Membranes + Transport (ch 10, 11) Flashcards
In what 4 ways can lipids move in the membrane bilayer?
- Lateral diffusion
- Flexion
- Rotation
- Flip-flop (rare)
What are the 2 types of phospholipids?
- Phosphoglycerides
2. Sphingolipids
What are the 3 most common types of phosphoglycerides?
- Phosphatidyl-choline (eukaryotes)
- Phosphatidyl-ethanolamine (prokaryotes)
- Phosphatidyl-serine
Describe the structure of a phosphoglyceride.
Two fatty acid tails connected to a phosphate group by glycerol.
Describe the structure of a sphingolipid.
One fatty group (not acid) connected to a phosphate group by sphingosine. Potential to add a fatty acid.
What is the resulting structure called when another sphingosine is added to an existing sphingolipid?
Sphingomyelin.
Describe the structure of a sterol.
Solid carbohydrate rings with small polar head group and non-polar hydrocarbon tail.
How can cholesterol in a membrane be beneficial?
Can act as a temperature buffer by stabilizing the membrane at extremely high or low temperatures.
Do prokaryotes have cholesterol? How do they protect against temperature fluctuations?
No, but they have other sterols which accomplish the same function.
Describe the structure of a glycolipid. Where on the membrane are they localized?
One fatty chain and one fatty acid connected to a sugar by sphingosine. Always on the exterior of the membrane.
How are archael lipids different from eukaryotic/prokaryotic lipids?
Archael: branched, ether-linked, monolayer
Pro-/Eukaryotes: unbranched, ester-linked, bilayer
In human cells, how many different types of lipids are there (give a range)?
Between 500 and 2000 types.
How does the ratio of saturated and unsaturated fatty acids in the bilayer change with temperature? What enzyme accomplishes the conversion?
Increased unsaturation in cold because of their looser association, keeping membrane fluid. Saturation/unsaturation done by enzyme desaturase.
Why is asymmetry of the lipid bilayer important?
Critical for cell recognition, apoptosis signalling, membrane protein function, etc.
What is the purpose of inositol phospholipids?
Involved in signalling pathways. They allow proteins to dock to the membrane.
Do eukaryotic cells ever have lipid monolayers? In what capacity?
Yes. Monolayers are used in storage vesicles/micelles for hydrophobic molecules. Often associated with the ER.
What 2 factors implicit to the association of proteins with the membrane affect protein function?
- Method of association
2. Location on the membrane
How are transmembrane proteins associated with the membrane?
They have a hydrophobic domain (or multiple) which crosses the membrane.
What is the purpose of β-barrels in the membrane?
Act as pores/channels for molecules to cross.
What are the 3 lipid anchors for lipid-anchored membrane proteins?
- myristoyl anchor
- palmitoyl anchor
- farnesyl anchor
What linkage allows a myristoyl anchor to bind a membrane protein? Is it reversible?
An amide linkage. Stable/irreversible.
What linkage allows a palmitoyl anchor to bind a membrane protein? Is it reversible?
A thioester linkage. Reversible.
What linkage allows a farnesyl anchor to bind a membrane protein? Is it reversible?
A thioether linkage (as in methionine). Stable/irreversible.
What is a glycoprotein?
A membrane bound protein with an attached sugar on the extracellular side of the membrane.
What differentiates the bound sugar of a proteoglycan from that of a glycoprotein?
Proteoglycan: long sugar with no branching, sugars added 1 at a time
Glycoprotein: branched sugars added 14 at a time
On which side of the membrane do integrated proteins have disulfide bonds? What about sulfhydryl groups?
Disulfide bonds = exterior
Sulfhydryl groups = interior
How can protein mobility in the lipid membrane be restricted?
By tight junctions or because of functional limitation.
How can proteins maintain their position in the membrane?
By forming complexes with anchored proteins, anchoring to cytoskeleton or extracellular matrix.
What are some characteristics of a lipid raft?
- Higher than normal cholesterol
- More GPI-anchored proteins
- More sphingolipids
- Slightly thicker membrane
How can the shape of a membrane be controlled?
Through the integration of membrane-bending proteins which modify the bilayer to a desirable shape.
