Practice Quiz 3 Chapters 10, 11, 13, 14, 16

Question 1

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

Answer 1

1. lipid bilayer: polar heads on the aqueous side; nonpolar fatty acid chains in the middle
2. fluid: lipids are free to move laterally but not across the bilayer
3. integral membrane proteins: span the bilayer, associating with lipid acyl chains by hydrophobic interactions and exhibiting lateral mobility
4. peripheral membrane proteins: associate noncovalently with the lipid head groups and protruding domains of integral membrane proteins; sometimes tethered to the membrane by a covalent lipid anchor

Question 2

Which one of the following statements 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

Answer 2

c

Question 3

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) have all of the properties listed above

Answer 3

e

Question 4

Which of these statements 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

Answer 4

a

Question 5

For the reaction A → B, ∆G’º=–60 kJ/mol. The reaction is started with 10 mmol of A; no B is initially present. After 24 hours, analysis reveals the presence of 2 mmol of B, 8 mmol of A. Which is the most likely explanation?

a) A and B have reached equilibrium concentrations
b) an enzyme has shifted the equilibrium toward A
c) B formation is kinetically slow; equilibrium has not been reached by 24 hours
d) formation of B is thermodynamically unfavorable
e) the result described is impossible, given the fact that ∆G’º is –60 kJ/mol

Answer 5

c

Question 6

For the following reaction, ∆G’º = +29.7 kJ/mol.

The reaction as written:

a) can never occur in a cell
b) can occur in a cell only if it is coupled to another reaction for which ∆G’º is positive
c) can occur only in a cell in which NADH is converted to NAD+ by electron transport
d) cannot occur because of its large activation energy
e) may occur in cells at some concentrations of substrate and product

Answer 6

e

Question 7

Which of the following reactions in glycolysis produces ATP as a product?

Answer 7

phosphoglycerate kinase

pyruvate kinase

Question 8

During strenuous exercise, the NADH formed in the glyceraldehyde 3-phosphate dehydrogenase reaction in skeletal muscle must be reoxidized to NAD+ if glycolysis is to continue. The most important reaction involved in the reoxidation of NADH is:

Answer 8

pyruvate → lactate

Question 9

Which one of the following statements about gluconeogenesis is false?

a) for starting materials, it can use carbon skeletons derived from certain amino acids
b) it consists entirely of the reactions of glycolysis, operating in the reverse direction
c) it employs the enzyme glucose 6-phosphatase
d) it is one of the ways that mammals maintain normal blood glucose levels between meals
e) it requires metabolic energy (ATP or GTP)

Answer 9

b) it consists entirely of the reactions of glycolysis, operating in the reverse direction

to bypass irreversible steps in glycolysis, pyruvate carboxylase (pyruvate kinase), PEP carboxylase (pyruvate kinase), fructose 1,6-bisphosphatase-1 (phosphofructokinase-1), and glucose 6-phosphatase (hexokinase) are used

Question 10

Which combination of cofactors is involved in the conversion of pyruvate to acetyl-CoA?

Answer 10

TPP, lipoic acid, and NAD+

Question 11

Which of the following statements about the oxidative decarboxylation of pyruvate in aerobic conditions in animal cells is correct?

a) one of the products of the reactions of the pyruvate dehydrogenase complex is a thioester of acetate
b) the methyl (–CH3) group is eliminated as CO2
c) the process occurs in the cytosolic compartment of the cell
d) the pyruvate dehydrogenase complex uses all of the following as cofactors: NAD+, lipoic acid, pyridoxal phosphate (PLP), and FAD
e) the reaction is so important to energy production that pyruvate dehydrogenase operates at full speed under all conditions

Answer 11

a

Question 12

What is gluconeogenesis and what useful purposes does it serve in people?

Answer 12

Gluconeogenesis is the biosynthesis of glucose from noncarbohydrate precursors like oxaloacetate or pyruvate.

During fasting periods, when glycogen stores have been exhausted, gluconeogenesis provides glucose for metabolism in the brain and erythrocytes that derive their energy primarily from glucose metabolism

Question 13

Why are phosphoglycerides capable of spontaneously assembling into the bilayer structure found in biological membranes but triacylglycerols are not?

What are the forces that drive bilayer formation?

