Question 1

Glycolysis is only partially reversible because of energy barriers at the reactions catalyzed by

Accepted Answer

Glycolysis is only partially reversible because of energy barriers at the reactions catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase. These are key regulatory steps in the pathway, and they all involve the transfer of a phosphate group from ATP to a substrate molecule. These reactions are highly exergonic, meaning that they release a lot of energy, and they are tightly controlled to ensure that the energy is used efficiently by the cell. These energy barriers make it difficult for the reverse reactions to occur spontaneously, which is why glycolysis is not completely reversible.

Question 2

An increase in glucagon will cause

Accepted Answer

Glucagon stimulates liver glycogenolysis, which means breaking down stored glycogen in the liver into glucose and releasing it into the bloodstream. This process increases blood glucose levels. Therefore, an increase in glucagon will cause an increase in liver glycogenolysis. Choice B is incorrect because an increase in glucagon will actually increase blood glucose levels, not decrease it. Choice C is also incorrect because glucagon activates rather than inactivates liver adenyl cyclase. Choice D is incorrect because glucagon does not stimulate muscle glycogenesis, but rather stimulates protein breakdown in muscle tissue to provide amino acids for gluconeogenesis in the liver.

Question 3

A substrate which transfers phosphate directly to ADP is

Accepted Answer

Phosphoenolpyruvate (PEP) transfers a phosphate directly to ADP, forming ATP, in the reaction catalyzed by pyruvate kinase during glycolysis. The other options mentioned are involved in glycolysis but do not directly transfer phosphate to ADP.

Question 4

The enzyme that catalyzes the conversion of fructose-l,-bisphosphate to fructose--phosphate + Pi is classified as

Accepted Answer

The answer of The enzyme that catalyzes the conversion of...

Question 5

Which statement about glycolysis is TRUE?

Accepted Answer

The phosphoglycerate kinase reaction is reversible under physiological conditions, while the hexokinase reaction is not.

Question 6

All of the following metabolic conditions favor the conversion of pyruvate to glucose EXCEPT

Accepted Answer

The conversion of pyruvate to glucose is a process known as gluconeogenesis. For gluconeogenesis to proceed efficiently, enzymes that catalyze the conversion steps must be active. Pyruvate carboxylase and phosphoenolpyruvate carboxykinase (PEPCK) are two key enzymes in this pathway. A low level of active PEPCK (choice D) would hinder the conversion of pyruvate to glucose, making it the correct exception. The other options either directly support gluconeogenesis or create conditions that favor the conservation of glucose or its synthesis from precursors like pyruvate.

Question 7

Which of the following is an enzyme which catalyzes a freely reversible reaction under physiological conditions?

Accepted Answer

Aldolase catalyzes the reversible reaction of fructose 1,6-bisphosphate to glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. This reaction is freely reversible under physiological conditions. The other enzymes listed catalyze irreversible reactions or reactions that are not freely reversible under physiological conditions.

Question 8

All the following statements about the enzyme glyceraldehyde -phosphate dehydrogenase are correct EXCEPT

Accepted Answer

Glyceraldehyde-phosphate dehydrogenase does not produce ATP as a product in the reaction. It produces NADH as a product, which can be used to produce ATP through cellular respiration.

Question 9

All of the following statements concerning gluconeogenesis are correct EXCEPT which one?

Accepted Answer

Gluconeogenesis from lactate primarily occurs in the liver and to a lesser extent in the kidneys, but not in muscle. Muscle lacks the necessary enzymes to convert lactate back into glucose, which is a key step in gluconeogenesis.

Question 10

What is the primary process by which epinephrine increases blood sugar?

Accepted Answer

Epinephrine activates the enzyme phosphorylase which converts glycogen into glucose through the process of glycogenolysis in the liver. This glucose is then released into the bloodstream, increasing blood sugar levels.

Question 11

All of the following can stimulate the activity of muscle glycogen phosphorylase EXCEPT

Accepted Answer

Glucagon primarily affects liver glycogen phosphorylase to stimulate glycogen breakdown in the liver, not in muscle. Muscle glycogen phosphorylase is activated by calcium ions, epinephrine, cyclic AMP, and phosphorylase kinase, but not directly by glucagon.

Question 12

Glucocorticoids such as cortisol and corticosterone have distinct effects on hepatic carbohydrate metabolism, such as promoting gluconeogenesis and glycogen deposition. The likely mechanism of these effects involves

Accepted Answer

Glucocorticoids exert their effects on hepatic carbohydrate metabolism primarily by inducing new enzyme synthesis for enzymes involved in gluconeogenesis and glycogen deposition. This includes activation of transcription factors that upregulate gene expression for these enzymes. Glucocorticoids do not directly inhibit enzymes in glycolysis or glycogenesis, and their effects are not mediated through membrane receptors or acute enzyme stimulation.

Question 13

The glycerol phosphate shuttle

Accepted Answer

The glycerol phosphate shuttle transfers electrons from cytosolic NADH to the respiratory chain of mitochondria, generating FADH2. This shuttle is important for ATP production in tissues that have high energy demands, such as muscle. The other choices are incorrect.

Question 14

If liver glucose -phosphatase were deficient, what would best characterize the expected change in metabolism relative to the normal state?

Accepted Answer

Liver glucose-6-phosphatase is responsible for releasing glucose into the bloodstream from glycogen stores or gluconeogenesis. If this enzyme is deficient, glucose cannot be made available between meals or during starvation, resulting in hypoglycemia.

Question 15

Cyclic AMP

Accepted Answer

Cyclic AMP modulates the level of fructose 2,6 bisphosphate.

Question 16

Which reaction is catalyzed by glycogen synthase?

Accepted Answer

Glycogen synthase catalyzes the reaction between glycogenin and UDP-glucose, forming glycogen chain and releasing UDP. Thus, the correct choice is D. The other choices either involve different substrates or products, or they do not involve glycogen synthase at all.

Question 17

During the breakdown of glycogen, free glucose is formed from

Accepted Answer

The answer of During the breakdown of glycogen, free glucose...

Question 18

All of the following statements about galactose and its metabolism are true EXCEPT:

Accepted Answer

Galactose is actually an aldohexose, not a ketose. The other statements are true: galactose is metabolized to UDP-glucose, galactokinase uses ATP, galactose can be converted to glucose in the liver, and digestion of lactose can contribute to an increase in blood glucose.

Question 19

In the following pathway for conversion of glucose to pyruvate, what is compound X? glucose --> --> --> --> glyceraldehyde -phosphate --> X --> phosphoglycerate --> --> --> pyruvate

Accepted Answer

1,3-bisphosphoglycerate is the compound that is formed after glyceraldehyde-3-phosphate in the glycolytic pathway, which eventually leads to the formation of pyruvate.

Question 20

All of the following statements concerning the conversion of pyruvate to glucose in mammals are true EXCEPT

Accepted Answer

The conversion of pyruvate to glucose, known as gluconeogenesis, involves multiple regulated enzymes, not just one. This process includes enzymes that are distinct from those used in glycolysis, as well as some that are shared but regulated differently to facilitate glucose production.

Quiz 15: Carbohydrate Metabolism I: Major Metabolic Pathways and Their Control