Biological Membranes Flashcards
What are lipids
Compounds which are primarily: Non-polar, Amphipathic, Hydrophobic, Insoluble in water
What do lipids include
Fatty acids, Triacylglycerols (Typically contain fatty acids), Membrane Lipids (Typically contain fatty acids) and Cholesterol
What are fatty acids
Long chain hydrocarbon carboxylic acids. Up to 24 carbons long (16 and 18 most common). General formula CH3(CH2)nCOO(-) Amphipathic (Amphiphilic) Polar and non-polar portions
What are saturated fatty acids
No double bonds
What are unsaturated fatty acids
Double bonds, mono- or polyunsaturated (one or more double bonds).
How does length affect fatty acids
Longer fatty acids melt at higher temperatures. Shorter fatty acids melt at lower temperatures.
How does saturation affect fatty acids
Saturated fatty acids melt at higher temperatures. Unsaturated fatty acids melt at lower temperatures. Saturation has a greater effect on melting point than length.
What is the difference between fatty acids packing together
Saturated can align closely, maximising Van der Waal’s interactions between them. In contrast, the fatty acids that have both saturated and unsaturated fatty acids. These molecules cannot pack as closely together because of the bend in the unsaturated hydrocarbon chain. Trans fatty acids do not adopt the same shape as cis one and are able to pack better.
What are Triacylglycerol (TAG)
Triacylglycerol (TAG) is a way of storing fatty acids. TAG is very hydrophobic (not amphipathic) Triacylglycerol: Three acyl chains are attached to glycerol. Acyl chains from fatty acids (ester-linked)
What are the possible makeups of a membrane lipids
Glycerophospholipids, Sphingolipids and Cholesterol
What are glycerophospholipids made up of
Like triacylglycerol, glycerol has fatty acyl groups covalently attached. Unlike triacylglycerol, the presence of a large polar group makes these molecules amphipathic. Variations exist in polar head groups and acyl chains, affecting size and melting points.
What are cholesterols made up of
Accounts for ~35% of mammalian membranes. Rigid, non-polar structure (hydrocarbon/ring structure). Weakly amphipathic (single -OH; weakly polar on the -OH side and very hydrophobic everywhere else). Most hydrophobic (27 carbons, 1 OH; very weakly amphipathic). Membrane lipid ~30% of mammalian plasma membranes. Maintains fluidity and rigidity. Does not form membranes alone. The -OH associates with polar head groups of other lipids. Non-polar portion is found in the membrane.
What happens when amphipathic molecules form Micelles or Bilayers in Water
These arrangements eliminate unfavourable contact between H2O and hydrophobic tails, yet permit solvation of polar head groups.
What do liposomes form
An inner and an outer leaflet which make a bilayer
What are bilayers formed of
Vary depending on lipid composition: Acyl chain and Polar head group. Non-covalent assembly: Fluid yet stable.
How are the dimensions of lipid bilayers variable
The lipid head groups have significantly different dimensions. The acyl trails vary between 16-20 carbon atoms in length. Cholesterol is almost entirely buried in the bilayer. Despite their fluidity, lipid bilayers are stable.
What happens when the lipid bilayer is below the transition temperature
Below the transition temperature, acyl chains pack together in Van der Waals contact, in a gel-like solid state.
What happens when the lipid bilayer is above the transition temperature
Above the transition temperature, the lipid molecules and their acyl chains move freely and rapidly.
What happens when the lipid bilayer is at the transition temperature
The melting temperature (transition temperature) of a lipid bilayer is the temperature of its transition from an ordered crystalline to a more fluid state and depends on acyl-chain unsaturation and length. Transition temperature may be very sharp for an artificial membrane. Homogenous preparation. However, for biological membranes the transition temperature is typically not sharp. Mixture of compounds (different lipids/proteins). Membranes must operate above gel temperature (melting point) but not be completely disordered.
How do they maintain membrane fluidity
Adapting to differences in temperature will affect the lipid composition of a biological membrane. With decreasing temperature, more unsaturated fatty acids and shorter fatty acids are incorporated into membrane lipids. With increasing temperature, more saturated fatty acid and longer fatty acids are incorporated into membrane lipids.
How does cholesterol impact membrane rigidity
In animals. Because it is rigid and planar, cholesterol limits the rotational movement of neighbouring acyl tails, thereby increasing Van der Waals interactions. Cholesterol increases membrane fluidity for a larger effective temperature range. Low temperature: prevents close packing between acyl chains. High temperature: decreases motion/disorder of acyl chains, increases rigidity. Cholesterol is not found in bacteria.
How can lipids move freely through the bilayer
Lipids move freely and rapidly in the bilayer, but only laterally.
How can it be difficult lipids to move through the bilayer
Lipids do NOT undergo transverse (flip-flop) at appreciable rates.
How do lipids flip from one side of the lipid bilayer to the other
“Flipases” (and other enzymes) increase the rate of transverse diffusion. Specific transport by these proteins allows for differences in lipid composition in the leaflets. A significant energy barrier is associated with desolvating a polar head group to move it through a hydrophobic bilayer.
What are the types of Membrane Proteins
Integral membrane protein, peripheral membrane protein and lipid-linked protein
Describe integral membrane proteins
Hydrophobic interactions. Polar surface on the outside of the membrane and hydrophobic surface on the inside on the membrane
Describe peripheral membrane protein
Polar/Ionic/Electrostatic interactions, on the outside of the membrane. Can be disrupted via changing the concentrations of the environment like salts.
