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
where will one never find carbohydrates on the membrane?
on the inside surface
why is it easy to maintain the asymmetry of the lipid membrane?
asymmetry for what can lipid not be maintained? why is this so?
transverse diffusion is slow
enzymes maintain symmetry but putting lipids back in their place (flippase and floppase) - flippase moves from outer to inner – floppase moves from inner to outer
asymmetry for cholesterol can not be maintained because it has a small polar head that enzymes can’t interact with
how much of a typical biological membrane’s mass does proteins make up?
40-50%
how do proteins associate with the biological membrane?
integral
peripheral
lipid linked
describe integral membrane proteins
also define transmembrane proteins
some portion of a polypeptide chain contacts the hydrophobic interior of the membrane. usually, the peptide chain of the integral membrane protein completely crosses the membrane at least once. a few don’t
integral membrane proteins don’t flip flop
transmembrane proteins: polypeptides having membrane spanning segments. they are exposed to aqueous compartments on both sides (including hydrophobic interior)
describe the structure of the transmembrane segment of a protein. why is it like this?
they have alpha-helical structures which maximizes the hydrogen bonding between the polar backbone groups while minimizing interactions between the backbone group and hydrophobic interior
how many hydrophobic amino acid residues does a single membrane-spanning alpha-helix consist of? why are they hydrophobic?
20
the hydrophobic side chains favourably interact with the bilayer’s interior: the alpha-helix doesn’t readily leave bilayer because doing so will expose the hydrophobic side chains to the water
describe the beta-barrel structure and its purpose
made of beta-strands in transmembrane proteins
the outside of the barrel is covered with hydrophobic side chains while the side chains that project toward the inside are hydrophilic (polar)
describe peripheral membrane proteins
not embedded into bilayer. they are associated with integral membrane proteins or with the hydrophilic head groups of the membrane lipid. it does not make contact with the hydrophobic interior
describe lipid linked proteins
one or more lipid molecules covalently attached to the end of the protein or to particular side chains (usually cysteine).
the lipids are embedded into the membrane but the protein itself and the sidechains are not exposed to the interior
how can the different types of proteins associated with the membrane be removed? which of these disrupt the bilayer?
integral and lipid linked protein removal disrupts the bilayer
integral: detergent, etc
lipid linked: detergent or removing attached lipid, etc
peripheral: can be done under relatively mild conditions that leave the bilayer intact
why are integral membrane proteins more difficult to study than water-soluble proteins?
their hydrophobic regions must always be covered with lipids or detergents. or else they hydrophobic regions cluster together, causing the protein to aggregate or even precipitate
use of detergent complicates the purification procedure as it may alter secondary, tertiary, and quaternary structure
what are glycoproteins? where could they never be found?
membrane proteins that are modified by the carbohydrates on asparagine, serine, or threonine side chains - these sugars are never attached to the cytosolic (inside) surface of the proteins
what are detergents?
amphipathic molecules that disrupt the membrane and coat the hydrophobic parts of the membrane proteins to keep them stable
why is the alpha-helix stable in the hydrophobic interior?
in the alpha helix, polar backbone molecules bond with each other. since there are no H-bonding groups to bond to, the structure remains stable (transmembrane proteins)
describe the cluster of transmembrane alpha-helices
hydrophilic interior
hydrophobic exterior
what’s needed to satisfy the hydrogen-bonding requirements of a transmembrane protein’s backbone?
beta-barrel
side chains of the beta-barrel will be arranged so that the hydrophobic side chains face the lipids while the inside of the beta-barrel may be hydrophilic
what are the different functional proteins associated with the biological membrane?
channels/transporters
receptors
enzymes
anchors
what are the different types of transporter proteins? what do they do?
uniporter: transports a single species in one direction
symporter: transports two species in one direction
antiporter: transports two species in opposite directions
define facilitated transport
in which transport/channel proteins move ions and polar molecules between cell’s compartments or inside/outside a cell.
differentiate between transporters and channel proteins
transporters: undergo conformational change to move ions/molecules across membrane
channels: when open, they form continuous pores through the bilayer. some channels can open and close through conformational changes
transporters can facilitate active transport; channels perform only passive transport
what are porins?
beta-barrel proteins that cross the membrane while acting as a pore, allowing molecules to diffuse through.
forms non-specific passageways
what can diffuse through a potassium channel? what is it impermeable to?
K+ and large monovalent cations such as Rb+
impermeable to small ions: Na+ & Li+
describe the electrochemical gradient
opposites attract :3
charged species tend to move towards side with opposite charge.
the side with a higher concentration would “leak” to the other side. net movement towards side with a lower concentration.
differentiate between passive and active transport
passive: if a pathway exists, ions/molecules will move down the electrochemical gradient
active: source of energy used to move ions/molecules. this energy can be from the hydrolysis of ATP (into ADP & phosphate) or other sources
describe receptor proteins
ions/molecules/ligands bind to the receptor at the binding site
they sense chemical signals on the outside of the cell and carry the message to the inside so the cell can react to the environment - they can recognize specific molecular signals
receptors can also have enzymatic activity. sometimes the receptor itself acts on the signal, or another protein will
describe enzyme proteins
catalyze chemical reactions
describe anchor proteins
structural proteins that provide stability to the membrane and helps control the cell’s shape and position relative to other cells
they bind to other macromolecules on one/both sides
example given: describe the steps a glucose transporter will take when a glucose binds to it
- glucose binds to site on the transporter’s exterior
- binding of glucose triggers conformational change, exposing the binding site to the interior
- glucose dissociates from the transporter
- transporter reverts to original conformation
why do phospholipids with unsaturated tails make the bilayer more fluid?
unsaturated tails are not as straight as saturated tails and thus are less able to pack close together
how do glycerophospholipids respond when placed in an aqueous solution?
they form spherical liposomes that enclose water
what distinguishes transmembrane proteins from lipid-linked integral membrane proteins?
all transmembrane proteins have amino acid side chains that are in direct contact with the hydrophobic part of the lipid bilayer; some lipid-linked integral membrane proteins do not.
how do membrane proteins move?
laterally
rotate across perpendicular axis
CANT tumble (“flip flop”)
what type of protein is bacteriorhodopsin (in regards to membrane)?
transmembrane
why do transmembrane regions of integral membrane proteins stay within the membrane?
the transmembrane regions contain hydrophobic side chains that would be exposed to water if they were to leave the membrane
