Glucose-6-phosphate (G6P) is the first molecule of the pentose phosphate pathway where it is acted upon by glucose-6-phosphate dehydrogenase. G6P is the result of the hexokinase (first) reaction in glycolysis. What is key here is that the tissue in question is muscle. Because muscle cells lack the glucose-6-phosphatase necessary to produce free glucose from G6P, they cannot be said to perform gluconeogenesis. They do, however, perform glycogenesis through conversion of G6P to glucose-1-phosphate followed by conversion to uridine diphosphateglucose for addition to a growing molecule of glycogen. 

Sucrose is the linking of glucose and fructose. Recall from the glycolytic pathway that fructose is further downstream than glucose, and therefore would allow for faster production of ATP.  

First, we must realize that the first committed step is the first irreversible reaction of glycolysis that is unique to glycolysis (cannot lead to another process, such as the pentose phosphate pathway). This is the third step, in which fructose-6-phosphate is converted to fructose-1,6-bisphosphate (the correct answer).  

Glucose is the beginning reactant of glycolysis, and pyruvate is the final product. Glucose-6-phosphate is the product of the first step of glycolysis overall, but not of the committed step.

Conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate is mediated by phosphoglycerate kinase. Conversion of phosphoenolpyruvate to pyruvate is mediated by pyruvate. In both these reactions adenosine diphosphate (ADP) is converted to ATP via substrate level phosphorylation. Conversion of 2-phosphoglycerate to phosphoenolpyruvate, mediated by enolase, does not produce ATP.

In this question, we're shown a portion of glycolysis. We're asked to identify an intermediate in glycolysis based on the intermediate that comes before it and the one that comes after it.

To answer this, we'll need to know the pathway of glycolysis. The first intermediate shown here, glyceraldehyde-3-phosphate, is acted on by the enzyme glyceraldehyde-3-phosphate dehydrogenase. The product of this reaction is 1,3-bisphosphoglycerate, which is thus the correct answer. This intermediate is then acted on by the enzyme phosphoglycerate kinase to produce 3-phosphoglycerate.

As glucose is introduced into the glycolytic pathway, it is first phosphorylated to create glucose-6-phosphate. That will then be converted to fructose-6-phosphate via phosphoglucose isomerase. That product will then be phosphorylated once more via phosphofructokinase-1 to create fructose-1,6-bisphosphate. Glucose-1,6-bisphosphate is never an observed intermediate in glycolysis.

Cellular respiration is a long process, and so it is easiest to break it into the following steps:

Step 1: Glycolysis

Step 2: Pyruvate decarboxylation

Step 3: Krebs cycle

Step 4: Oxidative phosphorylation

In the above steps, ATP is only produced by substrate-level phosphorylation in glycolysis and during the Krebs cycle.

In glycolysis, two molecules of pyruvate are produced for every molecule of glucose oxidized. During this process, two ATP molecules are consumed, but four are produced via substrate-level phosphorylation.

In the Krebs cycle, each pass of pyruvate through the cycle generates one molecule of GTP, which is subsequently used to generate a molecule of ATP via substrate-level phosphorylation. Thus, one molecule of ATP is produced via substrate-level phosphorylation per molecule of pyruvate oxidized. But remember that glycolysis produces two molecules of pyruvate for each molecule of glucose oxidized. Hence, the Krebs cycle will contribute a total of two molecules of ATP per glucose molecule oxidized.

Since we have a total of four moles ATP from glycolysis and two moles of ATP from the Krebs cycle (one per pyruvate), we have a cumulative production of six moles of ATP generated by substrate-level phosphorylation per mole of glucose oxidized.

The first and third steps of glycolysis involve energy consumption in the form of ATP. A phosphate group is added to glucose, and fructose-6-phosphate. In the seventh and tenth steps of glycolysis, ADP is phosphorylated at the level of the substrate into ATP. Since this is after glucose had been split into two three-carbon molecules, each molecule of glucose results in four ATP produced. However, since two were consumed early in glycolysis, the net ATP production is 2.

The first step of glycolysis is the addition of a phosphate group to glucose to form glucose-6-phosphate. The third step of glycolysis is the addition of another phosphate group to fructose-6-phosphate to form fructose-1,6-bisphosphate. The conversion of ATP to ADP is needed to supply the phosphate group in both of these reactions. These are the only two reactions in glycolysis where ATP is used to to add phosphate groups.

Glycolysis produces 4 ATP molecules. However, 2 ATP molecules are required to initiate glycolysis. Subtracting these two numbers gives the net ATP yield from glycolysis--2 ATP molecules.