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Tobin and Dusheck: Asking About Life, 3E Chapter 4
Chapter 4
Why Are AllOrganisms Made of
Cells?
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Tobin and Dusheck: Asking About Life, 3E Chapter 4
Key Questions
What are cells?What are the advantages and
disadvantages of being amulticellular organism?
What organelles and other partsof cells make them work?
How do membranes act asboundaries and regulate thecontents of cells?
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The Cell Theory
Works of Schleiden, Schwannand Virchow
The Cell TheoryAll organisms are composed of 1
or more cellsCells are alive and are the basic
living unitAll cells come from other cells
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Common characteristics ofprokaryotes and eukaryotes
Membrane separating interior (living)from exterior (non-living)
A thick semifluid material enclosed bythe membrane called protoplasmfluid + non-fluid contents within
protoplasm
Hereditary material in control center
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Prokaryotic and Eukaryotic CellsEukaryotic cells have a nucleus with a
nuclear membraneProkaryotic cells have no nuclear
membrane; considered more primitive
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How bacterial, animal, andplant cells differ
Eukaryotic cells arecompartmentalized by organellarstructures; prokaryotes, however,are less complicated
Eukaryotic cells have cytoskeleton.Eukaryotic cells have slower rates of
growth and division than that ofbacteria.
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Cell Boundary, Cell Bodyand a Set of Genes
Plasma Membrane is the cell’sboundary
Nucleus — contains DNAOrganelles — tiny structures
with specific tasksCytosol — part of cytoplasm not
in organellesCytoskeleton — network of
protein fibers
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Tobin and Dusheck: Asking About Life, 3E Chapter 4
How are cells alive?
Are organizedAre made of cellsPerform chemical reactionsObtain energyRespond to environmentChange over timeReproduceHave common evolutionary
history
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Cell maintains its internalenvironment
Cells must move substances acrossthe cell membrane, i.e. taking innutrients and getting rid of wastes,fast enough to meet their need forsurvival.
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Can a Cell Be Any Size?
Surface-to-volume ratio
How do largecells (i.e.,eukaryotes)increasetheirtransportingefficiency?
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Most cells are microscopically small
Why are most cells small?As cells grow larger, volume
increases faster than surface areaLarge cells have difficulty to obtain
nutrients and get ride of wastes.Nuclei control activities of smaller
cells more easily.
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Cellular Organization
Division of labor among specializedcells
Organisms outlive the cells thatcompose them — replacement ofcells
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Eukaryotic cells: An overview
All eukaryotic cells possess:Many internal organelles
(membranous or bacteria-derived)
Internal protein skeletalframework
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Nucleus
Chromosomes— DNA and protein
Nuclear envelope— double membrane
Nuclear pores— channels
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Cytosol
Half of the volume of the cellAqueous plus proteinConsistency of Jell-OThousands of enzymesRibosomesGlycogenFat
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Endoplasmic Reticulum (ER)Single convoluted sheet of membraneMakes proteins and lipids
•Rough ER (studded with ribosomes)•Smooth ER (no ribosomes)
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Endoplasmic Reticulum (ER)
Endoplasmic (means “within thecytoplasm”) reticulum (means “a little net”).
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Golgi Complex
Packaging center Traffic director Flattened disksSpherical vesiclesManages flow of proteins to
different destinations
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Lysosomes
Present ineukaryoticcells
Small vesiclesenclosed by asinglemembrane
Amoeba —phagocytosis
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Lysosomes
Lysosomes contain many differentdigestive enzymes, proteases,glycosidases, lipases, and nucleases,which are only active under theacidic condition (pH 4.5~5.2).
Lysosome helps cell to renew itsaged (or old) parts and to protectcell from the invasion.
