Week 7: Lipids, Membrane Structure, Energy of Membrane Transport Flashcards
The main energetic driving force for the formation of phospholipid bilayers is the _______ among the hydrocarbon tails of the phospholipids.
hydrophobic interactions
The whole bilayer arrangement is held together by noncovalent interactions, such as ______ and _______.
van der Waals
hydrophobic interactions
You have just isolated a pure lipid that contains sphingosine, a fatty acid, and a sugar moiety. To which class of lipids does it belong?
Glycolipid
A glycolipid is a lipid to which a _______ is bonded. Quite frequently, ceramides are the parent compounds for glycolipids, and the glycosidic bond is formed between the primary alcohol group of the ceramide and a sugar residue.
sugar moiety
Which of the following lipids is/are not found in animal membranes?
Triacylglycerols
Triaclyglycerols are typically used for the storage of _____; they do not appear in cell membranes of ______. They are mostly deposited in ______ and are also found in the ______ in the form of very low density lipoprotein (VLDL) and chylomicrons.
fatty acids
animals
adipose tissue
blood
_______ of the proteins associated with membranes span the bilayer from one side to the other.
Some
Membrane proteins can fully traverse the bilayer, they can penetrate a portion of the bilayer without crossing it, or they can be peripherally associated with the surface of the bilayer. The association of a particular protein with the membrane depends on the details of the protein’s structure.
The rate of transport of a species through a protein channel does not depend solely on the concentration gradient. _______ may be involved in bringing substances into cells.
Transport proteins
Transport of species into the cell can either be _______ depending on the direction the species is moving relative to its concentration gradient.
passive or active
Protein channels and pores are not necessary for all species to enter the cell. Small, uncharged molecules, such as O2, can pass through the membrane by _______.
simple diffusion
Although ATP hydrolysis to ADP and phosphate is one energy source for active transport processes, the energy stored in the ________ gradient for a second species is used in some cases to provide the energy for transport of the first species.
electrochemical
For membranes, the ______ head groups are in contact with water, and the ______ tails lie in the interior
polar
nonpolar
Proteins ______ in the lipid bilayers rather than being sandwiched between them.
“float”
Lipid bilayers are an important component of ______.
membranes
Waxes are mixtures of esters of long-chain ______ and long-chain ______.
carboxylic acids
alcohols
Waxes are found on the ______ of leaves.
outside
Waxes frequently form _______ for plants as well as animals.
protective coatings
Lipids can be grouped based on:
having a preponderance of ______ groups.
their _______ in nonpolar solvents.
nonpolar
high solubility
Steroids have a ______ consisting of _______. Steroids vary in the functional groups attached to this four-ring core and in the oxidation state of the ring carbons.
four fused ring system
three six-membered rings and one five-membered ring
Types of lipids:
Fatty acids
Triacylglycerols
Phospholipids
Glycolipids
Steroids
Fatty acids:
Triacylglycerols:
Phospholipids:
Glycolipids:
Steroids:
Fatty acids: simplest, fuels
Triacylglycerols: storage of energy
Phospholipids: membrane
Glycolipids: membrane
Steroids: component of membranes, signaling molecules
Fatty acids are classified based on:
length of chain (# of carbons in chain) and #/nature of double bonds
Saturated fatty acid means hydrocarbon chain…
has no double bonds
“saturated” with carbons, has as many carbons as possible
Unsaturated fatty acid means hydrocarbon chain…
has a double bond
18:1(∆9) means…
18 carbons
1 double bond at 9th carbon counting from the carbonyl carbon
“Omega-3” means the first double bond is on…
the 3rd carbon from the END of the chain
Humans can convert ALA (essential) to…
EPA and DHA
Saturated fatty acids solidify _____ and has a relatively _____ melting temperature because…
easily
high
dispersion forces (packed tight)
Mixture of saturated and unsaturated fatty acids: acyl chains are liquid, and solidify at a relatively ____ temperature
low
less strong IMFs
_______ determines the melting point of fatty acids
of double bonds
Adipocytes (fat cells) - The lipid droplet contains ________, and takes up most of the cell volume.
triacylglycerols
In fat (or ________), glycerol is esterfied with ______
triacylglyceride
three fatty acids
Fat can be used for metabolic ______ because its carbon atoms are _______
energy storage
highly reduced
Fat can act as a source for _______ production, and for ______ in organisms
energy or heat
thermal insulation
Because of the extremely small polar region, waxes are _______
completely water insoluble
Fatty acids are ____ and tend to form ______.
