Ch. 3: Plasma Membrane Structure and Function

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Ch. 3 Plasma Membrane Structure and Function
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Biochemistry The unique properties of water
d
Hydrogen bonding is when the partial charge on
Hydrogen is attracted to the partial charge of
another compound.
d
d -
Water molecules have polar covalent bonds.
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Properties of Phospholipids

• Phospholipids
• Glycerol with Phosphate Head 2 Fatty Acid
  Chains
• Amphiphilic (Both lover)
• Hydrophilic head
• Hydrophobic tail
• Forms 2 layers in water
• (negatively charged phosphate head attracted to
  end of polar water molecules)
• Makes up cell membranes

P.Membrane (PM) held together by weak hydrophobic
interactions
Phosphate
Glycerol
Fatty Acids
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Fluid Mosaic Model

• Fluidity (not rigid)
• P.Membrane (PM) held together by weak hydrophobic
  interactions (bi-lipid tails face each other,
  away from water)
• Lateral drifting ability of lipids
• Temperature Dependent

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Why is this rare?
Cholesterol puts gaps between phospholipids,
increasing fluidity
Unsaturated tails prevents packed phospho-lipid
rafts
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Mosaic

• PM is made up of a mosaic or collage of
• Phospholipids
• cholesterol
• integral and peripheral proteins
• glycolipids
• glycoproteins

Hey Sugar! Lets stick
Hey Suga-!
glycocalyx
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Integral or Transmembrane Proteins

• Penetrate hydrophobic core of membrane

Surface or Peripheral Proteins

• Loosely bound to surface
• Some attaches to cyto-skeleton or ECM
  (Extracellular matrix)

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Review What organelles are responsible for
creating membrane proteins?
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Membrane Transport

• Cells NEED to be able to
• remove waste
• take in necessary nutrients from interstitial
  fluids
• Send out signals to other cells
• Receive signals from other cells
• Transport Classified as
• Passive
• Active

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Selective Permeability of Plasma Membrane

• General rule like dissolves like
• Non-polar, hydrophobic solutes dissolve in lipid
• Ions, hydrophillic, or polar solutes dissolve in
  water

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Selective Permeability of Plasma Membrane

• Selective Permeability some substances can pass
  through lipid core or membrane more easily than
  others
• CO2, O2, non-polar molecules, and other lipids,
  are hydrophobic and can pass hydrophobic lipid
  membrane core easily
• Water, sugars, charged ions, or polar molecules
  cannot pass lipid core easily ? so must use
  hydrophillic transport proteins to pass (ex.
  Aquaporins)
• Small molecules are more permeable than larger
  ones

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Passive Transport

• Molecules move down gradient (from high to low
  concentration) until equilibrium is reached
• Spontaneous process
• No ATP needed uses Kinetic energy (KE) or
  Hydrostatic Pressure as E source
• Types of Passive Transport
• Simple Diffusion
• Facilitated Diffusion
• Osmosis
• Filtration

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Simple Diffusion

• Diffusion molecules of any substance moves down
  gradient, unassisted
• Ex. O2 in blood, CO2 in cells

Back to Types of PT
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Facilitated Diffusion
Back to Types of PT

• Assisted diffusion of molecules with help from
  channels or carriers

Channels specific for particular molecule, like
sugars, amino acids
Carriers move substances like ions, water.
Selective by size and charge
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Osmosis
Water always moves from hypotonic to hypertonic

• Diffusion of WATER across the membrane
• Tonicity dependent
• Isotonic solution solution in equilibrium to
  another solution across the membrane
• Hypotonic solution with less dissolved solute,
  higher water compared to another solution
• Hypertonic solution with more dissolved
  solute, lower water compared to another
  solution

Back to Types of PT
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Filtration

• Forcing of water and solutes through membrane
  by hydrostatic pressure
• Selective only by SIZE
• Ex. only blood cells/proteins too large to pass
  are held back

