UNLV BIOL 196 - Chapter 4: Cells the working units of life

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Chapter 4: Cells the working units of life4.1 Cells provide compartments for biochemical reactions 1) Cell theorya) Cells are fundamental units of lifeb) All living organisms are composed of cellsc) All cells come from preexisting cellsd) Conceptual implicationsi) Studying cell biology is in some sense the same as studying life(1) Principles that underlie the functions of a single bacterial cell are similar to those governing the approx. 60 trillion cells in an adult humanii) Life is continuous(1) All human cells came from single cell, zygote (or fertilized egg)2) Small cell size is a practical necessity arising from the decrease in the surface area-to-volume ratio of any object as it increases in sizea) Cells small b/c surface area must be large to accommodate changes between cell and environmentb) Volume of a cell determines amount of metabolic activity it carries out per unit of timec) Surface area of a cell determines the amount of substances that can enter it from the outside environment, and the amount of waste products that can exit to the environment3) Microscopesa) Light microscopesi) Use glass lenses and visible light to form imagesii) Smallest detail that can be seen is 0.2 micrometers in diameter, which is about 1,000 times smaller than an object the human eye can seeiii) Used to visualize living cells and general cell structureb) Electron microscopesi) Use electron beam focused by magnetsii) Size limit is 2 nanometer, which is 100,000 times smaller than something human eye can seeiii) Can be used to visualize most structures within preserved cells4) Chemical analysis of cells usually begins with breaking them open to make a cell-free extracta) If conditions are right the properties of cell-free extract are the same as inside the cell5) Plasma membrane consists of phospholid bilayers with proteinsa) There is both compositional and functional diversity within this general frameworkb) Rolesi) Acts as selectively permeable barrier, preventing some substances from crossing it while permitting other substances to enter and leave the cell(1) Allows cell to maintain homeostasis, stable internal environment that is distinct from surrounding environmentii) Important in communicating with adjacent cells and receiving signals from the environmentiii) Often has proteins protruding from it that are responsible for binding and adhering to adjacent cells or to a surface(1) Plays important structural role and contributes to cell shape6) Prokaryotesa) Archaea and Bacteriai) Have in common a prokaryoitc cellular organizationii) Typically does not have membrane-enclosed internal compartments; does not have a nucleusb) Eukaryotesi) Eukaryotic cell organization is found in members of domain Eukaryaii) Includes protists, plants, fungi, and animalsiii) Contain membrane-enclosed compartments called organelles where specific metabollic functions occur(1) Most notable is nucleus, where most of cell’s DNA located and where gene expression begins4.2 Prokaryotic Cells do not have a Nucleus1) Prokaryotic structurea) Plasma membrane encloses cell, regulates traffic in and out of cellb) Nucleoid is region in cell where DNA is located c) Rest of material enclosed in plasma membrane is cytoplasm (two components)i) Cystosol: consists mostly of water containing dissolved ions, small molecules, and solublemacromoleculesii) Ribosomes: complexes of RNA and proteins, are sites of protein synthesis d) Substances in cytoplasm are in constant motion2) Specialized structuresa) Cell walls (Many prokaryotes have one)i) Located outside plasma membraneii) Cell walls of bacteria contain peptidoglycan(1) Polymer of amino sugars cross-linked by covalent bonds to peptidesiii) Enclosing cell wall of some bacteria is capsule(1) Not essential to bacterial lifeb) Internal membranesi) System that helps with function, such as photosynthesis(1) Photosynthesis requires membranesii) Other prokaryotes have internal membrane folds that are attached to the plasma membrane(1) Bacterium with enclosed compartments would have several evolutionary advantages(a) Chemical Rxns more efficient(b) Certain biochemical activities could be segregated within compartmentsc) Flagellai) Made of protein called flagellinii) Complex motor protein spins each flagellum on its axis like a propelleriii) Motor protein anchored to plasma membrane and some in outer membrane of cell walld) Cytoskeletoni) Helical network of filamentous structures that extend down length of cell inside plasmamembrane4.3 Eukaryotic Cells have a Nucelus and other membrane bound compartments1) Eukaryotes have plasma membrane, ribosomes, and cytoplasm just like prokaryotes2) Contain many membrane-enclosed organelles that compartmentalize biochemical rxns3) Ribosomesa) Consist of one larger and one smaller uniti) Each subunit consists of 1-3 large RNA molecules called ribosomal RNA (rRNA)ii) Also consists of multiple smaller protein molecules that are bound noncovalently to oneanother and to the rRNAb) Role is to translate nucleotide sequence of a messenger RNA molecule into a polypeptide chain c) Not membrane-enclosed compartment. Float freely in cytoplasm4) Nucleusa) Contains most of cell’s DNAb) Location of DNA and DNA replicationc) Where DNA is transcribed to RNAd) Contains nucleolus, region where ribosomes begin to assemble from RNA and proteinse) Enclosed by two lipid bilayers that form nuclear envelopef) Seperates DNA transcription (occurs in nucleus) and translation ( in cytoplasm)g) Membrane perforated by nuclear pores, which connect interior of nucleus to cytoplasmi) Allows molecules to enter/leave nucleush) Inside nucleus, DNA+proteins form chromatini) Chromatin occurs in form of long, thin threads called chromosomes 5) Endomembrane systema) Includes nuclear envelope, ER, Golgi apparatus, and lysosomesb) Vesicles shuttle substances between various components of endomembrane system, as well asplasma membranec) Endoplasmic Reticulumi) Interior compartment (lumen) is separate and distinct from surrounding cytoplasmii) Rough ER(1) “Rough” because of the many ribosomes attached to outer surface of membrane(2) Ribosomes become attached when they begin synthesizing protein destined formodification within RER(a) Protein enter RER if it contains specific short sequence of amino acids(b) One inside, proteins chemically modified to alter functions and chemical “tag” fordelivery(c) RER participates in transporting proteins to other locations in

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UNLV BIOL 196 - Chapter 4: Cells the working units of life

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