Lipids and Cellular Transport Lecture 11

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Questions and Answers

What is a characteristic of membrane lipids?

They are amphipathic. (correct)

They are always unsaturated.

They are hydrophilic.

They are insoluble in water.

What is the typical number of carbon atoms in a fatty acid chain?

10 to 20

12 to 24 (correct)

15 to 30

18 to 36

What is the function of fat storage in animals?

Three distinct functions (correct)

Energy production

DNA replication

Protein synthesis

What is a characteristic of unsaturated fatty acids?

They contain one or more double bonds. Signup and view all the answers

What is the term for a fatty acid with no double bonds?

Saturated Signup and view all the answers

What is the structure of fats, or triacylglycerols?

Triester of fatty acids and glycerol Signup and view all the answers

What is a common feature of naturally occurring fatty acids?

They contain an even number of carbon atoms Signup and view all the answers

What is the term for the structures formed by lipids in contact with water?

Micelles, vesicles, and bilayers Signup and view all the answers

What is the result of adding 20 mol % cholesterol to a pure dipalmitoylphosphatidylcholine bilayer?

The transition is broadened. Signup and view all the answers

What is the typical state of biological membranes under physiological conditions?

Semi-fluid liquid crystalline state Signup and view all the answers

What is the effect of lipids with longer, saturated tails on the transition temperature (Tm) of a membrane?

They increase the Tm. Signup and view all the answers

What is the function of bacteriorhodopsin, an integral membrane protein?

It functions as a light-driven proton pump. Signup and view all the answers

What is notable about the two leaflets of a membrane?

They have different lipid compositions. Signup and view all the answers

What is the term for the sharp spike in heat absorption as the temperature is raised to a certain point in a pure dipalmitoylphosphatidylcholine bilayer?

Melting of the membrane. Signup and view all the answers

What is the effect of lipids with more cis double bonds and/or shorter tails on the transition temperature (Tm) of a membrane?

They decrease the Tm. Signup and view all the answers

What can be said about the lipid composition of the outer and inner leaflets of the plasma membrane?

They can be the same or different. Signup and view all the answers

What is the main function of waxes in some marine microorganisms?

Energy storage Signup and view all the answers

What is the shape of fatty acids that form micelles?

Wedge-shaped Signup and view all the answers

What is the thickness of the hydrophobic core of a bilayer membrane?

3 nm Signup and view all the answers

What is the purpose of waxes in beeswax?

Structural function Signup and view all the answers

How do phospholipids pack together to form a bilayer membrane?

Parallel to form extended sheets Signup and view all the answers

What is the main difference between the four major classes of membrane-forming lipids?

The nature of the head group Signup and view all the answers

What is the property of waxes that makes them useful as water repellents?

Insolubility in water Signup and view all the answers

What is the shape of phospholipids that allows them to form a bilayer membrane?

Cylindrical Signup and view all the answers

What is the function of the molecule retinal in the membrane?

It converts light into metabolic energy Signup and view all the answers

What percentage of the total protein mass of glycophorin A is constituted by oligosaccharides?

60% Signup and view all the answers

What is the characteristic of the single transmembrane helix of glycophorin A?

It is highly hydrophobic Signup and view all the answers

What is the function of the translocon in the insertion of integral membrane proteins?

It facilitates the insertion of integral membrane proteins into the membrane bilayer Signup and view all the answers

What happens to a segment of the nascent peptide if it is sufficiently hydrophobic?

It partitions into the lipid bilayer Signup and view all the answers

What is the orientation of the transmembrane segments in the translocon model?

They depend on the orientation of the hydrophilic sequences Signup and view all the answers

What is the characteristic of the cytosolic C-terminal domain of glycophorin A?

It is highly hydrophilic Signup and view all the answers

What is the significance of glycophorin A in the context of membrane proteins?

It is the first integral membrane protein to be sequenced Signup and view all the answers

What is the orientation of the transmembrane helices of wild-type leader peptidase (Lep) from E. coli?

