Cell Biology Chapter 4 - Phospholipids and Membrane Structure Flashcards
Phospholipids
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
Phospholipids Contain 4 Structural Elements
Phospholipids have a backbone which serves as an attachment site for different functional groups.
Glycerol
2 Fatty acids
Head group
Glycerol
is a three-carbon sugar-alcohol
Phospholipids constructed from a glycerol backbone are called phosphoglycerides
The Lipid Portion Of A Phospholipid Can Vary Widely In Structure
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
What Is The Consequence Of Saturated Versus Unsaturated
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
Polar head groups confer additional specificity on the structure of phospholipids
Ionic:
PC
PE
PS
Polar
PI
PG
bis-glycerol portion of CL
The Amphipathic Nature Of Phospholipids Allows Them To Form Lipid Bilayers In Aqueous Solution
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)
Lipid Bilayers Are Asymmetrical
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
Phospholipid Bilayers Are Semi-permeable Barriers
Small molecules are more permeable than large molecules
Nonpolar molecules are more permeable than polar molecules
Charged molecules do not diffuse
Maintaining a Chemical Imbalance Across a Membrane is Essential for Life
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
The Fluid–mosaic Model
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
Central Tenet:
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.
The Revised Fluid-Mosaic Model
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.
The Revised Fluid-Mosaic Model
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
Membrane Proteins Associate with Membranes in Three Different Ways
Integral membrane proteins
Lipid-anchored membrane proteins
Peripheral membrane proteins
Integral Membrane Proteins
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
Lipid-anchored Membrane Proteins
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
Peripheral Membrane Proteins
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)
Membrane proteins associate with membranes in 3 different ways
…
Transmembrane Proteins Typically Use Alpha Helices To Cross The Lipid Bilayer
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
Cellular Membranes Are Both Fluid And Static
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
Additional Membrane Lipids Are Synthesized In The ER And Golgi Apparatus
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
Most Membrane Assembly Begins In The SER And Is Completed In The Target Organelle
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
Eight Steps to Membrane Synthesis
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.
Generation Of Membrane Assymetry
Flippases & floppases
Eight Steps to Membrane Synthesis
) 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
Orientation and Insertion Of PM Lipids and Proteins
..
Eight Steps to Membrane Synthesis
) 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
Orientation and Insertion Of PM Lipids and Proteins
..
Eight Steps to Membrane Synthesis
6) Membrane proteins in other organelles are inserted in situ
Consider the following statements about biological membranes:
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
Which observation best illustrates the fact that a biological membrane acts as a two-dimensional fluid?
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.
Why does ethanol readily pass through phospholipid bilayers, while sugars (such as sucrose, which can be converted into ethanol) do not?
Ethanol is smaller than sucrose
What is the likely impact on the fluidity of a phospholipid if its monounsaturated fatty acid is converted to a fully saturated fatty acid?
Fluidity will decrease, because the mass of the phospholipid increases
Phosphotidyl inositol is mostly on the ________ of the plasma membrane and sugars are mostly on the ________ of the plasma membrane.
cytosolic leaflet, exoplasmic leaflet
The force that holds most molecules in a membrane together is ________.
hydrophobic interactions
Which of the following would most readily diffuse through the membrane
A) sugars B) amino acids C) K+ D) CO2 E) answers A and B Points Earned: 1.0/1.0 Correct Answer(s): D
Phosphoglycerides are composed of
a glycerol backbone with 2 fatty acyl groups
In order to make up the glycerol molecule, the carbons are modified by which of the following?
A) hydrogen B) phosphate C) nitrogen D) hydroxyl E) answer A and D Points Earned: 0.0/1.0 Correct Answer(s): E
Fatty acids are added to the glycerol backbone to form phosphoglycerides in a ________ reaction with the formation of a _________ bond.
A) dehydration, ester
Unsaturated fatty acids differ from saturated fatty acids in their
A) shape B) pH C) fluidity D) peptide bonds E) answer A and C Points Earned: 1.0/1.0 Correct Answer(s): E
_________ have structures with hydrophobic tails that cluster away from water.
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
Which of the following are found in the external leaf of the plasma membrane
) 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
Proteins that cross the membrane usually do so with a _________ structure.
quarternary
Glycerol and fatty acids are synthesized in the
cytosol
Phosphoglyceride synthesis begins in the ____________ with the help of the enzyme __________
smooth ER, acyl transferase
The peptide backbone of proteins is polar. So how can transmembrane proteins stay in membranes?
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
What is the likely impact on the fluidity of a phospholipid if its monounsaturated fatty acid is converted to a fully saturated fatty acid?
Fluidity will increase, because the density of the phospholid increases.
Inositol phospholipids are found exclusively in the cytoplasmic face of the plasma membrane because:
There are no inositol flippases in the ER
Flippases and Floppases use GTP energy to move phospholipids from one leaflet to the other in the plasma membrane.
False
What is the likely impact on the fluidity of a phospholipid if its monounsaturated fatty acid is converted to a fully saturated fatty acid?
Fluidity will increase, because the density of the phospholid increases.
Inositol phospholipids are found exclusively in the cytoplasmic face of the plasma membrane because:
There are no inositol flippases in the ER
Flippases and Floppases use GTP energy to move phospholipids from one leaflet to the other in the plasma membrane.
False
