208A Lipids and Membranes Flashcards
Why does life need membranes?
- it separates inside from outside
- generates an entropy barrier - prevent equilibrium with the environment ( if cells are in equilibrium w/its surroundings this would mean the cell or organism was not alive)
- creates a dynamic steady state - which drives reactions
what are some functions the membrane carries out?
cell growth and movement, gradients for generating energy, excreting toxins and waste products
Membranes are made up of what?
lipids and proteins but also carbohydrates are present in the form of glycoproteins or glycolipids.
The proportions of protein and lipid of a membrane depend on what?
vary and depend on the type of membrane and its biological function. For example, myelin sheath that surrounds axons consists primarily of lipids since the myelin sheath serves as an electrical insulator
What role do proteins in membranes play?
transport of solutes across the membrane, signal transduction, and generation of protein
Glycoproteins and glycolipids are involved in what?
cell signaling and cell differentiation
what does amphipathic mean?
a characteristic of lipid membranes, they have a hydrophilic surface comprised of the polar head groups and a hydrophobic surfaces comprised of the non-polar tails.
What happens to amphipathic membranes in an aqueous environment?
the lipid molecules aggregate to form either micelles, bilayers, or vesicles. The tendency for lipids to adopt each of these structures is dependent on the nature of the lipid molecules
What are the characteristics of fatty acids?
Fatty acids have a hydrocarbon chain made up of 4 ‐ 36 C‐atoms long and a carboxyl group at one end. The hydrocarbon chain can be fully saturated or it can contain one or more double bonds as in
The carboxylate group (with a negative charge) is polar; the hydrocarbon tail is hydrophobic.
What does it mean when a fatty acid is saturated vs unsaturated?
fully saturated means there are no double bonds in the hydrocarbon chain of the fatty acid.
unsaturated would indicate that there are one or more double bonds in the hydrocarbon chain of the fatty acid
what is the nomenclature fatty acid rule?
the total number of carbons, semicolon, number of double bonds. Normally numbering begins with the carboxylate carbon. One can also number the carbons from the methyl
E.g., stearic acid is an 18 carbon fatty acid with no double bonds, hence 18:-0
What is the difference between saturated vs unsaturated fatty acids?
SATURATED: very flexible, well packaged membrane, and higher melting T
UNSATURATED: cis double bonds, 30 degree bend, not conjugated double bonds, poorly packed membrane, and lower melting T
What does it mean when a saturated fatty acid is well packed?
This means the membrane is less fluid: there is maximized van der waals interactions and nearly crystalline. Higher temperature is required to disrupt and convert solid to liquid (the melting temperature is higher)
What does it mean when a unsaturated fatty acid is well packed?
this means the membrane is poorly packed, which means more fluid and fewer van der waals interactions; lower temperature is required to convert solid to liquid (the melting temperature is lower)
What are the types of lipids in membranes?
- Phospholipids
- Glycolipids
- Triacylglycerols
What are the role of lipids in membranes?
-storage as triacylglycerols, which are neutral but lipids can also be polar
What is the difference between fats and oils?
Fats = triacylglycerols that are solid at room temperature (butter); oils = triacylglycerols that are liquid at room temperature (palm oil)
Fats are more common in animals, oils in plants.
are triacylglycerols polar or non polar?
Triacylglycerols are very non‐polar since they have no free negatively‐charged carboxylate. Since triacylglycerols are hydrophobic, they don’t need extra water for hydration, but they are more difficult to transport from one tissue to another.
Why do fatty acids produce more energy compared to glucose?
Compared to glucose, fatty acids are more reduced and thus produce more energy per C atom when catabolized.
What are glycerolphopholipids (phosphoglycerides)?
Phosphoglycerides (= glycerolphospholipids) are major components of membranes. They are composed of glycerol esters of 2 carboxylic acids and phosphoric acid. A head group is attached to the phosphoryl group via a phosphodiester bond.
Isoprene
are C‐5 units that are also building blocks of steroids, vitamin A, ubiquinone, some hemes, etc.
Diphytanyl groups
are isoprene derived
8 isoprene units for the diphytanyl groups = C40. 32 of these C atoms are in the backbone that spans the entire membrane. I.e., the diphytanyl chain is twice the length of phospholipids and sphingolipids found in lipid bilayers of most organisms.
