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Chapter 13
Plasma Membrane
Undergraduate 4

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Describe the types of phospholipids

1. Outer
2. Inner
1. phosphatidylcholine and shingomyelin
2. phosphatidyl
-serine (apoptotic when outside)
-inositol (endocytosis)
*serine and inositol are both negative so they give the inner membrane a negative charge
Describe the nonlipids in the plasma membrane
1. Cholesterol
-present in similar molar amounts as lipids
-major role in fluidity

2. Glycolipids
-only in outer leaflet
-carbohydrate exposed on cell surface
Describe the effect of cholesterol on fluidity1

1. Hot
2. Cold
1. High temperature
-interferes with FA chains
-makes the outer membrane less fluid and reduces permeability

2. Low temperature
-interferes with interactions
-prevents freezing
-maintains fluidity
Describe Lipid Rafts
Cholesterol and sphingomyelin cluster in small patches on outer membrane
-more highly ordered than rest of membrane
-different melting points than other phosphos with glycerol
Describe the Fluid Mosaic Model
lipids, proteins, cholesterol move freely throughout
-don't flip over

50% lipid and 50% protein by weight
-1:50 protein to lipid
Describe the two types of membrane proteins

1. Peripheral
2. Integral
1. do not cross through the membrane
-indirectly associate through protein-protein interactions
-bonds can be broken by disrupting ionic bonds
ex. salts or extreme pH

2. inserted into lipid bilayer
-only released by reagents that disrupt lipids
ex. detergents
Describe Transmembrane proteins

1. Definitions
2. Glycoproteins
1. Integral proteins that span the lipid bilayer
-can be viewed via freeze fracture
-usually contain 20-25 hydophobic AA to form alpha helix
-inserted to ER during synthesis
ex. Band 3- transporter of bicarbonate and chloride
2. Glyco groups are added in ER and Golgi
-exposed on outer membrane
ex. glycophorin in RBCs
Describe the follow transmembrane proteins

1. Porins
2. GPI anchors
1. transmembrane proteins in outer membrane of bacteria
-cross membrane as beta barrels to form pore
-highly permeable to ions and small polar molecules

2. proteins are anchored to outer membrane by GPI
-added to C terminus
-binds to inositol
Describe movement of plasma membranes

1. Experiment
2. Methods of restriction
1. free movement was demonstrated by fusing human/mouse cells
-after incubation the proteins were evenly distributed throughout

2. not all membrane proteins/lipids can move freely
a. some are associated with cytoskeleton, other membrane proteins, or ECM
ex. Band 3 is bound to spectrin
b. many epithelial cells are polarized to apical and basolateral domains
-tight junction doesn't allow proteins to move between
Describe movement across the membrane

Passive Diffusion
molecules dissolve in the lipid bilayer and diffuse across
-transport is determined by concentrations inside and outside the cell
-net flow is always down the gradient

only small hydrophobic molecules can pass
-gases (O2 and CO2)
-small polars (H2O and ethanol)
Describe movement across the membrane

Facilitated Diffusion
1. Carrier proteins
2. Channel proteins
transport mediated by proteins that allow polar and charged molecules to cross
-still dependent on concentration gradient

1. bind and undergo conformational change
ex. sugars, AA, nucleosides
*glucose is phosphorylated once inside cell to gluc-6-phos by glucokinase

2. form open pores allowing free diffusion
-any molecule of appropriate size can pass
-very rapid
Describe movement across the membrane

Facilitated Diffusion: Types of Channels
1. Aquaporins
2. Ion channels
1. allow water to freely pass membrane and pass more rapidly
-impermeable to charged ions

2. highly selective to certain ions
-gates open in response to certain stimuli
a. Ligand gated- signaling molecule binds and opens channel
ex. neurotransmitter, cGMP
b. Voltage gated- electrical potential changes conformation
Describe the Patch Clamp Technique
micropipette is used to isolate a small patch of membrane, allowing the flow of ions through a single channel
-studies one specific channel
-measures voltage that can be altered by adding acetycholine
Describe the formation of the resting potential
*ion pumps use ATP to actively transport ions across concentration gradient

1. Na/K pump pumps out Na and pumps in K
2. for each ATP used, 3 Na go out and 2 K come in
-resulting in negative 1 net charge per ATP
3. Open K channels allow K to leak back and forth
-largest contributor to resting potential
4. even though there is more K inside the cell, K will leak through because of the negative charge inside the cell

Resting potential of squid axon= -60mV
Describe the Nernst Equation
describes relationship between ion concentration and charge


R- gas constant (8.314 J/mol*K)
T- temperature in K
z- charge
F- Faradays constant

Describe Action Potentials
1. Resting potential is -60mV
2. small changes (-40mV) lead to opening of Na channels
-Na rushes in and potential is +30mV
3. Na channels are inactivated and K opens
-K flows out of cell and potential is -75mV
4. K channel is inactivated and potential returns to -60mV from flow of K and other ions
Describe how Action Potentials relate to Nerve Impulses
1. depolarization of adjacent regions of membrane allows potential to travel down nerve cell
2. at the nerve end, neurotransmitters are released at the synapse
-ligand gated channels open
ex. GABA opens Cl- channel
Describe voltage gated channels: selectivity

1. Na
2. K
1. Na is smaller than K, so channel is only large enough for Na to pass through

2. Carbonyl atoms in the channel pore displace the water that is bound to K and allow passage
-Na is too small to interact with C=O group so water is not displaced
-Na is too large to pass through with water
Describe voltage gated channels: structure

1. Na and Ca
2. K
1. single polypeptide chain
2. four identical subunits

*alpha helix 4 mediates voltage gating
*globular domain of hydrophobic AA plug hole
-without domain ions can flow freely
Describe the importance of the Na/K pump
*consumes 25% of ATP used by animal cells
1. establishes concentration gradient in cells
-allows for transport of other molecules using Na gradient
2. establishes voltage gradient
3. maintains osmotic balance and cel volume
-kicks out Cl
Describe examples of active transport pumps
1. Na/K pump
2. Ca pump
-similar to Na/K and Ca is pumped out
3. H+ pumps
-stomach acid and lysosomes
-ATP transport
Describe Active Transport driven by ion gradients
1. Na is present in high concentrations in intestinal lumen
-glucose is in low concentration outside and high inside
2. Energy from Na concentration gradient is enough to transport glucose across its concentration gradient
3. Two Na bind to 1 glucose and pass through transporter at apical domain

4. glucose is transported out of basolateral domain by facilitated diffusion
5. Na/K pump in basolateral domain keeps Na concentrations low inside cell

lumen --> cell (active)
cell --> blood (facilitated)
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