Biology_Unit_5 (1110837), страница 4
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The rest of thechapter focuses on the cell components that are common to all orlarge groups of eukaryotic organisms.Eukaryotic Cells Have a True Nucleusand Cytoplasmic Organelles Enclosedwithin a Plasma MembraneThe cells of all eukaryotes have a true nucleus enclosed by membranes. The cytoplasm surrounding the nucleus contains a remarkable system of membranous organelles, each specialized tocarry out one or more major functions of energy metabolism andmolecular synthesis, storage, and transport.
The cytosol, the cytoplasmic solution surrounding the organelles, participates in en-FIGURE 5.8Research MethodCell FractionationPurpose: Cell fractionation partitions cells into fractions containing a single cell component,such as mitochondria or ribosomes. Once isolated, the cell component can be disassembled by the same general techniques to analyze its structure and function.Protocol:1. Break open intact cells by sonication (high-frequency sound waves), grinding in fine glass beads, or exposure to detergents that disrupt plasmamembranes.500 g(500 times theforce of gravity)Whole cellsCell fragmentsNucleiatantatant2. Use sequential centrifugations at increasing speeds to separate and purify cell structures.
The spinning centrifuge drives cellular structures to the bottomof tube at a rate that depends on their shape and density. With each centrifugation, the largest and densest components are isolated and concentratedinto a pellet; the remaining solution, the supernatant, is drawn off and can be centrifuged again at higher speed.rnpeSurnpeSu20,000 g150,000 gPelletMitochondria(and chloroplastsin a plant cell)Ribosomes,proteins,nucleic acids3.
Resuspend the pellet containing the isolated cell components and subfractionate using the same general techniques to examine the components oforganelles.Interpreting the Results: Many of the cell or organelle subfractions generated by cell fractionation retain their biological activity, making them useful in studiesof various cellular processes. For example, mitochondrial subfractions were used to work out the structure and function of the electron transfer system. Cell fractionation is still used to determine the cellular location of a protein or biological reaction, such as whether it is free in the cytosol or associated with a membrane.ergy metabolism and molecular synthesis and performs specialized functions in support and motility.
Researchers havediscovered much about the structures and functions of variouscellular organelles by using the research method of cell fractionation to isolate them and purify them (Figure 5.8).The eukaryotic plasma membrane carries out various functions through several types of embedded proteins. Some of theseproteins form channels through the plasma membrane thattransport substances into and out of the cell.
Other proteins inthe plasma membrane act as receptors; they recognize and bindspecific signal molecules in the cellular environment and triggerinternal responses. In some eukaryotes, particularly animals,plasma membrane proteins recognize and adhere to moleculeson the surfaces of other cells. Yet other plasma membrane proteins are important markers in the immune system, labeling cellsas “self,” that is, belonging to the organism. Therefore, the immune system can identify cells without those markers as beingforeign, most likely pathogens (disease-causing organisms orviruses).A supportive cell wall surrounds the plasma membrane offungal, plant, and many protist cells.
Because the cell wall liesoutside the plasma membrane, it is an extracellular structure(extra outside). Although animal cells do not have cell walls,they also form extracellular material with supportive and otherfunctions.Figure 5.9 presents a diagram of a representative animal celland Figure 5.10 presents a diagram of a representative plant cell toshow where the nucleus, cytoplasmic organelles, and other structures are located.
The following sections discuss the structureand function of eukaryotic cell parts in more detail, beginningwith the nucleus.The Eukaryotic Nucleus Contains Much MoreDNA Than the Prokaryotic NucleoidThe nucleus (see Figures 5.9 and 5.10) is separated from the cytoplasm by the nuclear envelope, which consists of two membranes, one layered just inside the other and separated by a narrow space (Figure 5.11).
A network of protein filaments calledlamins lines and reinforces the inner surface of the nuclear envelope in animal cells. Lamins are a type of intermediate filament(see later in this section). Unrelated proteins line the inner surface of the nuclear envelope in protists, fungi, and plants.Embedded in the nuclear envelope are many hundreds ofnuclear pore complexes. A nuclear pore complex is a large, octagonally symmetrical, cylindrical structure formed of manyCHAPTER 5THE CELL: AN OVERVIEW95MicrobodyMitochondrionNuclear porecomplexEnergymetabolismNuclearenvelopeChromatinNucleusMembrane-enclosedregion of DNA;hereditary controlNucleolusRough ERPair ofcentriolesRibosome (attached Endoplasmic reticulumto rough ER)Synthesis, modification,transport of proteins;Ribosome (freemembrane synthesisin cytosol)in cell centerLysosomeDegradation;recyclingSmooth ERMicrotubulesradiating fromcell centerMicrofilamentsVesicleGolgi complexModification, distributionof proteinsCytosolPlasmamembraneTransportFIGURE 5.9Diagram of an animal cell, highlighting the major organelles and their primary locations.CytosolMitochondrionEnergy metabolismGolgicomplexVesicleNuclear porecomplexCentralvacuoleNuclearenvelopeCell growth,support,storageChromatinNucleusMembrane-enclosedregion of DNA;hereditary controlNucleolusTonoplast(central vacuolemembrane)ChloroplastPhotosynthesis;some starchstoragePlasmodesmataMicrotubulesRough ER(componentsof cytoskeleton)Cell wallProtection;structuralsupportPlasma membraneTransportFIGURE 5.10Diagram of a plant cell, highlighting the major organelles and their primary locations.Ribosome (attachedEndoplasmic reticulumto rough ER)Synthesis, modification,transport of proteins;membrane synthesisRibosome (freein cytosol)Smooth ERNucleusCytoplasmRibosomes onouter surface ofnuclear envelopeNuclear porecomplexOuter nuclearmembrane(faces cytoplasm)Space betweennuclear membranesNuclearenvelopeNuclearenvelopeInner nuclearmembrane(faces nucleoplasm)ChromatinNuclear pore complexMartin W.
