Biology_Unit_5 (1110837), страница 9
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The polysaccharide materialCytoplasm ofone cellof the middle lamella, called pectin, is extracted from some plantsand used to thicken jams and jellies.Both primary and secondary cell walls are perforated byminute channels, the plasmodesmata (singular, plasmodesma;see Figure 5.28). A typical plant cell has between 1,000 and100,000 plasmodesmata connecting it to abutting cells. Thesecytosol-fi lled channels are lined by plasma membranes, so thatconnected cells essentially all have one continuous surface membrane.
Most plasmodesmata also contain a narrow tubelikestructure derived from the smooth endoplasmic reticulum of theconnected cells. Plasmodesmata allow ions and small moleculesto move directly from one cell to another through the connecting cytosol, without having to penetrate the plasma membranesor cell walls. Proteins and nucleic acids move through some plasmodesmata using energy-dependent processes.Cell walls also surround the cells of fungi and algal protists.Carbohydrate molecules form the major framework of cell wallsin most of these organisms, as they do in plants.
In some, the wallfibers contain chitin (see Figure 3.9D) instead of cellulose. Details of cell wall structure in the algal protists and fungi, as wellas in different subgroups of the plants, are presented in laterchapters devoted to these organisms.As noted earlier, animal cells do not form rigid, external,layered structures equivalent to the walls of plant cells. However,most animal cells secrete extracellular material and have otherstructures at the cell surface that play vital roles in the supportPlasmamembraneCell wallCytoplasm ofadjacent cellRay F. EvertPlasmodesmataSection through five plasmodesmata that bridge the middlelamella and primary walls of two plant cellsCell wallCytoplasmCytoplasmPlasma membraneSecondary cell wallCytoplasmBiophoto Associates/Photo Researchers, Inc.Primary cell wallMiddle lamellaPrimary cell wallSecondary cell wallPlasma membraneCytoplasmFIGURE 5.28Cell wall structure in plants.
The upper right diagram and electron micrograph show plasmodesmata, which form openings in the cell wall that directly connect thecytoplasm of adjacent cells. The lower diagram and electron micrograph show the successive layers in the cell wall between two plant cells that have laid downsecondary wall material.CHAPTER 5THE CELL: AN OVERVIEW109and organization of animal body structures.
The next section describes these and other surface structures of animal cells.STUDY BREAK 5.4<1. What is the structure and function of a chloroplast?2. What is the function of the central vacuole in plants?5.5 The Animal Cell SurfaceAnimal cells have specialized structures that help hold cells together, produce avenues of communication between cells, and organize body structures.
Molecular systems that perform thesefunctions are organized at three levels: individual cell adhesionmolecules bind cells together, more complex cell junctions sealthe spaces between cells and provide direct communication between cells, and the extracellular matrix (ECM) supports andprotects cells and provides mechanical linkages, such as those between muscles and bone.Cell Adhesion Molecules Organize Animal Cellsinto Tissues and OrgansCell adhesion molecules are glycoproteins embedded in theplasma membrane. They help maintain body form and structurein animals ranging from sponges to the most complex invertebrates and vertebrates. Rather than acting as a generalized intercellular glue, cell adhesion molecules bind to specific moleculeson other cells. Most cells in solid body tissues are held together bymany different cell adhesion molecules.Cell adhesion molecules make initial connections betweencells early in embryonic development, but then attachments arebroken and remade as individual cells or tissues change positionin the developing embryo.
As an embryo develops into an adult,the connections become permanent and are reinforced by celljunctions. Cancer cells typically lose these adhesions, allowingthem to break loose from their original locations, migrate to newlocations, and form additional tumors.Some bacteria and viruses—such as the virus that causes thecommon cold—target cell adhesion molecules as attachmentsites during infection. Cell adhesion molecules are also partiallyresponsible for the ability of cells to recognize one another asbeing part of the same individual or foreign. For example, rejection of organ transplants in mammals results from an immuneresponse triggered by the foreign cell-surface molecules.Cell Junctions Reinforce Cell Adhesionsand Provide Avenues of CommunicationThree types of cell junctions are common in animal tissues (Figure 5.29).
