Chapter 8 Membrane structure and function

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Chapter 8 Membrane structure and function
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Review of structure

• Lipids and proteins are the main components of
  the cell membrane
• Phospholipid is an amphipatic molecule has a
  water loving and a water fearing section
  (hydrophilic and hydrophobic)
• Carbs are also important, but usually as markers
  or components of ECM

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History of Membrane models

• 1895 Charles Overton membranes made of lipids
  because substances soluble in lipids move through
  quickly
• 1915 red blood cell membranes analyzed, found
  to be composed of lipids and proteins
• 1917 Irving Langmuir artificial membranes
  made, added phopholipids dissolved in benzene to
  water
• 1925 Gorter and Grendel bilayer
• 1935 Davson and Danielli sandwich model
• 1972 Singer and Nicolson fluid mosaic model

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Figure 8.2 Two generations of membrane models
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Figure 8.3 Freeze-fracture and freeze-etch
Gives ability to look at ultra-structure of cells
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Figure 8.4 The fluidity of membranes
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Figure 8.5 Evidence for the drifting of membrane
proteins
Some proteins can drift takes less than an
hour for the membrane proteins of the two species
above to completely intermingle in the membrane
of the hybrid cell
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Figure 8.6 The detailed structure of an animal
cells plasma membrane, in cross section
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Figure 8.7 The structure of a transmembrane
protein
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Figure 8.8 Sidedness of the plasma membrane
Cell membrane had 2 sides cytoplasmic and
extracellular Extracellular is equivalent to the
inside face of ER, Golgi, and vesicle membranes
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Figure 8.9 Some functions of membrane proteins
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Transport across membranesWhat determines
direction?

• All molecules have kinetic energy thermal
  motion (heat) each molecule is random, but the
  population of molecules may be directional
• One result Diffusion def tendency for
  molecules of any substance to spread out (in
  absence of other forces, substances move from
  areas of high concentration to areas of low
  concentration)
• So, is Passive Transport requires NO energy, no
  work is performed, follows the CONCENTRATION
  GRADIENT (concentr grad represents potential
  energy)

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• Diffusion is a spontaneous process because it
  decreases free energy (increases entropy)

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Figure 8.10 The diffusion of solutes across
membranes
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Diffusion exists in two formsDialysis and
Osmosis

• Dialysis movement of particles
• Osmosis movement of water molecules across a
  semi-permeable membrane

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Solutions and comparing their concentrations

• Solutes solids that are dissolved in a liquid
• Solvents liquids that dissolve the solids
• So, when comparing solutions, use three terms to
  differentiate
• hypertonic solution with more solutes,
  less water
• isotonic solutions with equal solute
  concentrations
• hypotonic solution with less solutes,
  more water

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Figure 8.11 Osmosis
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Osmoregulation

• The control of water balance
• Cells without rigid walls can tolerate neither
  excessive uptake nor excessive loss of water
• Ways to solve
• live under isotonic conditions
• develop ways to prevent water loss or
  excessive water uptake

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Figure 8.12 The water balance of living cells
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Facilitated Diffusion

• Allows polar molecules and ions to diffuse
  passively with the help of transport proteins
  that span the membrane still no energy
  involved!!!
• channel proteins ex. aquaporins
• gated channels electrical or chemical
  stimulus causes them to open or close (Ex.
  Neurotransmitters cause sodium channels to
  open)
• translocation of solute-binding site

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Figure 8.14 Two models for facilitated diffusion
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Active Transport

• Movement of solutes AGAINST the concentration
  gradient requires energy from cell
• Typically referred to as pumps
• Ex. Na-K pump (page 149)

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Figure 8.15 The sodium-potassium pump a
specific case of active transport
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Some Ion pumps generate voltage across membranes

• Voltage is electrical potential energy
  separation of opposite charges
• Voltage across the membrane
• MEMBRANE POTENTIAL
• In cells, ranges from -50 to -200 volts (negative
  because inside of cell is negative compared to
  the outside)
• Because the inside is negative, membrane
  potential favors passage of CATIONS INTO THE
  CELL,
• AND ANIONS OUT OF THE CELL

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• SO, WITH IONS
• ions do not simply diffuse down the
  concentration gradient, they diffuse down their
  electrochemical gradients
• (Ex. Nerve cells)
• Thus, Na-K Pump is REALLY an electrogenic pump,
  because it generates voltage across the cell
  membrane due to 3 Nas for every 2 Ks
• (this is the main electrogenic pump in animal
  cells in other kingdoms, Proton Pump is main
  one.)

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Figure 8.17 An electrogenic pump
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Figure 8.16 Review passive and active transport
compared
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Cotransport

• When one ATP-powered pump that transports a
  specific solute indirectly drives the active
  transport of several other solutes in a mechanism
• Ex. Proton pump in plants drives active transport
  of amino acids, sugars, and nutrients

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Figure 8.18 Cotransport
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Movement of LARGE molecules across the cell
membrane

• Exocytosis vesicles fuse with membrane and push
  materials OUT
• Endocytosis the taking IN of macromolecules and
  particulate matter by forming vesicles in the
  plasma membrane
• 3 types of endocytosis
• 1. phagocytosis cell eating particles taken
  in
• 2. pinocytosis cell drinking fluid taken in
• 3. receptor-mediated endocytosis very
  specific uses receptors to bind LIGANDS (this
  type allows a cell to stock-pile amounts of
  specific substances)

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Figure 8.19 The three types of endocytosis in
animal cells