Biology_Unit_5 (1110837), страница 8
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Despite the molecular diversity of intermediate fi laments, however, they all play similar roles in the cell, providingstructural support in many cells and tissues. For example, thenucleus in epithelial cells is held within the cell by a basketlikenetwork of intermediate fi laments made of keratins.Microfi laments (Figure 5.22C) are thin protein fibers 5 to7 nm in diameter that consist of two polymers of actin subunits wound around each other in a long helical spiral. Theactin subunits are asymmetrical in shape, and they are all oriented in the same way in the polymer chains of a microfi lament.
Thus, as for microtubules, microfi laments have a polarity; the two ends are designated (plus) and (minus). Andas for microtubules, growth and disassembly occur more rapidly at the end than at the end.Microfi laments occur in almost all eukaryotic cells and areinvolved in many processes, including a number of structuraland locomotor functions. Microfi laments are best known as oneof the two components of the contractile elements in muscle fibers of vertebrates (the roles of myosin and microfi laments inmuscle contraction are discussed in Chapter 41).
MicrofilamentsBase of flagellumor ciliumPlasma membrane(cell surface)Basalbody orcentriole10 6FIGURE 5.24Eukaryotic flagellum. (A) The relationship between the microtubules and the basal body of aflagellum. (B) Diagram of a flagellum in cross section, showing the 9 2 system of microtubules. The spokes and connecting links hold the system together. (C) Electron micrograph of aflagellum in cross section; individual tubulin molecules are visible in the microtubule walls.UNIT ONEMOLECULES AND CELLSWaving and bendingmechanism:BaseTipLennart NilssonFlagella:Flagella beat insmooth, S-shapedwaves that travelfrom base to tip.Cilia:The waves and bends areproduced by dynein motorproteins, which slide themicrotubule doublets overeach other.
An examinationof the tip of a bent cilium orflagellum shows that thedoublets extend farthertoward the tip on the sidetoward the bend, confirmingthat the doublets actuallyslide as the shaft of thecilium or flagellum bends.Bent>FIGURE 5.25ole moves to a position just under the plasma membrane.
Thentwo of the three microtubules of each triplet grow outward fromone end of the centriole to form the ring of nine double microtubules. The two central microtubules of the 9 2 complex alsogrow from the end of the centriole, but without direct connection to any centriole microtubules. The centriole remains at theinnermost end of a flagellum or cilium when its development iscomplete as the basal body of the structure (see Figure 5.24).Cilia and flagella are found in protozoa and algae, and manytypes of animal cells have flagella—the tail of a sperm cell is aflagellum—as do the reproductive cells of some plants. In humans,cilia cover the surfaces of cells lining cavities or tubes in someparts of the body.
For example, cilia on cells lining the ventricles(cavities) of the brain circulate fluid through the brain, and cilia inthe oviducts conduct eggs from the ovaries to the uterus. Cilia covering cells that line the air passages of the lungs sweep out mucuscontaining bacteria, dust particles, and other contaminants.Although the purpose of the eukaryotic flagellum is the sameas that of prokaryotic flagella, the genes that encode the components of the flagellar apparatus of cells of Bacteria, Archaea, andEukarya are different in each case.
Thus, as mentioned earlier inthe chapter, the three types of flagella are analogous, not homologous, structures, and they must have evolved independently.With a few exceptions, the cell structures described so far inthis chapter occur in all eukaryotic cells. The major exception islysosomes, which appear to be restricted to animal cells. Thenext section describes three additional structures that are characteristic of plant cells.THINK OUTSIDE THE BOOKOn your own or collaboratively, explore the Internet and theresearch literature to develop an outline of the molecularsteps that a protein with a nuclear localization signal followsfor nuclear import (that is, being transported through a nuclear pore complex).FIGURE 5.26Centrioles.
The twocentrioles of the pair atthe cell center usually lieat right angles to eachother as shown. Theelectron micrographshows a centriole froma mouse cell in crosssection. A centriolegives rise to the 9 2system of a flagellumand persists as thebasal body at the innerend of the flagellum.CentriolesTripletDr. Donald Fawcett & H. Bernstet/Visuals Unlimited, Inc.Flagellar and ciliary beating patterns. The micrographs show a few humansperm, each with a flagellum (top), and cilia from the lining of an airway in thelungs (bottom).STUDY BREAK 5.3LinkCNRICilia beat in anoarlike power stroke(dark orange) followedby a recovery stroke(light orange).Straight<1.
Where in a eukaryotic cell is DNA found? How is that DNAorganized?2. What is the nucleolus, and what is its function?3. Explain the structure and function of the endomembrane system.4. What is the structure and function of a mitochondrion?5. What is the structure and function of the cytoskeleton?CHAPTER 5THE CELL: AN OVERVIEW107some plants, such as the potato. Chromoplasts (chromo color)contain red and yellow pigments and are responsible for the colors of ripening fruits or autumn leaves.
