Chapter 11: Biological Membrane and Membrane Transport

Question 1

1. Which molecules make up nearly all the mass of biological membrane?
2. Each membrane type has… what does this imply?
3. How do these constituents vary?
4. How does the inner face of the membrane differ from the outer face?

Answer 1

1. Proteins, Polar Lipids, and Carbohydrates
2. Characteristic lipids, implies that there is regulatory controls on:
   
   1. Lipid synthesis
   2. Lipid transport
   3. Membrane assembly
   4. Vary based on climate but not on diet. Diet determines what is available, not how it’s used.
   5. Differ based on the distribution of membrane lipids.

Question 2

1. What is unique about membrane proteins?
2. When does membrane protein composition change?
3. In what ways do they associate with the membrane?
4. How thick is the membrane?
5. What is it permeable to?
6. What do Glycerophospholipids, sphingolipids and sterols form?
7. How do nonpolar and polar elements arrange themselves? What does this imply?

Answer 2

1. Content in/on membrane varies widely (varies with cell type).
2. Changes with conditions, such as diet.
3. Associate with membrane:
   
   1. Span both layers
   2. Only on one face
   3. Loosely associated with one face
4. 50-80 A (5-8 nm)
5. Semi-permeable, permeable to nonpolar compounds.
6. Form Liposomes (form a cell membrane).
7. Non-polar elements face each other internally. Polar head groups face outward. Implies asymmetry.

Question 3

1. What are the three biological membrane lipids?
2. What makes these lipids unique?
3. How do they form?
4. Micelle: What are their organization?
5. How do they form?

Answer 3

1. Glycerophospholipids, Sphingolipids, and Sterols
2. All are virtually insoluble in water.
3. Form spontaneously, the goal is to minimize contact with water. So, hydrophobic interactions drive the formation of the membrane.
4. Spherical, hydrophobic, acyl chains inside. Polar head groups outside.
5. Form easily from free fatty acids, lysophospholipids, and detergents.

Question 4

1. Bilayer: How are lipids arranged?
2. What do they form from?
3. Liposome: How is the structure arranged?
4. What make the liposome special?

Answer 4

1. Lipids are arranged as a sheet.
   
   1. Acyl chains apposed.
   2. Head groups outside and inside
   3. Bad because hydrophobic sides are exposed.
2. Forms from glycerophospholipids and sphingolipids.
3. A vesicle (sphere)
   
   1. Head groups in the center and on the outside.
   2. Inner head groups form a second, isolated aqueous environment.
   3. Closed surface that can interact all the way around when in aqueous solution.

Question 5

1. What does the degree of mobility of lipid in the membrane depend on?
2. If lipid composition does not change, then what happens?
   
   1. At low temperatures?
   2. At high temperatures?
   3. At intermediate temperatures?
3. What is phase change defined as?
   
   1. What lowers the transition temperature?
   2. What do sterols do ?

Answer 5

1. Depends on temperature and lipid composition.
2. No change in lipid composition:
   
   1. Lipids solidify into a Paracrystal
   2. Lipids in a fluid state (liquid disordered)
   3. Gel in most membranes (liquid ordered)
3. Phase Change: defined as transition temperature
   
   1. The more unsaturated fatty acids = lower transition temperature.
   2. Sterols change transition temperature,
      
      1. Adjust ordering of acyl chains.
      2. Promote liquid-ordered state.

Question 6

Three levels of membrane mobility

1. What is first level?
2. What is second level?
3. What is catalyzed movement?
   
   1. What does Flipases do?
   2. What does Floppases do?
   3. What do Scamblase do?

Answer 6

1. Lipids move very slowly from one leaflet to the other (A).
2. Lipid acyl chains move freely and rapidly through the plane of the membrane (B).
3. Catalyzed movement from one side of bilayer to the other (C).
   
   1. Moves lipid from outer to cytosolic leaflet via ATPases.
   2. Moves lipid from cytosolic to outer leaflet via ATPases.
   3. Scamblase moves lipids in either direction toward equilibrium.

Question 7

1. Peripheral Proteins:
   
   1. How are they associated with the membrane?
   2. What are they regulators of?
2. Integral Proteins:
   
   1. What are they anchored to?
   2. How are they removed?
3. What are transmembrane proteins?
   
   1. How are they oriented?
   2. Where is carbohydrate located?
4. Type I-V?
   
   1. Type V
   2. Type VI
5. What is an example of Peripheral Proteins?
   
   1. How does it function?
   2. Where is it found?
   3. What is its function?

Answer 7

1. Peripheral Proteins:
   
   1. Via H-bonds and electrostatic interactions with integral proteins, or polar head groups of lipids.
   2. Limit integral protein mobility.
2. Integral Proteins:
   
   1. Firmly anchored to the membrane
   2. Removed by detergents or organic reagents. Must disrupt the membrane bilayer and hydrophobic interactions.
3. A subclass of integral proteins. Span the lipid bilayer. Usually asymmetric and most have polar and nonpolar regions.
   
   1. Oriented asymmetrically within the membrane. Amino terminus: Outside, Carboxy terminus: Inside
   2. Carbohydrate is always on the outside of the plasma membrane.
4. Type I: One polypeptide, Amine group outside. Type II: One polypeptide, Amine group inside. BOTH have one transmembrane domains. Type III and IV BOTH have multiple transmembrane domains.
   
   1. Type V: One acyl anchor, Peripheral, Inside
   2. Type VI: Two acyl anchors, Integral, Outside
5. Annexin Family would be an example of Peripheral Proteins
   
   1. Binds acidic membrane phospholipids in a Ca²⁺ dependent manner.
   2. Found on the inner surface of the plasma membrane, outer surfaces of intracellular vesicles.

