Deck 11: Biological Membranes and Transport

Question 1

Which enzymes are NOT involved in moving phospholipids from one leaflet to another?

A) flippases that move phosphatidylethanolamine and phosphatidylserine
B) floppases that move phospholipids from the cytosolic leaflet to the extracellular leaflet
C) flip-floppases that allow phospholipids to move back and forth between the inner and outer leaflets
D) scramblases that allow phospholipids to move down their concentration gradient
E) phosphatidylinositol transfer proteins that play a role in lipid signaling

Answer 1

C

Question 2

Which step is NOT involved in neurotransmitter release at a synapse?

A) creation of a fusion pore
B) hemifusion induced by zipping and lateral tension on the bilayers
C) v-SNARE binding to t-SNARE
D) pore widening and release of neurotransmitter
E) flippase-mediated movement of phosphatidylserine from the inner to the outer leaflet

Answer 2

E

Question 3

The inner (plasma) membrane of E. coli is about 75% lipid and 25% protein by weight. How many molecules of membrane lipid are there for each molecule of protein? (Assume that the average protein is M_r 50,000 and the average lipid is 750.)

A) 1
B) 50
C) 200
D) 10,000
E) 50,000

Answer 3

C

Question 4

Peripheral membrane proteins:

A) are generally noncovalently bound to membrane lipids.
B) are usually denatured when released from membranes.
C) can be released from membranes only by treatment with detergent(s).
D) may have functional units on both sides of the membrane.
E) penetrate deeply into the lipid bilayer.

Question 5

Which statement about caveolin is FALSE?

A) Caveolin is an integral membrane protein.
B) Caveolin induces outward membrane curvature.
C) Caveolin is palmitoylated.
D) Caveolin associates with cholesterol rich regions.
E) Caveolin is involved in membrane trafficking and cellular signaling.

Question 6

A process NOT involving the fusion of two membranes or two regions of the same membrane is:

A) endocytosis.
B) entry of enveloped viruses into cells.
C) entry of glucose into cells.
D) exocytosis.
E) reproductive budding in yeast.

Question 7

Membrane proteins:

A) are sometimes covalently attached to lipid moieties.
B) are sometimes covalently attached to carbohydrate moieties.
C) are composed of the same 20 amino acids found in soluble proteins.
D) diffuse laterally in the membrane unless they are anchored
E) All of the answers are correct.

Question 8

When a bacterium such as E. coli is shifted from a warmer growth temperature to a cooler growth temperature, it compensates by:

A) increasing its metabolic rate to generate more heat.
B) putting longer-chain fatty acids into its membranes.
C) putting more unsaturated fatty acids into its membranes.
D) shifting from aerobic to anaerobic metabolism.
E) synthesizing thicker membranes to insulate the cell.

Question 9

The shortest $\alpha$ helix segment in a protein that will span a membrane bilayer has about _____ amino acid residues.

A) 5
B) 20
C) 50
D) 100
E) 200

Question 10

Which statement about the composition of biological membranes is FALSE?

A) In a given eukaryotic cell type (e.g., a hepatocyte), all intracellular membranes have essentially the same complement of lipids and proteins.
B) The carbohydrate found in membranes is virtually all part of either glycolipids or glycoproteins.
C) The plasma membranes of the cells of vertebrate animals contain more cholesterol than the mitochondrial membranes.
D) The ratio of lipid to protein varies widely among cell types in a single organism.
E) Triacylglycerols are not commonly found in membranes.

Question 11

A hydropathy plot is used to:

A) determine the water-solubility of a protein.
B) deduce the quaternary structure of a membrane protein.
C) determine the water content of a native protein.
D) extrapolate for the true molecular weight of a membrane protein.
E) predict whether a given protein sequence contains membrane-spanning segments.

Question 12

The fluidity of a lipid bilayer will be increased by:

A) decreasing the number of unsaturated fatty acids.
B) decreasing the temperature.
C) increasing the length of the alkyl chains.
D) increasing the temperature.
E) substituting 18:0 (stearic acid) in place of 18:2 (linoleic acid).

Question 13

The fluidity of the lipid side chains in the interior of a bilayer is generally increased by:

A) a decrease in temperature.
B) an increase in fatty acyl chain length.
C) an increase in the number of double bonds in fatty acids.
D) an increase in the percentage of phosphatidyl ethanolamine
E) the binding of water to the fatty acyl side chains.

Question 14

Which statement describes a general feature of the lipid bilayer in all biological membranes?

