Cellular respiration mcq Flashcards
(106 cards)
1
Q
A
2
Q
Which enzyme catalyzes the first step of glycolysis?
A
Hexokinase
The first step of glycolysis is the phosphorylation of glucose to glucose-6-phosphate, an irreversible reaction catalyzed by hexokinases.
3
Q
What is the general term for the anaerobic breakdown of glucose to gain energy?
A
Fermentation
Fermentation refers to the anaerobic degradation of glucose (and other sugars) to generate ATP.
4
Q
Which regulatory enzyme’s activity increases when a cell’s ATP levels are low?
A
Phosphofructokinase-1 (PFK-1)
PFK-1 is the major rate-limiting enzyme of glycolysis.
5
Q
Fructose-1,6-bisphosphate is split (aldolase reaction) into which products?
A
One aldose and one ketose
Aldolase cleaves fructose-1,6-bisphosphate into two triose phosphates: glyceraldehyde-3-phosphate (an aldose) and dihydroxyacetone phosphate (a ketose).
6
Q
Dihydroxyacetone phosphate (DHAP) is rapidly converted into which glycolytic intermediate?
A
Glyceraldehyde-3-phosphate
DHAP is reversibly isomerized to glyceraldehyde-3-phosphate by the enzyme triose phosphate isomerase.
7
Q
What is the first step of the ‘payoff’ phase of glycolysis?
A
Oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate
This step is catalyzed by glyceraldehyde-3-phosphate dehydrogenase.
8
Q
Which substrate is used in the final step of glycolysis to produce pyruvate?
A
Phosphoenolpyruvate
In the last step of glycolysis, phosphoenolpyruvate transfers its high-energy phosphate to ADP, forming ATP and pyruvate.
9
Q
High concentrations of glucose-6-phosphate have an inhibitory effect on which glycolytic enzyme?
A
Hexokinase
Hexokinase is allosterically inhibited by its product G-6-P.
10
Q
The first substrate-level phosphorylation in glycolysis produces which molecule?
A
3-Phosphoglycerate
This occurs when 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate by phosphoglycerate kinase.
11
Q
Glycolysis is best described as the conversion of:
A
Glucose into pyruvate
Glycolysis splits one molecule of glucose into two molecules of pyruvate.
12
Q
How many net ATP molecules are gained by the cell per molecule of glucose through glycolysis?
A
2 ATP
Glycolysis produces 4 ATP per glucose, but 2 ATP were invested, leaving a net gain of 2 ATP.
13
Q
How many molecules of O₂ are required during glycolysis of one glucose molecule?
A
0
Glycolysis does not require molecular oxygen.
14
Q
In which part of the cell does glycolysis take place?
A
Cytoplasm (cytosol)
Glycolysis occurs in the cytosol of the cell.
15
Q
The glycolytic pathway is also known as the:
A
Embden–Meyerhof–Parnas (EMP) pathway
Glycolysis is often called the Embden–Meyerhof pathway after its discoverers.
16
Q
Which of the following represents the net products of glycolysis (per glucose molecule)?
A
2 pyruvate, 2 ATP, 2 NADH
Glycolysis yields two pyruvate molecules along with a net gain of two ATP and two NADH.
17
Q
What is the first committed (rate-limiting) step of glycolysis?
A
Fructose-6-phosphate → Fructose-1,6-bisphosphate (PFK-1)
This step is highly regulated by cellular energy status.
18
Q
During which step of glycolysis is NADH formed?
A
Glyceraldehyde-3-phosphate → 1,3-bisphosphoglycerate
This is the only oxidation step in glycolysis.
19
Q
Under anaerobic conditions in human muscle, pyruvate is primarily converted into:
A
Lactate
Muscle cells regenerate NAD^+ by reducing pyruvate to lactate.
20
Q
Which of the following compounds is a known inhibitor of the glycolytic pathway?
