Exam 2 Unit 3 Membrane Composition and Characteristics

What are the three major types of membrane lipids?

Phosphoglycerides, sphingolipids, and sterols (including cholesterol).

What does amphipathic mean?

A molecule that has both a hydrophilic/polar region and a hydrophobic/nonpolar region.

What is the most abundant lipid in membranes?

Phosphoglycerides.

What are the three basic parts of a phosphoglyceride?

A glycerol backbone, two fatty acid tails, and a phosphate-containing head group.

What is the backbone of a phosphoglyceride?

Glycerol.

How many carbons does glycerol contain?

Three carbons.

How many hydroxyl groups does glycerol contain?

Three hydroxyl (-OH) groups.

Where are the two fatty acids attached to glycerol?

The fatty acids are esterified to C1 and C2 of glycerol.

What is attached to C3 of glycerol?

A phosphate group.

What is esterification?

A reaction in which a carboxylic acid reacts with an alcohol.

What part of a phosphoglyceride is hydrophilic?

The phosphate-containing head group.

What part of a phosphoglyceride is hydrophobic?

The two fatty acid tails.

What are common phosphoglyceride head groups?

Choline, ethanolamine, serine, and inositol.

What is phosphatidylcholine (PC)?

A phosphoglyceride with choline as its head group.

What is phosphatidylethanolamine (PE)?

A phosphoglyceride with ethanolamine as its head group.

What is phosphatidylserine (PS)?

A phosphoglyceride with serine as its head group.

What is phosphatidylinositol (PI)?

A phosphoglyceride with inositol as its head group.

How long are phosphoglyceride fatty acyl tails?

Approximately 14-24 carbons.

Can fatty acid tails be saturated or unsaturated?

Yes.

What does saturated mean?

A fatty acid tail with no carbon-carbon double bonds.

What does unsaturated mean?

A fatty acid tail with one or more carbon-carbon double bonds.

Why do phosphoglycerides spontaneously form bilayers in water?

Their amphipathic nature causes hydrophilic heads to interact with water while hydrophobic tails cluster away from water.

Are triglycerides membrane lipids?

No.

Why are triglycerides not membrane lipids?

They are not amphipathic and therefore do not form membrane bilayers.

What is the backbone of a sphingolipid?

Sphingosine.

How are sphingolipids different from phosphoglycerides?

Sphingolipids are built from a sphingosine backbone rather than glycerol.

Are all sphingolipids phospholipids?

No.

What is sphingomyelin?

A sphingolipid that is also a phospholipid and is found in cell membranes, especially the myelin sheath.

What is a glycolipid?

An amphipathic membrane lipid containing one or more sugars.

Are glycolipids phospholipids?

No.

What is the hydrophobic region of a glycolipid?

Two long hydrocarbon tails.

What is the hydrophilic region of a glycolipid?

One or more sugars.

Can glycolipids contain multiple sugars?

Yes. They can contain complex linkages of different simple sugars.

What type of membrane lipid is cholesterol?

A sterol.

What is the basic structure of a sterol?

A rigid ring structure related to steroids.

What polar group does cholesterol contain?

A single polar hydroxyl (-OH) group.

What nonpolar region does cholesterol contain?

A short nonpolar hydrocarbon chain and hydrophobic ring structure.

What does cholesterol do to membranes?

It stabilizes membranes and regulates membrane fluidity.

What are the two major categories of membrane proteins?

Integral membrane proteins and peripheral membrane proteins.

What is an integral membrane protein?

A protein that passes entirely through the lipid bilayer.

What is a single-pass transmembrane protein?

An integral membrane protein that spans the membrane once using a single hydrophobic alpha-helix.

Approximately how many amino acids are needed for an alpha-helix to span the membrane?

Approximately 20 amino acids.

Why is the transmembrane portion of an alpha-helix hydrophobic?

Its amino acid side chains interact favorably with the hydrophobic interior of the membrane.

Are single-pass membrane proteins entirely hydrophobic?

No. They have hydrophilic regions exposed to the aqueous extracellular and intracellular environments.

What is a multi-pass membrane protein?

An integral membrane protein that spans the lipid bilayer multiple times.

What are two structures that can form multi-pass membrane proteins?

Multiple alpha-helices or a beta-barrel.

What is a beta-barrel?

A structure formed when a beta-sheet closes on itself to form a barrel.

How are beta-barrels amphipathic?

Hydrophobic R-groups face the membrane while hydrophilic R-groups face the aqueous interior of the barrel.

What is a peripheral membrane protein?

A protein that associates with the membrane but does not pass entirely through it.

How can peripheral membrane proteins associate with membranes?

They can partially interact with the membrane, associate through attached lipids, or bind to integral membrane proteins.

How can a water-soluble protein become localized to a membrane?

A lipid can be covalently attached to the protein, allowing it to associate with the membrane.

What is a hydropathy plot used for?

