Archetypal Retrograde: Endocytosis

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Archetypal Retrograde Endocytosis
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Endocytosis

• Obtain raw materials for cell functions
• Defend organism from noxious chemicals and
  harmful biologicals

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Endocytosis

• Phagocytosis
• Macropinocytosis
• Pinocytosis
• Can involve specific receptors on surface or can
  be passive
• Receptors concentrate uptake
• In all cases some extracellular material
  internalized passively

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Endocytosis

• Plasma membrane invaginates to form pit
• Opening of pit constricts to form narrow neck
• Apposing membranes fuse to seal pit into vesicle

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Phagocytosis
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Pinocytosis
Material can be randomly captured or
concentrated by specific receptors Receptor
mediated endocytosis Membrane receptors
concentrate extracellular material for
specific uptake and trafficking
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Endosomes Lysosomes

• Membrane vesicles endosomes/phagosomes --
  deliver ingested microorganisms and other
  material destined for destruction to lysosomes
  (1).
• Fusion of lysosome with these vesicles creates a
  2 lysosome.
• Fusion exposes these endocytosed substrates to
  hydrolases which
• degrades the ingested material.

Secondary Lysosome
Endosome
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Lysosomal Storage Disease
Golgi
Primary Lysosome
Hydrolase
Digestion
Endo- cytosis
Secondary Lysosome
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Lysosomal Storage Disease
Golgi
Hydrolase
Mutant Enzyme
Primary Lysosome
Endo- cytosis
Secondary Lysosome
Cholesteryl Ester
Wolmans Fibroblast
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Lysosomal Storage Disease
Golgi
Hydrolase
Primary Lysosome
X
Abnormal Trafficking
Endo- cytosis
Secondary Lysosome
Cholesteryl Ester
Wolmans Fibroblast
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Smooth Endoplasmic Reticulum

• Short anastomosing tubules that are NOT
  associated with ribosomes
• Well developed in cells that synthesis and
  secrete steroids
• Referred to as sarcoplasmic reticulum in
  skeletal and cardiac muscle
• segregates Ca essential for contraction

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Smooth Endoplasmic Reticulum
Steroid synthesis Neutralization of poison
Lipid metabolism Phospholipid synthesis
Glycogen utilization
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Peroxisomes (Microbodies)

• Small, membrane-limited spherical bodies
• 0.5 to 1.0 um in diameter
• contain oxidative enzymes -- catalase
• Involved in the formation and breakdown of
  intracellular hydrogen peroxide
• used for killing phagocytosed bacteria
• Involved in beta oxidation of fatty acids

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Peroxisomes (Microbodies)
In many animals, but not as prominate in humans,
peroxisomes contain a central dark nucleoid body
of urate oxidase.
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Mitochondria

• Involved in aerobic energy production and storage
• Regulate ion content of cytoplasm.
• Store factors important in regulated cell death
  (apoptosis)
• Defects lead to cancer,
• neurodegenerative diseases and auto-immununity

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Mitochondria

• Vary in size, shape and number
• Move freely within cytosol and tend to aggregate
  in areas with high energy demands

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Mitochondria

• Outer membrane
• permeable
• contains porin, a pore-forming protein which
  allows free passage of small molecules
• contains enzymes that convert lipids into forms
  that can be metabolized by mitochondria

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Mitochondria

• Inner membrane
• separated from outer by intermembranous space
• thinner and thrown into folds -- cristae
• aerobic respiration and electron transport takes
  place here (contains cytochromes and enzymes
  involved in ATP production)

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Mitochondria

• Inner Matrix
• rich in proteins
• contains dense granules which are the storage
  site for divalent cations
• contains one or more strands of double stranded
  circular DNA
• enzymes for Krebs cycle and fatty acid beta
  oxidation abundant in matrix

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Mitochondria

• Inner Matrix DNA
• Maternally derived
• Produces only a small portion of the protein in
  mitochondria, rest come via nuclear DNA
  transcription of RNA and protein synthesis in
  cytosolic polyribosomes.
• Imported proteins have a mitochondrial targeting
  sequence.

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Mitochondria

• Cells with higher energy demands have more
  mitochondria and more cristae per mitochondria

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Non-membrane Limited Organelles
Cytoskeleton and Inclusions
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Cytoplasmic Matrix

• Proteins organized (microtrabecular lattice)
• Very little unbound water, cytoplasm more like
  runny jello than glass of water
• Some areas more solid than others

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Cytoskeleton

• Supporting framework which maintains shape and
  polarity of cell

Fluorescent staining of Actin (green) and lipid
inclusions (blue) in smooth muscle cells
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Cytoskeleton

• Important for cell movement
• Can be very dynamic structures under constant
  remodeling to accommodate shape changes necessary
  to confront environment
• Critical component of cilia and flagella (motile
  structures)

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Cytoskeleton

• Intracellular Movement
• Contraction can move cytoplasm
• Attach to other organelles and with molecular
  motors coordinate their movement
• Outside-In signaling
• Bind to integral membrane proteins and transduce
  signals from outside environment

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Components of the Cytoskeleton
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Cytoskeleton

• Microtubules
• Filaments
• Microfilaments
• Intermediate Filaments
• Microtrabecular components

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Microtrabecular Lattice

• Hypothetical
• Very little unbound water in cell
• Proteins and bound water organized in lattice
  structure
• Provides framework for organizing biochemical
  reactions

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Microfilaments

• Two recognized Types
• Thick (13-16 nm diameter) Filaments- Myosin
• Thin (6-8 nm diameter) Filaments- Actin
• Major proteins of muscle cells where they are
  critical components of contraction apparatus
• Actin (and probably myosin) found in almost all
  cell types

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Actin Microfilaments

• 2 strings of bead-like subunits twisted together
  in a rope
• globular subunits are stabilized by Ca and ATP
• form stable subunits with myosin

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Actin is a major component of microvilli

• Cylindrical, membrane-bound cytoplasmic
  projections
• Core of 25-30 actin microfilaments crosslinked by
  villin anchored into terminal web
• complex of actin and spectrin molecules

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Intermediate Filaments

• Intermediate in size (10-15 nm)
• Reasonably stable elements serving primarily
  structural function
• Bind other cytoskeletal and intracellular
  structures to one another

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Microtubules

• Elongate macromolecules made up of globular
  protein subunits (hollow cylinder)
• Composed of heterodimer of alpha tubulin and beta
  tubulin subunits in helix

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Microtubules

• Polymerization directed by microtubule organizing
  centers
• Cilia
• Basal bodies
• Centrosome

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Microtubules

• Polymerization can be highly dynamic (mitotic
  spindle) or can be relatively stable (cilia)
• Change in length accomplished by fast growth at
  one end () while other end grows slowly (-) or
  disassembly at negative end.
• Change in length controlled by environment and
  Microtubular Associated Proteins.

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Microtubules

• Functions
• Cell shape and movement (microtubules are stiff)
• Maintenance of cell polarity
• Intracellular transport of secretory granules
• Chromosome movement during division
• Beating of cilia and flagella

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Centrioles

• Small cylindrical paired structures located in
  centrosome
• 9 triplets of microtubules arranged around a
  central axis
• Each triplet consists of 1 complete and 2
  incomplete microtubules fused

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Centrioles
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Cilia

• Short, fine, hair-like, beating structures
• Associated with basal bodies
• thin, dark-staining band at base of cilia
• Similar to centriole
• result of centriole replication

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Cilia

• Contain an organized core of microtubules
• 9 2 arrangement
• Contains a pair of dynein arms which make a
  temporary bridge with the B microtubule of
  adjacent doublet