Cells as Units of Life

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Cells as Units of Life

• Chapter 3

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Cell Theory

• Cells represent the basic structural and
  functional unit of life.
• Important unifying concept in biology.
• All organisms are composed of one or more cells.
• All tissues organs are composed of cells.
• There is no life without cells!

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Cell Theory

• Cell theory states that all living organisms are
  composed of cells.
• Cells come from preexisting cells.

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Prokaryotic vs. Eukaryotic Cells

• All cells
• Have DNA
• Use the same genetic code
• Synthesize proteins
• Use ATP in similar ways
• This implies common ancestry.

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Prokaryotic vs. Eukaryotic Cells

• Prokaryotic cells no nucleus or other
  membrane-bound organelles.
• Kingdom Archaebacteria
• Kingdom Eubacteria
• Eukaryotic cells do have nucleus and
  membrane-bound organelles.
• Kingdom Protista
• Kingdom Fungi
• Kingdom Plantae
• Kingdom Animalia

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Components of Eukaryotic Cells

• The plasma membrane surrounds the cell.
• The nucleus is the largest organelle.
• Double layered nuclear envelope.

Cell Model
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Components of Eukaryotic Cells

• Cytoplasm refers to the cellular material between
  the cell membrane and nuclear envelope.
• Organelles such as the mitochondria, Golgi
  complex, centrioles, and endoplasmic reticulum
  are found in the cytoplasm.

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Plasma Membrane

• Plasma membrane structure is described using the
  fluid mosaic model.

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Plasma Membrane

• Two layers of phospholipid molecules oriented
  with hydrophilic heads toward the outside and
  hydrophobic tails inside.
• Fluid-like flexible

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Plasma Membrane

• The nonpolar nature of the hydrophobic ends in
  the interior of the membrane prohibit polar
  substances from crossing the membrane.
• Glycoproteins embedded in the membrane function
  in the transport of molecules across the
  membrane.

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Nucleus

• The nuclear envelope contains pores to allow
  molecules to move between nucleus cytoplasm.
• Chromosomes are contained in the nucleus.
• Chromatin refers to loosely condensed DNA
  proteins.

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Nucleus

• Nucleoli are specialized parts of certain
  chromosomes that carry multiple copies of the DNA
  used to synthesize ribosomal RNA.
• This rRNA combines with protein to from the two
  subunits of ribosomes.
• Ribosomes leave the nucleus through pores in the
  nuclear envelope.

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Endoplasmic Reticulum

• The nuclear envelope joins with a cytoplasmic
  membranous system the endoplasmic reticulum
  (ER).
• Rough endoplasmic reticulum (RER) is covered with
  ribosomes. Smooth (SER) is not.

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Endoplasmic Reticulum

• Ribosomes on the RER synthesize proteins that
  enter the ER that will either be incorporated
  into the plasma membrane, exported from the cell,
  or they may be bound for lysosomes.
• Lipids and phospholipids are synthesized in the
  SER.

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Golgi Complex

• The Golgi complex is a stack of membranous
  vesicles where storage, modification, and
  packaging of protein products occurs.

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Assembling Secreting Proteins
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Lysosomes

• Lysosomes contain enzymes (proteins) that can
  breakdown foreign material like bacteria or worn
  out cellular components.
• Contents of lysosome would kill cell if membrane
  ruptured.
• May pour enzymes into food vacuoles.

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Mitochondria

• Mitochondria are the powerhouses of cells they
  contain enzymes that carry out the
  energy-yielding steps of aerobic metabolism. ATP
  is produced here.
• Composed of a double membrane the inner
  membrane is folded into cristae.
• Mitochondria are self-replicating, containing
  their own circular DNA molecule.

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Cytoskeleton

• Eukaryotic cells have a cytoskeleton that
  provides support and often locomotion and
  movement of organelles.
• Composed of microfilaments, microtubules, and
  intermediate filaments.