A
Iodoacetate
Iodoacetate reacts with glyceraldehyde-3-phosphate dehydrogenase, blocking the glycolytic step.
21
Q
How many enzymatic reactions comprise the glycolysis pathway?
A
10
Glycolysis consists of 10 sequential enzyme-catalyzed reactions.
22
Q
Which of the following processes can occur in the absence of oxygen?
A
Glycolysis
Glycolysis can function anaerobically.
23
Q
Which of these is not an intermediate of glycolysis?
A
Oxaloacetate
Oxaloacetate is an intermediate of the citric acid cycle, not glycolysis.
24
Q
How many ATP molecules are expended (used) during the preparatory phase of glycolysis per glucose?
A
2
Two ATP molecules are invested in the early steps of glycolysis.
25
Glucose is split into two three-carbon sugars at which step of glycolysis?
Cleavage of fructose-1,6-bisphosphate by aldolase
## Footnote
This cleavage occurs at the fourth step of glycolysis.
26
The end product of glycolysis (under aerobic conditions) is:
Pyruvate
## Footnote
Pyruvate is the final product of the glycolytic pathway when oxygen is available.
27
Where in the cell does the oxidation of pyruvate to acetyl-CoA occur?
Mitochondrial matrix
## Footnote
In eukaryotes, pyruvate produced by glycolysis is transported into the mitochondrial matrix.
28
The multi-enzyme pyruvate dehydrogenase complex catalyzes the conversion of pyruvate into:
Acetyl-CoA, CO₂, and NADH
## Footnote
The pyruvate dehydrogenase complex irreversibly converts pyruvate into acetyl-CoA.
29
What does the multi-enzyme pyruvate dehydrogenase complex catalyze the conversion of pyruvate into?
Acetyl-CoA, CO₂, and NADH
## Footnote
The pyruvate dehydrogenase complex irreversibly converts pyruvate (3C) into acetyl-CoA (2C unit attached to CoA) with the release of one CO₂ and the reduction of NAD⁺ to NADH.
30
The conversion of pyruvate to acetyl-CoA by the PDH complex is an example of what?
Oxidative decarboxylation
## Footnote
In the PDH reaction, pyruvate is oxidized and decarboxylated – one carbon is removed as CO₂ and the remaining two-carbon acetyl unit is attached to CoA.
31
Which enzyme is not a component of the pyruvate dehydrogenase complex?
Glyceraldehyde-3-phosphate dehydrogenase
## Footnote
The PDH complex consists of three core enzymes: E1 (pyruvate dehydrogenase), E2 (dihydrolipoyl transacetylase), and E3 (dihydrolipoyl dehydrogenase).
32
How many molecules of acetyl-CoA are produced from one molecule of glucose via pyruvate oxidation?
2
## Footnote
Each glucose (6C) yields two pyruvate molecules via glycolysis. Each pyruvate (3C) is converted into one acetyl-CoA (2C), so two acetyl-CoA molecules are produced per glucose.
33
Which cofactor or coenzyme is NOT required by the pyruvate dehydrogenase complex?
Biotin
## Footnote
PDH requires five cofactors: TPP (from vitamin B₁), lipoic acid, CoA (from pantothenic acid, B₅), FAD (from riboflavin, B₂), and NAD⁺ (from niacin, B₃).
34
What vitamin is pantothenic acid (vitamin B₅) a precursor of, in the context of pyruvate oxidation?
Coenzyme A
## Footnote
Coenzyme A (CoA) is derived from pantothenic acid (vitamin B₅) and serves as an acyl carrier.
35
Which statement about the pyruvate dehydrogenase (PDH) reaction is FALSE?
It is reversible under physiological conditions
## Footnote
The PDH reaction is essentially irreversible in cells.
36
High levels of which compounds directly inhibit the pyruvate dehydrogenase complex?
NADH and acetyl-CoA
## Footnote
PDH is feedback-inhibited by its products: NADH and acetyl-CoA.
