Biology_Unit_5 (1110837), страница 7
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Beginning or intermediate stepsin the breakdown of some amino acids and alcohols also takeplace in microbodies, including about half of the ethyl alcoholthat humans consume. Many types of microbodies produce as aby-product the toxic substance hydrogen peroxide (H2O2), whichis broken down into water and oxygen by the enzyme catalase.Microbodies with this reaction are often termed peroxisomes.Microbodies in plants convert oils or fats to sugars that canbe used directly for energy-releasing reactions in mitochondriaor for reactions that require sugars as chemical building blocks.These microbody reactions are particularly important in plantembryos that develop from oily seeds, such as those of the peanutMitochondrionIntermembrane compartment(between outer and inner membranes)Cristae (folds ofinner membrane)OuterMatrixInnermitochondrial (inside both mitochondrialmembranes) membranemembraneChloroplast0.5 μmMicrobodyLargecentralvacuoleEldon Newcomb, University of WisconsinMitochondrionKeith R.
PorterCytosolChloroplastFIGURE 5.19Mitochondria. The electron micrograph shows a mitochondrionfrom bat pancreas, surrounded by cytoplasm containing rough ER.Cristae extend into the interior of the mitochondrion as folds fromthe inner mitochondrial membrane. The darkly stained granulesinside the mitochondrion are probably lipid deposits.FIGURE 5.20A microbody in the cytoplasm of a tobacco leaf cell.
The EM has been colorized to makethe structures easier to identify.CHAPTER 5THE CELL: AN OVERVIEW103a. Microtubulesb. Intermediate filamentsc. MicrofilamentsJennifer C. Waters/Photo Researchers, Inc.Courtesy of Mary OsbornCourtesy Dr.
Vincenzo Cerulli, Lab of Developmental Biology, The WhittierInst. for Diabetes, Univ. of Cal.-San Diego, La Jolla, CAFIGURE 5.21Cytoskeletons of eukaryotic cells, as seen in cells stained for light microscopy. (A) Microtubules (yellow) and microfilaments (red) in a pancreatic cell. (B) Intermediate filaments assembled from keratin proteins in cells of the kangaroo rat.
The nucleus is stained blue in these cells. (C) Microfilaments (red) in a migratingmammalian cell.or soybean. Depending on the particular reaction pathways theycarry out, plant microbodies are called peroxisomes, glyoxysomes, or glycosomes.The Cytoskeleton Supports and MovesCell StructuresThe characteristic shape and internal organization of each typeof cell is maintained in part by its cytoskeleton, the interconnected system of protein fibers and tubes that extends throughout the cytoplasm.
The cytoskeleton also reinforces the plasmamembrane and functions in movement, both of structures withinthe cell and of the cell as a whole. It is most highly developed inanimal cells, in which it fills and supports the cytoplasm from the plasmaA. Microtubulemembrane to the nuclear envelopeα-tubulin(Figure 5.21). Although cytoskeletal+ endβ-tubulinstructures are also present in plantTubulincells, the fibers and tubes of the systemdimersare less prominent; much of cellularsupport in plants is provided by the cell wall and a large centralvacuole (described in Section 5.4).The cytoskeleton of animal cells contains structural elements of three major types: microtubules, intermediate filaments, and microfilaments. Plant cytoskeletons likewise containthe same three structural elements. Microtubules are the largestcytoskeletal elements, and microfilaments are the smallest.Each cytoskeletal element is assembled from proteins—microtubules from tubulins, intermediate fi laments from a large andvaried group of intermediate filament proteins, and microfi laments from actins (Figure 5.22).
The keratins of animal hair,nails, and claws contain a common form of intermediate fi lament proteins known as cytokeratins. For example, human hairB.Intermediate filamentC.Microfilament+ endEach green line isan intermediatefilament proteinActinsubunit8–12nmFIGURE 5.22The major components of the cytoskeleton.(A) A microtubule, assembled from dimers of- and -tubulin proteins. (B) An intermediatefilament. Eight protein chains wind together toform each subunit shown as a green cylinder.(C) A microfilament, assembled from twolinear polymers of actin proteins, woundaround each other into a helical spiral.10 4UNIT ONE– endThirteen filamentsside by side in amicrotubuleMOLECULES AND CELLS– end15nm25nm5–7nmconsists of thick bundles of cytokeratin fibers extruded fromhair follicle cells. The lamins that line the inner surface of thenuclear envelope in animal cells are also assembled from intermediate fi lament proteins.Microtubules (Figure 5.22A) are microscopic tubes with anouter diameter of about 25 nm and an inner diameter of about 15nm; they function much like the tubes used by human engineersto construct supportive structures.
