# Critical Thinking Questions

24.

Why is it beneficial for cells to use ATP rather than directly using the energy stored in the bonds of carbohydrates to power cellular reactions? What are the greatest drawbacks to harnessing energy from the bonds of several different compounds?

1. ATP is readily available in the form of a single unit that provides a consistent, appropriate amount of energy. The cell would need to tailor each reaction to each energy source if it harvested energy from different compounds.
2. ATP energy cannot activate the ROS dependent stress response, whereas food molecules are responsible for activating ROS.
3. ATP is low in energy, but food molecules possess higher levels of energy that cells can use.
4. ATP is readily available to cells, unlike compounds that have to first be phosphorylated in order to release their energy.
25.

What role does NAD+ play in redox reactions?

1. NAD+, an oxidizing agent, can accept electrons and protons from organic molecules and get reduced to NADH.
2. NAD+, a reducing agent, can donate its electrons and protons to organic molecules.
3. NAD+, an oxidizing agent, can accept electrons from organic molecules and get reduced to NADH2.
4. NAD+, a reducing agent, can donate its electrons and protons to inorganic molecules.
26.

In the following general reaction, explain how electrons are transferred and the role of each species. Remember that R represents a hydrocarbon molecule and RH represents the same molecule with a particular hydrogen identified. RH + NAD+ → NADH + R

1. RH acts as a reducing agent and donates its electrons to the oxidizing agent NAD+, forming NADH and R.
2. NAD+, the oxidizing agent, donates its electrons to the reducing agent RH, forming R and NADH.
3. RH acts as an oxidizing agent and donates electrons to the reducing agent NAD+, producing NADH and R.
4. NAD+, the reducing agent, accepts electrons from the oxidizing agent RH, producing NADH and R.
27.

Nearly all organisms on Earth carry out some form of glycolysis. How does this fact support or not support the assertion that glycolysis is one of the oldest metabolic pathways?

1. To be present in so many different organisms, glycolysis was probably present in a common ancestor rather than evolving many separate times.
2. Glycolysis is present in nearly all organisms because it is an advanced and recently-evolved pathway that has been widely used as it is so beneficial.
3. Glycolysis is absent in a few higher organisms. This contradicts the fact that it is one of the oldest metabolic pathways.
4. Glycolysis is present in some organisms and absent in others, which may or may not support the assertion that it is one of the oldest metabolic pathways.
28.
Red blood cells (RBCs) do not perform aerobic respiration, but they do perform glycolysis. Why do all cells need an energy source and what would happen if glycolysis were blocked in a red blood cell?
1. Cells require energy to perform certain basic functions. Blocking glycolysis in RBCs causes imbalance in the membrane potential, leading to cell death.
2. Cells need energy to perform cell division. Blocking glycolysis in RBCs interrupts the process of mitosis leading to nondisjunction.
3. Cells maintain the influx and efflux of organic substances using energy. Blocking glycolysis stops the binding of $CO2$ to the RBCs, causing cell death.
4. Cells require energy to recognize attacking pathogens. Blocked glycolysis inhibits the process of recognition, causing invasion of the RBCs by a pathogen.
29.

What is the primary difference between a circular pathway and a linear pathway?

1. The reactant and the product are the same in a circular pathway, but different in a linear pathway.
2. The circular pathway components get exhausted, whereas those of the linear pathway do not and are continually regenerated.
3. Circular pathways are not suited for amphibolic pathways, whereas linear pathways are.
4. Circular pathways contain a single chemical reaction that is repeated, while linear pathways have multiple events.
30.
Cellular respiration breaks down glucose and releases carbon dioxide and water. Which step in the oxidation of pyruvate produces carbon dioxide?
1. Removal of a carboxyl group from pyruvate releases carbon dioxide. The pyruvate dehydrogenase complex comes into play.
2. Removal of an acetyl group from pyruvate releases carbon dioxide. The pyruvate decarboxylase complex comes into play.
3. Removal of a carbonyl group from pyruvate releases carbon dioxide. The pyruvate dehydrogenase complex comes into play.
4. Removal of an acetyl group from pyruvate releases carbon dioxide. The pyruvate dehydrogenase complex comes into play.
31.

What three steps are included in the breakdown of pyruvate?

1. Pyruvate dehydrogenase removes a carboxyl group from pyruvate, producing carbon dioxide. Dihydrolipoyl transacetylase oxidizes a hydroxyethyl group to an acetyl group, producing NADH. Lastly, an enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl-CoA.
2. Pyruvate dehydrogenase oxidizes a hydroxyethyl group to an acetyl group, producing NADH. It further removes a carboxyl group from pyruvate, producing carbon dioxide. Lastly, dihydrolipoyl transacetylase transfers enzymebound acetyl group to CoA, forming an acetyl-CoA molecule.
3. Pyruvate dehydrogenase transfers an enzyme-bound acetyl group to CoA, forming an acetyl CoA molecule. It then oxidizes a hydroxyethyl group to an acetyl group, producing NADH. Dihydrolipoyl transacetylase removes a carboxyl group from pyruvate, producing carbon dioxide.
4. Pyruvate dehydrogenase removes a carboxyl group from pyruvate, producing carbon dioxide. Dihydrolipoyl dehydrogenase transfers enzyme-bound acetyl groups to CoA, forming an acetyl-CoA molecule. Lastly, a hydroxyethyl group is oxidized to an acetyl group, producing NADH.
32.