Answer 13

Triacylglycerols have three fatty acyl groups in ester linkage with glycerol; they are hydrophobic because the carboxyl groups of the fatty acid chains cannot ionize (since they are involved in ester linkages)

phosphoglycerides have a polar head group (serine, choline, ethanolamine, etc); the phosphate in a phosphodiester linkage also bears a negative charge. the amphipathic phospholipid forms a bilayer spontaneously in water

The formation of lipid bilayers is driven by an entropic force and stabilized by hydrophobic effect. A hydrophobic chain in water results in a solvation shell, immobilizing the water molecules around it and decreasing entropy. When several hydrophobic chains cluster, the nonpolar surface area exposed to water decreases; with fewer water molecules forming the shell and increasing entropy

Question 14

What is the difference between ∆G and ∆G’º of a chemical reaction?

Describe quantitatively the relationship between them.

Answer 14

∆G’º is a physical constant that reflects the difference in free energies of the product and reactants at standard biological conditions for each chemical reaction.

∆G is a variable that depends on ∆G’º, the temperature, and the concentrations of products and reactants.

Question 15

In the Sanger (dideoxy) method for DNA sequencing, a small amount of a dideoxynucleoside triphosphate – say ddCTP – is added to the sequencing reaction along with a larger amount of the corresponding dCTP.

What result would be observed if the dCTP were omitted?

Answer 15

If dCTP is omitted, when the first G residue is encountered in the template, ddCTP will be added, and polymerization will halt. Only one band will be seen in the sequencing gel.

Question 16

How is the definition of “lipid” different from the types of definitions used for other biomolecules such as amino acids and nucleic acids?

Answer 16

“Lipid” does not specify a particular chemical structure. Compounds are categorized as lipids based on their greater solubility in organic solvents than in water.

Question 17

The melting points of a series of 18-carbon fatty acids are:

stearic acid, 69.6 ºC; oleic acid, 13.4 ºC; linoleic acid, –5 ºC; linolenic acid, –11 ºC.

a) What structural aspect of these 18-carbon fatty acids can be correlated with the melting point?
b) What are all the possible triacylglycerols that can be constructed from glycerol, palmitic acid, and oleic acid. Rank them in order of increasing melting point.
c) Branched-chain fatty acids are found in some bacterial membrane lipids. Would their presence increase or decrease the fluidity of the membrane? Why?

Answer 17

a) The number of cis double bonds. Each cis double bond causes a bend in the hydrocarbon chain, lowering the melting temperature.
b) OOO < OPO = OOP < PPO = POP < PPP
c) branched-chain fatty acids increase the fluidity of membranes because they decrease the extent of membrane lipid packing. (lowering melting point)

Question 18

Catalytic hydrogenation, used in the food industry, converts double bonds in the fatty acids of oil triacylglycerols to –CH2–CH2–.

How does this affect the physical properties of the oils?

Answer 18

It increases the melting point of the oil triglyceride because we reduced the double bonds (more lipid packing)

Question 19

Lipid bilayers formed between two aqueous phases have this important property: they form two-dimensional sheets, the edges of which close on each other, and undergo self-sealing to form vesicles (liposomes).

a) What properties of lipids are responsible for this property of bilayers? Explain.
b) What are the consequences of this property for the structure of biological membranes?

Answer 19

a) amphipathic property (hydrophilic head and hydrophobic tail)

To minimize the hydrophobic area exposed to water, these lipids form two-dimensional sheets, with the hydrophilic regions outside and the hydrophobic regions buried inside. To avoid exposing the hydrophobic edges of the sheet to water, the bilayers close on themselves.

b) formation of organelles

Question 20

What would you expect on the rate of diffusion if an experiment was conducted at 37 ºC and then at 10 ºC? Why?

Answer 20

The rate of diffusion would decrease. Movement of individual lipids in bilayers occurs much faster at 37 ºC when the lipids are in the “fluid” phase. At 10 ºC, lipids are in the “solid” phase.

Question 21

Cellular membranes are self-sealing – if they are punctured or disrupted mechanically, they quickly and automatically reseal. What properties of membranes are responsible for this important feature?

Answer 21

Hydrophobic property: interactions among membrane lipids are due to the hydrophobic effect, which is noncovalent and reversible, allowing membranes to spontaneously reseal.

Question 22

Membrane lipids in tissue samples obtained from different parts of a reindeer’s leg have different fatty acid compositions. Membrane lipids from tissue near the hooves contain a larger proportion of unsaturated fatty acids than those from tissue in the upper leg. What is the significance of this observation?

Answer 22

The temperature of body tissues at the extremities is lower than that of tissues closer to the center of the body. If lipid is to remain fluid at this lower temperature, it must contain a higher proportion of unsaturated fatty acids, which lower the melting point of lipid mixtures.

Question 23

The inner leaflet (monolayer) of the human erythrocyte membrane consists predominately of phosphatidylethanolamine and phosphatidylserine. The outer leaflet consists predominantly of phosphatidylcholine and sphingomyelin. Although the phospholipid components of the membrane can diffuse in the fluid bilayer, this sidedness is preserved at all times. How?