Question 8

1. Some peripheral proteins have covalently attached….?
   
   1. What Type is this?
   2. What does this provide?
   3. Where are they founds?
2. What Type is Glycosylated Phosphatidylinositols (GPI)?
   
   1. Describe GPI

Answer 8

1. Lipids
   
   1. Type V membrane protein.
   2. Provides a weak membrane anchor
   3. Found on the inner surface of the plasma membrane.
2. Type VI, found on the outer surface.
   
   1. Not released by alkali, integral by definition, only on extracellular face.

Question 9

1. What is an Integrin?
   
   1. What type are they?
   2. What do they bind?
2. What are Cadherins and Selectins?
3. What are all these proteins involved in?

Answer 9

1. Involved in cell signaling, adhesion, and cell-cell interactions. Heterodimeric: 14 alphas, 8 beta.
   
   1. Type IV membrane protein
   2. Extracellular domain binds specific extracellular ligands. Many possible alpha/beta combinations mean many potential ligand binding sites.
2. Cadherins, Immunoglobulin-like proteins and Selectins function alike.
3. Adhesion

Question 10

1. What are Microdomains (Rafts)?
   
   1. What do they contain?
   2. What is the combind function of these proteins?
2. What are caveolae?
   
   1. What do they span?
   2. What are they bounds to?
   3. What do they function in?

Answer 10

1. Thicker regions of the membrane enriched in certain proteins and lipids.
   
   1. Contain Sphingolipids, Cholesterol, Lipid-linked proteins with 2 palmitoyl or myristoyl groups, and GPI-linked proteins.
   2. Function as a unit to change membrane shape.
2. Curved areads of the membrane.
   
   1. Span both leaflets.
   2. Caveolin in high concentration, bound to 3 fatty acyl chains per molecule.
   3. Functions in endocytosis.

Question 11

1. What does Membrane Fusion allow?
2. What do Membrane Channels allow?
3. What does Passive Transport facilitated by?
4. What is Simple Diffusion?
5. What is Membrane Potential?
6. What do both Simple Diffusion and Membrane Potential define?
7. What does a selectively permeable membrane plus an unequal distribution of charged solute result in?

Answer 11

1. Membrane Fusion allows larger things in and out.
2. Membrane Channels allow smaller things in and out.
3. Membrane Proteins
   
   1. Two compartments, Unequal solute concentrations, Separation of compartments by permeable barrier.
4. Movement of particles until an equilibrium is achieved. Conc 1 = Conc 2
5. An electrical gradient caused by an unequal charge distribution. Charged particles move until they achieve charge equilibrium.
6. Define Biomembrane Electrochemical Gradient
7. Results in unequal solute concentration and charge concentration between both sides of the membrane.

Question 12

Solute Transport

1. What must a solute do to get through a membrane?
2. What are the three classes of membrane transport systems?
3. What are two fundamental mechanisms for transportation?
4. What is passive transport?
5. What proteins assist in passive transport?
6. Give an example

Answer 12

1. Strip away solvation layer, move past hydrophobic molecules and re-solvate.
   
   1. Physical barrier must be overcome with energy.
   2. So much energy that membranes are virtually impenetrable to polar solutes.
2. Uniport, Symport, Antiport.
3. Passive and Active Transport
4. Passage of polar solutes made thermodynamically favorable. Membrane proteins decrease required activation energy. Passive cannot go against the gradient.
5. Transporters or Permeases
   
   1. Span the membrane, often multiple times.
   2. Bind with specificity and stereo-specificity
   3. Bind using weak interactions.
6. Glucose Transporter: Polar groups in the middle of the membrane so they can interact with the glucose to help transport it across the cellular membrane. H-bonds helps with this transport.

Question 13

Passive (Facilitated Diffusion)

1. Give an examle
2. How does this mechanism work?

Answer 13

1. Erythrocyte Anion Exchanger
2. Co-Transport, an example of an antiport. Cl^- moves into the cell while HCO₃^- moves out of the cell (respiring tissues). Or vice-versa (lungs). Does not change Vm.

Question 14

1. What is Active Transport?
2. When is it thermodynamically favored?
3. What does the picture show?
4. What does the movement of ions create?
   
   1. What transport is this?

Answer 14

1. Movement against the electrochemical gradient.
2. Thermodynamically favored only when it’s coupled an exergonic process (Absorption of light, Oxidation reaction, Breakdown of ATP)
3. Primary active transport creates an electrochemical gradient which facilitates the movement of S2 through the secondary active transport, against its gradient.
4. Movement creates an electrical potential (Eukaryotic membranes have a potential of 0.05-2.0 V)
   
   1. This is electrogenic transport

Question 15

1. What are Ion-selective channels?
2. What are they coupled to?
3. How are they different from Ion transporters?
4. What is Ligand Gated?
   
   1. What is an example?
5. What is Voltage Gated?
   
   1. What is an example?

Answer 15

1. Move certain ions only.
2. Often coupled to rapid changes in membrane electrical potentials.
3. Move several orders of magnitude more molecules.
   
   1. Do not become saturated (rates are not maximal at very high substrate.
   2. Open and close in response to a cellular event.
4. Binding of molecules induces the opening of the channel.
   
   1. Acetylcholine Receptor: Bind is cooperative, allosteric effect causes twisting.
      
      1. Twisting is conformation changes and changes in weak interactions.
      2. Channels open
      3. Ion movement is very fast.
5. Protein conformation changes in respones to a threshold membrane potential that was achieved.