Membrane Structure & Proteins

Question 1

Why is a membrane’s characteristic of compartmentalization important?

Answer 1

Compartmentalization allows specialized activities to proceed without external interference and enables cellular activities to run independently of another.

Question 2

How does a membrane act as a scaffold for biochemical activities?

Answer 2

Membranes, though they compartmentalize other sections of the cell, are themselves a compartment. Because of their construction, membranes provide an extensive framework for effective interaction between reactants.

Question 3

Define signal transduction.

Answer 3

Signal transduction is the process by which a cell, using receptors on its membrane, binds to ligands. This specific signal causes the cell to perform some sort of action.

Question 4

Describe the fluid-mosaic model of the plasma membrane.

Answer 4

In this model, the lipid bilayer remains central to the structure, but attention is focused on the physical state of the lipid . The bilayer is present in a fluid state, capable of moving and changing as necessary. Other molecules and proteins in the membrane are also capable of interacting dynamically with the surroundings, each other, and the bilayer.

Question 5

Define membranes.

Answer 5

Membranes are lipid-protein assemblies in which the components are held together in a thin sheet by noncovalent bonds.

Question 6

What affects the lipid-protein ratio in a membrane?

Answer 6

The type of cellular membrane, the type of organism, and the type of cell. Mitochondria are the protein carriers of the electron-transport chain, meaning cells with many mitochondria will have high protein ratios.

Question 7

What are the three main types of membrane lipids, and what common characteristic do they share?

Answer 7

Phosphoglycerides, sphingolipids, and cholesterol. All of these lipids are amphipathic.

Question 8

Describe the composition of phosphoglycerides.

Answer 8

Phosphoglycerides are diglycerides built on a glycerol backbone with a phosphate group. However, most phosphoglycerides have an additional functional group linked to the phosphate group–usually choline, ethanolamine, serine, or inositol.

Question 9

Define head group.

Answer 9

A highly water-soluble domain at one end of the molecule.

Question 10

Describe sphingolipids.

Answer 10

These lipids are derivatives of sphingosine, an amino acid. Sphingolipids are made by attaching sphingosine to a fatty acid by its amino group.

Question 11

Describe the structure of cholesterol.

Answer 11

Cholesterol molecules are oriented with their small hydrophilic hydroxyl group toward the membrane surface and the remainder of the molecule embedded in the lipid bilayer. The hydrophobic rings of a cholesterol molecule are flat and rigid, and they interfere with the movements of the fatty acid tails of the phospholipids.

Question 12

Define liposomes.

Answer 12

Liposomes are fluid-filled spherical vesicles that play a part in the self-reassembly of membranes. Their walls consist of a continuous lipid bilayer that is organized in the same manner as that of the lipid bilayer in a natural membrane.

Question 13

What important functions have liposomes fulfilled in research?

Answer 13

Membrane proteins can be inserted into liposomes and their function studied in the lab. They can also be used as vesicles to deliver DNA or drugs into the body.

Question 14

How are membrane lipids asymmetric?

Answer 14

The lipid bilayer consists of two distinct leaflets that have a distinctly different lipid composition. It can be thought of as composed of two more-or-less stable, independent monolayers having different physical and chemical properties.

Question 15

Which leaflet are glycolipids found within, and what function do they have there?

Answer 15

Glycolipids are found in the outer leaflet where they serve as receptors for extracellular ligands.

Question 16

Which leaflet is phosphatidylethanolamine found within, and what function does it have there?

Answer 16

It is mostly found in the inner leaflet, where it tends to promote the curvature of the membrane, which is important in membrane budding and fusion.

Question 17

Which leaflet is phosphatidylserine found within, and what function does it have there?

Answer 17

It is mostly found in the inner leaflet. It has a net negative charge at physiologic pH, which means it can bind positively charged lysine and arginine residues.

Question 18

Which leaflet is phosphatidylinositol (PI) found within, and what function does it have there?

Answer 18

It is mostly found in the inner leaflet. It can be phosphorylated at different sites on the inositol ring, which converts the lipid into a phosphoinositide (this molecule plays a role in the transfer of stimuli from the plasma membrane to the cytoplasm).

Question 19

Define glycosylation.

