Cell Biology Chapter 4 - Phospholipids and Membrane Structure

Question 1

Phospholipids

Answer 1

are the Basic Building Blocks of Cellular Membrane

Acquire structural diversity through branching and chemical modification
ALL phospholipids are branched
Unsaturation of carbon bonds as a chemical modification
Are not polymers of a simpler structure and therefore are considerably less diverse
MW of most is <1,000 Daltons
Eukaryotes contain ~1,000 different lipids (less than one-tenth the estimated # of genes or proteins
Have fewer unique functions than other biomolecules

Question 2

Phospholipids Contain 4 Structural Elements

Answer 2

Phospholipids have a backbone which serves as an attachment site for different functional groups.
Glycerol
2 Fatty acids
Head group

Question 3

Glycerol

Answer 3

is a three-carbon sugar-alcohol

Phospholipids constructed from a glycerol backbone are called phosphoglycerides

Question 4

The Lipid Portion Of A Phospholipid Can Vary Widely In Structure

Answer 4

Phospholipids contain 2 lipids called fatty acids (have a carboxylic grp at one end)

2 structural classes of fatty acids:
Saturated (“fully stocked with hydrogens”)
Unsaturated (containing double bonds in tail)
-Can be further categorized into polyunsaturated or monounsaturated

The two fatty acids do not have to be the same in each phospholipid

Question 5

What Is The Consequence Of Saturated Versus Unsaturated

Answer 5

When Carbon binds 4 atoms it is tetrahedral
When Carbon binds 3 atoms it adopts a planar, triangular arrangement

Trans versus Cis arrangements around the double bond

Trans adopt a more linear configuration similar to a saturated fatty acid

Cis fatty acids have kinks and bends

Question 6

Polar head groups confer additional specificity on the structure of phospholipids

Answer 6

Ionic:
PC
PE
PS

Polar
PI
PG
bis-glycerol portion of CL

Question 7

The Amphipathic Nature Of Phospholipids Allows Them To Form Lipid Bilayers In Aqueous Solution

Answer 7

Amphipathic
attract and repel water
Having hydrophobic AND hydrophilic parts

Aggregate into groups/ clusters (energetically favorable)
When added to water, they form 3 
characteristic structures:
Micelle
Liposome
Bilayer (monolayer)

Question 8

Lipid Bilayers Are Asymmetrical

Answer 8

Lipids cannot usually flip spontaneously from one side of the membrane to the other

The relative concentration of individual phospholipids on two sides of the same membrane can differ
Phosphatidylcholine (PC) is concentrated in the external leaflet of the plasma membrane

Phosphatidylethanolamine (PE), phosphatidylserine (PS), and phosphatidylinositol (PI) in the cytoplasmic leaflet

Question 9

Phospholipid Bilayers Are Semi-permeable Barriers

Answer 9

Small molecules are more permeable than large molecules

Nonpolar molecules are more permeable than polar molecules

Charged molecules do not diffuse

Question 10

Maintaining a Chemical Imbalance Across a Membrane is Essential for Life

Answer 10

Cells are constantly fighting to maintain themselves AWAY from equilibrium

At equilibrium, life would cease to exist!

Prokaryotes use proton and sodium pumps to pump ions out to the extracellular environment

Eukaryotes use a protein that creates two gradients simultaneously: Na2+ is pumped out while K+ is pumped in

Question 11

The Fluid–mosaic Model

Answer 11

Three components of membranes:
Phospholipids
Other Lipids
And Proteins
Almost nothing else in nature is arranged in a bilayer
Early models were not correct

1972 Singer and Nicholson Propose the Fluid Mosaic Model

Question 12

Central Tenet:

Answer 12

A membrane resembles a mosaic pattern, wherein the phospholipid bilayer acts like a planar fluid, with membrane proteins floating randomly within it, supported by hydrophobic interactions between phospholipid tails and hydrophobic amino acids.

Question 13

The Revised Fluid-Mosaic Model

Answer 13

Membrane proteins are extremely large in comparison to phospholipids and they cluster to form large patches (lipid rafts)

Lipid rafts are chemically and physically distinct from the surrounding membrane and contain protein, phospholipids and typically cholesterol, yet they are not covalently linked.

Hydrophilic portions of phospholipids also associate with membrane proteins allowing them to spread out over the membrane surface.

