Biology_Unit_5 (1110837)
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55.15.25.35.45.5Basic Features of CellStructure and FunctionProkaryotic CellsEukaryotic CellsSpecialized Structuresof Plant CellsThe Animal Cell SurfaceDavid Becker/SPL/Photo Researchers, Inc.study outlineCells fluorescently labeled to visualize their internal structure (confocal light micrograph). Cell nuclei are shown in blue and parts of thecytoskeleton in red and green.The Cell: An OverviewWhy It Matters.
. . In the mid-1600s, Robert Hooke, Curator of Instruments forthe Royal Society of England, was at the forefront of studies applying the newly invented light microscopes to biological materials. When Hooke looked at thinly sliced cork from a mature tree througha microscope, he observed tiny compartments (Figure 5.1A). He gave them the Latin name cellulae,meaning “small rooms”—hence, the origin of the biological term cell. Hooke was actually looking atthe walls of dead cells, which is what cork consists of.Reports of cells also came from other sources.
By the late 1600s, Anton van Leeuwenhoek (Figure 5.1B), a Dutch shopkeeper, observed “many very little animalcules, very prettily a-moving,”using a single-lens microscope of his own construction. Leeuwenhoek discovered and described diverse protists, sperm cells, and even bacteria, organisms so small that they would not be seen byothers for another two centuries.In the 1820s, improvements in microscopes brought cells into sharper focus.
Robert Brown, anEnglish botanist, noticed a discrete, spherical body inside some cells; he called it a nucleus. In 1838,a German botanist, Matthias Schleiden, speculated that the nucleus had something to do with thedevelopment of a cell. The following year, the zoologist Theodor Schwann of Germany expandedSchleiden’s idea to propose that all animals and plants consist of cells that contain a nucleus. He alsoproposed that even when a cell forms part of a larger organism, it has an individual life of its own.However, an important question remained: Where do cells come from? A decade later, the Germanphysiologist Rudolf Virchow answered this question. From his studies of cell growth and reproduction, Virchow proposed that cells arise only from preexisting cells by a process of division.88B.Hooke’s microscopeLeeuwenhoek and microscopeArmed Forces Institute of PathologyA.Bettmann/CORBISNational Libraryof MedicineFIGURE 5.1Investigations leading to the first descriptions of cells.
(A) The cork cells drawn by RobertHooke and the compound microscope he used to examine them. (B) Anton van Leeuwenhoek holding his microscope, which consisted of a single, small sphere of glass fixed in aholder. He viewed objects by holding them close to one side of the glass sphere and lookingat them through the other side.Thus, by the middle of the nineteenth century, microscopicobservations had yielded three profound generalizations, whichtogether constitute what is now known as the cell theory:1.2.3.All organisms are composed of one or more cells.The cell is the basic structural and functional unit of allliving organisms.Cells arise only from the division of preexisting cells.These tenets were fundamental to the development of biologicalscience.This chapter provides an overview of our current understanding of the structure and functions of cells, emphasizing both thesimilarities among all cells and some of the most basic differencesamong cells of various organisms.
The variations in cells that helpmake particular groups of organisms distinctive are discussed inlater chapters. This chapter also introduces some of the modernmicroscopes that enable us to learn more about cell structure. <5.1 Basic Features of Cell Structureand FunctionAs the basic structural and functional units of all living organisms,cells carry out the essential processes of life. They contain highlyorganized systems of molecules, including the nucleic acids DNAand RNA, which carry hereditary information and direct the manufacture of cellular molecules.
Cells use chemical molecules orlight as energy sources for their activities. Cells also respond tochanges in their external environment by altering theirinternal reactions. Further, cells duplicate and pass ontheir hereditary information as part of cellular reproduction. All these activities occur in cells that, in mostcases, are invisible to the naked eye.Some types of organisms, including almost allbacteria and archaeans, some protists, such as amoebas, and some fungi, such as yeasts, are unicellular.Each of these cells is a functionally independent organism capable of carrying out all activities necessaryfor its life. In more complex multicellular organisms,including plants and animals, the activities of life aredivided among varying numbers of specialized cells.However, individual cells of multicellular organismsare potentially capable of surviving by themselves ifplaced in a chemical medium that can sustain them.If cells are broken open, the property of life is lost:They are unable to grow, reproduce, or respond to outside stimuli in a coordinated, potentially independentfashion.