How do the roles of ubiquinone and cytochrome c differ from the other components of the electron transport chain?

1. CoQ and cytochrome c are mobile electron carriers, while NADH dehydrogenase and succinate dehydrogenase are bound to the inner mitochondrial membrane.
2. CoQ and cytochrome covalently bind electrons, while NADH dehydrogenase and succinate dehydrogenase are bound to the inner mitochondrial membrane.
3. CoQ and cytochrome c are bound to the inner mitochondrial membrane, while NADH dehydrogenase and succinate dehydrogenase are mobile electron carriers.
4. CoQ and cytochrome c covalently bind electrons, while NADH dehydrogenase and succinate dehydrogenase are mobile electron carriers.
33.

What accounts for the different number of ATP molecules that are formed through cellular respiration?

1. Transport of NADH from cytosol to mitochondria is an active process that decreases the number of ATP produced.
2. The ATPs produced are utilized in the anaplerotic reactions that are used for the replenishment of the intermediates.
3. Most of the ATPs produced are rapidly used for the phosphorylation of certain compounds found in plants.
4. A large number of ATP molecules are used in the detoxification of xenobiotic compounds produced during cellular respiration.
34.

Which of the following best describes complex IV in the electron transport chain?

1. Complex IV consists of an oxygen molecule held between the cytochrome and copper ions. The electrons flowing finally reach the oxygen, producing water.
2. Complex IV contains a molecule of flavin mononucleotide and iron-sulfur clusters. The electrons from NADH are transported here to coenzyme Q.
3. Complex IV contains cytochrome b, c, and Fe-S. Here, the proton motive Q cycle takes place.
4. Complex IV contains a membrane-bound enzyme that accepts electrons from FADH2 to make FAD. This electron is then transferred to ubiquinone.
35.

What is the primary difference between fermentation and anaerobic respiration?

1. Fermentation uses only glycolysis and its final electron acceptor is an organic molecule, whereas anaerobic respiration uses glycolysis, TCA, and the ETC, but finally gives electrons to an inorganic molecule.
2. Fermentation uses glycolysis, TCA, and ETC, but finally gives electrons to an inorganic molecule, whereas anaerobic respiration uses only glycolysis and its final electron acceptor is an organic molecule.
3. Fermentation uses glycolysis and its final electron acceptor is an inorganic molecule, whereas anaerobic respiration uses glycolysis, TCA, and ETC, but finally gives electrons to an organic molecule.
4. Fermentation uses glycolysis, TCA, and ETC, but finally gives electrons to an organic molecule, whereas anaerobic respiration uses only glycolysis and its final electron acceptor is an inorganic molecule.
36.

What type of cellular respiration is represented in the following equation, and why?

$CO2+H2+NADH→CH4+H2O+NAD+$
1. Anaerobic respiration, because the final electron acceptor is inorganic.
2. Aerobic respiration, because oxygen is the final electron acceptor.
3. Anaerobic respiration, because NADH donates its electrons to a methane molecule.
4. Aerobic respiration, because water is being produced as a product.
37.
Would you describe metabolic pathways as inherently wasteful or inherently economical, and why?
1. Metabolic pathways are economical due to feedback inhibition. Also, intermediates from one pathway can be utilized by other pathways.
2. Metabolic pathways are wasteful as they perform uncoordinated catabolic and anabolic reactions that wastes some of the energy that is stored.
3. Metabolic pathways are economical due to the presence of anaplerotic reactions that replenish the intermediates.
4. Metabolic pathways are wasteful as most of the energy produced is utilized in maintaining the reduced environment of the cytosol.
38.
What lipids are connected to glucose catabolism pathways and how are they connected?
1. Cholesterol and triglycerides can be converted to glycerol-6-phosphate that continues through glycolysis.
2. Glucagon and glycogen can be converted to 3-phosphoglyceraldehyde that is an intermediate of glycolysis.
3. Chylomicrons and fatty acids get converted to 1,3-bisphosphoglycerate that continues in glycolysis, forming pyruvate.
4. Sphingolipids and triglycerides form glucagon that can be fed into glycolysis.
39.
How does citrate from the citric acid cycle affect glycolysis?
1. Citrate and ATP are negative regulators of phosphofructokinase-1.
2. Citrate and ATP are negative regulators of hexokinase.
3. Citrate and ATP are positive regulators of phosphofructokinase-1.
4. Citrate and ATP are positive regulators of hexokinase.
40.

Why might negative feedback mechanisms be more common than positive feedback mechanisms in living cells?

1. Negative feedback mechanisms maintain homeostasis, whereas positive feedback drives the system away from equilibrium.
2. Positive feedback mechanisms maintain a balanced amount of substances, whereas negative feedback restricts them.
3. Negative feedback turns the system off, making it deficient of certain substances. Positive feedback balances out these deficits.
4. Positive feedback brings substance amounts back to equilibrium, while negative feedback produces excess amounts of the substance.