With the termini in the periplasm and the loop between the helices in the cytoplasm Signup and view all the answers

What happens to the bilayer thickness in membrane rafts compared to the surrounding membrane?

It becomes thicker Signup and view all the answers

What is the result of hydrophobic mismatch in a membrane?

The protein undergoes conformational change or the bilayer changes composition Signup and view all the answers

What is the function of the dynamic raft domains in membranes?

To facilitate protein-lipid interactions Signup and view all the answers

What is the role of cholesterol in membrane rafts?

To stabilize raft domains Signup and view all the answers

How do membrane rafts associate with each other?

Through dynamic associations Signup and view all the answers

What happens to the membrane potential in the presence of a mutant Lep?

It becomes more positive Signup and view all the answers

What is the effect of actin fibers on raft domains?

They stabilize the raft domains Signup and view all the answers

Study Notes

Lipid Molecules

Lipid molecules are insoluble in water but can associate to form water-soluble structures such as micelles, vesicles, and bilayers.

Fatty Acids

Fatty acids have a hydrophilic carboxylate group attached to a hydrocarbon chain, which typically contains 12 to 24 carbons.
Saturated fatty acids have carbons in the tail that are saturated with hydrogen atoms, whereas unsaturated fatty acids contain one or more double bonds.
Stearic acid is an example of a saturated fatty acid, while oleic acid is an example of an unsaturated fatty acid.

Membrane Lipids

Membrane lipids are amphipathic, meaning they tend to form surface monolayers, bilayers, micelles, or vesicles when in contact with water.
Most naturally occurring fatty acids contain an even number of carbon atoms, and if double bonds are present, they are usually cis.

Fats (Triacylglycerols)

Fats are triesters of fatty acids and glycerol, serving as major long-term energy storage molecules in many organisms.
Fat storage in animals serves three distinct functions.

Waxes

Waxes are formed by esterification of fatty acids and long-chain alcohols, resulting in complete water insolubility.
Waxes often serve as water repellents, as in the feathers of some birds and the leaves of some plants.

Membrane-Forming Lipids

There are four major classes of membrane-forming lipids: glycerophospholipids, sphingolipids, glycosphingolipids, and glycoglycerolipids.
These lipids have a large head group attached to a double tail, yielding a roughly cylindrical molecule that can pack in parallel to form extended sheets of bilayer membranes.

Phospholipids and Membrane Structure

Phospholipids are cylindrical and pack together to form a bilayer structure, with the hydrophilic head groups facing outward into the aqueous regions on either side.
The bilayer is roughly 6 nm thick, with ~1.5 nm of interface on either side of the ~3 nm hydrophobic core.

Transition Temperature (Tm) of Membranes

The transition temperature (Tm) of a membrane depends on its lipid composition.
Lipids with longer, saturated tails tend to increase the Tm, while those with more cis double bonds and/or shorter tails reduce the Tm.

Integral Membrane Proteins

Examples of integral membrane proteins include bacteriorhodopsin, which functions as a light-driven proton pump in certain bacteria, and glycophorin A, which is the MNS blood group protein in human erythrocytes.
These proteins have hydrophobic transmembrane helices that span the membrane and hydrophilic domains that face the aqueous regions.

Co-Translational Insertion and Folding of Transmembrane Helices

The translocon, a protein channel, facilitates the insertion of integral membrane proteins into the membrane bilayer.
Hydrophilic sequences (loops) partition to either side of the bilayer, depending on the orientation of the transmembrane segments.

Membrane Rafts

Membrane rafts are rich in cholesterol, sphingolipids, and GPI-linked proteins.
Rafts are thicker than the surrounding membrane and can coalesce to form larger structures ("platforms") that may be stabilized by interactions with actin fibers.

Transport Across Membranes

Adaptation to hydrophobic mismatch in a membrane involves either the protein undergoing conformational change or the bilayer changing composition until the dimensions of the hydrophobic regions match.

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Description

This quiz covers the molecular structure and behavior of lipids, including their solubility in water and formation of micelles, vesicles, and bilayers.