BIOL 304 Chapter 12: Lipids and Biological Membranes Part 2

What is FRAP recovery?

a process resulting in an increase in fluorescence intensity

When does FRAP recovery occur?

occurs if the labeled component is mobile because bleached molecules leave and unbleached molecules enter

What does the rate of FRAP recovery depend on?

the lateral mobility of the fluorescence-labeled component

Overall, why is FRAP important?

it can be used to measure lateral diffusion rates

What is the fluid mosaic model?

describes biological membrane organization as two-dimensional solutions of oriented lipids and globular proteins

In the fluid mosaic model, what role does the lipid bilayer play?

acts as both a solvent and a permeability barrier

What do lipids do in the fluid mosaic model?

they rapidly diffuse laterally in membranes

Does transverse diffusion (flip-flopping) move fast or slow?

it moves very slow

Does lateral diffusion move fast or slow?

moves faster than transverse diffusion

What controls membrane fluidity?

fatty acid composition/properties and cholesterol content

Where does the transition from a rigid to a fluid state take place?

above the melting temperature (Tm)

What does the melting temperature of a fluid membrane depend on?

the degree of unsaturation

Under what condition is the rigid state favored?

when straight hydrocarbon chains of saturated fatty acid residues interact favorably with one another

Why do cis double bonds (unsaturated) lower the melting temperature (Tm)?

the cis double bond produces a bend in the hydrocarbon chain, which interferes with a highly order packing of fatty acid chains

What happens to the Tm as the degree of unsaturation increases?

the Tm decreases (lowers)

How does fatty acid chain length impact the melting temperature?

long hydrocarbon chains interact more strongly than the short ones do

What can disrupt the highly ordered packing of fatty acid chains?

the presence of cis double bonds

How does cholesterol disrupt the tight packing of the fatty acid chains?

the bulky steroid nucleus of the cholesterol disrupts the regular interactions between fatty acid chains

Why is it important for cholesterol to disrupt the fatty acid chains?

helps maintain proper membrane fluidity

What type of complexes does cholesterol form with lipids?

complexes that contain sphingosine backbone and with lipid-anchored membrane proteins

What are lipid rafts?

membrane domains containing cholesterol/lipid complexes that exhibit reduced fluidity

What is one function that lipid rafts have?

induce conformational changes in membrane proteins to regulate their functional activities

What is another function that lipid rafts have?

function in signal transduction by providing a favorable environment for specific protein-protein interactions

True or False: all biological membranes are asymmetric

True! this is because the outer and inner surfaces of all biological membranes have different components and different enzymatic activities

What are gram-positive bacteria?

bacteria that have a single membrane surrounded by a thick cell wall

What are gram-negative bacteria?

bacteria that have two membranes separated by periplasm (which contains the cell wall)

How can bacteria membranes be distinguished?

through gram staining

What is the first step in gram staining?

crystal violet stain (purple) is added to bacteria

What is the second step in gram staining?

iodine is added to trap the stain in the cell

What is the third step in gram staining?

alcohol is used to wash out the stain

What is the fourth step in gram staining?

a secondary stain (pink) is added to stain cells

What color do gram positive bacteria turn?

purple, due to their thick cell wall that retain the violet stain

What color do gram negative bacteria turn?

pink, since their thin cell wall does not retain the violet stain

True or False: eukaryotic cells have cell walls

False! they have plasma membranes composed of a single lipid bilayer

What specific feature do eukaryote cells have?

they have membranes inside the cell that form internal compartments to compartmentalize cellular functions

What is receptor-mediated endocytosis?

process by which cells take up molecules from their environment

What does receptor-mediated endocytosis cause?

receptor binding induces membrane invagination by the action of intracellular clathrin and dynamin

Why is the fusion of a vesicle to a membrane important?

plays a key role in the release of neurotransmitters from a neuron into the synaptic cleft

What molecule is free iron in the blood bound to?

transferrin

How is receptor-mediated endocytosis initiated?

complex formation between transferrin receptor and iron-bound transferrin

What does receptor-mediated endocytosis result in?

internalization of these transferrin complexes within vesicles (endosomes)

What are SNARE proteins?

guide membrane fusion that ensures specificity

What is the main function of SNARE proteins?

draw appropriate lipid bilayers together through the formation of tightly coiled four-helical bundles

What do SNARE protein determine?

the compartment with which a vesicle will fuse (determines specificity)