How do small hydrophobic molecules move across membranes?
Diffusion.
How do inorganic ions and small organic polar molecules move across membranes?
Transporters/Channels.
How do large macromolecules move across membranes?
Endo-/exocytosis.
What are the 2 types of passive transport?
- Channel-mediated
2. Transporter-mediated
What differentiates the kinetics of a transporter from the kinetics of a channel?
Transporter: has a max saturation (transport rate)
Channel: no max saturation (transport rate)
What are the 3 types of active transporters?
- Coupled transporters
- ATP-driven pump
- Light-driven pump
What differentiates a symporter from an antiporter?
Symporter: 2 molecules are moved together in the same direction
Antiporter: 2 molecules move in opposite directions for transport
What are the 3 types of ATP-driven pumps?
- P-type pump
- ABC transporter
- V-type pump / F-type ATP synthase
How are the functions of a V-type pump and an F-type ATP synthase linked?
V-type: uses ATP to move protons out of the cell
F-type: moves protons into the cell to produce ATP
What is required for a P-type ATPase to function?
ATP must be used to facilitate (anti)transport of molecules.
How do ABC transporters work?
Use 2 ATP to move solute molecules into/out of the cell.
What does the ABC in ABC transporter stand for?
ATP-binding casette.
How do prokaryotes use ABC transporters differently than eukaryotes?
Prokaryotes: for import and export
Eukaryotes: mainly for export
What are the 4 main characteristics of ion channels?
- Always passive
- Have high ion selectivity
- Super high efficiency
- Gated (open or closed)
What are the 4 types of ion channels?
- Voltage-gated
- Ligand-gated (extracellular ligand)
- Ligand-gated (intracellular ligand)
- Mechanically gated
How is the membrane potential in animal cells created?
Through the movement of Na+ and K+ ions across the membrane through diffusion or transport.
How many of ions are moved across the membrane by an Na+/K+ exhanger per molecule of ATP?
3 Na+ out, 2K+ in.
K+ diffuses via K+ leak channel according to chemical gradient until _______.
Membrane potential approaches 0 (never reaches neutral charge though).
How is the K+ channel impermeable to Na+?
The K+ leak channel has a selectivity filter made up of carbonyl oxygens which can interact with the K+ ion (after hydrate shell lost) but not the Na+ ion.
How come the Na+ ion channel is impermeable to K+ even though it is larger than the K+ channel?
The channel is large enough to allow the passage of Na+ with its hydrate shell but too small to allow passage of hydrated K+. Loss of K+ shell would be unfavourable though.
What do aquaporins transport? What do they not transport?
Only water. Nothing else.
How do aquaporins remain impermeable to ions?
Small enough that they only allow H20 to pass one at a time.
How do aquaporins remain impermeable to protons?
Because of a double Asn region which breaks up the H20 chain (which would otherwise relay H+).
Why do voltage-gated channels have lateral portals?
To allow the central cavity to be influenced by other factors (ex: hydrophobic drugs).
What is the purpose of the inactivation gate on a voltage-gated channel?
Plugs the open state to facilitate membrane potential propagation (closes during refractory period).
What differentiates patch current from aggregate current?
Patch: multiple small depolarizations
Aggregate: sum of the patch currents
In a transmitter-gated ion channel, what is the purpose of an excitatory neurotransmitter?
Opens cation channels, depolarizes.
In a transmitter-gated ion channel, what is the purpose of an inhibitory neurotransmitter?
Opens K+ or Cl- channels, repolarizes.
What is an example of a transmitter-gated ion channel?
Neuron synapses.
What might be the purpose of drugs which target (neuro)transmitter-gated ion channels?
Muscle relaxants, anti-depressants, insomnia/anxiety/schizophrenia treatments.
Organize the following by how easily they cross a membrane, from easiest to hardest:
large polar molecules (glucose), small polar molecules (H₂O, urea), hydrophobic molecules (steroids, O₂), ions (Na⁺, Ca²⁺)
- hydrophobic molecules (steroids, O₂)
- small polar molecules (H₂O, urea)
- large polar molecules (glucose)
- ions (Na⁺, Ca²⁺)
