208A Lipids and Membranes

Question 1

Why does life need membranes?

Answer 1

• 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

Question 2

what are some functions the membrane carries out?

Answer 2

cell growth and movement, gradients for generating energy, excreting toxins and waste products

Question 3

Membranes are made up of what?

Answer 3

lipids and proteins but also carbohydrates are present in the form of glycoproteins or glycolipids.

Question 4

The proportions of protein and lipid of a membrane depend on what?

Answer 4

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

Question 5

What role do proteins in membranes play?

Answer 5

transport of solutes across the membrane, signal transduction, and generation of protein

Question 6

Glycoproteins and glycolipids are involved in what?

Answer 6

cell signaling and cell differentiation

Question 7

what does amphipathic mean?

Answer 7

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.

Question 8

What happens to amphipathic membranes in an aqueous environment?

Answer 8

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

Question 9

What are the characteristics of fatty acids?

Answer 9

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.

Question 10

What does it mean when a fatty acid is saturated vs unsaturated?

Answer 10

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

Question 11

what is the nomenclature fatty acid rule?

Answer 11

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

Question 12

What is the difference between saturated vs unsaturated fatty acids?

Answer 12

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

Question 13

What does it mean when a saturated fatty acid is well packed?

Answer 13

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)

Question 14

What does it mean when a unsaturated fatty acid is well packed?

Answer 14

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)

Question 15

What are the types of lipids in membranes?

Answer 15

• Phospholipids
• Glycolipids
• Triacylglycerols

Question 16

What are the role of lipids in membranes?

Answer 16

-storage as triacylglycerols, which are neutral but lipids can also be polar

Question 17

What is the difference between fats and oils?

Answer 17

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.

Question 18

are triacylglycerols polar or non polar?

Answer 18

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.

Question 19

Why do fatty acids produce more energy compared to glucose?

Answer 19

Compared to glucose, fatty acids are more reduced and thus produce more energy per C atom when catabolized.

Question 20

What are glycerolphopholipids (phosphoglycerides)?

Answer 20

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.

Question 21

Isoprene

Answer 21

are C‐5 units that are also building blocks of steroids, vitamin A, ubiquinone, some hemes, etc.

Question 22

Diphytanyl groups

Answer 22

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.

Question 23

How are archaeal lipids linked to glycerol?

Answer 23

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

Question 24

What are sphyingolipids?

Answer 24

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

Question 25

What are sphingomyelins?

Answer 25

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.

Question 26

What are glycosphingolipids?

Answer 26

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.

Question 27

What is cholesterol?

Answer 27

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

Question 28

What is the driving force for formation of membranes?

Answer 28

bury hydrophobic groups away from aqueous solvent

• van der waals interactions
• increase in solvent entropy

Question 29

What are micelles?

Answer 29

cone shaped lipids that form monolayers. Individual units are wedge shaped

Question 30

What are the uses of micelles?

Answer 30

detergents and adsorption of fat-soluble vitamins and non-polar lipids in the small intestine

Question 31

What is a lipid bilayer?

Answer 31

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.

Question 32

liposomes and vesicles are what type of lipids? c

Answer 32

cylindrical lipids and liposomes are self-sealed, solvent-filled sphere

Question 33

why is the membrane described as a fluid mosaic model?

Answer 33

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.

Question 34

Are hydrophilic protein surfaces and head groups soluble in the lipid phase?

Answer 34

no, such interactions must be catalyzed

Question 35

what are the ways proteins can move through a membrane?

Answer 35

1) lateral diffusion - does not require catalysis

2) transverse diffusion/flip flip - is very slow in the absence of a catalyst.

Question 36

What is transverse diffusion? why is a catalyst needed?

Answer 36

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.

Question 37

What are the three enzymes involved in transverse diffusion?

Answer 37

1) flippase
2) floppase
3) scramblase

Question 38

What happens to membrane fluidity when the temperature is low?

Answer 38

it is gel like an ordered - movement of individual molecules is constrained

Question 39

What happens to membrane fluidity when the temperature is high?

Answer 39

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

Question 40

What helps regulate fluidity of a membrane?

Answer 40

1) chain length
2) degree of saturation
3) cholesterol

Question 41

When it comes to membrane fluidity, shorter lipids cause what?

Answer 41

decrease the transition temperature (are desaturated) making the lipid membrane more fluid

Question 42

How does cholesterol affect membrane fluidity ?

Answer 42

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

Question 43

What is frap?

Answer 43

fluorescence recovery after photobleaching

Question 44

What does frap measure?

Answer 44

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.