Overview of Cellular Organization and Functions

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Overview of Cellular Organization and Functions

• Cathleen Pettepher
• Cancer Biology
• 776 PRB
• 3-3427
• cathy.pettepher_at_vanderbilt.edu

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Objectives

• Get general idea of cell organization
• Describe the major cellular components
• Understand what changes in organelles tell you
  about dynamics of cells.
• Develop global perspective on cell organelles
• Structure molecular organization and appearance
• How structure relates to function

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Biological Organization
Atoms Molecules Cells
Tissues
Organs
Cell Biology
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The Cell

• Human body comprises more than 200 different
  types of cells, each of which
• is surrounded by a bi-lipid plasma membrane
• possesses organelles structural subunits that
  permit it to discharge its functions.
• synthesizes macromolecules (proteins) for its own
  use or for export
• The specific proteins produced are the 1 way
  that cells regulate function.
• produces energy
• is capable of communicating with other cells

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Importance of Studying Cell Structure
Changes in cell structure provide important clues
about changes in cell function.
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Changes in structure can indicate a great deal
about what functional changes are occurring
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Importance of Studying Cell Structure
Changes in cell structure provide important clues
about pathologic changes. In pathologic
conditions cells exhibit either too much or not
enough of a normal function.
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Liver Normal vs. Pathologic
Normal
Necrotic
Uniform Size
Round, open nuclei
Uniform, pink cytoplasm
Swollen Cells
Pyknotic Nuclei
Condensed Cells
Dead Cells
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Simplistic View of Cell Function
Heart of control of cell function is synthesis

• General (all cells)
• Protein Factory
• Reproduction
• Specific (variation on above)
• Liver cell detoxifies noxious substances
• Neuron transmits nerve impulses

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Protein Synthesis is at Heart of Everything the
Cell Does.
Cell as Protein Factory
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Simplistic View of Cell Function
Specificity of function is product of what is
synthesized (i.e what genes are turned on).
DNA RNA Protein
Cellular control via mix of Proteins produced
Other Molecules
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Evolution Cell Function is Reproduction
Reproduction requires protein synthesis.
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Simplistic Regions of Cell
Inside
Nucleus
Outside
Nuclear Boundary
Outside Boundary
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Lipid Bilayer Membrane forms boundary between
inside and out
Inside
Outside
(Plasma membrane is Archetype Cellular Membrane)
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Cytoplasm

• Protoplasm
• Living substance of the cell
• Divided into two compartments

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Cytosol

• Cytoplasmic matrix
• mostly water
• organelles -metabolically active structures that
  perform distinctive functions
• inclusions - metabolic byproducts, storage forms
  of nutrients or inert crystals pigments

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Central hydrophobic portion of membrane provides
barrier to diffusion of water and small water
soluble molecules
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Material on the inside stays in and things on the
outside stay out except by specific and
controllable functions
Plasma Membrane is Selectively Permeable! Meaning,
it lets some materials pass through but not
others!
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Plasma Membrane

• Not visible with LM
• By TEM
• 8-10 nm thick
• trilaminar structure -- unit membrane
• two thin, dense lines
• intervening light area
• inner leaflet
• outer leaflet

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Plasma Membrane Molecular Composition
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Fluid Mosaic Model
Proteins in cellular membranes float and move
laterally within the lipid bilayer
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Phospholipid Bilayer

• Phospholipids have a polar head located at
  surface of membrane
• Nonpolar tail of 2 long fatty acid chains
• total molecule is amphipathic

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Membrane Structure
Hydrophillic Surfaces
Integral Membrane Proteins have Hydrophobic
Sequences
Hydrophobic Center
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Phospholipids are Asymmetrically Distributed
Phosphatidylcholine and Sphingomyelin more
abundant in outer leaflet
Phosphatidylserine, Phosphatidylethanolamine
preferentially on inner leaflet
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• Integral proteins- span the entire lipid bilayer
• Peripheral proteins- looser connection, usually
  cytoplasmic, often associated with 2
  messenger system or cytoskeletal apparatus

Lateral but not perpendicular movement possible
(without energy)
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Cholesterol and unsaturated hydrocarbons of
phospholipids help determine fluidity
Hydrocarbon Chains
Cholesterol
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Glycocalyx

• Carbohydrate chains that are covalently linked
  to integral proteins or phospholipids
• Form fuzzy coat on surface of cell

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Glycocalyx

• Added measure of protection to cells
• Bind to substances outside of cell

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Plasma Membrane Functions

• Maintain structural integrity
• Regulate permeability
• Mediate cell-cell interactions

• Recognize outside environment
• Regulate transport of material in and out
• Transduce signals

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Plasma Membrane Protein Functions
Structural Transducers Receptors
Enzymes Channels Pumps
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Transport Across Plasma Membranes

• Exchange of materials and information is mediated
  between the internal and external cellular
  environments
• Enables the cell to control the quality of its
  intercellular environment
• Four Principle Mechanisms
• passive diffusion active transport
• facilitated diffusion bulk transport

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Passive Diffusion

• Entirely dependent on the presence of a
  concentration gradient across the membrane
• Requires no input of energy
• Allows lipids and lipid-soluble molecules to pass
  freely hydrophilic molecules are impermeable

