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Chapter 5

Chapter 5: Cells: The Working Units of Life.docx

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CAS BI 108
Francis Monette

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BI108 Chapter 5 Notes: Cells: The Working Units of Life 5.1: Cells are the Fundamental Units of Life Cell Theory: • Cells are the fundamental units of life • All living organisms are composed of cells • All cells come from preexisting cells • Evolution through natural selection explains the diversity of modern cells *Implications: • Principles that underlie functions of single cell of bacterium similar to cells of body • Life is continuous; cells derived from ancestors • Origin of life on Earth marked by origin of first cells Cell size is limited by the surface area-to-volume ratio • Most cells are tiny; exception: eggs of birds (single cell)—enormous, some algae and bacteria • Surface area-to-volume ratio: as an object increases in volume, its surface area also increases; surface area increase proportionally to r , while volume increase r • As cell grows, chemical activity (need for resources/waste production) increases faster than surface area • Cells must move from one site to another; smaller easier (?)Thus, cells must be small in volume in order to maintain a large enough surface area- to-volume ratio and an ideal internal volume; surface area (large) enables it to carry out different functions for survival Microscopes reveal features of cells • Resolution: allows detail to be seem; minimum distance two objects can be apart and still be seen as two objects • Resolution better in electron microscopy, however, only dead cells visualized; light microscopes can be used to see living cells The plasma membrane forms the outer surface of every cell • Plasma membrane: best observed with electron microscope; consists of phospholipid bilayer with proteins embedded o Selectively permeable barrier; prevents some substances from crossing o Polarity and size determine molecule’s ability to cross plasma membrane; macromolecules can’t normally but small like O ca2  Nonpolar can cross more easily than polar • Allows cell to maintain more/less constant internal environment (homestasis); by, actively regulating transport of substances across • Communicates with outside; receives signals from environment • Contributes to cell shape (proteins protruding from it) BI108 Chapter 5 Notes: Cells: The Working Units of Life Cells are classified as either prokaryotic or eukaryotic • Archaea/bacteria: prokaryotic • Prokaryotes: no membrane enclosed internal compartments; no nucleus • Protists, plants, fungi, animals: Eukaryotic • Eukaryotes: membrane-enclosed compartments (organelles), nucleus—DNA is located and gene expression begins • Organelles provide compartment that seperates molecules/biochem rxns from rest of cell: “division of labor”—explains complexity of eukaryotes to prokaryotes 5.2: Characteristics of Prokaryotic Cells -Most successful organisms on Earth Prokaryotic cells share certain features • Plasma membrane encloses the cell • Nucleoid: region where DNAis located; DNA—hereditary material that controls growth, maintenance, reproduction • Cytoplasm: enclosed in membrane; liquid component, insoluble filaments • Cytosol: dissolved ions, small molecules, soluble macromolecules • Ribosomes: complexes of RNAand proteins; sites of protein synthesis Specialized features are found in some prokaryotes As they evolved, developed specialized structures that gave selective advantage; cells with structures better able to survive and reproduce in particular environments • Cell Walls: located outside the plasma membrane o Rigidity of cell supports the cell and determines shape o Bacteria cell contain peptidoglycan: a polymer of amino sugars that is linked at regular intervals to short peptides  Some have outer membrane (polysaccharide rich phospholipid membrane) encloses peptidoglycan layer; but outer is not a barrier to movement of molecules across it o Capsule: slimy layer composed of polysaccharides; protect bacteria from attack by white blood cells in animal they infect; help cell from drying out, help bacteria attach to other cells; not essential to prokaryotic life • Internal Membranes: contains molecules needed for photosynthesis (cyanobacteria); other prokaryotes have internal membrane folds that are attached to plasma membrane • Flagella: appendage used to swim; complex motor protein spins flagellum on its axis like a propeller; anchored to plasma membrane; *causes motion (if removed, cells don’t move) • Pili: structures of protein that project from surface; conjugative pili (sex pili) help bacteria join to one another to exchange genetic material; Fimbriae composed of same proteins (shorter) help cells adhere to surfaces (animal cells for food/protection) BI108 Chapter 5 Notes: Cells: The Working Units of Life • Cytoskeleton: protein filaments that play