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Cytoskeleton

• Microfilaments are made of the proteins actin and
  myosin and function in a cells ability to
  contract as seen in muscle cells.
• Actin microfilaments move molecules and
  organelles through the cytoplasm.

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Cytoskeleton

• Microtubules are larger tubular structures
  composed of the protein tubulin.
• Move chromosomes during cell division.
• Part of the structure of cilia flagella.

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Cytoskeleton

• Microtubules radiate out from the centrosome
  the microtubule organizing center.
• Located near nucleus.
• Not membrane bound.
• Centrioles are found in the centrosome.
• Centrioles composed of 9 triplets of
  microtubules.
• Replicate before cell division.

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Cytoskeleton

• Intermediate fibers fall in between
  microfilaments and microtubules in size.
• There are five biochemically distinct types of
  intermediate fibers.

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Cilia Flagella

• Cilia flagella are motile extensions of the
  cell surface.
• In many single celled organisms they are a source
  of locomotion.
• In multicellular animals they usually sweep
  material past the fixed cell.
• Nine pairs of microtubules enclose a central
  pair.
• At the base is a basal body - identical to a
  centriole.

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Pseudopodia

• Some single-celled organisms, migrating cells in
  embryos, and white blood cells show ameboid
  movement.
• Cytoplasmic streaming through the action of actin
  microfilaments extends a pseudopodium outward.
• Some have specialized pseudopodia with
  microtubules that are assembled disassembled to
  allow movement.

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Junctions

• Tight junctions form when cell membranes
  ofadjacent cells fuse.
• Function as seals.
• Adhesion junctions occur under tight junctions.
  Transmembrane proteins link across a small space
  and connect to microfilaments.

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Junctions

• Desmosomes act as spot welds and increase the
  strength of the tissue.
• Hemidesmosomes are found at the base of cells and
  anchor them to connective tissue.
• Gap junctions are canals between cells that
  provide intercellular communication.

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Microvilli

• Microvilli are small fingerlike projections that
  have bundles of actin microfilaments.
• They serve to increase the surface area of the
  tissue as in the intestine.

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Membrane Function

• Membranes surround the outside of the cell and
  the organelles inside it.
• The plasma membrane acts as a selective
  gatekeeper.
• A substance may cross the membrane
• By diffusion
• By a mediated transport system
• By endocytosis

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Diffusion Osmosis

• Diffusion is the movement of molecules from an
  area of high concentration to an area of low
  concentration. This tends to equalize the
  concentration.
• Down the concentration gradient.
• Solutes are molecules (e.g. salt) that are found
  in a solution.

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Diffusion Osmosis

• Cell membranes are selectively permeable water
  can pass through, but not most solutes.
• Gases (oxygen carbon dioxide), urea, lipid
  soluble solutes can cross the membrane.

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Diffusion Osmosis

• Osmosis - if there is a membrane between two
  solutions with unequal concentration of solutes
  that can not cross the membrane, water will flow
  toward the side with less water / more solute
  until the two sides have equal concentrations.

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Diffusion Osmosis
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Diffusion Osmosis

• Animals utilize osmosis to control internal fluid
  and solute levels.
• The blood of marine fishes has 1/3 the salt
  content of the water. They are hypoosmotic to
  seawater.
• Freshwater fishes have blood that is saltier than
  the water. They are hyperosmotic to the water.
• If the solute concentrations were the same, the
  two solutions would be isoosmotic.

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Diffusion Through Channels

• Charged substances, like water and dissolved
  ions, cant simply diffuse across the cell
  membrane.
• They pass through channels created by
  transmembrane proteins.
• Some channels always open.
• Some are gated channels.

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Diffusion Through Channels

• Gated channels require a signal to open or close
  them.
• Chemically-gated channels open or close when a
  signaling molecule binds to a binding site on the
  transmembrane protein.
• Voltage-gated channels open or close when the
  ionic charge across the membrane changes.