Phospholipids are more _______ and pack together to form ______.
wedge-shaped
spherical micelles
cylindrical
a bilayer structure
3 common types of membrane lipids:
Phospholipids
Glycolipids
Cholesterol
_______ are the major class of membrane lipids
Phospholipids
Phospholipids are composed of four components:
fatty acids (2 or more)
a platform
a phosphate
an alcohol
Two common platforms are _____ and ______.
glycerol
sphingosine
Phospholipids with a glycerol platform are called ______
phosphoglycerides or
phosphoglycerols
The major phospholipids are derived from ______.
phosphatidate
On a sphingosine platform, the ____ of Serine can react with a fatty acid and _____ of Serine is where phosphate will bond
NH3+
OH
Glycolipids are _________ lipids.
carbohydrate-containing
The carbohydrate components of glycolipids are on the _______ of the cell membrane, where they play a role in cell-cell interactions.
extracellular surface (outer leaflet)
Some sphingolipids have ______ as head groups
monosaccharides
Cholesterol is in a class of compounds called ______
steroids
Cholesterol is the most common _____ and plays a role in maintaining _______
steroid
membrane fluidity
Spontaneous formation of bilayers is driven by the _______.
hydrophobic effect.
Saturated fatty acid chains have ____ packing, therefore ______ fluid
tighter
less
Unsaturated fatty acid chains have _____ packing, therefore ______ fluid
looser packing
more
Cholesterol disrupts ______ packing of fatty acid chains, therefore _____ fluidity but can also complex with lipids and proteins to form a ______, therefore ______ fluidity
straight chain
increasing
“lipid raft”
decreasing
Diffusion of lipids in membranes differs according to ______.
direction
Transverse diffusion (flip-flop):
Outer to inner leaf
very slow
Lateral diffusion
ex. changes position on outer leaf
rapid
Integral proteins are also called _____ protein and they go ______
transmembrane
all the way through bilayer
Three modes of membrane transport processes:
1) Nonmediated transport
2) Facilitated (passive) transport
3) Facilitated (active) transport
1) Nonmediated transport
* Diffusion across a membrane is more rapid for those solutes that are _______, and significantly slower for ______.
hydrophobic (lipophilic)
polar/charged solutes (membranes are effective barriers to the diffusion of polar/charged molecules)
Diffusion typically from ____ concentration to _____ concentration
higher, lower
2) Facilitated (passive) transport
* Diffusion of certain solutes is accelerated by _______
specific transporter proteins (channels, pores, carriers)
usually polar solutes
still spontaneous
3) Facilitated (Active) transport
* Transporter proteins that couple a _______ to achieve transport against a concentration gradient
thermodynamically favorable process
For diffusion, where we have the same starting and ending
molecule, the gibbs free energy equation is:
∆G = RTlnQ
Q = final concentration/initial concentration
There is a charge difference across the semi-permeable membrane. More _____ inside, more ______ outside
This separation of charge causes a ______ across the membrane space
negative, positive
voltage
Free energy equation when there is a cell (membrane potential):
∆Gt = RTln (C2/C1) + ZF∆Ψ
Active transport against a concentration gradient can be driven by ______; such direct coupling is seen in _____, such as the ______
ATP hydrolysis
ion pumps
Na + -K+ ATPase
Why high release of energy from ATP?
