Lecture 5
1. Antibody-induced patching and capping
a. Bivalent/Polyclonal antibodies
i. Y-shaped
ii. Can bind two proteins
b. Patching
i. Clusters of proteins starting to appear in membrane
ii. Antibodies are conjugating together
c. Capping
i. Large protein dense section of cell membrane
1. Caused by antibodies
ii. Capping requires
1. Energy (ATP)
2. Actin filaments
2. Patterns of movement of integral membrane proteins
a. Free Random diffusion
b. Immobilized by cytoskeleton
c. Directed motion
d. Transient confinement by obstacle clusters
e. Confinement by cytoskeleton
3. Permeability properties of a lipid (only) bilayer
a. Barrier for most polar molecules
b. Diffusion rate across the membrane depends primarily on
i. Size
ii. Solubility in oil
c. Some molecules are highly membrane permeant
i. Small nonpolar molecules
1. O2,N2,NO,CO
ii. Small uncharged polar molecules
1. H2O, Urea, Ethanol
d. Some molecules are essentially membrane impermeant
i. Charged molecules
1. Ions
ii. Large uncharged polar molecules
1. Glucose, sucrose
4. Membrane Transport Proteins
a. 20% of bacterial genes identified so far are associated with membrane
transport processes.
b. Most membrane transport proteins are multipass transmembrane proteins.
c. 2 major classes of membrane transport proteins
i. Transporter proteins (carriers)
1. Mediate passive or active transport
2. Undergo conformational change
3. Bind specific solutes from one side or the other
ii. Channel proteins
1. Form narrow hydrophilic pores
2. Transport rates usually much higher
a. 1000X
b. 1 million ions/channel/second
3. Mediate only passive transport
4. Always open
5. Don’t go through conformational change
5. General Types of membrane transport
a. Passive transport
i. Passage of solutes not requiring energy
b. Modes of passive transport
i. Simple diffusion
1. Not mediated by proteins
ii. Channel-Mediated diffusion
1. Channels always open
a. Closed off by a gate tho
b. 3 types of gated channels
i. voltage-gated channels
1. depends on ionic charge on both sides of
the membrane
ii. Ligand-gated channels
1. controlled by
a. binding of a molecule
i. To the outer surface of
cation channels
ii. i.e. acetylcholine
iii. To the inside of a channel
iv. camp
iii. Mechano-gated channels
1. Conformation state depends on
a. Mechanical forces applied to
membrane
i. i.e. stretch tension when ur
stuffed
2. Specific conformation for specific molecules
3. No conformational change
iii. Transporter-mediated diffusion
1. Opening closes after entry of molecule
2. Other end opens to release the molecule
3. Not always open
6. Types of transporter-mediated transport (can be active or passive)
a. Uniporter
i. 1 binding site
ii. transports a single solute from one side of the membrane to the other
iii. GLUT1
1. Facilitated diffusion
2. Opens and grabs glucose
3. Closes and then opens other side
a. Closes after glucose left and opens back up the other
side
i. Called recovery
ii. Not all can do that
b. Coupled Transporters
i. Symport
1. Two solutes are transported in the same direction
2. Both binding sites must be occupied for transport to occur
ii. Antiporter
1. Two solutes are transported in opposite directions
a. 1 molecule at a time
7. Driving force behind passive transport
a. Electrochemical gradient
i. Net driving force for each solute
1. 2 components
a. Concentration gradient
i. Move from [HIGH] to [LOW]
1. Driving force for both uncharged and
charged solutes
b. Electrical membrane potential
i. Driving force only for charged solutes
b. Many plasma membranes have an electrical potential difference, with the
inside negative
8. Transport of water through membranes- Osmosis
a. Water moves from region of high water concentration to region of low
concentration
b. Impermeable solute concentrations
i. Sum of molar concentrations= osmolarity
c. Response of cell gives tonicity
i. Cell swells in hypotonic solution
1. High water influx
a. Due to low solute concentration
ii. Cell shrinks in hypertonic solution
1. High water efflux
a. Due to high concentration of solute
iii. Permeant solute can give diff. response
1. Cell in hyperosmotic solution of permeant solute may shrink
then return to normal size as solute enters, followed by water;
solution is then isotonic (overtons experiments)
2. I think that’s what deplasmolysis is
d. Aquaporin proteins allow rapid transport of water across membranes.