Describe lipid-linked proteins
Lipid prosthetic group inside the membrane that is hydrophobic, tightly associated with the bilayer but less so than the integral membrane protein. Hydrophobic interactions via prosthetic group.
Where is the contact points of the integral membrane proteins
The portion of an integral membrane protein that is in contact with the acyl tails of the bilayer must have hydrophobic amino acid side chains on its surface. Interactions with water occurs with the polar surface.
What side chains are the K+ Channel
Nonpolar side chains make up most of the helix surfaces that face the lipid tails. Polar side chains are more prominent in the loops, interacting with the lipid head groups and solvent. Very polar side chains (Asp, Glu, Lys, and Arg) often flank these regions, interacting with solvent.
What role do regular secondary structures cross membran
Regular structures satisfy the hydrogen bond potential of the polypeptide backbone
Do amino acids form a transmembrane alpha-helix that is generally hydrophobic
True
What is the Fluid Mosaic Model of Membrane Structure
Dynamic, non-covalent, complex assembly of lipids and proteins (and carbohydrates). Both lipids and proteins move laterally but cannot (easily) undergo transverse movement. The movement of proteins may be limited by the cytoskeleton. Carbohydrate chains are attached to the extracellular surface of some proteins and lipids.
How do gases and hydrophobic molecules pass through the bilayer
Gases and Hydrophobic molecules pass freely across the bilayer
How do small polar molecules pass through the bilayer
Small polar molecules often need assistance to pass through the bilayer but they have an easier time passing through than polar and charged molecules
How do large polar and charged molecules pass through the bilayer
Large polar molecules and charged molecules must have assistance because they won’t traverse the membrane wit some kind of assistance
What does the rate of simple unmediated diffusion depend on?
Size of molecule, concentration gradient and lipid solubility
How does the size of molecule affect diffusion
Smaller molecules move faster than larger ones
How does the concentration gradient affect diffusion
Larger gradient increase rate of diffusion. Smaller gradient decreases rate of diffusion
How does the lipid solubility affect diffusion
Largest impact of the other factors. Greater solubility increases diffusion rate. Less solubility decreases diffusion rate.
What direction does simple diffusion go in
Simple diffusion is always down the concentration gradient
What are the two MAJOR types of transport
Passive and active
What happens when Delta G is negative
The motion is spontaneous, passive transport
What happens when Delta G is positive
Energy must be provided to make transport occur, active transport
What are porins
Porins contain a relative non-specific, water-filled pore in the center of a beta-barrel. Most porins are trimers (3 subunits). Each subunit contains a pore. Non-selective – free diffusion up to 1.5K Daltons
What are ion channels
The channel is formed between subunits. They are highly selectrive
What occurs in a K+ ion channel
Selectivity depends on the size of the pore and the properties of the side chains/functional groups found there. Potassium ions interacts with the carbonyl subunit groups surrounding.
What happens when Sodium ions go through a K+ channel
Sodium ions are smaller but cannot interact with all the carbonyl subunits simultaneously, so this results in a slow transport through this channel.
What is the conformational change transport
Transporter (carrier) proteins. Do not have membrane-spanning pores. Conformational change alternates openings from one side of the membrane to other. Selective for substrate transported. May be passive or active.
What is the graph for passive transport by carrier proteins
It is hyperbolic
What is a uniport
One molecule can be transported in one direction
What is a symport
Many molecules can be transported in one direction
What is an antiport
Many molecules can be transported in two directions
What is the energy source for primary active transporters
Primary active transporters typically uses ATP as the source of free energy
What is the energy source for secondary active transporters
Secondary active transporters uses an ion gradient (Na+) as the source of free energy
Is there a transport protein in simple diffusion
No
Is there a transport protein in channels and pores
Yes
Is there a transport protein in passive transport
Yes
Is there a transport protein in primary active transport
Yes
Is there a transport protein in secondary active transport
Yes
Is simple diffusion saturable with substrate
No
Are channels and pores saturable with substrate
No
Is passive transports saturable with substrate
Yes
Is primary active transport saturable with substrate
Yes
Is secondary active transport saturable with substrate
Yes
What is the movement relative to concentration gradient in simple diffusion
Down
What is the movement relative to concentration gradient in channels and pores
Down
What is the movement relative to concentration gradient in passive transport
Down
What is the movement relative to concentration gradient in primary active transport
Up
What is the movement relative to concentration gradient in secondary active transport
Up
Is there energy input required for simple diffusion
No
Is there energy input required for channels and pores
No
Is there energy input required for passive transport
No
Is there energy input required for primary active transport
Yes, from a direct source
Is there energy input required for secondary active transport
Yes, from the ion gradient
How is Na + and K + ATPase a primary active transporter
In each cycle: 3 Na + ions exported and 2 K+ ions imported. ATP + H2O = ADP + Pi + H(+). The two concentration gradients generated across the cell membrane are used as the source of energy for a variety of secondary active transport processes.
How is the Na + Glucose Transporter a secondary active transporter
Glucose is higher in the cytosol. Sodium is higher in the exterior. Even though glucose is a neutral compound it still requires positive energy to move across the concentration gradient. The net of the sodium glucose transport must be less than 0 for glucose import.
