MEMBRANES

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• MEMBRANES
• from Chapter 4 and more

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most important concept

• All cells have lipid membranes with protein
  channels and enzymes
• So how are the lipids and proteins involved in
  membrane function (controlling the entry/exit of
  cellular materials)?

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Do the last two rows on your own
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Figure 4.10
DIFFUSION ACROSS A SEMI-PERMEABLE MEMBRANE
1. Start with two different molecules on
opposite sides of a semipermeable membrane (a
phospholipid bilayer).
2. Molecules diffuse across the membrane - each
along its own concentration gradient.
3. Equilibrium is established. Molecules
continue to move back and forth across the
membrane but at equal rates.
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Figure 4.5
Lipid micelles
Lipid bilayers
Water
No water
Serine
Phosphate
Hydrophilic heads interact with water
Hydrophobic tails interact with each other
Hydrophilic heads interact with water
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Figure 4.6c
Artificial membrane experiments
How rapidly can different solutes cross the
membrane (if at all) when.
1. Different types of phospholipids are used to
make the membrane?
Solute (ion or molecule)
?
2. Proteins or other molecules are added to the
membrane?
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Figure 4.7b
Summary of relative permeabilities
Phospholipid bilayer
Hydrophobic molecules
O2, CO2, N2
Small, uncharged polar molecules
H2O, glycerol
Large, uncharged polar molecules
Glucose, sucrose
Ions
H,Na,NCO3, Ca2,CL-,Mg2,K
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Figure 4.8c
Kinks change the permeability of membranes.
Lipid bilayer with no unsaturated fatty acids
Low permeability
Lipid bilayer with many unsaturated fatty acids
High permeability
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Figure 4.9a
Cholesterol fills spaces between phospholipids.
Polar
Nonpolar
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Are liposomes and artificial membranes
biotech tools?
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Diffusion is a spontaneous, passive process

• Molecules and ions move downhill along
  electrochemical gradients.
• Facilitated diffusion assisted by a type of
  membrane protein
• Osmosis Diffusion of water across a membrane
  towards regions lower water concentration
  (because of higher solute concentration)
• Activity 4.1 Diffusion and Osmosis

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Activity 4.1 Diffusion and Osmosis
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Figure 4.11
OSMOSIS
1. Start with more solute on one side of the
lipid bilayer than the other using molecules
that cannot cross the semipermeable membrane.
Lipid bilayer
2. Water moves from the region of
low concentration of solutes (high
concentration of water)to the region of
high concentration of solutes (low
concentration of water).
Osmosis
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Figure 4.12
HYPERTONIC, HYPOTONIC, AND ISOTONIC SOLUTIONS
Start with
Hypertonic solution
Hypotonic solution
Isotonic solution
Arrows represent direction that water moves
via osmosis
Lipid bilayer
No change
Membrane swells or even bursts
Membrane shrinks
Result
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Figure 4.18
HOW RESEARCHERS MAKE RED BLOOD CELL GHOSTS
2. In hypotonic solution, cells swell as water
enters via osmosis. Eventually the cells burst.
1.Normal blood cells in isotonic solution.
3. After the contents of the cells have spilled
out, all that remains are cell ghost, which
consist entirely of cell membranes.
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Osmosis Nobel Prize

• Believe it or not, we really didnt know how
  water moved through membranes so fast until

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Osmosis Nobel Prize for aquaporin (water channel)

• 2003 Nobel Prize in chemistry 
  http//www.sciencemag.org/cgi/content/full/302/564
  4/383/
• mutations of aquaporin http//www.jci.org/cgi/cont
  ent-nw/full/109/11/1395/F3
• aquaporin structure animations of water flow 
  http//www.mpibpc.gwdg.de/abteilungen/071/bgroot/p
  resentations/aqp1_dyn/md_glpf.html

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Osmosis Nobel Prize

• Believe it or not, we really didnt know how
  water moved through membranes so fast until

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All three use protein channels or carriers,
usually with active sites like enzymes have
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Activity 4.2 Facilitated Diffusion
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Figure 4.16a
Electrochemical gradients across membranes
Cl
Na
Cl
Na
Cl
Cl
Na
Na
Gramicidin
Cl
Cl
Cl
Cl
Cl
Cl
Na
Na
Na
Na
Na
Na
Electrochemical gradient for sodium ions (Na)

