Topic 1 - Module 4 Flashcards
(47 cards)
Describe 3 broad areas for lipid functions and provide examples.
- Storage, such as fatty acids, triacylglycerols, and waxes.
- Structural lipids, such as phospholipids, glycolipids, cholesterol, sphingolipids, and glycerolphospholipids.
- Signaling and cofactor lipids, such as phospholipid derivates, steroid hormones, lipid-soluble vitamins and eicosanoids.
Define “lipids” and distinguish them from other biological molecules.
They are generally non-polar and have low solubility in water, and high solubility in organic solvents.
Predominantly used for energy storage, structural components and to a lesser extent, in signalling and as cofactors.
Proteins are more abundant as signaling and receptor molecules, compared to lipids.
What formation does the addition of free fatty acid molecules to an aqueous solution at physiological temperatures tend to favour and why?
Formation of micelles.
The hydrophobic regions are buried deeply in the core, and tightly packed fatty acid tails will exclude water from the region - increasing entropic gain due to release of ‘ordered’ water, thermodynamically stabilising the structure.
As a bilayer, the hydrophobic edges are exposed to aqueous solution, making it energetically unstable.
What is the difference between phospholipids and fatty acids?
Fatty acids are wedge-shaped units (cross-section of phosphate head is greater than their single side chain) that will form micelles when packed together.
Phospholipids are individual units that are cylindrical units (phosphate head = side chains (2)) and forms lipid bilayers.
For a fully saturated fatty acid chain, describe its physical and chemical properties.
As the length of the hydrocarbon chain increases, the melting point increases, alongside decrease in water solubility.
This is because its composition enables it to tightly pack into a crystalline array (liquid) while stabilised by extensive interactions with the acyl chain with one another.
Release of ordered water will lead to an increase in entropic gain, via condensation reactions, leading to the spontaneity of reactions.
Explain the physical properties of unsaturated fatty acids by describing the natural conformation.
Generally, the unsaturated fatty acids will have their double bonds in cis formation, as the biosynthetic pathway via enzymes are unable to produce “trans” isomers.
This then creates less favourable steric interactions, and hence, will require less thermal energy to disrupt the disordered packing of the acids - resulting in lower melting point.
Differentiate trans fatty acids from typically unsaturated cis fatty acids.
Trans fatty acids are formed by the partial dehydrogenation of unsaturated fatty acids in food processing. The trans double bond enables the molecule to pack more regularly, resulting in a higher melting point than their cis counterparts.
Define triacylglycerols and then describe the different types of triacylglycerols.
They are fatty acid esters of glycerol, involving one glycerol molecule bonded to 3 fatty acid chains.
- simple triacylglycerols have the same, identical fatty chains.
- mixed triacylglycerols have different fatty acids
Describe the properties of triacylglycerols
They provide stored energy and insulation, and are generally uncharged, hydrophobic molecules.
advantages include higher energy yield per molecule (compared to oxidation of other sources such as glycogen or starch) due to their hydrophobic nature - making them lighter - while expending less energy on releasing water.
Explain the functional significance of membrane lipids and their formation.
They are generally a lipid bilayer with a hydrophobic core, this means that they are also a barrier to the movement of hydrophilic molecules into the cell.
They are formed via spontaneous reaction whereby the burial of the hydrophobic core leads to entropic gain as order water is being released.
Differentiate the two types of phospholipids, which both have distinctive signalling roles.
- glycerolphospholipids have an alcohol group attached to the phosphate group
- the charge is individually determined, and dependent on identity of the substituent alcohol group. - sphingolipids have a choline group attached to their phosphate group, and sphingosine backbone (derived from amino alcohol sphingosine)
- contains amide linkage, instead of ester linkage
- will contribute to 1 out of the 3 tails (like glycerol)
Which type of membrane lipid is used in the outer membrane leaflet and as determinants of the blood groups (ABO) - then explain why.
Glycolipids are components of the outer membrane leaflets as it can contribute to sites of biological recognition (as a signalling molecule) while also as part of glycosphingolipids to be determinants of antigens.
Describe the chemical structural properties of sterols
They have 4 fused carbon rings, which constrains their conformation, making them almost planar and relatively rigid.
Predominantly hydrophobic, enabling it to interact with various other non-polar groups, such as lipids. But they are amphipathic molecules due to polar -OH group.
Explain the role of cholesterol as a signalling molecule.
In addition to its structural role in membranes, cholesterol can act as a precursor for the synthesis of steroid hormones.
Summarise the membrane composition and properties, in terms of the fluid mosaic model.
Proteins embedded in the bilayer are held by hydrophobic interactions, which is what allows fluid dynamic properties.
Charges of the lipid head groups contribute significantly to surface properties of the membrane, as it allows recognition of membrane components.
Define the phase transition temperature and explain it in terms of membrane fluidity.
The temperature at which the membrane goes from paracrystalline into a fluid state.
Increasing temperature will mean that membrane fluidity increases as acyl chains have become fluid, while the lipid itself can laterally diffuse into another position within the bilayer.
At 37 degrees, all biological membranes are fluid.
Explain the difference between lateral and transverse lipid diffusion.
Uncatalyzed lateral diffusion occurs rapidly, whereas transverse “flip-flop” diffusion occurs very slowly.
This is because the polar head group needs to travel or be pushed through nonpolar acyl chains, a movement that is highly energetically unfavorable.
How is transverse “flip-flop” diffusion facilitated?
Through the process of catalysis, such as the use of specialised proteins embedded in the bilayer, the trans-bilayer movement can be made energetically more favourable and faster than uncatalyzed lateral diffusion.
What is the Fluorescence recovery after photobleaching (FRAP) technique useful for?
The FRAP technique measures the rate of fluorescence return over a given area of phospholipids, which determines the rate of lipid diffusion across the membrane.
Define ‘sidedness’ in terms of the bilayer membrane and explain its significance.
A ‘sidedness’ is a difference in lipid composition on either side of the bilayer.
Catalysis involves moving down a concentration gradient, hence sidedness of lipid composition will inherently facilitate catalysis and movement of substances across the membrane.
By detecting this difference in composition, you can easily identify potential cell death when inner monolayer components are showing an increasing concentration.
What are membrane rafts made of? Explain the functional significance of these membrane rafts.
They are mostly made of clustered sterol and sphingolipids, with different localized proteins (would form transient interactions with lipids) that promotes efficient signalling both in and out of the cell.
Compare the properties of peripheral and integral membrane proteins
Peripheral proteins are typically attached to integral membrane proteins, through ionic interactions and H-bonding, while associating with outside surfaces.
Integral membrane proteins themselves are firmly attached to the membrane, usually embedded within it. They have extensive hydrophobic interactions with AA residues at the surface of membranes, and also with the acyl chains of lipids within the membranes.
Describe how peripheral membrane proteins can be experimentally released from biological membranes.
Peripheral proteins are released from membrane by reagents that disrupt the ionic interactions formed with integral proteins, such as high salt concentrations (disrupts salt bridges), changes in pH (changes charged state of groups in salt bridges) and chelating agents that can add/remove metal that forms base of salt-bridge interactions.
Explain the differences and similarities between active and passive transport across a biological membrane
The major distinction between passive and active transport is still whether it is against the concentration gradient, and whether the transport requires energy (i.e. ATP).
Passive transporters are known to be able to undergo conformational changes during the transport process and participate in facilitated diffusion of specific molecules.
Active transporters will also undergo conformational changes that isn’t solely driven by hydrolysis of ATP to ADP, phosphorylation stabilizes this conformational changes.