A) Individual lipid molecules are free to diffuse laterally in the surface of the bilayer.
B) Individual lipid molecules in one face (monolayer) of the bilayer readily diffuse (flip-flop) to the other monolayer.
C) Polar, but uncharged, compounds readily diffuse across the bilayer.
D) The bilayer is stabilized by covalent bonds between neighboring phospholipid molecules.
E) The polar head groups face inward toward the inside of the bilayer.

Question 15

Which statement about the composition of membranes is TRUE?

A) All biological membranes contain cholesterol.
B) Free fatty acids are major components of all membranes.
C) The inner and outer membranes of mitochondria have different protein compositions.
D) The lipid composition of all membranes of eukaryotic cells is essentially the same.
E) The lipid:protein ratio varies from about 1:4 to 4:1.

Question 16

Integrins are:

A) membrane proteins that are involved in ion transport.
B) membrane proteins that are involved in sugar transport.
C) membrane proteins that mediate cell adhesion.
D) proteins of the extracellular matrix that bind to cell surface proteins.
E) proteins that are found at the membrane-cytoplasm interface.

Question 17

An integral membrane protein can be extracted with:

A) a buffer of alkaline or acid pH.
B) a chelating agent that removes divalent cations.
C) a solution containing detergent.
D) a solution of high ionic strength.
E) hot water.

Question 18

Which statement about membranes is TRUE?

A) Most plasma membranes contain more than 70% proteins.
B) Sterol lipids are common in bacterial plasma membranes.
C) Sterol lipids are common in human cell plasma membranes.
D) Sterol lipids are common in plant cell plasma membranes.
E) The plasma membranes of all cell types within a particular organism have basically the same lipid and protein composition.

Question 19

Which statement is generally TRUE of integral membrane proteins?

A) The secondary structure in the transmembrane region consists solely of $\alpha$ helices or $\beta$ sheets.
B) The domains that protrude on the cytoplasmic face of the plasma membrane nearly always have covalently attached oligosaccharides.
C) They are unusually susceptible to degradation by trypsin.
D) They can be removed from the membrane with high salt or mild denaturing agents.
E) They undergo constant rotational motion that moves a given domain from the outer face of a membrane to the inner face and then back to the outer.

Question 20

Membrane fusion leading to neurotransmitter release requires the action of:

A) cadherins.
B) selectins.
C) flipases.
D) tSNARE and vSNARE.
E) None of the answers is correct.

Question 21

Facilitated diffusion through a biological membrane is:

A) driven by a difference of solute concentration.
B) driven by ATP.
C) endergonic.
D) generally irreversible.
E) not specific with respect to the substrate

Question 22

Glucose transport into erythrocytes is an example of:

A) active transport.
B) antiport.
C) electrogenic uniport
D) facilitated diffusion.
E) symport.

Question 23

An electrogenic Na⁺ transporter:

A) catalyzes facilitated diffusion of Na⁺ from a region of high Na⁺ concentration to one of lower Na⁺ concentration.
B) must catalyze an electron transfer (oxidation-reduction) reaction simultaneously with Na⁺ transport.
C) must transport both Na⁺ and a counterion (Cl^-, for example).
D) transports Na⁺ against its concentration gradient.
E) transports Na⁺ without concurrent transport of any other charged species.

Question 24

The type of membrane transport that uses ion gradients as the energy source is:

A) facilitated diffusion
B) passive transport.
C) primary active transport.
D) secondary active transport.
E) simple diffusion.

Question 25

A ligand-gated ion channel (such as the nicotinic acetylcholine receptor) is:

A) a charged lipid in the membrane bilayer that allows ions to pass through.
B) a membrane protein that permits a ligand to pass through the membrane only when opened by the appropriate ion.
C) a membrane protein that permits an ion to pass through the membrane only when opened by the appropriate ligand.
D) a molecule that binds to the membrane thereby allowing ions to pass through.
E) always requires a second ligand to close the channel once it is opened.

Question 26

Movement of water across membranes is facilitated by proteins called:

A) annexins.
B) aquaporins.
C) hydropermeases.
D) selectins.
E) transportins.

Question 27

Which statement is TRUE regarding lipid anchors?

A) Palmitoylations and prenylations both occur via thioester linkages.
B) GPI anchors and prenylations are both attached to the C-terminal end of the mature protein.
C) Myristoylations and palmitoylations are both reversible modifications.
D) Prenylations occur exclusively on the outside leaflet of the plasma membrane.

