Biological Membranes Flashcards
Name the four lipid classes
F.a., triacylglycerols, membrane lipids, cholesterol
Most naturally occurring double bonds are in what conformation?
cis
Most common length of f.a.?
16 or 18
Fatty acids are amphipathic, meaning?
Polar and nonpolar portions
What is the problem with unsaturated f.a.?
Cannot pack together effectively. When packed together, VDW interactions are maximized
What affects f.a. melting point?
Length and unsaturation
Longer fatty acids lead to _____ melting point
higher
Unsaturated f.a. melt at _______ temperatures
lower (less VDW interactions bc kinks)
Which double bonds alter f.a. shape?
cis
How are f.a. stored?
As fat (triacylglycerols or TAGS)
What are the three classes of membrane lipid?
Glycerophospholipids, Sphingolipids, cholesterol
Characteristics of cholesterol
Weakly amphipathic (sterol), not found in bacteria. Mostly hydrophobic, membrane lipid in mammalian plasma membranes
What does cholesterol do
Maintains fluidity and rigidity
When in water, what happens to amphipathic molecules
They form a micelle (when wedge-shaped) or bilayer (when cylindrical)
Bilayer can assemble into closed vesicle (liposome)
Can cholesterol form membranes alone?
No, OH associates with polar headgroups while non-polar portion is found in the membrane
Bilayers vary depending on…
Lipid composition (f.a. chain and polar head)
How does lipid composition vary
Size of head group, acyl tails vary in length (16-20 atoms), amount of cholesterol varies
Lipid bilayers are _____ yet _____
fluid, stable
What happens at a lipid bilayers melting temperature
Lipid molecules and chains move freely and rapidly. Ordered gel phase to fluid crystalline phase
What affects membrane fluidity
Temperature, lipid composition (presence of cholesterol, f.a. chain length, degree of saturation)
What is a bilayers transition temperature
The melting temperature
How does cholesterol regulate membrane fluidity at high and low temperatures
High = rigid and planar, limits rotational movement of acyl tails, increasing VDW interactions Low = inserts btw phospholipids and prevents them from clustering together and stiffening
If temperatures decrease, how would you maintain membrane fluidity
More unsaturated f.a. and shorter chain length
How can lipids move in a bilayer
Move freely and rapidly laterally, no flip-flop movement (significant energy barrier to moving polar head group through bilayer)
What are the three types of membrane proteins
Integral, peripheral, lipid-linked
What is the portion of an integral membrane (transmembrane) protein that is in contact with the acyl tails
Hydrophobic a.a. sidechains
What are the two forms of transmembrane protein
a-helices and B-barrels (antiparallel)
How can you tell how many times a protein has passed through the membrane
The number of different secondary structures
How do small molecules cross the lipid bilayer
Simple diffusion
What are the two major types of transport across biological membranes
Active and passive
What kind of transport are porins and ion channels
Passive
What are the differences between porins and ion channels
Porins = trimers (3 subunits; each contains a pore), non-selective
Ion channels = form between subunits, highly selective
What are transportER proteins
Do not have pores, selective for substrate, requires energy
