Membrane Structure and Function

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Chapter 7

• Membrane Structure and Function

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• Current model is the fluid mosaic model
• proposed by Singer and Nicolson
• Functions
• boundary selectively permeable
• contain enzymes capable of breaking down
  nutrients and producing ATP
• Membrane function is determined by its structure
• Membranes with different functions differ in
  chemical composition

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• Phospholipid bilayer
• Amphipathic molecules
• Polar (hydrophilic) heads
• Non-polar (hydrophobic) tails
• A mosaic of proteins bobbing in the fluid
  bi-layer
• Proteins are individually inserted
• Integral, trans-membrane or peripheral
• Functions of membrane proteins include
• transporters, enzymes, cell surface receptors,
  cell surface identity markers, cell adhesion
  proteins, and attachments to the cytoskeleton

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• Membrane is fluid not static
• Phospholipids and proteins move within the
  lateral plane, but rarely flip transversely
• Fluidity is dependent on temperature
• Saturated phospholipids solidify at lower
  temperatures
• Unsaturated phospholipids remain fluid at lower
  temps
• Steroid rings reduces fluidity and lowers
  solidification temperature

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Membrane Carbohydrates

• Function in movement, adhesion and cell-cell
  recognition
• Located on the outer surface of the membrane
• combine with lipids (glycolipids) or proteins
  (glycoproteins)
• Vary between species, among individuals and even
  among cells of an individual
• human blood groups

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Traffic Across Membrane

• Plasma membranes are selectively permeable
• The selective permeability of a membrane depends
  on
• membrane solubility characteristics
• presence of specific integral transport proteins

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• The ability of a substance to cross hydrophobic
  core depends on the chemical characteristics of
  the substance
• nonpolar molecules dissolve into the phospholipid
  bilayer
• polar molecules may not pass as easily
• Transport proteins are integral proteins that
  provide a passageway for large polar molecules
  and ions across the membrane
• channels
• binding to proteins cause conformational change

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• 2 types of transport across membrane
• Passive transport
• Simple diffusion
• Facilitated diffusion
• Osmosis
• Active transport
• Active transport
• Group translocation (prokaryotes only)
• Endocytosis and exocytosis (eukaryotes only)

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Simple Diffusion

• Molecules move down a concentration gradients
• high concentration to low concentration
• Equilibrium
• Small uncharged molecules move directly through
  the membrane
• Rate of diffusion varys
• size, polarity, charge and concentration

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Facilitated Diffusion

• Molecules that have a charge (positive or
  negative) it will tend to move to the side of the
  membrane that have the opposite electrical
  potential
• Proteins form channels through membrane that
  transfer molecules in accordance with electrical
  or chemical gradients
• transporter (carrier) protein is required

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• transmembrane proteins
• uniport - which is where one molecule is
  transported
• Co-transport - where 2 molecules are transferred
• two basic types of cotransport
• Symport- two molecules are transported in the
  same direction
• Antiport - molecules are transported opposite
  directions through the membrane

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Osmosis

• movement of solvent molecules across a semi
  permeable membrane from high to low solvent
  concentration (low to high solute concentration)
• Osmotic concentration is the total solute
  concentration of a solution
• Osmotic pressure is the measure of the tendency
  for a solution to take up water
• Water balance of a cell is vital

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Cells without cell walls can not tolerate
excessive changes in water concentrations live
in isotonic environments osmoregulate in
extreme environments
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• Cells with cell walls have a limit to the amount
  of water the cell can absorb
• turgid firmness or tension such as found in
  walled cells that are in a hypoosmotic
  environment
• plant cells become flaccid or limp in isotonic
  environments
• plasmolysis walled cell shrivels and the plasma
  membrane pulls away from the cell wall as the
  cell loses water to a hypertonic environment

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Active Transport

• Requires transporter proteins
• Transport into the cell is against concentration
  gradient
• Energy from ATP is required
• Group translocation (prokaryotes only)
• A form of active transport in which the
  transported substance is modified so that it
  cannot cross the membrane in the opposite
  direction
• Example phosphorylation of glucose

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Active Transport
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• The Na/K pump is an active transport system
• The energy for the active transport comes from
  the ATP molecule which is used to phosphorylate
  the transporter
• Pump requires a continuous source of ATP
• For every ATP molecule used, three Na ions are
  pumped out of the cell and 2 K ions are pumped
  into the cell

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The sodiumpotassium pump Sodium ion
concentration is high outside the cell and low
inside Potassium ion concentration is low outside
the cell and high inside The pump oscillates
between two conformational states in cycle that
translocates three sodium ions out of the cell
for every two potassium ions pumped into the cell
ATP powers the changes in conformation by
phosphorylating the transport protein
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• Some ion pumps generate voltage across membranes
• Diffusion is driven by chemical and electrical
  forces
• Electrochemical gradient
• Membrane potential
• Energy source that effects the movement of all
  charged substances
• Inside of cell is more negative so it favors the
  movement of cations into cell and anions out

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• Exocytosis process of exporting molecules from a
  cell by fusion of vesicles with the plasma
  membrane
• Endocytosis process of importing molecules into
  a cell by forming vesicles derived from the
  plasma membrane
• Phagocytosis
• Pinocytosis
• Receptor-mediated endocytosis

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Endocytosis and Exocytosis
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Cell engulfs a particle by wrapping pseudopodia
around it then forming a vesicle. The vesicle
fuses with a lysosome forming a phagolysosome.
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Cell gulps droplets of extra cellular fluid into
tiny vesicles. Unspecific sampling method.
(b) Pinocytosis
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• Receptor-mediated endocytosis is very specific
• Extracellular substances called ligands bind to
  proteins with specific receptor sites
• These proteins are clustered in regions of the
  membrane called coated pits that have a fuzzy
  protein coat
• This protein coat deepens and forms a vesicle
• Allows a cell to take in bulk quantities of a
  specific substance