Answer 23

The energetic cost of moving the highly polar (sometimes charged) head group through the hydrophobic interior is super high. Movement is prohibitive because it’s thermodynamically unfavorable.

Question 24

At pH 7, tryptophan crosses a lipid bilayer at about one-thousandth the rate of indole, a closely related compound:

Suggest an explanation for this observation.

Answer 24

At neutral pH, tryptophan bears a positive and negative charge, but indole is uncharged. The movement of the less polar indole through the hydrophobic core of the bilayer is energetically more favorable.

Question 25

Aldolase catalyzes the glycolytic reaction:

The standard free-energy change for this reaction in the direction written is +23.8 kJ/mol. The concentrations of the three intermediates in the hepatocyte of a mammal are: fructose 1,6-bisphosphate, 1.4 x 10^-4 M; glyceraldehyde 3-phosphate, 3 x 10^-6 M; and dihydroxyacetone phosphate, 1.6 x 10^-5 M.

At body temperature (37 ºC), what is the actual free-energy change for the reaction?

Answer 25

∆G = –8.6 kJ/mol

Question 26

During strenuous activity, the demand for ATP in muscle tissue is vastly increased. In rabbit leg muscle or turkey flight muscle, the ATP is produced almost exclusively by lactic acid fermentation. ATP is formed in the payoff phase of glycolysis by two reactions, promoted by phosphoglycerate kinase and pyruvate kinase.

Suppose skeletal muscle were devoid of lactate dehydrogenase. Could it carry out the strenuous physical activity; that is, could it generate ATP at a high rate by glycolysis? Explain.

Answer 26

NAD+ must be regenerated from NADH in order for glycolysis to continue.

some tissues, such as skeletal muscle, obtain almost all their ATP from glucose and are capable of short-term exercise only. In order to generate ATP at a high right, the NADH formed during glycolysis must be oxidized. In the absence of O2, lactate dehydrogenase converts pyruvate and NADH to lactate and NAD+.

In the absence of this enzyme, NAD+ could not be regenerated and glycolytic production of ATP would stop – and as a consequence, muscle activity could not be maintained.

Question 27

Adults engaged in the strenuous physical activity require an intake of about 160 g of carbohydrate daily but only about 20 mg of niacin for optimal nutrition.

Given the role of niacin in glycolysis, how do you explain the observation?

Answer 27

Dietary niacin is used to synthesize NAD+. NAD+ is reused as an electron carrier in glycolysis via the oxidation-reduction cycle (NAD+/NADH). Because of this cycling, one NAD+ can oxidize many molecules of glucose, and thus the dietary requirement for the precursor vitamin (niacin) is relatively small.

Question 28

What is the cost (in ATP equivalents) of transforming glucose to pyruvate via glycolysis and back again to glucose via gluconeogenesis?

Answer 28

glycolysis: consume 2 ATP and produce 4 ATP
gluconeogenesis: consumes 4 ATP and 2 GTP

so the overall energy cost is 4 ATP equivalents per glucose molecule.

Question 29

Why is it important that gluconeogenesis is not the exact reversal of glycolysis?

Answer 29

If gluconeogenesis were simply the reverse of the reactions in glycolysis, the process would be energetically unfeasible (highly endergonic) because of the 3 reactions with large, negative standard free-energy changes in the catabolic direction.

If the same enzymes were used for all reactions in the two pathways, it would be impossible to regulate the two processes separately. Anything that stimulated (or inhibited) the forward reaction for a given enzyme would stimulate (or inhibit) the reverse reaction to the same extent.

Question 30

Explain in bioenergetic terms how the conversion of pyruvate to phosphoenolpyruvate in gluconeogenesis overcomes the large, negative, standard free-energy change of the pyruvate kinase reaction in glycolysis.

Answer 30

In a stepwise fashion:

step 1) pyruvate carboxylase uses 2 ATP to convert 2 pyruvates to 2 oxaloacetate

step 2) PEP carboxykinase uses 2 GTP to convert oxaloacetate to phosphoenolpyruvate

by coupling the expenditure of two ATP equivalents to the conversion of pyruvate to PEP, the gluconeogenic process is made exergonic.

Question 31

Individuals with a thiamine-deficient diet have relatively high levels of pyruvate in their blood. Explain this in biochemical terms.

Answer 31

Thiamine is essential for the formation of TPP (one of the cofactors in the pyruvate dehydrogenase reaction). Without TPP, the enzyme activity reduces, and pyruvate generated by glycolysis accumulates in cells and enters the blood.