Net Charge High concentration of Na
Net Charge Low concentration of Na

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Figure 4.16c
Gramicidin is an ion channel.
Hydrophilic interior
Hydrophobic exterior
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Figure 4.13b
Amphipathic proteins can integrate into lipid
bilayers.
Glu
Tyl
Met
Pro
Ile
Pro
Gly
Ser
Asp
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K-channel won a Nobel 2003

• http//www.sciencemag.org/cgi/content/full/302/56
  44/383

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Figure 4.19
HOW GLUT-1 FACILITATES GLUCOSE DIFFUSION
Outside cell
Outside cell
Glucose
GLUT-1
Inside cell
Inside cell
2. Glucose binds to GLUT-1 from outside the
cell.
3. A conformational change results, transporting
glucose to the interior.
1. GLUT-1 is a membrane protein, shown with its
binding site facing outside the cell.
4. Glucose is released inside of cell.
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http//www.sciencemag.org/cgi/content/full/295/556
0/1658/F1
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ACTIVE TRANSPORT DIFFERS1. Requires cell
energy source, usually ATP directly or
indirectly 2. May move against the diffusion
gradient
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Open in another windowhttp//rsb.info.nih.gov/Neu
roChem/biomach/IONpmp.html

• Na /K pump uses ATP to move K in and Na out
  of cell

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co-transport is possible once a cell has a
gradient (not equilibrium) of H or K or Na

• www.cco.caltech.edu/lester/neurotra.htm

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http//www.sciencemag.org/cgi/content/full/301/563
3/603 
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CASE STUDY CFTR

• Cystic fibrosis transmembrane-conductance
  regulator CFTR, an ATP-dependent active
  transport pump for Cl-

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Essay 4.1, Figure 1
Sweatduct
Skin cell membrane
Skincell
Skincell
Sweatduct
Skin cell membrane
Na
Na
Cl
Cl
Cl
Cl
Na
Cl
Na
Cl
Cl
Na
Cl
Cl
Na
Duct of sweat gland
Cl
Na
Cl
Na
Cl
Cl
Na
Na
Cl
Na
Cl
Cl
Na
Na
Cl
Na
Cl
Cl
Cl
Cl
Cl
Na
Na
Na follows along electrochemical gradient
sweat becomes less salty
Sweat moves up duct movement of Cl form sweat
through CFTR, into skin cells
CFTR
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http//www.emory.edu/WHSC/MED/PHYSIOLOGY/NMCC/Stru
cture.htm
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Theoretical Model of NBD1. The deletion of
Phenylalanine 508 (Phe 508) is the most common
cause of CF.http//www.ornl.gov/hgmis/posters/chr
omosome/cftr.html
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ACTIVE SITE The glove and the hand are
near-perfect matches

• geometrically
• electrochemically
• for hydrophobicity etc.
• for motions and vibrations
• for van der Walls forces.

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MEMBRANES more

• http//www.queens.edu/faculty/jannr/bio103/helpPag
  es/c04membrane.htm
• http//stke.sciencemag.org/content/sigtrans/vol200
  0/issue29/images/data/pe1/DC1/pe1M1.swf?cknck
• check http//www.biologie.uni-hamburg.de/lehre/bza
  /kanal/ionch/1msl/e1mslm.htm
• animations of channels
• http//www.utexas.edu/ftp/depts/pharmacology/gonza
  les/spiral.mov
• http//www.utexas.edu/ftp/depts/pharmacology/gonza
  les/l-g_ch.mov
• active transport cartoon animation
  http//rsb.info.nih.gov/NeuroChem/biomach/IONpmp.
  html

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most important concept

• All cells have lipid membranes with protein
  channels and enzymes

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These are hypotheses can you think of
experiments to test them?

• Small, nonpolar molecules pass through membrane
  phospholipid layers because they can be dissolved
  in lipid.
• Active transport is essential for the intake of
  amino acids.
• Exocytosis is accomplished by the fusion of
  vesicle membranes with plasma membranes.
• Glucose intake requires carrier molecules but not
  active transport.

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