Question 28

In one catalytic cycle, the Na⁺/K⁺ ATPase transporter transports:

A) 2 Na⁺ out, 3 K⁺ in, and converts 1 ATP to ADP + P_i.
B) 3 Na⁺ out, 2 K⁺ in, and converts 1 ATP to ADP + P_i.
C) 3 Na⁺ in, 2 K⁺ out, and converts 1 ATP to ADP + P_i.
D) 1 Na⁺ out, 1 K⁺ in, and converts 1 ATP to ADP + P_i.
E) 2 Na⁺ out, 3 K⁺ in, and converts 1 ADP + P_i to ATP.

Question 29

The specificity of the potassium channel for K⁺ over Na⁺ is mainly the result of the:

A) differential interaction with the selectivity filter protein.
B) hydrophobicity of the channel.
C) phospholipid composition of the channel.
D) presence of carbohydrates in the channel.
E) presence of cholesterol in the channel.

Question 30

Which statement does NOT describe a characteristic of the fluid mosaic model of membranes?

A) Hydrophobic interactions predominate between lipids in the membrane.
B) Proteins and lipids are able to move laterally at different rates.
C) Protein structures are able to move from one leaflet to another.
D) Noncovalent forces predominate in intermolecular interactions.

Question 31

Integral membrane proteins tend to be enriched for regular secondary structures in the transmembrane portions of their structures. Which statement explains this observation?

A) Regular secondary structures allow for polar amino acids to interact with lipid head groups.
B) Regular secondary structures allow the polypeptide to interact with the membrane core.
C) Regular secondary structures are energetically favorable for polypeptides.
D) Regular secondary structures satisfy hydrogen-bonding potential of the backbone in a hydrophobic environment.

Question 32

For the process of solute transport, the constant K_t is:

A) analogous to K_a for ionization of a weak acid.
B) analogous to K_m for an enzyme-catalyzed reaction.
C) analogous to V_max for an enzyme reaction
D) proportional to the number of molecules of glucose transporter per cell.
E) the maximum rate of glucose transport.

Question 33

Which statement does NOT describe a process by which the specificity of aquaporins is ensured?

A) The channel contains a negatively charged Asp residue to scavenge protons and H₃O⁺.
B) The diameter of the channel narrows to 2.8 Å.
C) There are carbonyl backbone residues in the channel that hydrogen bond with water.
D) Arg and His residues in the channel repel protons and H₃O⁺.
E) Electric dipoles of short $\alpha$ helices repel protons and H₃O⁺.

Question 34

Which statement is NOT true about the chloride-bicarbonate exchanger (AE)?

A) The AE protein increases the rate of bicarbonate transport across the membrane.
B) The AE protein uses ATP as an energy source to drive bicarbonate transport.
C) The AE protein transports chloride ions across the membrane.
D) The AE protein is classified as an anti-porter.
E) The AE protein spans the membrane at least 12 times.

Question 35

Syndecans are proteoglycans that have glycosaminoglycans attached to the N-terminal domain, a single transmembrane helix and a C-terminal domain that can be phosphorylated. What type of membrane proteins are these?

A) Type I
B) Type II
C) Type III
D) Type IV
E) Type V

Question 36

Choline-containing lipids are enriched in the outer leaflet of erythrocyte plasma membranes. This includes:

A) cardiolipin.
B) phosphatidylinositol.
C) sphingomyelin.
D) phosphatidylcholine.
E) both sphingomyelin and phosphatidylcholine.

Question 37

Consider the transport of glucose into an erythrocyte by facilitated diffusion. What is the free-energy change for glucose uptake into the cell when the glucose concentrations are 5 mM on the outside and 0.1 mM on the inside? (These values may be of use to you: R = 8.315 J/mol · K; T = 298 K;(Faraday constant) = 96,480 J/V; N = 6.022 × 10²³/mol.)

A) less than 2 kJ/mol
B) about 10 kJ/mol
C) about 30 kJ/mol
D) about -30 kJoule/mol
E) It is impossible to calculate without knowledge of the membrane potential.

Question 38

Consider the transport of K⁺ from the blood (where its concentration is about 4 mM) into an erythrocyte that contains 150 mM K⁺. The transmembrane potential is about 60 mV, inside negative relative to outside. What is the free-energy change for this transport process? (These values may be of use to you: R = 8.315 J/mol ^. K; T = 298 K; (Faraday constant) = 96,480 J/V; N = 6.022 × 10²³/mol.)

