Membrane Structure and Function

Question 1

What percentage of cell membranes is made up of proteins? Lipids?

Answer 1

50% lipids and 50% proteins

Although depending on the cell type, the proteins can range from 25% (in neurons) and 75% (in mitochondria)

Question 2

What features of fatty acid tails dictate membrane fluidity?

Answer 2

Length and saturation

Question 3

How does a cis-double bond impact fatty acid structure?

Answer 3

Cis-double bonds induce a kink that prevents phosphoglycerides from packing too tightly

Question 4

Describe the general structure of phospholipids.

Answer 4

A glycerol molecule has two fatty acid chains bound to adjacent carbons, and a phosphatidyl group bound to the other carbon

Question 5

What does cholesterol incorporation into a lipid bilayer cause?

Answer 5

Increased rigidity of the membrane

Decreased permeability of the bilayer to water soluble molecules

Question 6

What is the maximum ratio of cholesterol to phospholipids in a cell membrane?

Answer 6

1:1

Question 7

What is self-aggregation?

Answer 7

Amphipathic molecules automatically organize themselves so that the hydrophilic portions are in contact with water and the hydrophobic portions are protected from water

Example: Micelles

Question 8

What structure makes all life on earth possible?

Answer 8

The lipid bilayer

Question 9

Describe the ways that phospholipids can move throughout lipid membranes.

Answer 9

Lateral diffusion

Flexion

Rotation

Flip flop (rarely occurs)

Question 10

What is membrane fluidity?

Answer 10

The relative mobility of lipids within a membrane

Question 11

How is fluidity related to transition temperature?

Answer 11

More fluid membranes have lower transition temperatures

Question 12

Two phospholipids have the same length fatty acid tails, but one has double bonds and the other doesnt.

Which will have the lower transition temperature?

Answer 12

The phospholipid with more double bonds will have a lower transition temprature, and thus more fluidity

Question 13

How does length of hydrocarbon tails affect fluidity?

Answer 13

Shorter hydrocarbon tails will have more fluidity (lower transition temperature)

Question 14

What are lipid rafts?

Answer 14

Membrane microdomains that contain protein complexes associated with specific lipids that move around as one unit

Question 15

What molecules are found only on the exterior face of the plama membrane contributing to the asymmetry of the lipid bilayer?

Answer 15

Glycolipids are only found on the extracellular side of the cell membrane.

These are branched structures made up of several sugar moieties

Question 16

What does translocation of the PS protein cause?

Answer 16

PS on the outside layer of the lipid membrane tells macrophages to “eat me”

A scramblase enzyme flips the PS from the intracellular side to the extracellular side following apoptotic signals

Question 17

What role does phosphatidylserine play in membrane structure/function?

Answer 17

It is found on the interior side of the plasma membrane which leads to a net negative charge on the inner membrane compared to the outer memrane

Question 18

What does phospholipase C do?

Answer 18

Cleaves inositol phospholipids to generate relay

Signal and scaffold for protein kinase C

Question 19

How can Galectin 3 be used for imaging?

Answer 19

Galectins contain a carbohydrate recognition domain (CRD) that allows them to bind sugars present on surface proteins

Galectins are usually only found within the cytoplasm, where no sugars are accessible

When viruses, bacterio or other pathological agents rupture phagosomes/endosomes/lysosomes, the sugars normally present on the inside of the vesicle become accessible to Gal3

Question 20

What secondary structure is commonly found spanning the lipid bilayer?

Answer 20

Alpha helices with hydrophobic residues throughout

Question 21

What does a hydropathy plot show?

Answer 21

The plot shows the hydrophobicity of an entire amino acid chain

Positive peaks correspond with hydrophobic regions that most likely span the membrane

Question 22

What are the different ways that a protein can associate with the cytoplasmic leaflet of the plasma membrane?

Answer 22

Myristoyl anchor via an amide linkage

Palmitoyl anchor via a thioester linkage

Farnesyl anchor via a thioether linkage

Question 23

Would you expect the inside of a beta barrel to have many hydrophillic or hydrophobic residues?

Answer 23

Hydrophilic

These structures often function as channels through which molecules can enter the cytosol from the extracellular space

Question 24

What feature of membrane proteins does FRAP experiments demonstrate?

Answer 24

FRAP = fluorescence recovery after photobleaching

These experiments demonstrate the many membrane proteins diffuse laterally within the plasma membrane

Question 25

Describe the immunological synapse

Answer 25

Cell to cell contact induces clustering of cellular components (transmembrane proteins, actin and lipid rafts) at the point of contact