds eukaryotic cells as the outer limit of thecytoplasm.The remainder of this chapter surveys the components of prokaryotic and eukaryotic cells in moredetail.The plasma membrane, which forms the outer limit of a cell’s cytoplasm. The plasma membrane consists of a phospholipid bilayer, an arrangement of phospholipids two molecules thick,which provides the framework of all biological membranes. Water-soluble substances cannotpass through the phospholipid part of the membrane. Instead, they pass through proteinchannels in the membrane; two proteins that transport substances across the membrane areshown.
Other types of proteins are also associated with the plasma membrane. (Inset) Electronmicrograph showing the plasma membranes of two adjacent animal cells.92UNIT ONEMOLECULES AND CELLSDon W. Fawcett/Photo Researchers, Inc.HydrophilicheadCells Occur in Prokaryotic and EukaryoticForms, Each with Distinctive Structuresand OrganizationSTUDY BREAK 5.1<What is the plasma membrane, and what are its mainfunctions?5.2 Prokaryotic CellsMost prokaryotic cells are relatively small, usually not much morethan a few micrometers in length and a micrometer or less in diameter.
A typical human cell is about ten times larger in diameter andover 8,000 times larger in volume than an average prokaryotic cell.The three shapes most common among prokaryotes arespherical, rodlike, and spiral. Escherichia coli (E. coli), a normalinhabitant of the mammalian intestine that has been studied extensively as a model organism in genetics, molecular biology,and genomics research, is rodlike in shape. Figure 5.7 shows anEM and diagram of E. coli to illustrate the basic features of prokaryotic cell structure. More detail about prokaryotic cell structure and function, as well as about the diversity of prokaryoticorganisms, is presented in Chapter 25.The genetic material of prokaryotes is located in the nucleoid;in an electron microscope, that region of the cell is seen to containa highly folded mass of DNA (see Figure 5.7). For most species,the DNA is a single, circular molecule that unfolds when releasedfrom the cell.
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