A) about 5 J/mol
B) about 15 J/mol
C) about 5 kJ/mol
D) about 15 kJ/mol
E) It is impossible to calculate with the information given.

Question 39

Which statement about facilitated diffusion across a membrane is TRUE?

A) A specific membrane protein lowers the activation energy for movement of the solute through the membrane.
B) It can increase the size of a transmembrane concentration gradient of the diffusing solute.
C) It is impeded by the solubility of the transported solute in the nonpolar interior of the lipid bilayer.
D) It is responsible for the transport of gases such as O₂, N₂, and CH₄ across biological membranes.
E) The rate is not saturable by the transported substrate.

Question 40

ABC transporters are NOT known to facilitate moving:

A) cancer drugs out of cancer cells.
B) antibiotics out of bacteria.
C) membrane lipids from the inner leaflet to the outer leaflet.
D) chloride ions in the lung.
E) vitamin E into lipocytes.

Question 41

FRAP experiments are generally used in membrane studies to examine:

A) transverse diffusion rates.
B) active transport rates.
C) lateral diffusion rates.
D) both transverse diffusion rates and active transport rates.
E) both transverse diffusion rates and lateral diffusion rates.

Question 42

Bacterial cells change the fluidity of their lipid membranes to adapt to growth at lower temperatures by increasing the synthesis of:

A) cholesterol.
B) long-chain fatty acids and increasing the saturation of their membrane lipids.
C) both long- and short-chain fatty acids.
D) long-chain fatty acids and decreasing the saturation of their membrane lipids.
E) short-chain fatty acids and decreasing the saturation of their membrane lipids.

Question 43

Membrane lipids in tissue samples obtained from different parts of the leg of an arctic reindeer have different fatty acid compositions. Logically, what difference would you predict?

A) Membrane lipids from tissue near the hooves (feet) contain a greater proportion of unsaturated fatty acids relative to that in the upper leg.
B) Membrane lipids from tissue in the upper leg contain a greater proportion of unsaturated fatty acids relative to that in the hooves (feet).
C) Both answers A and B are equally likely.
D) Neither answer A nor B is likely.

Question 44

In humans, the fatty acid composition of membrane lipids in skin cells is different from the fatty acid composition of membrane lipids in the internal organs because the internal organs function at a slightly higher temperature. Which difference in fatty acid composition is MOST likely to be observed?

A) Skin cells show lower levels of 18:0 relative to 18:1 AND higher levels of 16:0 relative to 18:0.
B) Skin cells show higher levels of 20:0 relative to 18:0 AND higher levels of 18:2 relative to 18:0.
C) Skin cells show lower levels of 20:0 relative to 18:0 AND lower levels of 18:0 relative to 18:2.
D) Skin cells show higher levels of 18:2 relative to 18:0 AND lower levels of 16:0 relative to 18:0.
E) Both answers A and C are true.

Question 45

Which process represents the HIGHEST energy state in membrane fusion?

A) formation of coiled-coils in SNARE proteins
B) formation of the fusion pore
C) the hemifusion state
D) the expansion of the fusion pore
E) lipid mixing in both outer and inner leaflets

Question 46

Why are hydropathy plots NOT usually used to detect transmembrane $\beta$ strands in porins?

A) The length of a transmembrane $\beta$ strand is typically shorter than that of an $\beta$ helix.
B) Amino acids in transmembrane $\beta$ strands in porins may not be exclusively hydrophobic.
C) Amino acid side chains project away from the polypeptide in a $\beta$ sheet
D) Answers A and B are both correct.
E) All of the answers are correct.

Question 47

What kind of bond can link a myristoyl anchor to a polypeptide?

A) an ester bond to an internal serine
B) an amide bond to a glycine
C) a thioester to a cysteine
D) an ester bond to the C-terminus
E) a thioether bond to a cysteine side chain

Question 48

What is meant by the term "positive-inside rule"?

A) The typical transmembrane potential for plasma membranes is positive on the inside.
B) Integral plasma membrane proteins commonly have positively charge amino acids on the cytosolic side.
C) Concentrations of cations are usually higher inside than outside of cells.
D) All of the answers are correct.
E) None of the answers is correct.

Question 49

What is the meaning of the term "annular lipid"?

A) lipids that tend to form micelle structures
B) lipids that are formed from the degradation of phospholipids
C) lipids that are prosthetic groups in lipid-anchored proteins
D) lipids that are found in close association with integral membrane proteins.
E) None of the answers is correct.