This fact confirms the second tenet of the celltheory: Life as we know it does not exist in units moresimple than individual cells. Viruses, which consistonly of a nucleic acid molecule surrounded by a protein coat, cannot carry out most of the activities of life.Their only capacity is to infect living cells and directthem to make more virus particles of the same kind.(Viruses are discussed in Chapter 17.)Cells Are Small and Are Visualized Usinga MicroscopeCells assume a wide variety of forms in different prokaryotes andeukaryotes (Figure 5.2). Individual cells range in size from tinybacteria to an egg yolk, a single cell that can be several centimetersin diameter.
Yet, all cells are organized according to the same basicplan, and all have structures that perform similar activities.Most cells are too small to be seen by the unaided eye: Humans cannot see objects smaller than about 0.1 mm in diameter.The smallest bacteria have diameters of about 0.5 m (a micrometer is 1,000 times smaller than a millimeter). The cells of multicellular animals range from about 5 to 30 m in diameter. Yourred blood cells are 7 to 8 m across—a string of 2,500 of thesecells is needed to span the width of your thumbnail.
Plant cellsrange from about 10 m to a few hundred micrometers in diameter. (Figure 5.3 explains the units of measurement used in biology to study molecules and cells.)To see cells and the structures within them we use microscopy, a technique for producing visible images of objects, biological or otherwise, that are too small to be seen by the human eye(Figure 5.4). The instrument of microscopy is the microscope.The two common types of microscopes are light microscopes,which use light to illuminate the specimen (the object beingviewed), and electron microscopes, which use electrons to illuminate the specimen. Different types of microscopes give different magnification and resolution of the specimen.
Just as for acamera or a pair of binoculars, magnification is the ratio of theCHAPTER 5THE CELL: AN OVERVIEW89A.B.BacteriumC.ArchaeanD.ProtistAlgaeF.Animal cellsMANFRED KAGE/Peter ArnoldWim van Egmond/Visuals Unlimited, Inc.E.Dr Tony Brain/SPL/Photo ResearchersDr. Terry Beveridge/Visuals UnlimitedMichael Abbey/Visuals Unlimited, Inc.G.Plant cellsFungal cellsiStockphoto.com/Nancy NehringiStockphoto.com/Oliver Sun KimFIGURE 5.2Examples of the varied kinds of cells: (A) and (B) are prokaryotes, the others are eukaryotes. (A) A bacterial cell with flagella, Pseudomonas fluorescens.(B) An archaean, the extremophile Sulfolobus acidocaldarius.
(C) Trichonympha, a protist that lives in a termite’s gut. (D) Two cells of Micrasterias, an algalprotist. (E) Fungal cells of the bread mold Aspergillus. (F) Cells of a surface layer in the human kidney. (G) Cells in the stem of a sunflower, Helianthusannuus.object as viewed to its real size, usually given as something like1,200. Resolution is the minimum distance two points in thespecimen can be separated and still be seen as two points. Resolution depends primarily on the wavelength of light or electronsused to illuminate the specimen; the shorter the wavelength, the1 micrometer (µm) =1/1,000,000 meter1 nanometer (nm) =1/1,000,000,000 meterLight microscopes1 millimeter (mm) =1/1,000 meterElectron microscopesUnaided human eye1 centimeter (cm) =1/100 meteror 0.4 inch3 cm1 mmChicken egg(the “yolk”)Frog egg, fish egg100 µmHuman egg10–1005–302–101–55Typical plant cellTypical animal cellChloroplastMitochondrionAnabaena(cyanobacterium)1100 nm257–1020.1Escherichia coliLarge virus (HIV,influenza virus)RibosomeCell membrane(thickness)DNA double helix(diameter)Hydrogen atom1 meter = 102 cm = 103 mm = 106 µm = 109 nmFIGURE 5.3Units of measure and the ranges in which they are used in the study ofmolecules and cells.
The vertical scale in each box is logarithmic.90UNIT ONEMOLECULES AND CELLSbetter the resolution. Hence, electron microscopes have higherresolution than light microscopes. Biologists choose the type ofmicroscopy technique based on what they need to see in thespecimen; selected examples are shown in Figure 5.4.Why are most cells so small? The answer depends partly onthe change in the surface area-to-volume ratio of an object as itssize increases (Figure 5.5). For example, doubling the diameter ofa cell increases its volume by eight times but increases its surfacearea by only four times. The significance of this relationship isthat the volume of a cell determines the amount of chemical activity that can take place within it, whereas the surface area determines the amount of substances that can be exchanged between the inside of the cell and the outside environment.Nutrients must constantly enter cells, and wastes must constantly leave; however, past a certain point, increasing the diameter of a cell gives a surface area that is insufficient to maintain anadequate nutrient–waste exchange for its entire volume.Some cells increase their ability to exchange materials withtheir surroundings by flattening or by developing surface foldsor extensions that increase their surface area.
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