Microtubules vary widely inlength from less than 200 nm to several micrometers. The wall ofthe microtubule consists of 13 protein fi laments arranged side byside. A fi lament is a linear polymer of tubulin dimers, each dimerconsisting of one -tubulin and one -tubulin subunit boundnoncovalently together. The dimers are organized head-to-tail ineach fi lament, giving the microtubule a polarity, meaning thatthe two ends are different. One end, called the (plus) end, has-tubulin subunits at the ends of the fi laments; the other end,called the (minus) end, has -tubulin subunits at the ends ofthe fi laments.
Microtubules are dynamic structures, changingtheir lengths as required by their functions. This is seen readilyin animal cells that are changing shape. Microtubules changelength by the addition or removal of tubulin dimers; this occursasymmetrically, with dimers adding or detaching more rapidlyat the end than at the end. The lengths of microtubules aretightly regulated in the cell.Many of the cytoskeletal microtubules in animal cells areformed and radiate outward from a site near the nucleus termedthe cell center or centrosome (see Figure 5.9).
At its midpoint aretwo short, barrel-shaped structures also formed from microtubules called the centrioles (see Figure 5.26). Often, intermediatefi laments also extend from the cell center, apparently held in thesame radiating pattern by linkage to microtubules. Microtubulesthat radiate from the cell center anchor the ER, Golgi complex,lysosomes, secretory vesicles, and at least some mitochondria inposition. The microtubules also provide tracks along which vesicles move from the cell interior to the plasma membrane and inthe reverse direction.
The intermediate fi laments probably addsupport to the microtubule arrays.Microtubules play other key roles, for instance, in separating and moving chromosomes during cell division, determining the orientation for growth of the new cell wall duringplant cell division, maintaining the shape of animal cells, andmoving animal cells themselves. Animal cell movements aregenerated by “motor” proteins that push or pull against microtubules or microfi laments, much as our muscles producebody movements by acting on bones of the skeleton.
One endof a motor protein is fi rmly fi xed to a cell structure such as avesicle or to a microtubule or microfi lament. The other endhas reactive groups that “walk” along another microtubule ormicrofi lament by making an attachment, forcefully swivelinga short distance, and then releasing (Figure 5.23). ATP suppliesthe energy for the walking movements. The motor proteinsthat walk along microfi laments are called myosins, and theones that walk along microtubules are called dyneins and kinesins.
Some cell movements, such as the whipping motions ofsperm tails, depend entirely on microtubules and their motorproteins.FIGURE 5.23A.“Walking” end of a kinesin moleculeThe microtubulemotor protein kinesin.(A) Structure of the end of akinesin molecule that “walks”along a microtubule, with -helicalsegments shown as spirals and strands as flat ribbons. (B) How akinesin molecule walks along thesurface of a molecule by alternatelyattaching and releasing its “feet.”B.Connects to cell structuresuch as a vesicleOne “foot” ofmotor proteinHow a kinesin molecule “walks”CHAPTER 5THE CELL: AN OVERVIEW105A.Eukaryotic flagellumB.are involved in the actively flowing motion of cytoplasm calledcytoplasmic streaming, which can transport nutrients, proteins,and organelles in both animal and plant cells, and which is responsible for amoeboid movement.
When animal cells divide,microfi laments are responsible for dividing the cytoplasm (seeChapter 10 for further discussion).Flagella Propel Cells, and Cilia Move Materialsover the Cell SurfaceFlagella and cilia (singular, cilium) are elongated, slender, motilestructures that extend from the cell surface.
They are identical instructure except that cilia are usually shorter than flagella andoccur on cells in greater numbers. Whiplike or oarlike movements of a flagellum propel a cell through a watery medium, andcilia move fluids over the cell surface.A bundle of microtubules extends from the base to the tip ofa flagellum or cilium (Figure 5.24). In the bundle, a circle of ninedouble microtubules surrounds a central pair of single microtubules, forming what is known as the 9 2 complex.
Dyneinmotor proteins slide the microtubules of the 9 2 complex overeach other to produce the movements of a flagellum or cilium(Figure 5.25).Flagella and cilia arise from the centrioles. These barrelshaped structures contain a bundle of microtubules similar tothe 9 2 complex, except that the central pair of microtubules ismissing and the outer circle is formed from a ring of nine triplerather than double microtubules (compare Figure 5.24 and Figure 5.26). During the formation of a flagellum or cilium, a centri-C.Cross section of flagellum9 + 2 systemPlasma membraneDynein armTwo centralmicrotubulesCentral sheathSpokeLinks of theconnective systemMicrograph of flagellumDon W.
Fawcett/SPL/Photo Researchers, Inc.Intermediate filaments (Figure 5.22B) are fibers with diameters of about 8 to 12 nm. (“Intermediate” signifies, in fact, thatthese fi laments are intermediate in size between microtubulesand microfi laments.) These fibers occur singly, in parallel bundles, and in interlinked networks, either alone or in combinationwith microtubules, microfi laments, or both. Intermediate fi laments are only found in multicellular organisms. Moreover,whereas microtubules and microfilaments are the same in all tissues, intermediate fi laments are tissue-specific in their proteincomposition.
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