Anchoring junctions form buttonlike spots, or belts,that run entirely around cells, “welding” adjacent cells together.For some anchoring junctions known as desmosomes, intermediate filaments anchor the junction in the underlying cytoplasm;110UNIT ONEMOLECULES AND CELLSin other anchoring junctions known as adherens junctions,microfilaments are the anchoring cytoskeletal component.Anchoring junctions are most common in tissues that are subjectto stretching, shear, or other mechanical forces—for example,heart muscle, skin, and the cell layers that cover organs or linebody cavities and ducts.Tight junctions, as the name indicates, are regions of tightconnections between membranes of adjacent cells (see Figure 5.29).The connection is so tight that it can keep particles as small asions from moving between the cells in the layers.Tight junctions seal the spaces between cells in the cell layers that cover internal organs and the outer surface of the body,or the layers that line internal cavities and ducts.
For example,tight junctions between cells that line the stomach, intestine, andbladder keep the contents of these body cavities from leakinginto surrounding tissues.A tight junction is formed by direct fusion of proteins on theouter surfaces of the two plasma membranes of adjacent cells.Strands of the tight junction proteins form a complex networkthat gives the appearance of stitch work holding the cells together. Within a tight junction, the plasma membrane is notjoined continuously; instead, there are regions of intercellularspace. Nonetheless, the network of junction proteins is sufficientto make the tight cell connections characteristic of thesejunctions.Gap junctions open direct channels that allow ions andsmall molecules to pass directly from one cell to another (seeFigure 5.29).
Hollow protein cylinders embedded in the plasmamembranes of adjacent cells line up and form a sort of pipelinethat connects the cytoplasm of one cell with the cytoplasm of thenext. The flow of ions and small molecules through the channelsprovides almost instantaneous communication between animalcells, similar to the communication that plasmodesmata providebetween plant cells.In vertebrates, gap junctions occur between cells within almost all body tissues, but not between cells of different tissues.These junctions are particularly important in heart muscle tissues and in the smooth muscle tissues that form the uterus, wheretheir pathways of communication allow cells of the organ to operate as a coordinated unit.
Although most nerve tissues do nothave gap junctions, nerve cells in dental pulp are connected bygap junctions; they are responsible for the discomfort you feel ifyour teeth are disturbed or damaged, or when a dentist pokes aprobe into a cavity.The Extracellular Matrix Organizesthe Cell ExteriorMany types of animal cells are embedded in an ECM that consistsof proteins and polysaccharides secreted by the cells themselves(Figure 5.30).
The primary function of the ECM is protection andsupport. The ECM forms the mass of skin, bones, and tendons; italso forms many highly specialized extracellular structures such asthe cornea of the eye and filtering networks in the kidney. The ECMalso affects cell division, adhesion, motility, and embryonic development, and it takes part in reactions to wounds and disease.CellsChannel in acomplex of proteinsDon W.
Fawcett/Photo ResearchersDr. Donald Fawcett/Visuals Unlimited, Inc.IntermediatefilamentsSPL/Photo Researchers, Inc.PlaqueAnchoring junction:Adjoining cells adhere at a mass of proteins(a plaque) anchored beneath their plasmamembrane by many intermediate filaments(adherens junction) or microfilaments(desmosome) of the cytoskeleton.Gap junction:Cylindrical arrays of proteins form directchannels that allow small molecules andions to flow between the cytoplasm ofadjacent cells.Tight junction:FIGURE 5.29Tight connections form between adjacent cells byfusion of plasma membrane proteins on theirouter surfaces.
A complex network of junctionproteins makes a seal tight enough to preventleaks of ions or molecules between cells.Anchoring junctions, tight junctions, and gap junctions, which connect cells in animal tissues. Anchoring junctions reinforce the cell-to-cell connections made by celladhesion molecules, tight junctions seal the spaces between cells, and gap junctions create direct channels of communication between animal cells.Glycoproteins are the main component of theECM. In most animals, the most abundant ECMglycoprotein is collagen, which forms fibers withgreat tensile strength and elasticity. In vertebrates,the collagens of tendons, cartilage, and bone arethe most abundant proteins of the body, making upabout half of the total body protein by weight.
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