1) separation of negative charges
2) ATP –> ADP + Pi, increase in entropy
3) more resonance from inorganic phosphate, stabilizing
What structural features do a triacylglycerol and a phosphatidyl ethanolamine have in common? How do the structures of these two types of lipids differ?
In both types of lipids, glycerol is esterified to carboxylic acids, with three such ester linkages formed in triacylglycerols and two in phosphatidyl ethanolamines. The structural difference comes in the nature of the third ester linkage to glycerol. In phosphatidyl ethanolamines, the third hydroxyl group of glycerol is esterified not to a carboxylic acid but to phosphoric acid. The phosphoric acid moiety is esterified in turn to ethanolamine.
What structural features do a sphingomyelin and a phosphatidyl choline have in common? How do the structures of these two types of lipids differ?
Both sphingomyelins and phosphatidylcholines contain phosphoric acid esterified to an amino alcohol, which must be choline in the case of a phosphatidylcholine and may be choline in the case of a sphingomyelin. They differ in the second alcohol to which phosphoric acid is esterified. In phosphatidylcholines, the second alcohol is glycerol, which has also formed ester bonds to two carboxylic acids. In sphingomyelins, the second alcohol is another amino alcohol, sphingosine, which has formed an amide bond to a fatty acid.
You have just isolated a pure lipid that contains only sphingosine and a fatty acid. To what class of lipids does it belong?
This lipid is a ceramide, which is one kind of sphingolipid.
Sphingolipids contain _____ bonds, as do proteins. Both can have _______ parts, and both can occur in _______, but their functions are different.
amide
hydrophobic and hydrophilic
cell membranes
Steroids contain a characteristic ______, which other lipids do not.
fused-ring structure
Which is more hydrophilic, cholesterol or phospholipids? Defend your answer.
Phospholipids are more hydrophilic than cholesterol. The phosphate group is charged, and the attached alcohol is charged or polar. These groups interact readily with water. Cholesterol has only a single polar group, an -OH.
Succulent plants from arid regions generally have waxy surface coatings. Suggest why such a coating is valuable for the survival of the plant.
The waxy surface coating is a barrier that prevents loss of water.
Membranes contain glyco-
glycolipids and glycoproteins.
In lipid bilayers, there is an order–disorder transition similar to the melting of a crystal. In a lipid bilayer in which most of the fatty acids are unsaturated, would you expect this transition to occur at a higher temperature, a lower temperature, or the same temperature as it would in a lipid bilayer in which most of the fatty acids are saturated? Why?
The transition temperature is lower in a lipid bilayer with mostly unsaturated fatty acids compared with one with a high percentage of saturated fatty acids. The bilayer with the unsaturated fatty acids is already more disordered than the one with a high percentage of saturated fatty acids.
Suggest a reason why animals that live in cold climates tend to have higher proportions of polyunsaturated fatty acid residues in their lipids than do animals that live in warm climates.
The higher percentage of unsaturated fatty acids in membranes in cold climates is an aid to membrane fluidity.
Are the relative amounts of cholesterol and phophatidylcholine the same in all the kinds of membranes found in a typical mammalian cell?
The relative amounts of cholesterol and phosphatidylcholine can vary widely in different types of membranes in the same cell
Do all proteins associated with membranes span the membrane from one side to another?
Proteins that are associated with membranes do not have to span the membrane. Some can be partially embedded in it, and some associate with the membrane by noncovalent interactions with its exterior.
Suppose that you are studying a protein involved in transporting ions in and out of cells. Would you expect to find the nonpolar residues in the interior or the exterior? Why? Would you expect to find the polar residues in the interior or the exterior? Why?
In a protein that spans a membrane, the nonpolar residues are the exterior ones; they interact with the lipids of the cell membrane. The polar residues are in the interior, lining the channel through which the ions enter and leave the cell.
Some proteins and lipids undergo ______ along the inner or outer surface of the membrane.