Answer 19

The attachment of a carbohydrate to a protein.

Question 20

Why is the kink in one of the fatty acid chains of a phospholipid important?

Answer 20

It restricts rotation, causing space to form. This works to maintain the fluid properties of the membrane.

Question 21

How do groups bind to the head of a phospholipid?

Answer 21

They bind covalently by a condensation reaction (hydrolysis).

Question 22

Define micelle.

Answer 22

A micelle is an aggregate of 50-100 lipid molecules with polar heads facing outwards.

Question 23

What is the function of micelles?

Answer 23

Micelles can act as emulsifying agents; hydrophobic compounds are sequestered in the micelle interior, allowing them to be dispersed in water as an emulsion. This has been developed as a drug delivery system for hydrophobic compounds.

Question 24

Define liposomes and describe how they form.

Answer 24

Liposomes are small membrane-bounded vesicles that form from the lipid bilayer. Open sheets of membranes are energetically unfavorable because the hydrophobic lipid tails are exposed to water at the edges.

Question 25

Which types of molecules can get through the semi-permeable barrier of the lipid bilayer?

Answer 25

Small hydrophobic molecules (quickly) and small uncharged polar molecules (slowly) and benzene (slowly).

Question 26

Which types of molecules cannot get through the semi-permeable barrier of the lipid bilayer?

Answer 26

Larger polar molecules such as amino acids, nucleotides, and sugars; ions; and water (which requires the assistance of channels called Aquaporins)

Question 27

How do cells regulate the fluidity of the membrane?

Answer 27

1. The cell can control the length of the fatty acid tails. Shorter fatty acids are more fluid at room temperature; they have lower melting points.
2. The cell can control the saturation of the fatty acids. Unsaturated fatty acids are more fluid at room temperature, which means they have lower melting points.

Question 28

How does cholesterol affect the cellular membrane?

Answer 28

It stiffens the membrane, making it less fluid and less permeable to small molecules at normal temperatures, but broadens the range of transition temperature, keeping the membrane more fluid at a lower temperature. This is because it sits in between phospholipid molecules, keeping them from getting too close in colder temperatures. Essentially, it buffers fluidity.

Question 29

Define lipid rafts and describe what purpose they accomplish.

Answer 29

Lipid rafts are very concentrated domains of sphingolipids and cholesterol. They are formed due to the asymmetric distribution of lipids within a single leaflet. They are responsible for membrane trafficking, cellular adhesion, targets for antibodies, and are a common point at which bacteria enter the cell.

Question 30

Define peripheral membrane proteins.

Answer 30

These proteins are weakly associated to the membrane. They do not penetrate the lipid bilayer; can be removed by salt or other agents that do not disrupt the lipid bilayer.

Question 31

Define integral membrane proteins.

Answer 31

These proteins are anchored in the membrane. They can only be removed by disrupting the lipid bilayer with strong detergents or by physically rupturing the bilayer.

Question 32

What does scanning electron microscopy accomplish?

Answer 32

This technique reveals the cell surface and membrane-associated proteins by shooting electrons onto the specimen.

Question 33

What does freeze fracture TEM reveal and how does it work?

Answer 33

It reveals the inside of a membrane and the proteins found there. It works by freezing a cell and fracturing it along a point of weakness (which is usually the inside of the lipid bilayer since the phospholipids are only held together by the hydrophobic effect).

Question 34

How do proteins embed within the cellular membrane?

Answer 34

They are embedded through the formation of a hydrophobic alpha helix. This helix surrounds a polar protein that needs to be embedded, shielding it from the hydrophobic interior of the cellular membrane. About 20 hydrophobic amino acids are needed to span the 3 nm membrane to form a transmembrane domain. Often, these multipass proteins contain multiple a-helices which are amphipathic. They associate to form a hydrophilic channel through the lipid bilayer.

Question 35

How do B-sheets interact to form a gateway into the cellular membrane for polypeptides?

Answer 35

Nonpolar amino acid side chains interact with lipids in the membrane; polar amino acid side chains form an aqueous pore through the membrane. This process creates such a large hole (using about 8-22 transmembrane segments of 8-10 amino acids each) that B-sheet transmembrane domains are rare in biology.