37
Which of the following is not an allosteric inhibitor of the pyruvate dehydrogenase complex?
Citrate
## Footnote
ATP, NADH, and acetyl-CoA signal a high-energy state and inhibit PDH activity.
38
A deficiency of thiamine (vitamin B₁) would primarily impair which enzyme complex involved in glucose oxidation?
Pyruvate dehydrogenase complex
## Footnote
Thiamine pyrophosphate is an essential cofactor for the PDH complex.
39
During the pyruvate dehydrogenase reaction, which molecule is not produced?
ATP
## Footnote
The PDH complex produces acetyl-CoA, NADH, and CO₂ from pyruvate, CoA, and NAD⁺.
40
The pyruvate dehydrogenase complex requires multiple vitamin-derived cofactors. Which one of these coenzymes is derived from riboflavin (vitamin B₂)?
FAD (Flavin adenine dinucleotide)
## Footnote
FAD is synthesized from riboflavin and is used by the E3 component of PDH.
41
Which enzyme converts pyruvate to acetyl-CoA, connecting glycolysis to the citric acid cycle?
Pyruvate dehydrogenase
## Footnote
Pyruvate dehydrogenase catalyzes the oxidative decarboxylation of pyruvate to acetyl-CoA.
42
The reaction catalyzed by pyruvate dehydrogenase: Pyruvate + CoA + NAD⁺ → Acetyl-CoA + NADH + CO₂. Which of the following statements about this reaction is correct?
It is inhibited by a high ATP/ADP ratio
## Footnote
When energy charge is high, PDH is inhibited to prevent excess acetyl-CoA production.
43
Which metabolic intermediate accumulates in the blood when the pyruvate dehydrogenase complex is not functioning?
Pyruvate (and lactate)
## Footnote
If PDH is inactive, pyruvate from glycolysis cannot be oxidized to acetyl-CoA and instead accumulates.
44
The PDH complex shares a similar mechanism and cofactor requirements with which citric acid cycle enzyme?
α-Ketoglutarate dehydrogenase
## Footnote
α-Ketoglutarate dehydrogenase is very similar to PDH and uses similar cofactors.
45
What is the fate of the acetyl-CoA produced by the pyruvate dehydrogenase complex?
It enters the citric acid cycle to be oxidized for energy
## Footnote
Acetyl-CoA produced from pyruvate is the substrate for the Krebs cycle.
46
The enzyme pyruvate dehydrogenase is activated by which of the following?
Dephosphorylation by PDH phosphatase (high Ca²⁺, insulin)
## Footnote
PDH is regulated by reversible phosphorylation.
47
The reaction linking glycolysis to the Krebs cycle is often called the 'gateway' or 'bridge' reaction. It refers to what?
The oxidative decarboxylation of pyruvate to acetyl-CoA by PDH
## Footnote
This reaction is the gateway from anaerobic glycolysis to aerobic metabolism.
48
Which of the following is another name for the citric acid cycle?
Krebs cycle
## Footnote
The citric acid cycle is also known as the Krebs cycle or the tricarboxylic acid (TCA) cycle.
49
In the first step of the TCA cycle, acetyl-CoA (2C) combines with which 4-carbon molecule to form citrate (6C)?
Oxaloacetate
## Footnote
Oxaloacetate condenses with acetyl-CoA to form citrate in a reaction catalyzed by citrate synthase.
50
How many molecules of NADH are produced per acetyl-CoA oxidized in one turn of the citric acid cycle?
3
## Footnote
For each acetyl-CoA that enters the cycle, three NAD⁺ molecules are reduced to NADH.
51
Which enzyme of the citric acid cycle produces FADH₂?
Succinate dehydrogenase
## Footnote
Succinate dehydrogenase oxidizes succinate to fumarate, reducing FAD to FADH₂.