Question 14

The Revised Fluid-Mosaic Model

Answer 14

Phospholipid interacts with hydrophilic and hydrophobic membrane proteins distorts these lipids from an ideal planar arrangement causing differences in the thickness of membrane

The membrane is very dynamic. The level and location of lipid rafts changes over time. Proteins enter and exit the membrane

Question 15

Membrane Proteins Associate with Membranes in Three Different Ways

Answer 15

Integral membrane proteins

Lipid-anchored membrane proteins

Peripheral membrane proteins

Question 16

Integral Membrane Proteins

Answer 16

Are embedded in the Bilayer
Can be partially embedded = monotonic proteins
Can span the membrane entirely = membrane-spanning or transmembrane proteins
Membrane-spanning proteins can be single-pass or multi-pass

Question 17

Lipid-anchored Membrane Proteins

Answer 17

Don’t penetrate the lipid bilayer, but rather have a lipid covalently attached to the end of a cysteine
Example lipid modifications are:
- Prenyl groups, farnesyl, and geranylgeranyl groups
- Glycophosphatidylinositol (GPI) can attach to the carboxy terminus of some membrane proteins

Anchor the protein to the membrane

Question 18

Peripheral Membrane Proteins

Answer 18

Don’t come in direct contact with the membrane

Bond integral membrane proteins, and this association is stable enough that it effectively immobilizes them at or near the surface of the membrane.

Typically they remain associated with the membrane when the cell is lysed (broken apart)

Question 19

Membrane proteins associate with membranes in 3 different ways

Answer 19

…

Question 20

Transmembrane Proteins Typically Use Alpha Helices To Cross The Lipid Bilayer

Answer 20

Most common 2º structure formed by the amino acids in interior of membrane is α-helix

• AAs with hydrophobic side chains interact with the lipid membrane

Antiparallel β-sheets bend to create a cylindrical structure called a β-barrel which can serve as membrane channels

Hydrophilic interior and hydrophobic exterior

Question 21

Cellular Membranes Are Both Fluid And Static

Answer 21

Membranes vary in structure and function
Bacterial outer membranes are quite porous
Mitochondrial and Chloroplast inner membranes are only permeable to a few molecules
Golgi and ER membrane allow phospholipids to diffuse quite easily

Question 22

Additional Membrane Lipids Are Synthesized In The ER And Golgi Apparatus

Answer 22

farnesyl and geranylgeranyl lipids - attached to lipid-anchored membrane proteins are synthesized in the ER membrane from the same building blocks used to make fatty acids
Are precursors to cholesterol

Question 23

Most Membrane Assembly Begins In The SER And Is Completed In The Target Organelle

Answer 23

Once the phosphoglycerides have been synthesized at cytosolic face of SER, they are joined by additional membrane proteins synthesized in the RER to form a fluid-mosaic structure that closely resembles a mature cellular membrane
Once all proteins and lipids are together, in ER, eight final tasks need to be completed for membrane assembly

Question 24

Eight Steps to Membrane Synthesis

Answer 24

1) Membrane proteins in the endomembrane system are inserted in the ER
2) Non-specific flippases and floppases transport phosphoglycerides from one membrane leaflet to the other in the ER

“Flipping” is normal, but slow to occur. Enzymes facilitate this process. Floppases use ATP to take lipids to the outer leaflet. Flippases use ATP to take lipids to the inner leaflet.

3) Membrane vesicles transport new phosphoglycerides and membrane proteins from the ER to other organelles in the endomembrane system

When membrane is accepted by a new organelle, it maintains the membrane assymmetry of the parent organelle.

Question 25

Generation Of Membrane Assymetry

Answer 25

Flippases & floppases

Question 26

Eight Steps to Membrane Synthesis

Answer 26

) Glycolipid synthesis is completed on the exoplasmic face of the Golgi apparatus
This adds to the membrane assymmetry
Final stage of Synthesis: all subsequent stages involve modifying or moving components to the proper destination (Ch 9)

5) Final orientation of plasma membrane lipids occurs in situ (“in place”)

This reorganization happens right at the plasma membrane and not in the parent compartments of the ER and Golgi

Question 27

Orientation and Insertion Of PM Lipids and Proteins

Answer 27

..

Question 28

Eight Steps to Membrane Synthesis

Answer 28

) Glycolipid synthesis is completed on the exoplasmic face of the Golgi apparatus
This adds to the membrane assymmetry
Final stage of Synthesis: all subsequent stages involve modifying or moving components to the proper destination (Ch 9)

5) Final orientation of plasma membrane lipids occurs in situ (“in place”)

This reorganization happens right at the plasma membrane and not in the parent compartments of the ER and Golgi

Question 29

Orientation and Insertion Of PM Lipids and Proteins

Answer 29

..

Question 30

Eight Steps to Membrane Synthesis

Answer 30

6) Membrane proteins in other organelles are inserted in situ

Question 31

Consider the following statements about biological membranes:

Answer 31

i. Membranes can contain both saturated and unsaturated fatty acids in their phospholipids
ii. The α-helix is the most common secondary structure found in the transmembrane domain of membrane-spanning proteins
iii. Ethanol readily penetrates biological membranes because it passes through the hydrophobic core of membrane spanning proteins.
iv. Multispanning membrane proteins are generally more fluid than lipid-anchored membrane proteins
v. A lipid raft contains only lipids
Which of these statements is/are true?
A) i, iii, and v
B) i and iv
C) i and ii
D) iii and iv
E) v only
Points Earned: 0.0/1.0
Correct Answer(s): C

Question 32

Which observation best illustrates the fact that a biological membrane acts as a two-dimensional fluid?

