topic 06 biological membranes Flashcards
what are a cell’s membrane composed of?
lipids, proteins, and carbohydrates
what are the functions of a biological membrane?
separates the content of a cell/organelle from its environment
control import and export of molecules using proteins
contains sensors/receptors that allow cells to respond to external stimuli
involved in cell movement
what are lipids insoluble in? soluble in?
insoluble in water
soluble in organic solvents
what are the functions of lipids?
structural components of biological membranes
energy storage
enzyme cofactors
signaling molecules
pigment
what are the major classes of lipids?
fatty acids
triacylglycerol/triglycerides
glycerophospholipids
sphingolipids
steroids
describe the structure of a fatty acid. draw one out with no double bonds!
hydrocarbon chain ending in a carboxylic acid group (COOH). usually contains an even number of carbons (4-24)
what are the levels of saturation for hydrocarbon chains?
saturated: no double bonds
monosaturated: one double bond
polysaturated: more than one double bond
on a fatty acid, where would the first double bond be? what about additional double bonds? knowing this, are the double bonds cis or trans?
draw a fatty acid with 2 or 3 double bonds
the first double bond is USUALLY between carbon 9/10 from the carboxylic acid group
any other double bonds after would usually be added at every 3 carbons. after drawing it out, one can see that the double bonds have a cis conformation and form kinks in the chain
in nutrition literature, how are carbons counted in fatty acid?
from the methyl end
describe the structure of a glycerol. draw it.
describe the structure of a triacylglycerol/triglyceride.
glycerol is a 3 carbon chain with hydroxyl groups at the end
triacylglycerol has fatty acids attached to the hydroxyl groups through an ester linkage.
what is the function of a triacylglycerol?
to store fatty acids as energy reservoirs in adipocytes (fat cells)
describe the structure of glycerophospholipids
similar to triacylglycerol but one of the fatty acids replaced with a phosphate group
describe the structure of a sphingolipid
based on a sphingosine molecule which contains a large (hydrophobic) hydrocarbon chain. a second chain, a fatty acid chain, is added to its amine group to form a ceramide. this ceramide is modified to make sphingomyelins which form a sphingolipid with monosaccharide groups
describe the structure of a steroid
based on a system of 4 fused rings. 3 of these rings have 6 carbons. 1 of these rings has 5 carbons. the ring system is almost planar
describe the structure of a sterol
a steroid with a hydroxyl group at C3
why are fatty acids, sphingolipids, and glycerophospholipids ideal for forming biological membranes?
they are amphipathic - they have both hydrophobic and hydrophilic parts.
when placed in water they will spontaneously arrange themselves so hydrophilic regions contact water and hydrophobic parts will cluster together in an area protected from water
what happens if the hydrophilic group of the lipid bilayer has a larger diameter than the hydrophobic group?
what if the groups are of equal size?
if the hydrophilic portion is larger, the lipid will have be triangular or wedge shaped. these lipids will form a micelle, a circle, so water is excluded from the center
if the portions are of equal size, the lipid bilayer can form in which the hydrophilic heads form two surfaces that contact water and protect the hydrophobic group in the interior
describe the flexibility and fluidity of a lipid bilayer
they are flexible bc of the many individual molecules. the flexibility allows them to prevent hydrophobic edges from contacting the water by spontaneously closing to form spherical vesicles or liposomes that enclose the aqueous interior.
the fluidity is caused by the individual lipid molecules that move and diffuse within the plane of the lipid bilayer. the composition of the bilayer is adjusted to maintain optimal fluidity
what are the four factors that determine the fluidity/diffusion rate of the lipid bilayer?
temperature: fluidity is high at certain temperatures. below a certain transition temperature, the lipid bilayer will exist in a gel state where its movement is reduced
proportion of unsaturated fatty acyl chains within the phospholipids and glycolipids: CIS double bonds prevent tight packing of the fatty acyl chains, increasing fluidity
length of fatty acyl chains in the phospholipids and glycolipids: shorter chains = more mobility & fluidity
amount of cholesterol in the bilayer: because of the presence of a hydroxyl group, cholesterol is mildly amphipathic. at higher temperatures, increasing the amount of cholesterol in the membrane reduces the fluidity of the bilayer because the cholesterol packs between the other lipid molecules and restricts motion. at lower temperatures approaching phase transition, cholesterol increases fluidity in the bilayer by interfering with the orderly packing of other lipids
describe the composition of the biological lipid bilayer and how this is established
it is asymmetrical. the composition of 1/2 of the bilayer’s surface will be different than the other half. however, cholesterol is evenly distributed
the asymmetry is established as lipids are synthesized. the asymmetry is regulated by phospholipid translocases that are either ATP-independent (scramblase) or ATP-dependent (flippases or floppases).
what are the types of diffusion a lipid could undergo within a bilayer?
lateral
rotation
flexion
transverse
describe the fluid mosaic model
proteins are free to diffuse laterally within the lipid bilayer unless restricted by cellular components