52
During one turn of the Krebs cycle, how many molecules of CO₂ are released and at what steps?
2 CO₂ – one at isocitrate dehydrogenase and one at α-ketoglutarate dehydrogenase
## Footnote
CO₂ is released during oxidative decarboxylation steps in the cycle.
53
What enzyme oxidizes succinate to fumarate in the citric acid cycle?
Succinate dehydrogenase
## Footnote
It is the only TCA enzyme that uses FAD and is embedded in the inner mitochondrial membrane.
54
During one turn of the Krebs cycle, how many molecules of CO₂ are released, and at what steps?
2 CO₂ – one at isocitrate dehydrogenase, one at α-ketoglutarate dehydrogenase
## Footnote
The cycle oxidizes the two-carbon acetyl group to two CO₂.
55
Which step of the citric acid cycle involves substrate-level phosphorylation?
Succinyl-CoA → Succinate (succinyl-CoA synthetase)
## Footnote
This step couples the reaction to phosphorylation of GDP to GTP.
56
Which enzyme of the TCA cycle is bound to the inner mitochondrial membrane?
Succinate dehydrogenase
## Footnote
It participates in both the TCA cycle and the electron transport chain.
57
Which citric acid cycle enzyme is not located in the mitochondrial matrix?
Succinate dehydrogenase
## Footnote
It is embedded in the inner membrane.
58
How many turns of the citric acid cycle occur per molecule of glucose oxidized?
2
## Footnote
Each glucose yields two acetyl-CoA from glycolysis.
59
Which citric acid cycle intermediate can be transaminated to form an amino acid?
α-Ketoglutarate → Glutamate
## Footnote
Oxaloacetate can also be transaminated to aspartate.
60
What is the rate-limiting enzyme of the citric acid cycle?
Isocitrate dehydrogenase
## Footnote
It is allosterically stimulated by ADP and Ca²⁺.
61
Malonate is a competitive inhibitor of which citric acid cycle enzyme?
Succinate dehydrogenase
## Footnote
It leads to an accumulation of succinate.
62
Which TCA cycle intermediate is a substrate for gluconeogenesis?
Oxaloacetate
## Footnote
It is converted to malate for transport to the cytosol.
63
Excess citrate from an active citric acid cycle can serve as a precursor for:
Fatty acid synthesis
## Footnote
Citrate is cleaved to acetyl-CoA and oxaloacetate in the cytosol.
64
What are the overall main purposes of the citric acid cycle?
To oxidize acetyl-CoA to CO₂ and generate NADH/FADH₂ for ATP production
## Footnote
It also provides intermediates for biosynthetic pathways.
65
The citric acid cycle is considered amphibolic because:
It functions in both the oxidative breakdown of molecules and the synthesis of biomolecules
## Footnote
It provides precursors for anabolic pathways.
66
Which citric acid cycle intermediate is directly involved in heme synthesis?
Succinyl-CoA
## Footnote
It combines with glycine to initiate heme biosynthesis.
67
What type of reaction does aconitase catalyze in the citric acid cycle?
An isomerization (citrate to isocitrate)
## Footnote
It involves a dehydration-rehydration mechanism.
68
If one acetyl-CoA enters the Krebs cycle, how many ATP (or GTP) are directly produced?
1
## Footnote
One GTP is produced, which can be converted to ATP.
69
Where in the cell do the reactions of the Krebs cycle occur in eukaryotes?
Mitochondrial matrix
## Footnote
Enzymes of the cycle are dissolved in the matrix.
70
On which membrane do the electron transport chain components reside in eukaryotic cells?
Inner mitochondrial membrane
## Footnote
This is crucial for establishing the proton gradient.
71
What is the final electron acceptor in the mitochondrial electron transport chain?
Oxygen (O₂)
## Footnote
It is reduced to H₂O at Complex IV.
72
Where do protons become concentrated during oxidative phosphorylation?