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Carrier Mediated Transport

• Sugars amino acids must be able to enter cells
  and waste products must be able to leave.
• These molecules cross the membrane with the help
  of transporter proteins.
• Transporter proteins are specific.
• Facilitated diffusion
• Active transport

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Facilitated Diffusion

• In facilitated diffusion, the transporter protein
  binds to the substrate molecule on one side of
  the plasma membrane then changes shape to release
  it on the other side.
• Takes place in the direction of the concentration
  gradient.

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Active Transport

• Active transport requires energy (ATP) to
  transport molecules in the direction opposite the
  concentration gradient.

http//www.youtube.com/watch?vSTzOiRqzzL4
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Endocytosis

• Endocytosis is the ingestion of material by
  cells.
• Phagocytosis cell eating method of feeding by
  single-celled organisms.
• Pinocytosis small molecules or ions are
  enclosed in vesicles called caveolae.
• Receptor-mediated endocytosis method of
  bringing large molecules into a cell with the
  help of the protein clathrin.

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Endocytosis and Exocytosis
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Exocytosis

• Exocytosis - membranes of a vesicle inside the
  cell can fuse with the plasma membrane to
  discharge the contents of the vesicle outside the
  cell.
• Transcytosis a substance may be picked up on
  one side of the cell, transported completely
  across the cell and discharged on the other side.

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Mitosis and Cell Division

• Mitosis is the process of nuclear cell division
  in nonreproductive, or somatic, cells.
• A fertilized egg, or zygote, divides by mitosis
  to produce a multicellular organism.
• Damaged cells are replaced by mitosis.

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Chromosomes

• In cells that are not dividing, the DNA is
  loosely organized so that individual chromosomes
  cant be distinguished it is now referred to as
  chromatin.
• Before division, chromatin becomes more compact
  and chromosomes can be recognized.

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Chromosomes

• All nonreproductive cells in a species have the
  same number of chromosomes.
• 46 in humans
• Half of these chromosomes come from each parent.
• Result is two sets of chromosomes.
• Diploid
• Chromosome 1 from Mom and chromosome 1 from Dad
  are called homologous chromosomes.

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The Cell Cycle

• Cells come from preexisting cells through the
  process of cell division.
• Cell division mitosis and cytokinesis occupy
  a very small portion of the cell cycle.

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The Cell Cycle

• Interphase includes
• G1 growth phase where RNA and functional
  proteins are synthesized.
• S DNA replication.
• G2 growth phase where structural proteins are
  made.
• Mitosis
• Cytokinesis

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Chromosome Structure

• During S phase, each of the 2 homologues
  replicates, resulting in identical copies called
  sister chromatids.
• Chromatids remain connected at a linkage site
  called the centromere.

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Cell Division

• There are two phases of cell division
• Mitosis nuclear cell division
• Prophase
• Metaphase
• Anaphase
• Telophase
• Cytokinesis division of the cytoplasm
• Multiple nuclear divisions not accompanied by
  cytokinesis result in a multinucleate cell.

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Cell Division

• Prophase Chromosomes condense enough to be seen
  with a light microscope.
• Spindle forms between the 2 centrioles.
• Spindle fibers attach to kinetochores.

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Cell Division

• Metaphase Alignment of the chromosomes along
  center of cell (metaphase plate).
• Fibers attached to kinetochores on both sides of
  each chromosome.

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Cell Division

• Anaphase Separation of the sister chromatids.
• Centromere splits apart sister chromatids move
  toward opposite poles.
• Disassembly of the tubulin subunits shortens the
  microtubules.

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Cell Division

• Telophase re-formation of the nuclei once the
  chromosomes are at opposite poles.
• Chromosomes unwind.

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Cell Division

• Cytokinesis division of the cytoplasm.
• Two complete, diploid cells that are identical to
  the original cell.

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Cytokinesis

• During cytokinesis in animal cells, the cell
  pinches in two.
• A cleavage furrow produced by microfilaments
  deepens until the cell splits.

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http//www.youtube.com/watch?vVGV3fv-uZYI