Carbohydrates are ______ bonded to the outside of the membrane.
lateral diffusion
covalently
Biological membranes are highly _____ environments. Charged ions tend to be ______ from such environments rather than dissolving in them, as they would have to do to pass through the membrane by simple diffusion.
nonpolar
excluded
Compared to unsaturated fatty acids, saturated fatty acids have ________.
Tighter packing of their hydrocarbon chains
Integral membrane proteins tend to have a higher concentration of ______ amino acids on their surface
hydrophobic
Integral membrane proteins:
Extends through ______ the membrane
Can act as a ______ through the cell membrane
Peripheral Membrane Proteins:
Attaches only at the ______ of a cell’s membrane
_____ associated with the membrane
all or part of
tunnel
surface
Loosely
K+ channel is specific for K+ and not for other ions because the channel ______ to the point where large ions cannot pass through.
narrows
What would be ∆G in the transport of a negatively charged ion out of cell that has a negative membrane potential? Assume that there is no difference in concentration across the cell membrane.
∆G < 0
a spontaneous process
steps of the mechanism of the sodium-potassium ATPase:
3 sodium ions bind
Phosphorylation of an asp residue
Conformational change releases 3 Na+ ions outside of the cell
2 K+ ions bind
Release of the phosphate group
2 potassium ions are released inside the cell
For naming fatty acids, delta nomenclature starts counting carbons from _____, whereas omega nomenclature starts counting carbons from ______.
the carbonyl carbon
the end carbon
A defining feature of cholesterol is a ______
fused-ring structure
Gonane structure (fused
cyclopentane/cyclohexane rings)
A defining feature of phospholipids is a _______
phosphate group
Phospholipids have a ______ backbone
glycerol
A defining feature of glycolipids is a _______
Sugar (carbohydrate ring)
Glycolipids have a _____ backbone
sphingosine
The potassium channel has a ‘selectivity filter’ in place that allows the channel to not only be able to allow potassium through, but keep other molecules out. Here are its features that allow it to do so:
- Size: The filter is physically restrictive to molecules _____ than K+.
- Charges in the filter: The filter commonly contains acidic amino acids (not usually shown in diagrams) and also has the ______ of several residues within the filter to repel ______.
- Coordination with carboyl oxygens: We have selected against opposing charges and molecules bigger than K+, but there is a big question mark with the one atom that gets past both of these filters: the sodium ion. The filter selects against sodium due to its smaller atomic radius that has trouble forming coordination bonds with the carbonyl oxygens present within the filter. This causes the sodium to greatly prefer its hydration shell (that cannot fit through the filter), whereas potassium readily forms the
coordination bonds with the carbonyl oxygens and easily gives up its hydration shell, allowing it to pass through the filter.
larger
carbonyl oxygens
anions
increased stability = ____ free energy.
lower
The hydrolysis of ATP provides substantial energy for organisms. Describe three reasons why.
- Increase in entropy: One molecule becoming two!
- Resonance stabilization of the inorganic phosphate
- Decreased internal repulsion; all of the negative charges from the phosphate groups make the molecule
unstable due to having many repelling charges near each other
The ATPase mechanism of moving Na+ into a cell and K+ out
of a cell. Is this an example of a uniporter, symporter, or antiporter?
This mechanism is an antiporter since it transports two molecules in opposite directions.
1) The protein “begins” open to the cytosol side of the membrane with an ATP bound to the protein. Upon 3 sodium ions binding, it can then accept a phosphate group from ATP, releasing ADP.
2) When this phosphate binds to the pump, there is an induced conformational change that causes the protein to now open to the extracellular side of the membrane.
3) The sodium ions will be released. This conformation has binding that is more favorable for potassium ions as opposed to sodium ions.
4) Two potassium ions will now bind to the protein.
5) When the potassium ions bind, another ATP binds, which will cause the protein to let go of the
phosphate. As this happens, the protein will then return to its original form.
6) Once the pump opens back up to the cytosolic side of the membrane, the potassium ions are released and we are back where we started.