In the intermembrane space of mitochondria
## Footnote
Complexes I, III, and IV pump protons into this space.
73
Proton motive force (PMF) is the combination of which two gradients?
A pH (H⁺ concentration) gradient and an electrical (voltage) gradient
## Footnote
These drive protons back into the matrix.
74
Who proposed the chemiosmotic hypothesis for ATP generation in mitochondria?
Peter Mitchell
## Footnote
This theory was confirmed and earned him a Nobel Prize.
75
Which electron carrier transfers electrons from Complex I to Complex III?
Ubiquinone (coenzyme Q)
## Footnote
It accepts electrons from both Complex I and II.
76
Which complex of the electron transport chain does not pump protons?
Complex II
## Footnote
It feeds electrons to CoQ without contributing to the gradient.
77
Cyanide inhibits which component of oxidative phosphorylation?
Cytochrome c oxidase (Complex IV)
## Footnote
It prevents the transfer of electrons to oxygen.
78
What is the effect of an uncoupler like DNP on mitochondria?
Electron transport continues but ATP synthesis stops
## Footnote
Energy is released as heat due to dissipated proton gradient.
79
What is the effect of an uncoupler like DNP on mitochondria?
Electron transport continues but ATP synthesis stops (energy is released as heat)
## Footnote
Uncouplers like DNP dissipate the proton gradient, leading to increased oxygen consumption while halting ATP production.
80
If rotenone is added to mitochondria, what happens?
NADH can no longer be oxidized, but FADH₂ (succinate oxidation) can still feed electrons into the chain
## Footnote
Rotenone blocks Complex I, preventing NADH oxidation, but electrons from FADH₂ can still enter at CoQ.
81
What is the immediate effect of oligomycin on mitochondrial respiration?
Proton flow through ATP synthase is blocked, so ATP synthesis stops and the ETC slows or stops
## Footnote
Oligomycin plugs the proton channel of ATP synthase, preventing ATP production.
82
Which statement best describes oxidative phosphorylation?
The process by which the energy from electrons in NADH/FADH₂ is used to generate ATP via a proton gradient
## Footnote
Oxidative phosphorylation involves the ETC creating a proton gradient that drives ATP synthesis.
83
During oxidative phosphorylation, the pH of the mitochondrial matrix is:
Higher (more alkaline) than the pH of the intermembrane space
## Footnote
The matrix becomes alkaline due to proton pumping into the intermembrane space.
84
What is required for the maintenance of the proton motive force across the inner mitochondrial membrane?
The inner membrane must be impermeable to protons
## Footnote
Proton permeability would collapse the gradient and halt ATP synthesis.
85
Why does FADH₂ produce fewer ATP molecules than NADH during oxidative phosphorylation?
FADH₂ electrons enter the ETC at Complex II, bypassing Complex I and pumping fewer protons
## Footnote
NADH donates electrons to Complex I, which pumps protons, while FADH₂ feeds into Complex II.
86
Approximately how many ATP molecules are synthesized from the complete aerobic oxidation of one glucose molecule?
~36–38 ATP
## Footnote
Yield varies, but traditionally about 36–38 ATP are produced per glucose, including ATP from glycolysis and the TCA cycle.
87
What is the role of cytochrome c in the electron transport chain?
It transfers electrons from Complex III to Complex IV
## Footnote
Cytochrome c is a mobile electron carrier in the intermembrane space.
88
Which of these would directly increase the rate of electron transport and oxygen consumption in mitochondria?
An uncoupling agent that dissipates the proton gradient
## Footnote
Uncouplers increase electron flow to maintain the proton gradient.
89
Which of the following is not involved in the mitochondrial electron transport chain?
Coenzyme A
## Footnote
Coenzyme A is involved in metabolic reactions but not in the electron transport chain.
90
During oxidative phosphorylation, ADP is converted to ATP by which enzyme complex?
