Membrane Structure and Function

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Membrane Structure and Function

• Chapter 8

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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).
• Carbohydrates are also important, but usually as
  markers or components of ECM.

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Figure 8.4 The fluidity of membranes
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Figure 8.6 The detailed structure of an animal
cells plasma membrane, in cross
sectionhttp//www.susanahalpine.com/anim/Life/mem
b.htm
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Figure 8.7 The structure of a transmembrane
protein
Ribbon model highlights the a-helical secondary
structure of the hydrophobic parts of a protein
lie mostly inside core of membrane. Non-helical
portions of protein are in contact w/ aqueous
solution on either side of cell membrane.
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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
Page 144
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TRANSPORT ACROSS MEMBRANES What 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
• 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)
• This is a form of Passive Transport requires NO
  energy, no work is performed, follows the
  CONCENTRATION GRADIENT (concentration gradient
  represents potential energy)

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Diffusion

• Diffusion is a spontaneous process because it
  decreases free energy (increases entropy)
• Diffusion is the movement of particles FROM AN
  AREA OF HIGH CONCENTRATION TO AN AREA OF LOW
  CONCENTRATION (WITH THE CONCENTRATIO GRADIENT)
  REQUIRING NO ENERGY!

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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 the PASSIVE movement of particles
  across a semi-permeable membrane from an area of
  high concentration to an area of low
  concentration (no energy)
• Osmosis the PASSIVE movement of water molecules
  across a semi-permeable membrane from an area of
  high concentration to an area of low
  concentration (no energy)

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Molecules that Diffuse Easily

• Hydrophobic molecules such as hydrocarbons,
  carbon dioxide, and oxygen can easily diffuse the
  lipid bilayer of the cell membrane
• BUTthe hydrophobic core of the membrane impedes
  the transport of ions and polar molecules, which
  are hydrophlic.
• These molecules may require protein channels for
  passage through the cell 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
Water diffuses from less concentrated (hypotonic)
solution to the more concentrated (hypertonic)
solution. In this example2 sugar solutions of
different concentrations are separated by
membrane permeable to solvent (water) but not
solute (sugar). The passive transport of water,
or osmosis, reduces the difference in sugar
concentrations.
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Osmoregulation

• Osmoregulation is the control of water balance in
  cells.
• 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
In plants water pressure against cell wall
provides turgor pressure.
In plants isotonic cell environment promotes
limp cells.
In plants lack of water pressure causes
shrinking of cytoplasm away from cell wall
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Osmotic Water Potential

• Osmotic Potential the tendency of water to move
  across a permeable membrane into a solution.
• Water Potential (?) A way to look at the
  movement of water quantifies the tendency of
  water to diffuse from one area to another.
• Results from 2 factors solute concentration and
  pressure.
• The water potential for PURE WATER is ZERO and
  the addition of solutes lowers water potential to
  a value less than zero.
• Water will move across a membrane from the
  solution with the higher water potential to the
  solution with the lower water potential.

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Water Potential (?)

• Quantifies the tendency of water to diffuse from
  one area to another
• Remember the water potential of pure water is
  zero.
• 2 components solute potential (?s) pressure
  potential (?p)
• If a substance is dissolved in water, the water
  potential drops BELOW zero because as solute
  concentration goes up, the water concentration
  goes downso there are fewer water molecules
  available to diffuse.
• SOas solute concentration increases, solute
  potential decreases.

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The Importance of Water Potential to Plants

• As related to water movement through plants
• Ensures water moves into plant root
• Helps movement of water within plant
• Is a factor involved in transpiration
• Cell wall allows for increased pressure (turgor
  pressure)
• Pressure might counteract osmolarity

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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 (transport
  protein in plasma membrane of plant or animal
  cell that specifically facilitates the diffusion
  of water across the membrane)
• gated channels electrical or chemical stimulus
  causes them to open or close (Ex.
  Neurotransmitters cause sodium channels to
  open)
• translocation of solute-binding site a subtle
  change in shape of protein that translocates the
  solute-binding site across the membrane.

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Figure 8.14 Two models for facilitated diffusion
(A) Transport protein forms a channel through
which water molecules or a specific solute can
pass. (B) Transport protein alternates between
two conformations, moving a solute across the
membrane as the shape of the protein changescan
transport in either directionbut net movement
MUST BE DOWN CONCENTRATION GRADIENT!
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Active Transport

• Movement of solutes AGAINST the concentration
  gradient requires energy from cell!
• Movement from area of low concentration to area
  of high concentration.
• 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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Sodium Potassium Pump Animation

• http//highered.mcgraw-hill.com/sites/0072495855/s
  tudent_view0/chapter2/animation__how_the_sodium_po
  tassium_pump_works.html

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Some Ion Pumps Generate Voltage Across Membranes

• Voltage is electrical potential energy
  separation of opposite charges
• Voltage across the membrane is referred to as
  MEMBRANE POTENTIAL
• In cells, it 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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Contd

• 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
Using ATP for powera proton pump translocates
positive charge in the form of hydrogen ions
(these are the main electrogenic pumps of plants,
fungi, and bacteria)work by generating voltage
(charge separation) across membranes! PROTON
PUMPS ARE MEMBRANE PROTEINS THAT STORE ENERGY BY
GENERATING VOLTAGE ACROSS MEMBRANES.
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Figure 8.16 Review passive and active transport
compared
DIFFUSION hydrophobic and very small uncharged
polar molecules can diffuse through the lipid
bilayer w/out the use of energy. FACILITATED
DIFFUSION hydrophyllic substances, including
water molecules, diffuse through membranes with
the assistance of transport proteinsthis happens
DOWN the concentration gradient w/out the use of
energy. ACTIVE TRANSPORT some transport proteins
act as pumps, moving substances across membranes
AGAINST their concentration gradientsthis
requires ENERGY!
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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 Cotransporthttp//www.northland.cc.m
n.us/biology/biology1111/animations/active1.swf
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Movement of LARGE Molecules Across the 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
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Animated Tutorials Membrane Transport

• http//www.wiley.com/legacy/college/boyer/04700037
  90/animations/membrane_transport/membrane_transpor
  t.htm