Practice Quiz 3 Chapters 10, 11, 13, 14, 16 Flashcards
(31 cards)
What are the principal features of the fluid mosaic model of membranes?
- lipid bilayer: polar heads on the aqueous side; nonpolar fatty acid chains in the middle
- fluid: lipids are free to move laterally but not across the bilayer
- integral membrane proteins: span the bilayer, associating with lipid acyl chains by hydrophobic interactions and exhibiting lateral mobility
- 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
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
c
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
e
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
a
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
c
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
e
Which of the following reactions in glycolysis produces ATP as a product?
phosphoglycerate kinase
pyruvate kinase
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:
pyruvate → lactate
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)
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
Which combination of cofactors is involved in the conversion of pyruvate to acetyl-CoA?
TPP, lipoic acid, and NAD+
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
a
What is gluconeogenesis and what useful purposes does it serve in people?
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
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?
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
What is the difference between ∆G and ∆G’º of a chemical reaction?
Describe quantitatively the relationship between them.
∆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.
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?
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.
How is the definition of “lipid” different from the types of definitions used for other biomolecules such as amino acids and nucleic acids?
“Lipid” does not specify a particular chemical structure. Compounds are categorized as lipids based on their greater solubility in organic solvents than in water.
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?
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)
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?
It increases the melting point of the oil triglyceride because we reduced the double bonds (more lipid packing)
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?
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
What would you expect on the rate of diffusion if an experiment was conducted at 37 ºC and then at 10 ºC? Why?
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.
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?
Hydrophobic property: interactions among membrane lipids are due to the hydrophobic effect, which is noncovalent and reversible, allowing membranes to spontaneously reseal.
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?
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.
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?
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.
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.
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.