Goldberg, Durham University, UKNucleolusNucleoplasmFIGURE 5.11The nuclear envelope, which consists of a system of twoconcentric membranes with nuclear pore complexesembedded. Nuclear pore complexes are octagonally symmetrical protein structures with a channel—the nuclear pore—through the center. They control the transport of moleculesbetween the nucleus and cytoplasm.Enlargedregion showingphospholipidbilayertypes of proteins, called nucleoporins.
Probably the largest protein complex in the cell, it exchanges components between thenucleus and cytoplasm and prevents the transport of materialnot meant to cross the nuclear membrane. A channel throughthe nuclear pore complex—a nuclear pore—is the path for theassisted exchange of large molecules such as proteins and RNAmolecules with the cytoplasm, whereas small molecules simplypass through unassisted. A protein or RNA molecule (called thecargo) associates with a transport protein acting as a chaperoneto shuttle the cargo through the pore.Some proteins—for instance, the enzymes for replicatingand repairing DNA—must be imported into the nucleus to carryout their functions. Proteins to be imported into the nucleus aredistinguished from those that function in the cytosol by thepresence of a special, short amino acid sequence called a nuclearlocalization signal.
A specific protein in the cytosol recognizesand binds to the signal and moves the protein containing it to thenuclear pore complex where it is then transported through thepore into the nucleus. Figure 5.12 shows how researchers discovered the nuclear localization signal.The liquid or semiliquid substance within the nucleus iscalled the nucleoplasm. Most of the space inside the nucleus isfi lled with chromatin, a combination of DNA and proteins. Bycontrast with most prokaryotes, most of the hereditary information of a eukaryote is distributed among several to manylinear DNA molecules in the nucleus.
Each individual DNAmolecule with its associated proteins is a eukaryotic chromosome. The terms chromatin and chromosome are similar but0.1 μmhave distinct meanings. Chromatin refers to any collection ofeukaryotic DNA molecules with their associated proteins.Chromosome refers to one complete DNA molecule with its associated proteins.Eukaryotic nuclei contain much more DNA than do prokaryotic nucleoids. For example, the entire complement of 46chromosomes in the nucleus of a human cell has a total DNAlength of about 2 meters (m), compared with about 1,500 mm inprokaryotic cells with the most DNA. Some eukaryotic cells contain even more DNA; for example, a single frog or salamandernucleus, although of microscopic diameter, is packed with about10 m of DNA!A eukaryotic nucleus also contains one or more nucleoli(singular, nucleolus), which look like irregular masses of smallfibers and granules (see Figures 5.9 and 5.10).
These structuresform around the genes coding for the rRNA molecules of ribosomes. Within the nucleolus, the information in rRNA genes iscopied into rRNA molecules, which combine with proteins toform ribosomal subunits. The ribosomal subunits then leave thenucleoli and exit the nucleus through the nuclear pore complexesto enter the cytoplasm, where they join on mRNAs to form complete ribosomes.The genes for most of the proteins that the organism canmake are found within the chromatin, as are the genes for specialized RNA molecules such as rRNA molecules. Expression ofthese genes is carefully controlled as required for the function ofeach cell. (The other proteins in the cell are specified by DNA inthe mitochondria and chloroplasts.)CHAPTER 5THE CELL: AN OVERVIEW97Experimental ResearchFIGURE 5.12Discovery of the Nuclear Localization SignalQuestion: How are proteins that are imported into the nucleus identified by the importmachinery?Experiment: Alan Smith and his colleagues at the National Institute for Medical Research, Mill Hill, London, studied a viral protein that normally is found inthe nucleus after the virus infects a cell.
They mutated the 708-amino-acid–long protein, changing one or more specific amino acids in the protein or deletingsegments of the protein, and determined whether the alterations affected the location of the viral protein in rodent or monkey cells in culture.Results: The researchers obtained the following results:CytoplasmNucleusProteinNormal protein: Localized tonucleus.The amino acid at position 128(thought to be important for theprotein to bind to DNA) was mutated from lysine to threonine: Mutated protein localized in cytoplasm. The researchers interpretedthis result to mean that the mutated amino acid was important forlocalizing the protein to the nucleus.
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