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Responsible for the cell’s abilityto obtain energy
Make ATPDouble MembraneHave their own DNA
Mitochondria
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PlastidsOrganelles surrounded by double
membrane present in plant cellsChloroplastChromoplastsAmyloplasts
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Tobin and Dusheck: Asking About Life, 3E Chapter 4
Cytoskeleton
Complex network of proteinfilamentsTypes of filaments:•Microtubules: hollow cylinders 9+2useful in cell division•Actin filaments: finer tubes,involved in shape changes•Intermediate filaments — fibrous,add strength
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Functions of Cytoskeleton
Microfilaments and intermediatefilaments help to support and shape thecell.
Microtubule extends from the center ofthe cell the plasma membrane and actsmainly as the intracellular traffickingnetwork.
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A Plasma Membrane
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Principle components ofmembrane
Principle components of membrane:phospholipids, cholesterol, andproteins
Phospholipids have both ahydrophilic head and hydrophobictail that interacts laterally withother phospholipids.
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Plasma membrane is a double-layeredstructure containing phospholipids as major
components.
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Structure of Membrane
Fluid Mosaic Model Lipid bilayer with 2 sheets of
phospholipids arranged tail to tail Proteins are dispersed through the
membraneBoth protein and lipid molecules
move freely within membrane (fluidaspect)
Serves as a hydrophobic barrierSome proteins help transport
molecules
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Membrane Permeability
Lipid bilayerallows only afew smalluncharged polarmolecules, suchas water (H2O)and ammonia(NH3), and somelipid solublesubstances,vitamines A, D,and E, to passthrough.
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Function of Membranes
Serve as boundaries that separatethe inside from the outside
Regulate contents of the enclosedspaces
Serve as a “workbench”forbiochemical reactions
Participate in energy conversions
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Regulation
Membrane is selectively permeableGeneral rules:
Diffusion — movement of moleculesfrom an area of high concentration tolow concentration (concentrationgradient)
Osmosis — movement of wateracross any selectively permeablemembrane in response to aconcentration gradient
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Predicting Movements
Hypotonic — more water movesinto cell than out
Isotonic — water movements inand out are balanced
Hypertonic — more water movesout of the cell than in
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Vacuoles
The word “vacuole”means “empty”.Vacuoles are most often found in plant
cells. It makes plant tissues very rigidand holds up the whole structure.
In fresh water protists, the vacuoles maytake up extra water that tends to flow intothe organism. Periodically, thesevacuoles expel their contents by fusingwith the plasma membrane.
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Turgor PressureOccurs in plant cellsCell wall encloses all plant cellsWhen cells take on water,pressure is applied against cellwall — turgor pressure
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Passive and Active Transport
Passive transport occursspontaneously, resulting in equalconcentrations of a molecule on the 2sides of the membrane
Active transport moves moleculesagainst a concentration gradient andrequires energy
Facilitated Diffusion uses a protein toincrease rate of diffusion
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Membrane Interactions
Extracellular MatrixNetwork of carbohydrates and
proteinsOccurs in both plant and animals
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Import and Export of Particles
Phagocytosis — cells engulf largeparticles, membrane extendsoutward
Endocytosis — cells take in tinyamounts by inward folding
Types: pinocytosis, receptormediated endocytosis
Exocytosis — reverse ofpinocytosis
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Cell Communication — Plants
Immediateneighbors
Plasmodesmata— narrowchannelsbetween plantcells; thinstrands ofcytoplasmstream through
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Cell Communication — Animals
Gap Junctions —membranes ofanimal cells areclose together,rivet-like
AdheringJunctions —connect internalcell skeletons
Tight Junctions —membranes arefused
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Cell Communication —Distant Cells
Signaling moleculesReceptors — specialized proteins
that lie either on the surface of acell or within cells
Examples: hormones such asestrogen and testosterone
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Tobin and Dusheck: Asking About Life, 3E Chapter 4
Key Concepts
Cells have a boundary, a bodyand a set of genes
Eukaryotic cells have a nucleusand other organelles enclosed bymembranes
Cytoskeleton serves as aninternal communication system
Cell membranes establish theboundaries of the cell