Membrane Structure and Function

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Membrane Structure and Function

• Chapter 8

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Membrane Structure

• Composed of
• Phospholipids
• Amphipathic molecules- having both hydrophilic
  and hydrophobic regions
• Carbohydrates
• Proteins

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Fluid mosaic model

• The membrane is a fluid structure
• Different proteins are embedded in the double
  layer of phospholipids

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Membranes are Fluid!

• Lipids move laterally in a membrane.
• Flip-flopping of lipids across membrane is rare
• Unsaturated hydrocarbon tails of phospholipids
  have kinks
• This allows the membrane to be more fluid
• Cholesterol within the membrane reduces fluidity
  at moderate temperatures but hinders
  solidification at low temperatures (phospholipids
  cannot pack together regularly).

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What is a mosaic?

• A collage of different proteins embedded in the
  fluid matrix of the lipid bilayer.
• The phospholipids make up most of the bilayer but
  the proteins are responsible mostly for specific
  functions.
• There are two major types of membrane proteins
• Integral proteins
• Peripheral proteins

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Membrane Proteins

• Integral Proteins
• These go through the hydrophobic layer of the
  lipid bilayer.
• Peripheral Proteins
• These are not embedded in the lipid bilayer, but
  are rather appendages bound to the membrane
  surface.

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Transmembrane Proteins (Integral)
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Sidedness of the Plasma Membrane

• There are inside and outside faces of the
  membrane.
• May differ in lipid composition
• Proteins have directional orientation in the
  membrane
• Carbohydrates are restricted to exterior surface

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Major functions of membrane proteins

• Transport
• Enzymatic activity
• Signal transduction
• Intercellular joining
• Cell-cell recognition
• Attachment of the cytoskeleton and extracellular
  matrix

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Transport

• Transmembrane proteins may provide a channel
  across the membrane
• Some actively pump substance across the membrane
  using ATP

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Enzymatic activity

• May be useful for steps in a metabolic pathway.

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Signal transduction

• The protein may have a binding site with a
  specific shape that fits the shape of the
  chemical messenger (i.e. a hormone)
• Signal may cause a change in the protein to relay
  the message inside the cell

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Intercellular joining

• Membrane proteins of adjacent cells may be hooked
  together

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Cell-cell recognition

• Some glycoproteins (proteins with short chains of
  sugars) serve as identification tags that can be
  recognized by other cells

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Attachment of the cytoskeleton and extracellular
matrix

• Microfilaments or other parts of the cytoskeleton
  may bind to membrane proteins
• This maintains cell shape
• Proteins that adhere to the ECM can coordinate
  extracellular and intracellular changes

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A Review - Cell Membrane Functions

• Compartmentalization
• Control substance flow selectively permeable
• Serve to localize and organize cell functions
• Detections and transmission of extracellular
  signals

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Traffic Across Membranes

• Pages 144-152

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Selective Permeability

• Regulation of what crosses the membrane of the
  cell
• Depends on the plasma membrane and the specific
  transport proteins
• Two types Passive and Active Transport
• There are several different methods by which
  molecules can enter and leave the cell

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Transport Proteins

• Proteins that span the membrane
• Some have hydrophilic channel (because some ions
  are hydrophilic and cannot pass through
  hydrophobic core)

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Passive Transport Diffusion

• No energy is required
• Diffusion molecules spread out into available
  space
• Concentration gradient diffusion happens from
  high to low concentration without other forces
• Equilibrium Concentration is the same on both
  sides and molecules cross membrane at equal rates
  on both sides.

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Passive Transport Osmosis

• Different types of solutions
• Hypertonic- higher concentration of solutes
• Hypotonic- lower concentration of solutes
• Isotonic- same concentration of solutes
• Osmosis
• The diffusion of water across a selectively
  permeable membrane

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Without Walls

• Animal cells have a hard time adjusting and
  surviving in extremely different solutions.
• Some have adaptations to control water balance.
• Osmoregulation control of water balance
• Contractile vacuole of Parameciuim pumps out water

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With walls

• Plant cells have elastic walls that expand and
  exert pressure on the cell to stop more water
  uptake
• Turgid very firm healthy plant
• Flaccid limp wilting
• Plasmolysis hypertonic environment causes plant
  cell to shrivel the membrane pulls from the
  wall, and is usually fatal to the cell

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Facilitated diffusion

• The diffusion of molecules with the help of
  transport proteins
• Transport proteins are specialized for what they
  transport- binding sites

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Two types of facilitated diffusion

• Aquaporins- channel proteins that quickly
  transport water molecules or small ions to flow
  across the membrane
• Gated channels- open or close due to stimulus
  (electrical or chemical)

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Active Transport

• This is the pumping of molecules across a
  membrane against the concentration gradient
• This requires energy
• Common transport system sodium potassium pump
• This works by exchanging sodium ions for
  potassium ions across the plasma membrane.

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Voltage across the membrane

• Membrane potential the voltage across the
  membrane created by unequal distribution of
  anions and cations on opposite sides of the
  membrane.
• Acts like battery- energy for transport of
  charged substances.
• Inside negative passive transport of cations
  into the cell and anions out (electrochemical
  gradient)

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Different Pumps

• Electrogenic pump transport protein that
  generates voltage across membrane
• Proton pump main electrogenic pump of plants,
  bacteria and fungi that pumps hydrogen ions
  (protons) out of the cell

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Cotransport

• The active transport of several solutes
• Substance pumped across membrane does work as it
  leaks back by diffusion escorting another
  substance

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Transport of Large Molecules

• Exocytosis The transport of large molecules to
  the outside of the membrane by the fusion of
  vesicles with the plasma membrane.
• Endocytosis the taking in of macromolecules by
  forming new vesicles from the plasma membrane
• Phagocytosis engulfing a particle
• Pinocytosis cell drinks extracellular fluid
  and its dissolved solutes

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