Predicting transmembrane portions of proteins.

What does the hydropathy index measure?

The hydrophobicity of regions of a protein based on amino acid composition.

What does a positive hydropathy index indicate?

A hydrophobic portion of a protein.

What pattern identifies a likely transmembrane region?

A high hydropathy index spanning approximately 20 amino acids.

Why is approximately 20 amino acids important in a hydropathy plot?

An alpha-helix of approximately 20 amino acids can span the lipid bilayer.

Why do membranes spontaneously self-assemble?

The amphipathic nature of membrane lipids causes hydrophilic portions to interact with water and hydrophobic portions to cluster away from water.

Where do the polar portions of membrane lipids face?

Toward the aqueous environment.

Where do the nonpolar portions of membrane lipids face?

Inward, toward one another and away from water.

What shape do cone-shaped amphipathic lipids form?

Micelles.

What shape do cylinder-shaped lipids such as phospholipids form?

Bilayers.

What drives hydrophobic molecules to cluster together?

Clustering reduces the unfavorable interactions between hydrophobic molecules and water.

Why is bilayer closure energetically favorable?

It prevents hydrophobic hydrocarbon tails from being exposed to water.

Why do bilayers close into sealed compartments?

Closure eliminates free edges where hydrophobic tails would otherwise be exposed to water.

How do lipid bilayers behave?

Like two-dimensional liquids.

What does "2D liquid" mean for a membrane?

Lipids can diffuse, rotate, and flex within their monolayer.

Can phospholipids freely flip from one leaflet to the other?

No. This is normally energetically unfavorable.

Why is flipping between membrane leaflets energetically unfavorable?

The polar head group would have to pass through the hydrophobic core of the membrane.

What enzymes catalyze phospholipid flipping?

Flippases.

What type of enzymes are flippases?

ATPases.

Why do flippases use ATP?

ATP hydrolysis provides energy to drive the unfavorable movement of phospholipids between leaflets.

What happens to membrane fluidity when temperature increases?

Membrane fluidity increases.

What happens to membrane fluidity when temperature decreases?

The membrane becomes more rigid.

What happens when a membrane freezes?

The fatty acid tails form a more rigid crystalline structure and the membrane becomes more susceptible to rupturing.

What happens to fluidity when fatty acid tails become more saturated?

Fluidity decreases and the membrane becomes more rigid.

Why do saturated fatty acids make membranes more rigid?

Their tails can pack together more tightly.

What happens when fatty acid tails become more unsaturated?

Fluidity increases.

Why do unsaturated fatty acids increase fluidity?

Double bonds make it more difficult for fatty acid tails to pack tightly.

Why can cells increase unsaturated fatty acids at low temperatures?

Unsaturated fatty acids prevent tight packing and help prevent the membrane from freezing.

What are the three major factors that affect membrane fluidity?

Temperature, lipid saturation, and cholesterol content.

What does cholesterol do at high temperatures?

It reduces excessive fluidity by packing between regions of unsaturated fatty acids.

What does cholesterol do at low temperatures?

It prevents freezing by interfering with interactions between fatty acid chains and preventing tight packing.

Why can cholesterol be considered a regulator of membrane fluidity?

It restrains excessive fluidity at high temperatures and prevents excessive rigidity at low temperatures.

What is selective permeability?

The ability of a membrane to allow some substances to cross more easily than others.

What types of molecules cross lipid bilayers most easily?

Small, nonpolar molecules.

What types of molecules cross lipid bilayers poorly?

Polar and charged molecules.

Why do charged molecules have difficulty crossing the membrane?

The hydrophobic interior of the lipid bilayer is energetically unfavorable for charged molecules.

How does molecular size affect membrane permeability?

Generally, smaller molecules diffuse more easily across the membrane.

How does hydrophobicity affect membrane permeability?

More hydrophobic molecules generally diffuse more easily across the membrane.

What two factors are especially important for selective permeability?

Size and charge.

Can K+ freely cross the lipid bilayer?

No. Transporters or channels are required.

What does membrane asymmetry mean?

The inner and outer leaflets of a membrane have different lipid compositions.

Are the inner and outer leaflets chemically identical?

No. Their lipid compositions are different.

Which lipids are enriched in the outer leaflet of human red blood cells?

Phosphatidylcholine (PC) and sphingomyelin (SM).

Which lipids are enriched in the inner leaflet of human red blood cells?

Phosphatidylserine (PS) and phosphatidylethanolamine (PE).

Which leaflet is more negatively charged?

The inner/cytosolic leaflet.

Why is the inner leaflet more negatively charged?

It contains more negatively charged lipids, especially phosphatidylserine (PS).

What does asymmetric lipid distribution contribute to?

An electrochemical gradient across the membrane.

Where are glycolipids generally found?

On the extracellular face/outer leaflet.

Where is most membrane glycosylation found?

On the extracellular surface.