Question 50

Which transporter will move lipids from the cytosolic leaflet to the outer leaflet of a plasma membrane?

A) a flippase
B) a floppase
C) a scramblase
D) both a flippase and a floppase
E) both a floppase and a scramblase

Question 51

What is the CORRECT order of steps in vesicle fusion to a cell?

A) outer leaflet lipid mixing, content release, inner leaflet lipid mixing
B) outer leaflet lipid mixing, inner leaflet lipid mixing, content release
C) inner leaflet lipid mixing, content release, outer leaflet lipid mixing
D) inner-leaflet lipid mixing, outer leaflet lipid mixing, content release
E) content release, inner leaflet lipid mixing, outer leaflet lipid mixing

Question 52

Which bond or functional group is NOT found in a GPI anchor?

A) a phosphodiester bond
B) a glycosidic bond
C) a hydroxyl group
D) an ester bond
E) a thioester bond

Question 53

Which compound(s) is/are NOT enriched in lipid rafts?

A) sphingolipids
B) cholesterol
C) GPI-linked proteins
D) farnesylated proteins
E) All of these molecules are enriched in lipid rafts.

Question 54

What kind of bond can link a prenyl anchor to a polypeptide?

A) an ester bond to an internal serine
B) an amide bond to a glycine
C) a thioester to a cysteine
D) an ester bond to the C-terminus
E) a thioether bond to a cysteine side chain

Question 55

P-type ATPase transporters such as the SERCA pumps can be classified as enzymes. As what enzyme class would they be classified?

A) oxidoreductases
B) transferases
C) hydrolases
D) lyases
E) ligases

Question 56

Which phrase BEST describes the term "amphitropic"?

A) cytosolic lipid-linked proteins
B) GPI-linked proteins
C) proteins that are reversibly membrane-associated or cytosolic
D) proteins that are either integral or lipid-linked

Question 57

As the temperature for a membrane lipid preparation is raised, what would be the order of states that would be adopted?

A) liquid-ordered, gel phase, liquid-disordered
B) liquid-disordered, liquid-ordered, gel phase
C) liquid-disordered, gel phase, liquid-ordered
D) gel phase, liquid-disordered, liquid-ordered
E) gel phase, liquid-ordered, liquid-disordered

Question 58

Which transporters can be classified as active transporters?

A) flippases
B) floppases
C) scramblases
D) both flippases and floppases
E) both flippases and scramblases

Question 59

What kind of bond can link a palmitoyl anchor to a polypeptide?

A) an ester bond to an internal serine
B) an amide bond to a glycine
C) a thioester to a cysteine
D) an ester bond to the C-terminus.
E) both an ester bond to an internal serine and a thioester to a cysteine

Question 60

Which transporter will move lipids from the outer leaflet to the cytosolic leaflet of a plasma membrane?

A) a flippase
B) a floppase
C) a scramblase
D) both a flippase and a floppase
E) both a flippase and a scramblase

Question 61

Which structure describes an integral membrane protein containing a $\beta$ barrel?

A) maltoporin
B) aquaporin
C) E. coli OmpF
D) both maltoporin and E. coli OmpF
E) both maltoporin and aquaporin

Question 62

Which factor MOST significantly determines the direction in which an ion moves through an ion channel in a membrane?

A) the electrochemical gradient across the membrane
B) the size and shape of the channel
C) the ion selectivity of the channel.
D) both the electrochemical gradient across the membrane and the size and shape of the channel
E) both the size and shape of the channel and the ion selectivity of the channel

Question 63

Which statement is TRUE regarding the SERCA pump?

A) The E1 conformation has a higher K_d value for Ca²⁺ than the E2 conformation.
B) The P domain contains a high-energy acyl phosphate structure when it is phosphorylated.
C) Three calcium ions are moved in each cycle of the transporter.
D) This protein can be classified as an antiport.
E) None of the statements is true.

Question 64

Which statement is TRUE for passive transport across a biological membrane?

A) Passive transport is driven by a solute electrochemical gradient.
B) Passive transport is driven by ATP.
C) Passive transport is irreversible.
D) Passive transport is driven by both a solute electrochemical gradient ATP.
E) Passive transport is driven by a solute electrochemical gradient and is irreversible.

Question 65

What is an amphipathic compound? Explain how such compounds contribute to the structure of biological membranes.