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Active Transport

• Molecules move up or against gradient (from low
  to high) to create an electrochemical gradient
• Nonspontaneous
• Requires ATP as E source
• Types
• Primary Active Transport (T)
• Secondary Active (T)
• Clathrin-coated Vesicular (T)
• Endocytosis
• Exocytosis

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Active Transport generates an electrochemical
gradient charge difference (disequilibrium)
between both sides of the membrane
Back to Types of AT
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Primary Active Transport
Back to Types of AT

• Uses ATP E directly
• Ex 1 Sodium-Potassium Pump 3-D overview
• Pump keeps Na moving out of the cell, against
  its gradient, building its concentration
  (disequilibrium)
• Pump keeps K moving into the cell against its
  gradient, building its concentration
• NaK ratio 3 out 2 in
• NaK pump uses high concentration gradients to
  store PE for future cellular work or for
  secondary AT
• Ex 2 Pumping H ions into lysosome to create
  acidic envt for cellular digestion

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Secondary Active Transport(Coupled Transport)
Back to Types of AT
Ex. Hydrogen gradient creating PE
Symport two transported substances move in the
same direction Antiport two transported
substances move in opposite direction (wave to
each other)

• Involves the transport of a substance against a
  concentration gradient powered indirectly by an
  ATP powered pump

ATP
ADP Pi
Sucrose transported against gradient into cell,
using KE stored from H gradient, falling back
down gradient
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Endocytosis
Clathrin protein coat on PM helps with 1)
specifying cargo 2) membrane deformation

• The engulfing of substances by pseudopods
  extensions of the plasma membrane
• Three types
• Phagocytosis (cell eating lg. particles
  engulfed)
• Pinocytosis (cell drinking sm. ions and liquids
  engulfed)
• Receptor Mediated Endocytosis (use of surface
  proteins to engulf a specific substrate)

Often hijacked by pathogens that mimic a needed
substance by the cell
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Exocytosis

• Fusing of vesicles to the plama membrane, thus
  releasing its contents

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Function of Membrane Proteins

• TRANSPORT
• protein channels or carriers for passive
  transport
• protein pumps for active transport
• Clathrin-lined membrane for vesicular transport

• SIGNAL TRANSDUCTION
• substrates bind to protein surface ? sends a
  signal within the cell to start a chemical chain
  reaction or cell response

• INTERCELLULAR JOINING
• Ex. Gap Junctions, Tight Junctions, Desmosome

• CELL to CELL RECOGNITION
• Sugar on glycoproteins or glycolipids act as
  name tags for cells. Recognition of invaders,
  helps with cell communication and coordination

• ENZYMATIC
• Catalysis of Chemical Reactions at the Membrane
  Surface

• CYTOSKELETON and ECM ATTACHMENT
• Maintenance of Cell Shape

End of Slide Show
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Signal Transduction

• 3 Stages of Signal Transduction
• Reception A ligand or substrate binds to
  receptor protein. Receptor proteins can be on the
  cell surface, but not always. Receptor protein
  changes shape
• Transduction Amplifies and sends the signal
  through chemical relay
• Cell Response Specific response is triggered

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Examples of Signal Transduction
Why is this hormone-receptor protein not found on
the surface of the plasma membrane?
Steroids and Hormones are types of lipids, which
can pass through phospholipid membranes easily.
Back to Function of Membrane Proteins
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Cell Junctions
Back to Function of Membrane Proteins
Tight Junctions -Integral proteins of
neighboring cells fuse -prevents leakage btwn
cells into extracellular space (ex. Digestive
tract)
Gap Junctions communicating junction Hollow,
transmembrane protein cylinders, connexons, that
provide cytoplasmic channels btwn cells

• Disc-shaped plaque w/ linker protein fibers
  zipping tissues together to prevent separation

• Desmosomes
• -anchoring junctions
• Intermediate filaments extend from disc-shaped
  plaque to reduce tearing due mechanical stress to
  prevent separation

Transports ions, simple sugars, small
molecules Abundant in ion-dependent excitable
cells (ex. neurons)