F₀F₁-ATP synthase (Complex V)
## Footnote
ATP synthase uses the proton motive force to synthesize ATP.
91
What happens to the energy released as electrons flow down the ETC if it is not captured in ATP due to uncoupling?
It is released as heat
## Footnote
Uncoupling results in energy dissipation as heat instead of ATP production.
92
In the absence of oxygen, what happens to the electron transport chain and oxidative phosphorylation?
Electron transport stops and no further ATP is produced by oxidative phosphorylation
## Footnote
Oxygen is required for electron acceptance; without it, the chain backs up and ATP synthesis halts.
93
Which molecule has the highest reduction potential in the respiratory chain?
O₂ (oxygen)
## Footnote
Oxygen has the highest redox potential, driving electron flow toward it.
94
If a toxin made the inner mitochondrial membrane freely permeable to protons, what would occur?
ATP synthesis would cease, and the energy from electron transport would be lost as heat
## Footnote
A permeable membrane would collapse the H⁺ gradient, uncoupling ATP synthesis.
95
Complete oxidation of a molecule of NADH by the electron transport chain yields enough energy to produce how many ATP?
3 ATP
## Footnote
Traditionally, mitochondrial NADH is estimated to generate ~3 ATP via oxidative phosphorylation.
96
What is the role of ADP in regulating the rate of oxidative phosphorylation?
ADP availability controls the rate of ATP synthesis (respiratory control)
## Footnote
When ADP is abundant, ATP synthase accelerates ATP production and O₂ consumption increases.
97
Which component of the oxidative phosphorylation machinery directly utilizes the proton gradient to synthesize ATP?
The F₁ subunit of ATP synthase
## Footnote
The F₁ subunit catalyzes the conversion of ADP and inorganic phosphate into ATP.
98
What drives the import of ADP into mitochondria and export of ATP to the cytosol?
The voltage difference across the inner membrane (matrix negative)
## Footnote
The electrical component of the proton motive force favors the exchange of ADP and ATP.
99
If mitochondrial inner membrane vesicles contain only Complex IV and ATP synthase, will ATP synthesis occur when electrons are passed through Complex IV?
No, because a complete proton circuit (proton pumping elsewhere) is required
## Footnote
ATP synthesis requires a source of electrons and sustained proton pumping to maintain a gradient.
100
What is required for ATP synthesis involving Complex IV and ATP synthase?
A complete proton circuit (proton pumping elsewhere) is required.
## Footnote
Complex IV alone cannot sustain ATP synthesis without a source of electrons and additional proton pumping mechanisms.
101
What happens to the oxygen we breathe in during oxidative phosphorylation?
It is reduced to form water (H₂O).
## Footnote
Oxygen serves as the final electron acceptor in the electron transport chain, resulting in the production of water.
102
How does ATP synthase produce ATP from ADP and P_i?
By using the energy of proton flow from the intermembrane space back into the matrix.
## Footnote
The movement of protons down their electrochemical gradient drives the phosphorylation of ADP.
103
Which condition would increase the efficiency of oxidative phosphorylation?
A higher proton impermeability of the inner membrane (less leakage).
## Footnote
Less leakage of protons maximizes the proton motive force, enhancing ATP synthesis efficiency.
104
What is the effect of activating thermogenin in brown adipose tissue?
Heat generation with less ATP production.
## Footnote
Thermogenin uncouples electron transport from ATP synthesis, releasing energy as heat.
105
What is the function of Complex III in the electron transport chain?
It accepts electrons from ubiquinol (QH₂) and transfers them to cytochrome c, while pumping protons.
## Footnote
Complex III is crucial for mediating electron transfer and contributing to the proton gradient.
106
The majority of ATP synthesized during oxidative phosphorylation results from what process?
Chemiosmotic coupling – the flow of protons driving ATP synthase.
## Footnote
Most ATP is produced by converting the energy from the proton gradient into chemical energy via ATP synthase.