All plastids contain DNAgenomes and molecular machinery for gene expression and thesynthesis of proteins on ribosomes. Some of the proteins withinplastids are encoded by their genomes; others are encoded by nuclear genes and are imported into the organelles.Chloroplasts, like mitochondria, are usually lens- or discshaped and are surrounded by a smooth outer boundary membrane and an inner boundary membrane, which lies just insidethe outer membrane (Figure 5.27).
These two boundary membranes completely enclose an inner compartment, the stroma.Within the stroma is a third membrane system that consists offlattened, closed sacs called thylakoids. In higher plants, the thylakoids are stacked, one on top of another, forming structurescalled grana (singular, granum).The thylakoid membranes contain molecules that absorb lightenergy and convert it to chemical energy in photosynthesis. Theprimary molecule absorbing light is chlorophyll, a green pigmentthat is present in all chloroplasts. The chemical energy is used byenzymes in the stroma to make carbohydrates and other complexorganic molecules from water, carbon dioxide, and other simpleinorganic precursors. The organic molecules produced in chloroplasts, or from biochemical building blocks made in chloroplasts,are the ultimate food source for most organisms.
(The physical andbiochemical reactions of chloroplasts are described in Chapter 9.)The chloroplast stroma contains DNA and ribosomes thatresemble those of certain photosynthetic bacteria. Because ofthese similarities, chloroplasts, like mitochondria, are believedto have originated from ancient prokaryotes that became permanent residents of the eukaryotic cells ancestral to the plant lineage (see Chapter 24 for further discussion).5.4 Specialized Structuresof Plant CellsChloroplasts, a large and highly specialized central vacuole, andcell walls give plant cells their distinctive characteristics, but thesestructures also occur in some other eukaryotes—chloroplasts inalgal protists and cell walls in algal protists and fungi.Chloroplasts Are Biochemical FactoriesPowered by SunlightChloroplasts (chloro yellow–green), the sites of photosynthesis in plant cells, are members of a family of plant organellesknown collectively as plastids.
Other members of the family include amyloplasts and chromoplasts. Amyloplasts (amylo starch) are colorless plastids that store starch, a product of photosynthesis. They occur in great numbers in the roots or tubers ofFIGURE 5.27Chloroplast structure. The electronmicrograph shows a maize (corn)chloroplast.ChloroplastInner boundarymembraneOuter boundarymembraneCentral Vacuoles Have Diverse Roles in Storage,Structural Support, and Cell GrowthGranumStroma(fluid interior)Dr. Jeremy Burgess/Photo Researchers, Inc.Thylakoids1.0 μm10 8UNIT ONEMOLECULES AND CELLSCentral vacuoles (see Figure 5.10) are large vesicles identified asdistinct organelles of plant cells because they perform specializedfunctions unique to plants.
In a mature plant cell, 90% or more ofthe cell’s volume may be occupied by one or more large centralvacuoles. The remainder of the cytoplasm and the nucleus of thesecells are restricted to a narrow zone between the central vacuoleand the plasma membrane. The pressure within the central vacuole supports the cells.The membrane that surrounds the central vacuole, the tonoplast, contains transport proteins that move substances into andout of the central vacuole. As plant cells mature, they grow primarily by increases in the pressure and volume of the central vacuole.Central vacuoles conduct other vital functions. They storesalts, organic acids, sugars, storage proteins, pigments, and, insome cells, waste products.
Pigments concentrated in the vacuoles produce the colors of many flowers. Enzymes capable ofbreaking down biological molecules are present in some centralvacuoles, giving them some of the properties of lysosomes. Molecules that provide chemical defenses against pathogenic organisms also occur in the central vacuoles of some plants.Cell Walls Support and Protect Plant CellsThe cell walls of plants are extracellular structures because theyare located outside the plasma membrane (Figure 5.28). Cell wallsprovide support to individual cells, contain the pressure producedin the central vacuole, and protect cells against invading bacteriaand fungi.Cell walls consist of cellulose fibers (see Figure 3.9C), whichgive tensile strength to the walls, embedded in a network ofhighly branched carbohydrates.
The initial cell wall laid downby a plant cell, the primary cell wall, is relatively soft and flexible. As the cell grows and matures, the primary wall expandsand additional layers of cellulose fibers and branched carbohydrates are laid down between the primary wall and the plasmamembrane. The added wall layer, which is more rigid and maybecome many times thicker than the primary wall, is the secondary cell wall. In woody plants and trees, secondary cellwalls are reinforced by lignin, a hard, highly resistant substanceassembled from complex alcohols, surrounding the cellulosefibers. Lignin-impregnated cell walls are actually stronger thanreinforced concrete by weight; hence, trees can grow to substantial size, and the wood of trees is used extensively in humancultures to make many structures and objects, includinghouses, tables, and chairs.The walls of adjacent cells are held together by a layer of gellike polysaccharides called the middle lamella, which acts as anintercellular glue (see Figure 5.28).
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