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.