Answer 32

The fluorescent signal emitted by a fluorescently tagged phospholipid on the outer surface of the plasma membrane moves over the cell surface without flipping to the inner surface of the plasma membrane.

Question 33

Why does ethanol readily pass through phospholipid bilayers, while sugars (such as sucrose, which can be converted into ethanol) do not?

Answer 33

Ethanol is smaller than sucrose

Question 34

What is the likely impact on the fluidity of a phospholipid if its monounsaturated fatty acid is converted to a fully saturated fatty acid?

Answer 34

Fluidity will decrease, because the mass of the phospholipid increases

Question 35

Phosphotidyl inositol is mostly on the ________ of the plasma membrane and sugars are mostly on the ________ of the plasma membrane.

Answer 35

cytosolic leaflet, exoplasmic leaflet

Question 36

The force that holds most molecules in a membrane together is ________.

Answer 36

hydrophobic interactions

Question 37

Which of the following would most readily diffuse through the membrane

Answer 37

A) sugars 
  B) amino acids 
  C) K+ 
  D) CO2 
  E) answers A and B 
Points Earned: 	1.0/1.0 	
Correct Answer(s):	D

Question 38

Phosphoglycerides are composed of

Answer 38

a glycerol backbone with 2 fatty acyl groups

Question 39

In order to make up the glycerol molecule, the carbons are modified by which of the following?

Answer 39

A) hydrogen 
  B) phosphate 
  C) nitrogen 
  D) hydroxyl 
  E) answer A and D 
Points Earned: 	0.0/1.0 	
Correct Answer(s):	E

Question 40

Fatty acids are added to the glycerol backbone to form phosphoglycerides in a ________ reaction with the formation of a _________ bond.

Answer 40

A) dehydration, ester

Question 41

Unsaturated fatty acids differ from saturated fatty acids in their

Answer 41

A) shape 
  B) pH 
  C) fluidity 
  D) peptide bonds 
  E) answer A and C 
Points Earned: 	1.0/1.0 	
Correct Answer(s):	E

Question 42

_________ have structures with hydrophobic tails that cluster away from water.

Answer 42

A) Lipid bilayers 
  B) Micelle 
  C) Liposomes 
  D) All of the above 
  E) None of the above 
Points Earned: 	1.0/1.0 	
Correct Answer(s):	D

Question 43

Which of the following are found in the external leaf of the plasma membrane

Answer 43

) phosphatidyl choline (PC) 
  B) phosphatidyl ethanolamine (PE) 
  C) phosphatidyl serine (PS) 
  D) phosphatidyl inositol (PI) 
  E) none since all phosphoglycerides are found in the inner leaf of the plasma membrane 
Points Earned: 	0.0/1.0 	
Correct Answer(s):	A

Question 44

Proteins that cross the membrane usually do so with a _________ structure.

Answer 44

quarternary

Question 45

Glycerol and fatty acids are synthesized in the

Answer 45

cytosol

Question 46

Phosphoglyceride synthesis begins in the ____________ with the help of the enzyme __________

Answer 46

smooth ER, acyl transferase

Question 47

The peptide backbone of proteins is polar. So how can transmembrane proteins stay in membranes?

Answer 47

A) The R groups in the transmembrane region are mostly hydrophilic, and they form a ring around the helical backbone
B) The hydrophobic nature of the backbone allows it to exist in both polar and non-polar environments
C) Nonpolar amino acids neutralize the backbone and it too becomes nonpolar
D) The helical backbone is held together by covalent bonds, which make it stable
E) None of the above statements explain this fact.
Points Earned: 0.0/1.0
Correct Answer(s): E

Question 48

What is the likely impact on the fluidity of a phospholipid if its monounsaturated fatty acid is converted to a fully saturated fatty acid?

Answer 48

Fluidity will increase, because the density of the phospholid increases.

Question 49

Inositol phospholipids are found exclusively in the cytoplasmic face of the plasma membrane because:

Answer 49

There are no inositol flippases in the ER

Question 50

Flippases and Floppases use GTP energy to move phospholipids from one leaflet to the other in the plasma membrane.

Answer 50

False

Question 51

What is the likely impact on the fluidity of a phospholipid if its monounsaturated fatty acid is converted to a fully saturated fatty acid?

Answer 51

Fluidity will increase, because the density of the phospholid increases.

Question 52

Inositol phospholipids are found exclusively in the cytoplasmic face of the plasma membrane because:

Answer 52

There are no inositol flippases in the ER

Question 53

Flippases and Floppases use GTP energy to move phospholipids from one leaflet to the other in the plasma membrane.

Answer 53

False