Exam 3 cell bio

• Membrane-enclosed organelles


  Internal membranes that create enclosed compartments and segregate different metabolic processes in eukaryotic cells

  
• Membrane-enclosed organelles


  • Held in locations by attachment to cytoskeleton
  • Organelles occupy nearly half the volume of a eukaryotic cell

  
• Endosomes


  • Membrane-bound vesicles formed via endocytosis
  • Vesicles larger than 100 nanometers in diameter are typically referred to as vacuoles
• Smooth ER


  • Lacks ribosomes
  • Site for steroid hormone synthesis
  • Site for detoxification
  • Sequesters Ca2+ from the cytosol
• Organelle evolution


  1. Nuclear membranes and ER may have evolved through invagination of the plasma membrane

  2. Mitochondria and chloroplasts evolved from bacteria engulfed by primitive pre-eukaryotic cells

  
• Endomembrane system


  • Includes nuclear membrane, ER, Golgi, endosomes, peroxisomes, lysosomes
  • These individual membrane systems communicate with each other by means of vesicles
• Protein sorting


  1. As cells grow, membrane enclosed organelles enlarge by incorporation of new molecules

  2. Organelles divide and then are distributed between 2 daughter cells

  3. Proteins are continually being produced

  
• Protein transport mechanisms


  • Nuclear pores
  • Protein translocators
  • Transport vesicles
• Sorting signal/signal sequence


  Stretch of amino acids that directs a protein to a specific destination in a cell

  
• Nuclear pores


  • Penetrate the double membrane of the nuclear envelope
  • Proteins are transported in their fully folded conformation and ribosomes as assembled particles
• Nuclear Localization Signal (NLS)


  • Sequence that directs a protein from the cytosol into the nucleus
  • Contains several positively charged lysine or arginine residues

  
• Nuclear transport


  1. Requires energy supplied by GTP hydrolysis

  2. Ran GTPase exists in two conformations and is converted between them by accessory proteins

  3. Ran-GTP concentration is higher in the nucleus
• Protein entry into mitochondria and chloroplasts


  1. Proteins must unfold to enter

  2. Signal sequences and protein translocators are required

  
• Cytoskeleton

  Protein filaments
• Cytoskeleton components

  • Tubulin dimers
  • Actin monomers
• Cytoskeleton composition

  • Fibrous
  • Globular
  • Globular

  
• Intermediate Filaments

  • Provides tensile strength; resist mechanical stress
  • Most durable among the cytoskeletal filaments
  • Not defined by composition but by its diameter's size
• Intermediate Filament structure

  Tetramers pack together end-to-end and assemble in a helical array containing 8 strands of tetramers that generates the final ropelike intermediate filament
• Intermediate Filaments

  • Without polarity
• Where are Intermediate Filaments found?

  Intermediate filaments support and strengthen the nuclear envelope

  
• The nuclear lamina disassemble & reform at each cell division


  When the lamins are phosphorylated it weakens
• Centrosome

  The major microtubule-organizing center in animal cells
• Basal body


  Microtubules grow out from the small structure near the center of the cell called the centrosome

  
• Cilia and Flagella

  • Contain stable microtubules moved by dynein

  
• Defects in dynein causes Kartagener's syndrome
• Microtubules

  • Hollow tubes with structurally distinct ends
  • Have polarity which gives directionality
  • Built from sub-units called tubulin dimers
• Microtubule polymerization

  Tubulin polymerizes from nucleation sites on a centrosome
• Microtubule stabilization

  Microtubules can be stabilized by attachment to capping proteins
• Microtubule dynamics


  Dynamic instability driven by GTP hydrolysis
• Microtubule-modifying drugs


  • Anti-mitotic drugs; anti-cancer drugs
  • Colchicine binds to free tubulin and prevents polymerization, arresting cell division
  • Taxol binds to microtubules and prevents depolymerization, also arresting dividing cells in mitosis
• Microtubule functions


  • Organize the cell interior
  • Allow transport of cell components
  • Position organelles in the cytoplasm

  
• Motor proteins

  • Drive intracellular transport
  • Conformational changes result in movement, powered by ATP hydrolysis
  • Different motor proteins transport different cargo along microtubules
• Actin Filaments

  • Essential for cell movements
  • Key to changing cell shape
  • Structures formed depend on actin filaments association with different proteins
• Actin filament polymerization

  • Growth is by addition of actin monomers
  • ATP to ADP binding reduces binding between monomers, decreasing polymer stability
• Actin filament treadmilling

  When the rates of addition and loss are equal, the filament stays the same length
• Actin-specific drugs

  • Phalloidin binds and stabilizes filaments
  • Cytochalasin caps filament plus ends, preventing polymerization
  • Latrunculin binds actin monomers and prevents their polymerization
• Cell crawling

  • Depends on actin filaments
  • Involves extension, adhesion, translocation (contraction), and de-adhesion
• Actin filament nucleation

  1. Lamellipodia nucleation mediated by ARP (actin related proteins)

  2. Filopodia nucleation mediated by formin
• Extracellular signals control actin filament arrangement

  Activation of receptor proteins transduces signals to Rho-family GTPases
• Cell crawling


  Depends on actin