roles in cell division or in maintaining shapes of cells 5.3: Characteristics of Eukaryotic Cells -Human cells arise by differentiation of stem cells; results in different types of cells with specialized functions in human body Compartmentalization is the key to eukaryotic cell function • Compartments interiors separated from cytosol by membranes = organelles • Organelles have specific roles; factories that make specific products, power plants that convert energy to useful forms (defined by chemical rxns within cells) Organelles can be studied by microscopy or isolated for chemical analysis • Organelles first detected by light, then electron microscopy • Stains targeted to specific macromolecules determine chemical composition of organelles • Fractionation: take cells apart; destroys the plasma membrane, to allow cytoplasmic components to flow out Ribosomes are factories for protein synthesis • Consist of special RNA(ribosomal RNA)—rRNA • In prokaryotes: float freely in cytoplasm • In eukaryotes: cytoplasm (free or attached to ER), mitochondria, chloroplasts • Molecular factories where proteins are synthesized The nucleus contains most of the genetic information • Hereditary info stored in sequence of nucleotides in DNAmolecules • Information encoded in DNAtranscribed into proteins at the ribosomes • Functions: o Location of most DNAand site of DNAreplication o Site where gene transcription turned on/off o Nucleolus—ribosomes begin to be assembled from RNA and proteins • Contents of nucleus: nucleoplasm • Nuclear envelope: double membrane structure that surrounds the nucleus; separates genetic material from cytoplasm; separates DNAtranscription (in nucleus) from translation (in cytoplasm) • The two membranes of the nuclear envelope have nuclear pores—connect nucleoplasm with cytoplasm o Pores regulate traffic between by allowing molecules to enter/exit; allows nucleus to regulate its info-processing functions o Ions (small substances) can freely diffuse through the pores; larger molecules (proteins from cytoplasm) cannot get through without a specific short sequence of amino acids BI108 Chapter 5 Notes: Cells: The Working Units of Life • Inside nucleus, DNAcombined with proteins to form chromatin; occurs in long, thin threads—chromosomes o Prior to cell division chromatin becomes tightly compacted and condensed; facilitates distribution of DNA o Chromatin attached to protein meshwork—nuclear lamina; maintains shape of nucleus by its attachment to chromatin and nuclear envelope The endoplasmic membrane is a group of interrelated organelles • Endomembrane system: interconnected system of membrane-enclosed compartments that are flattened into sheets • Plasma membrane, nuclear envelope, endoplasmic reticulum, golgi apparatus, lysosomes • Vesicles: tiny membrane-surrounded droplets; shuttle substances between the various components of the endomembrane system Endoplasmic Reticulum: Networks of interconnected membranes branching throughout the cytoplasm; form tubes and flattened sacs • Interior compartment (lumen) is separate and distinct from surrounding cytoplasm Rough ER (RER): ribosomes attached to outer surface o Receives newly synthesized proteins, segregating them away from cytoplasm, transports proteins to other locations o While inside RER, proteins can be chemically modified to alter their functions “tag” for delivery to cellular destinations o Shipped elsewhere enclosed within vesicles that pinch off from RER o Most membrane-bound proteins are made in RER; via sequence of amino acids o Some proteins covalently linked to carbohydrate groups in RER glycoproteins o Protein form disulfide bridges and fold into tertiary structure o Directing system is very important because enzymes within lysosomes are very destructive—needs to go to right organelles Smooth ER (SER) o Lacks ribosomes and is more tubular o Certain proteins that are synthesized in RER are chemically modified in lumen of SER o Important Roles:  Chemical modification of small molecules taken in by cell that may be toxic  Site for glycogen degradation in animal cells  Lipids and steroids are synthesized  Stores calcium ions (muscle contraction) BI108 Chapter 5 Notes: Cells: The Working Units of Life • Synthesize a lot of protein  packed with RER; ex—glandular cells that secrete digestive enzymes, WBC secrete antibodies • Cells that carry out less protein synthesis (storage cells) contain less RER; ex—liver cells have abundant SER The GolgiApparatus • Flattened membrane sacs (cisternae) that are piled up and small membrane-enclosed vesicles • Roles: o Recieves protein-containing vesicles o Modifies, concentrates, packages, sorts proteins before sent to cellular/extracellular destinations o Adds carbohydrates to proteins and modifies carbohydrates o Polysaccharides for plant cell wall are synthesized • Cis region—nearest to nucleus or patch of RER; trans region—closest to plasm
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