When glycogen is broken down, the individual units that are removed are glucose-1-phosphate units. These are then transformed into glucose-6-phosphate molecules which are of extreme biological importance because of their ability to enter various different pathways. These pathways include glycolysis and the pentose phosphate pathway. The urea cycle, however, has to do with amino acids/proteins.

The non reducing end of a glycogen branch is the end from which glucose units are removed during degradation of glycogen.

A catabolic reaction is defined as a reaction used to break down a large molecule into smaller subunits. Of the following options, glycogenolysis is the only option where a larger molecule (glycogen) is broken down into smaller subunits (individual glucose molecules).

Glycogen is the storage form of glucose, and is more readily accessible than starches or fats. It is used for short-term supply of glucose and in starvation conditions is used up in a matter of hours. It is mainly stored in the liver and skeletal muscle. Glycogenolysis in the liver results in glucose release into the bloodstream, whereas in the muscle the glucose is immediately used up. The highest demand for the glucose is in the muscle, and that is where most of it is stored.

In the first step of gluconeogenesis, pyruvate carboxylase (with ATP and bicarbonate) converts pyruvate to oxaloacetate. Then phosphoenolpyruvate carboxykinase (PEPCK) (with GTP) releases carbon dioxide to give phosphoenolpyruvate.

GTP and ATP are used to drive the reactions that make pyruvate into PEP. Specifically, ATP catalyzes PEP carboxylase and GTP catalyzes PEP carboxykinase. A mutase moves phosphate groups already on a molecule, so it cannot be used to introduce one. The citric acid cycle can make pyruvate into any of the citric acid intermediates, but PEP isn't one of them. Pyruvate kinase is the enzyme that makes pyruvate into PEP, which is only favorable in the forward direction.

Answering this question requires knowledge of the pathway of gluconeogenesis. In this pathway, non-carbohydrate carbon substrates such as lactate, pyruvate, and certain amino acids are used to generate glucose as the final product. Much of this pathway utilizes the same enzymes used in glycolysis, which is essentially the reverse of gluconeogenesis. However, it is critical to note that there are 3 reactions in glycolysis that are irreversible. Therefore, gluconeogenesis is not an exact reverse of glycolysis, and instead there are a few different enzymes in gluconeogenesis that bypass these irreversible reactions. One of the irreversible steps in glycolysis is the formation of pyruvate from phosphoenolpyruvate (PEP), catalyzed by the glycolytic enzyme pyruvate kinase. To bypass this irreversible reaction, gluconeogenesis makes use of two enzymes. First, the enzyme pyruvate carboxylase converts pyruvate into oxaloacetate, which requires the input of one molecule of ATP per molecule of pyruvate used. Next, the gluconeogenic enzyme PEP carboxykinase converts oxaloacetate into PEP, using one molecule of GTP per molecule of oxaloacetate used. The other step that requires an investment of energy is by a reaction that is reversible. The conversion of 3-phosphoglycerate into 1,3-bisphosphoglycerate (1,3-BPG) by the enzyme phosphoglycerate kinase utilizes one molecule of ATP per molecule of 1,3-BPG generated. This is a reversible reaction. Now, we can add up the energy requirements. Since each of these reactions need to occur twice in order to generate a single molecule of glucose, we'll need to multiply the energy investment by two in each step. Thus, we have two molecules of ATP from the reaction catalyzed by pyruvate carboxylase. We also have two molecules of GTP from the reaction catalyzed by PEP carboxykinase. And lastly, we have two molecules of ATP used from the reaction catalyzed by phosphoglycerate kinase. Adding all of these up, we have a total of four molecules of ATP and two molecules of GTP.

Oxaloacetate contributes to fatty acid synthesis, but it’s not a lipid, because, among other reasons, it’s not hydrophobic. This is also why it cannot cross mitochondrial membranes. Glucose can be synthesized from glycerol, but this process occurs via dihydroxyacetone phosphate, and doesn’t involve oxaloacetate. Oxaloacetate is synthesized from pyruvate, which is the end product of glycolysis, so oxaloacetate cannot be an intermediary in glycolysis. However, lactate can be converted to pyruvate, which is the principle substrate for gluconeogenesis (sometimes called “reverse glycolysis”). In gluconeogenesis, oxaloacetate is an intermediary in the conversion of pyruvate to phosphoenolpyruvate, and so that makes it an intermediary, too, in the synthesis of glucose from lactate.

Futile cycling occurs when two metabolic processes occur in opposite directions, and thus result in no net change. This is very wasteful, and not ideal. The only example of the answer choices of metabolic processes occurring in opposite directions is glycolysis and gluconeogenesis occurring simultaneously. Other possible examples could include: glycogenesis and glycogenolysis, beta-oxidation and fatty acid synthesis, etc.

Gluconeogenesis occurs in times of starvation, fasting, and low access to sources of glucose. This is the cells way of creating its own precursor when none is available, albeit at a high energetic cost (one cycle of gluconeogenesis can cost 6 ATP). Thus, it is untrue that gluconeogenesis occurs during high consumption of carbohydrates and fatty acids. Each other selection is true regarding gluconeogenesis.