Membrane Structure and Function Chapter 8 by 2ca6u9z

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									Membrane Structure and
      Function
        Chapter 8
             Membrane Structure
•   A boundary separating the cell from its environment
•   About 8 nm thick
•   Controls chemical traffic in and out of the cell
•   Selectively permeable
•   Unique structure determines function and solubility
    characteristics
Artificial Membranes
    Two Generations of Membrane
              Models




1935                    1972
Always same thickness   Proteins span the membrane
Coated with proteins    Not all the same
     The Fluidity
    of Membranes
Membranes rarely flip-flop –
too unstable
Frequent lateral movement

Double bonds in unsaturated
lipid tails cause kinks which
contribute to fluidity

Cholesterol reduces fluidity at
high temps.
Increases fluidity at low temps.
        Membrane Proteins
• Membrane proteins contribute to the
  mosaic quality of the structure.
• Different proteins convey different
  properties to each membrane.
• Integral proteins - within the membrane.
• Peripheral proteins - attached to membrane
  surface
• Proteins attach to cytoskeleton or to extracellular
  fibers to help give animal cells a stronger
  framework
Evidence of the Drifting of
   Membrane Proteins
         Membrane Carbohydrates
•   Found only on the outside of the membrane.
•   Function in cell to cell recognition.
•   Sorting embryonic cells into tissues.
•   Immune defense.
•   Usually oligosaccharides (15 or less sugar units)
•   Glycolipids or Glycoproteins
Cell Plasma Membrane
Membrane Function Animation 8.1
      Structure of a Transmembrane
                  Protein

Helical secondary
Structure of the
non-polar portion

Bacteriorhodopsin
has 7 transmembrane
Domains and is a
specialized
transport protein.
Functions of
 Membrane
  Proteins
Sidedness of
 the Plasma
 Membrane

Membranes have
distinct cytoplasmic
and extracellular sides.
    Permeability of the Lipid Bilayer
• Nonpolar (hydrophobic) molecules
  – hydrocarbons, oxygen, carbon dioxide
  – cross with ease
  – the smaller the faster

•    Polar (hydrophilic) molecules
     - small molecules, water, ethanol, pass easily
     - larger, glucose, will not pass through the lipid
       membranes
     - all ions have difficulty passing
Selective Permeability
       Transport Proteins

Provide hydrophilic
tunnel for ions.

They are specific
for the substances
they transport
 Diffusion of
 Salts Across
 Membranes
Each dye diffuses down
its own concentration
gradient
         Passive Transport
• Diffusion across a membrane
• Down a concentration gradient (High to
  Low)
• Net directional movement
• Spontaneous, no energy required,
  decreases free energy
    Diffusion Animation 8.2

Diffusion is the net movement down a
concentration gradient until a dynamic
equilibrium is reached (there is still
movement, no NET movement).
Osmosis The Passive Transport of Water
The Selectively Permeable
       Membrane
Osmosis Animation 8.3
Towards Equilibrium
  Direction of Osmosis is Determined
   by the Difference in Total Solute
             Concentration
               Which direction will the water molecules move?




                                     0.2M fructose
1M glucose                           0.7M sucrose




Semi-permeable membrane, only water can pass through
The Water Balance of Living Cells
       Evolutionary Adaptations for
      Osmoregulation in Paramecium
    Filling vacuole




Fresh water organisms
are hypertonic to their
environment. They
take up water by
osmosis. A system is
needed to remove
excess water

  Contracting vacuole
Osmoregulation in Saltwater Fish
While seawater is isotonic to many organisms, it is
hypertonic to some, they will loose water by osmosis.




Fish conserve water and pump out salts
          Water Potential Y
• Measures the tendency of water to leave
  one place in favor of another.

• Water will always move from an area of
  high water potential to an area of low
  water potential.
• Water potential is affected by two physical
  factors

1.Addition of solute, always lowers the water
  potential
2.Pressure, an increase in pressure will
  increase the water potential
               Y = Yp + Ys
• Pressure potential, Yp, is usually positive in
  living cells, negative in dead xylem.
• Water potential, Y, is zero for pure water.
• Cell sap has a negative solute potential, Ys, as
  it always contains solute.
• Water potential, Y, can be negative, zero or
  positive, depending on the Ys and Yp
  values.
    Osmosis Lab 1B, Figure 1.2

      Beginning                             End

           Y = 0 (water)                        Y = 0 (water)

           Y = -3 (potato)                      Y = 0 (potato)

Movement of water into the cell causes the cell to swell and
the cell contents to push against the cell wall to produce an
increase in pressure potential Yp. When Yp = 3, Y = 0.
There is no NET movement of water.
Facilitated Diffusion
         Facilitated Diffusion
• Diffusion of solutes with the help of
  transport proteins.
• Passive - no energy required
• These solutes need a protein to facilitate
  their diffusion because they are too polar
  to pass through the lipid bilayer.

                Animation 8.4
           Active Transport
• Pumps molecules across the membrane
  against their concentration gradients.
• Requires energy, in the form of ATP
• Used to help maintain ionic gradients
  across membranes.
• These ionic gradients represent potential
  energy.

                         Animation 8.5
     Electrochemical Gradient

• Two forces drive the movement of ions
  into a cell, the ion’s conc. gradient and the
  electrical force – together called the
  electrochemical gradient.
                  Nerve Cells
Upon activation, Na+ enters the
cell by diffusion down its
electrochemical gradient.

In order to be ready for the next
signal, Na+ must be actively
pumped out against its gradient.
The Sodium Potassium Pump




Animal electrogenic pump.
Passive and Active Transport
Some Ion Pumps Generate Voltage
       Across Membranes
• All cells have voltages across their plasma membranes
• The cytoplasm of a cell is negative compared to the
  extracellular fluids.
• This voltage is called a membrane potential, ranges from
  -50 to -200 mV
• Membrane potential can act like a battery.
        A Plant Electrogenic Pump
Energy can be stored
by creating a voltage
across membranes.

Using ATP, H+ is
pumped out of the
cell to create a
gradient.

Proton pumps are
the main electrogenic
pumps of plants,
fungi and bacteria
                               Cotransport

A membrane protein couples
the transport of one product
to another.

The ATP-driven pump stores
energy by concentrating a
substance on one side of
the membrane.

H+ falls back down its conc.
gradient taking sucrose with it.
Sucrose accumulation in a
plant cell.
         Exocytosis




Secretion of macromolecules by the
fusion of vesicles with the plasma
membrane
Endocytosis
Exocytosis and Endocytosis
      Animation 8.6
    Cholesterol Enters Cells by
  Receptor Mediated Endocytosis
• In the blood, cholesterol is bound to lipid and
  protein complexes called LDLs

• The LDLs bind to LDL receptors on cell
  membranes initiating endocytosis.

• In the disease, familial hypercholesterolemia,
  the LDL receptors are defective, cholesterol
  cannot enter the cell and accumulates in the
  blood causing atherosclerosis.
     Aquaporin – facilitates water
         diffusion (osmosis)

Tetrameric protein,
four identical
subunits

Each subunit forms
a water channel
Osmotic Potential

								
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