L1 Membrane Structure

Question 1

Functions of biological membranes

Answer 1

Maintain environment separation

Provide support for membrane bound proteins

Import/export molecules

Expansion and retraction for movement

Question 2

Mammalian plasma membrane

Answer 2

Maintains different composition btw cytosol and extracellular environment
i.e. conc gradient, pH

Facilitates communication
i.e. ion gradients, support membrane bound proteins

Question 3

Components of biological membranes are

Answer 3

Lipids

Proteins

Question 4

Lipids

Answer 4

Phospholipids
Cholesterol
Glycolipids

Question 5

Phospholipids form from what components?

Answer 5

Lipid bilayer

Glycerol - 3 hydroxyl groups replaced by
Two fatty acid 14-24C
A polar/charged head group (containing phosphate and functional molecule)

Question 6

How many phospholipids make up 50% of most mammalian lipids membranes? I

Answer 6

4

All are neutral except

Phosphatidylserine (neg charge)

Question 7

Where is phosphatidylserine mostly found?

Answer 7

On the inside of the cell

Cytosolic side

(More neg side due to charge)

Question 8

Phospholipids form bilayers spontaneous due to?

Answer 8

Their amphipathic nature

Hydrophobic and hydrophilic interactions

Question 9

What organelle synthesizes phospholipids and in what monolayer?

Answer 9

Smooth endoplasmic reticulum

In the cytosolic monolayer

Question 10

What does scramblase do?

Answer 10

Non-specifically moves phospholipids from one monolayer to the other

Some phospholipids just synthesized in cytosolic will be moved to lumenal monolayer by scramblase

Maintains lipids bilayer asymmetry

Question 11

Newly synthesize bilayer can

Answer 11

Bud, pinch off, and be transported to target membrane

Cytosolic facing monolayer will remain cytosolic facing

Lumenal monolayer will face lumen of organelle

Question 12

Which layers fuse first during membrane fusion?

Answer 12

Cytosolic

Question 13

In Vesicle to organelle membrane fusion which membranes fuse second?

Answer 13

Lumenal layers

Question 14

In vesicles to plasm membrane fusion which membranes fuse second?

Answer 14

Lumenal and extracellular layers

Question 15

Flippase

Answer 15

“Flip” specific phospholipids to the cytosolic layer to create membrane asymmetry

Requires ATP

Question 16

Floppase

Answer 16

Move selected lipids to the extracellular/lumenal monolayer

Requires ATP

Question 17

What’s an example of membrane asymmetry and cell signaling?

Answer 17

Signaling programmed cell death (apoptosis)

Under stress phosphatidylserine is translocated from cytosolic to extracellular monolayer after:
Activation of scramblase and/or
Inactivation of flippase

Cell surface phosphatidylserine signals macrophages to phagocytosis and digest the cell

Question 18

Charge, size, and shape of phospholipids allow for?

Answer 18

Specific recruitment of enzymes, receptors, and other proteins

Proper environment for function of membrane proteins

Question 19

What does cholesterol do?

Answer 19

Maintains optimal fluidity

Question 20

How does level of phospholipid tail saturation affect membrane fluidity?

Answer 20

Higher saturation = more ordered = more easily packed = less fluid

More cis bonds = less saturation = less ordered = less packed = more fluid

Question 21

How does length of acyl chain affect membrane fluidity?

Answer 21

Longer tail = less fluid

Shorter tail = more fluid

Question 22

How does temperature affect membrane fluidity?

Answer 22

High temp = more fluid

Question 23

How does level of cholesterol affect membrane fluidity?

Answer 23

Maintains optimal

Question 24

What is the general structure of cholesterol?

Answer 24

Rigid steroid ring (provides stability)

Shorter fatty acid tail (creates space btw hydrophobic chains)

Polar head group

Question 25

What happens if plasma membrane is too fluid?

Answer 25

Integral membrane protein structure and function can be compromised

Permeability increases and ions can flow through

Question 26

What happens if plasma membrane is too rigid?

Answer 26

Permeability decreases

Proteins lose ability to undergo conformational changes

Flippase and floppase cannot work

Question 27

What are lipids rafts?

Answer 27

Plasma membrane microdomains where proteins aggregate to facilitate specific functions

Enriched in sphingolipids
Requires cholesterol
Less fluid due to presence of phospholipids with long, saturated hydrocarbon tails

Question 28

What are lipid rafts ideal for?

Answer 28

Vesicle formation
Protein protein interaction
Signal transduction

Question 29

No cholesterol

Answer 29

No lipid raft

Question 30

Where are glycolipids and glycoproteins added?

Answer 30

In the ER, golgi lumen

Occur on extracellular monolayer

Interact with extracellular molecules

Contain self associating H bonds (btw sugars)

Question 31

Integral membrane proteins are?

Answer 31

Amphipathic

Won’t leave membrane

Question 32

How mobile in the membrane are soluble membrane proteins that are covalently bound to membrane lipids directly or via an extracellular oligosaccharide?

Answer 32

More mobile than integral membrane

Won’t come off on their own but an enzyme can get them off

Question 33

How mobile in the membrane are soluble membrane proteins that are non-covalently interacting with integral membrane proteins?

Answer 33

Loosest association with membrane

Can be dissociated by changes in pH, ionic strength

Question 34

What do disulfide bonds between cysteine residues do?

Answer 34

Stabilize structure

Facilitate protein-protein association

Question 35

Where do disulfide bonds occur? And why?

Answer 35

Extracellular side

Because the cytosolic pH is low enough (excess protons) so that reactive groups in cytosol are reduced

S binds to protons instead of other S