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Facilitated Diffusion

• Concentration gradient dependent
• Involves the movement of large hydrophilic
  molecules
• Requires the presence of protein carrier
  molecules
• No energy required

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Active Transport

• Operates against extreme concentration gradients
• Sodium pump which exchanges Na for K
• transmembrane protein complex (Na-KATPase)
• ATP ADP to generate energy

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Plasma Membrane Lateral Domains
Receptors transduce signal from outside cell to
inside cell (or vice versa) Once inside cell must
effect some change to be useful
General Paradigm of Receptor Initiated Cascades
Ligand
Physical change receptor protein Activation of Rx
(phosphorylation/dephosphorylation GDP/GTP etc.)
Active
Inactive
Influence on cellular function or DNA regulation
Amplification/control cascade
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Vesicular Transport (Bulk)

• Involves vesicle formation from membrane
• Secretion of cell products into the external
  environment -- exocytosis
• Transport of large molecules or small particles
  into the cell -- endocytosis
• phagocytosis -- ingestion of particulate matter
• pinocytosis
• ingestion of substances in molecular dispersion
• ingestion of bulk fluid by ionic changes

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Endocytosis
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Phagocytosis
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Clathrin Coated Pits

• Plasma membrane invaginates to form small pits
  -- caveolae -- which project into cell
• Opening of pit constricts to form a narrow neck
  with further constriction resulting in separation
  of the vesicle from the membrane
• Smooth walled or clathrin coated
• Trap and concentrate specific receptor proteins
• Function to bind and internalize ligands

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Pinocytotic Vesicles
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Plasma Membrane Lateral Domains

• Junctional Specializations
• macromolecular clusters of membrane constituents
  to provide specific environment for enhanced
  function

Tight Junction
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Membrane Limited Organelles?
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Membrane Limited Organelles can be thought of as
Continuous with Outside
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Cordoning off of areas allows for localization of
specific functions to unique areas.

• Specific Functions
• Efficiency
• Control

Molecular makeup of membranes different to
facilitate specific functions
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Vesicular shuttling of contents between specific
regions (organelles) produces functional
continuity
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Intracellular Transport
Anterograde
Plasmalemma Vesicles TGN Golgi RER
Retrograde
Plasmalemma Vesicles TGN Golgi RER
Other Organelles
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Archetypal Anterograde Protein Synthesis
Secretion
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Ribosomes and Protein Synthesis

• Ribosomes catalyze the synthesis of proteins
  using the nucleotide sequence of mRNA to specify
  the sequence of AAs
• If protein being synthesized has a signal
  sequence specific for rER, the ribosome binds to
  it
• If no rER signal sequence, the ribosomes remain
  free in the cytoplasm and the newly synthesized
  protein enters the cytoplasm

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Protein Synthesis Secretion
Step 1 Rough Endoplasmic Reticulum
Extensive rER is evidence of intense protein
synthesis
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Rough Endoplasmic Reticulum

• rER consists of interconnecting network of
  membrane-limited flattened sacs (cisternae)
• Specialized for protein processing lipid
  biosynthesis
• rER is continuous with the outer membrane of the
  nuclear envelope

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Ribosomes

• Stud the outsides of rER
• Small particles (15-20 nm in diameter) composed
  of proteins and RNA
• Composed of a large and small subunit that dont
  combine until protein synthesis begins

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Protein Synthesis

• Within the rER, proteins
• As polypeptide chains grow, their signal sequence
  determines whether the protein folds up in the
  lipid bilayer (folded to form their 2 structure)
  or translocates into the lumen of the ER
• Forms intra-chain di-sulfide bonds
• are glycosylated

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Proteins that remain in cytosol do not need to
cross membrane.
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Step 2 Golgi Apparatus (Complex)
Protein Synthesis Secretion
rER
Movement of membrane and cargo between rER and
Golgi is primarily via vesicular transport
ERGIC
Golgi
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Golgi Apparatus

• Involved in post-translational modification,
  packaging and sorting of proteins
• By EM, stacks of flattened membranes that are
  closely associated with vesicles

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Golgi Apparatus

• Polarized
• forming face (FF) --outer or cis-Golgi
• maturing face (MF) -- inner or trans-Golgi
• As a result of a series of vesicle budding and
  fusion events, the membrane molecules and soluble
  proteins in the lumen pass through the stacks

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Golgi Apparatus

• Different Enzymes (Integral proteins) localized
  to different regions of stack

Trans
Medial
Cis
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Golgi Compartmentalization allows sequential
processing
Maturation of carbohydrate chains by removal of
some sugars and addition of others
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Step 3 Sorting in Trans Golgi Network (TGN)
Protein Synthesis Secretion

• Secretory path is default Pathway in absence of
  specific sorting signal
• Some sorting may use specific receptors

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Secretory Granules

• Vesicles appear granular because of coagulation
  of their contents
• Secretory products are released from cell by
  exocytosis

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Step 4 Trafficking of vesicle to plasma membrane
and release of contents
Protein Synthesis Secretion
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Lysosomal Sorting in TGN

• Lysosomal enzymes packaged and delivered to
  lysosomes

Many lysosomal enzymes have mannose 6-phosphate.
TGN has M-6-P receptor. This concentrates the
enzyme in small area for movement to lysosome.
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Lysosomes
Membrane limited organelle containing hydrolytic
enzymes and having a low pH