Question 66

(a) When relatively high concentrations of fatty acids are suspended in water, they form structures known as _____. (b) When relatively high concentrations of membrane phospholipids are dissolved in water, they form structures known as _____. (c) Why are the structures listed in your answers to (a) and (b) above energetically favored?

Question 67

(a) Explain why phosphoglycerides are capable of spontaneously assembling into the bilayer structure found in biological membranes but triacylglycerols are not. (b) What are the forces that drive bilayer formation?

Question 68

Which process does NOT require a protein facilitator?

A) simple diffusion
B) ionophore-mediated diffusion
C) a primary active uniport
D) a passive antiporter
E) both simple diffusion and ionophore-mediated diffusion

Question 69

What prevents proton translocation through aquaporins?

A) Asp and Glu residues bind to protons to prevent their translocation.
B) Carbonyl groups in the selectivity filter interact with water molecules.
C) Two side chain amide groups act as hydrogen bond donors to a single water molecule.
D) All of the answers are correct.
E) None of the answers is correct.

Question 70

You have discovered a protein that transports Ca²⁺ up a concentration gradient, from the cytoplasm into the endoplasmic reticulum. No other ions move during this transport. What type of transport protein does this appear to be?

A) a Ca²⁺ channel
B) a Ca²⁺ ion porin
C) a primary active uniporter
D) a secondary active uniporter
E) an ionophore

Question 71

Which statement does NOT describe a characteristic of valinomycin?

A) This can act as an antibiotic.
B) This molecule is a small protein.
C) This compound preferentially transports potassium ions over sodium ions.
D) This compound has carbonyl groups that form coordination bonds to ions.

Question 72

GLUT1 has a K_t of 1.5 mM for glucose and a K_t of 30 mM for galactose. What concentration of galactose would be required to reach the same rate of import as 5mM glucose?

A) 3 mM
B) 10 mM
C) 15 mM
D) 30 mM
E) 100 mM

Question 73

(a) List three different major components of eukaryotic membranes. (b) When a preparation of mitochondrial membranes was treated with high salt (0.5 M NaCl), it was observed that 40% of the total protein in this preparation was solubilized. What kind of membrane proteins are in this soluble extract, and what forces normally hold them to the membrane? (c) What kind of proteins constitute the insoluble 60%, and what forces hold these proteins in the membrane?

Question 74

Which statement is FALSE regarding the major facilitator superfamily of transporters?

A) These proteins are all secondary active transporters.
B) Both GLUT1 and lactose permease belong to this family.
C) These proteins contain two domains within the membrane.
D) These proteins contain 12 or 14 transmembrane $\alpha$ helices

Question 75

The plasma membrane of an animal cell consists of 45% by weight of phospholipid and 55% protein. What is the mole ratio (moles of lipid/moles of protein) if the average molecular weight of phospholipids is 750 and the average molecular weight of membrane proteins is 50,000?

Question 76

If a monovalent cation is being imported into a cell in the absence of a concentration gradient (i.e., the concentration for the transported cation is equal on both sides of the membrane), what could you predict about $\Delta$ G_t? Assume the transmembrane potential is 50mV (inside negative).

A) The value of $\Delta$ G_t will have a positive sign.
B) The value of $\Delta$ G_t will have a negative sign.
C) The value of $\Delta$ G_t will likely be close to zero.
D) It is impossible to predict anything about the value of $\Delta$ G_t.

Question 77

Which statement is TRUE concerning the chloride-bicarbonate exchanger?

A) This transporter is electrogenic.
B) The exchanger is a passive antiporter.
C) This protein is involved in carbon dioxide transport in the blood stream.
D) The exchanger is a passive antiporter, and this protein is involved in carbon dioxide transport in the blood stream.
E) This transporter is electrogenic, and the exchanger is a passive antiporter.

Question 78

What are the principle features of the fluid mosaic model of membranes?

Question 79

Which factor contributes to the selectivity pore of the potassium channel from S. lividans?

A) backbone carbonyl groups in irregular secondary structures
B) negatively charged amino acids near the channel opening
C) the N-terminal ends of 4 $\alpha$ helices
D) both backbone carbonyl groups in irregular secondary structures and negatively charged amino acids near the channel opening
E) both backbone carbonyl groups in irregular secondary structures and the N-terminal ends of 4 $\alpha$ helices

Question 80

Describe the structure of a biological membrane as proposed by the fluid mosaic model. Indicate the positions and orientations of phospholipids, cholesterol, integral and peripheral membrane proteins, and the carbohydrate moieties of glycoproteins and glycolipids.