How are archaeal lipids linked to glycerol?
via ether bonds rather than the usual ester bonds. Ether bonds are much more stable than ester and this helps explain how extremophiles can survive and thrive under conditions of extreme pH or temperature
What are sphyingolipids?
they are a second major component of biological membranes. Sphingolipids do not have a glycerol moiety, but rather a polar head group and 2 non‐polar chains. It contains a sphingosine molecule (a C‐18 amino alcohol), a fatty‐acid chain (of differing length) and a polar head group. sphingolipids can have different head groups
What are sphingomyelins?
phosphodiester bonded to headgroup either ethanolamine or choline.
They are present in plasma membranes of animal cells, especially prominent in myelin and insulation around some axons.
What are glycosphingolipids?
Glycosphingolipids are mostly found in the outer leaflet of plasma membranes. They have one or more sugars lined to ceramide. Since there is no phosphate groups, it is neutral.
What is cholesterol?
Cholesterol is amphipathic with a polar hydroxyl head group and a non‐polar hydrocarbon body. It is almost planar. and relatively rigid. It is synthesized from C‐5 isoprene groups, which are, in turn, synthesized from acetyl coenzyme A.
Cholesterol also alters membrane fluidity and is a precursor to many important steroids e.g., steroid hormones, biles acid in the small intestine: detergents for absorbing non-polar nutrients
What is the driving force for formation of membranes?
bury hydrophobic groups away from aqueous solvent
- van der waals interactions
- increase in solvent entropy
What are micelles?
cone shaped lipids that form monolayers. Individual units are wedge shaped
What are the uses of micelles?
detergents and adsorption of fat-soluble vitamins and non-polar lipids in the small intestine
What is a lipid bilayer?
the basic structural element of membranes. When mixed with water, lipids will try to bury the hydrophobic moieties and have the hydrophilic parts interact with the water. It is this hydrophobic interaction that is the thermodynamic driving force. By burying the hydrophobic groups it reduces the exposed hydrophobic surface to water, thus reducing the number of molecules in the shell of ordered water, resulting in an increase in entropy.
liposomes and vesicles are what type of lipids? c
cylindrical lipids and liposomes are self-sealed, solvent-filled sphere
why is the membrane described as a fluid mosaic model?
b/c proteins are embedded in the bilayer and held by hydrophobic interactions between the membrane lipids and the hydrophobic domains in the protein. there is an asymmetric distribution of lipids and proteins in the membrane. These individual lipids and proteins form a mosaic like a ceramic‐tile mosaic, but unlike the ceramic‐tile mosaic where the pattern is fixed, the membrane is free to change constantly. Hence the term fluid moaic model. It is often likened to a lipid ocean with floating protein icebergs.
Are hydrophilic protein surfaces and head groups soluble in the lipid phase?
no, such interactions must be catalyzed
what are the ways proteins can move through a membrane?
1) lateral diffusion - does not require catalysis
2) transverse diffusion/flip flip - is very slow in the absence of a catalyst.
What is transverse diffusion? why is a catalyst needed?
transverse diffusion requires that that a polar head group move from an aqueous environment into the hydrophobic interior of the bilayer, a process which is not energetically favorable. However, a catalyst can reduce that energy barrier and make the translocation possible.
What are the three enzymes involved in transverse diffusion?
1) flippase
2) floppase
3) scramblase
What happens to membrane fluidity when the temperature is low?
it is gel like an ordered - movement of individual molecules is constrained
What happens to membrane fluidity when the temperature is high?
membrane is fluid (disordered) - hydrocarbon chains are in constant motion and there is lateral diffusion of individual lipid molecules w/in the bilayer.
name: fluid or liquid-disordered state
What helps regulate fluidity of a membrane?
1) chain length
2) degree of saturation
3) cholesterol
When it comes to membrane fluidity, shorter lipids cause what?
decrease the transition temperature (are desaturated) making the lipid membrane more fluid
How does cholesterol affect membrane fluidity ?
Cholesterol can either increase or decrease fluidity
- w/lipids that have long saturated acyl chains - cholesterol increases fluidity
- w/lipids that have unsaturated fatty acyl chains, cholesterol causes the acyl chains to be more constrained in their movement thus reducing fluidity and fatty acid movement
- cholesterol also increases the width of a membrane
What is frap?
fluorescence recovery after photobleaching
What does frap measure?
This method is used to measure the lateral diffusion rate of lipids. Lipids in the outer leaflet of a cell are labeled with a membrane‐impermeable fluorescent probe so that there is uniform labeling of the surface. A small area is then bleached by irradiation with an intensive laser beam; the probe now does not fluoresce. With time, labeled phospholipids diffuse into the bleached area. This time course of fluorescence return can be measured and is a function of time of diffusion.
