Practice - Glycolysis 1 - Online Tutor, Practice Problems & Exam Prep

Glycolysis converts glucose into pyruvate, yielding 2 ATP and 2 NADH. Starting with fructose 1,6-bisphosphate increases ATP yield to 4. In fermentation, pyruvate is converted to acetaldehyde, retaining carbon-14 from glucose, which is then reduced to ethanol. The regeneration of NAD⁺ is crucial for glycolysis to continue under anaerobic conditions. Key enzymes include hexokinase, which uses ATP, and pyruvate kinase, which produces ATP. Triosephosphate isomerase facilitates the conversion of DHAP to G3P, demonstrating isomerization in glycolysis.

Practice - Glycolysis 1

Moving on to glycolysis. If you haven't answered question 19 yet, pause the video now and give it a shot. Alright, converting 1 mole of Fructose 1,6-bisphosphate to 2 moles of pyruvate by glycolysis results in 2 moles of NADH and 4 moles of ATP. Now, if we started with glucose and were to yield 2 moles of pyruvate, we would actually only make 2 moles of NADH and 2 moles of ATP. And the reason is because if we start with glucose, we have to use 2 ATP before we get to fructose 1,6-bisphosphate, which is why the net result for glycolysis is that you make 2 ATP and 2 NADH per molecule of glucose. Here, we're starting with fructose 1,6-bisphosphate, which is the product of phosphofructokinase. Now remember, hexokinase, the first enzyme of glycolysis, is the first one to spend ATP. So it's going to use ATP and then enzyme number 3, phosphofructokinase, also spends ATP to generate fructose 1,6-bisphosphate. Here, we're starting with that product. We're starting with the product of phosphofructokinase meaning that we're actually going to be yielding the full 4 moles of ATP. Moving on to question 20, glucose labeled with carbon 14 at the C1 and C6 positions gives rise to pyruvate labeled in its methyl carbon. Now, let's take a look at the molecule pyruvate. Here it is. You might recall, from the numbering exercises that we went through, while we were tracing the path of glycolysis that, depending on where this pyruvate came from whether it came from the DHAP, or the G3P after the aldolase reaction, this methyl carbon here will either be carbon 1 or carbon 6 depending on which one it was. This here will be 2 or 5, and this here will be 3 or 4. So, the answer here is that the pyruvate will have its methyl carbon, this position, will have the carbon 14 because that will be carbon 1 or carbon 6 from glucose. Now, if glucose was labeled with carbon 14 in the C1 position and fed to yeast carrying out ethanol fermentation, so we're not talking about lactic acid here. Ethanol fermentation, where is that carbon 14 going to be labeled in the products? Alright. So, let's take pyruvate again. Here's pyruvate. And in the process of ethanol fermentation, pyruvate gets converted to acetaldehyde, which is this molecule here. So, this is pyruvate, acetaldehyde. And in this process, it gets decarboxylated. It loses CO₂. So remember this is 1 or 6, this is 2 or 5, and this is 3 or 4. Well, this is what comes off as CO₂. So the CO₂ is either going to be carbon 3 or 4 from glucose, meaning that we retain 2, 5, and 1, and 6 in acetaldehyde and that thanks to NADH that produces ethanol, right? NADH reduces acetaldehyde to ethanol, but it doesn't lose any carbons. So, there you go. There's our carbon 1, 6, and our carbon 2, 5 meaning that the methyl or its C2 position of ethanol is going to have that carbon 14, right? Because this 1, 6, that's referring to the carbon of position 1 or 6 from the original glucose molecule. In ethanol, this is actually carbon 1 and that's carbon 2 because carbon 1 is attached to the hydroxyl group, right? So it gets the priority in terms of numbering. So that's why the answer is C2 of ethanol or the methyl carbon there because we're talking about this position on ethanol. Alright. Moving away from carbon numbering because I think we've had enough of that at this point.

The purpose of fermentation in cells is to regenerate NAD+, right? If you are doing fermentation, then that means that there is either no oxygen or not enough oxygen to be, putting all of your pyruvate towards cellular respiration. And it means that you need to rely on glycolysis for a lot of that energy demand. So or all of your energy demand depending on whether or not there's any oxygen present. So, the point is, glycolysis can't continue indefinitely because it's going to use up all of the available NAD+ and turn it all into NADH unless there's some process that can convert some of that NADH back into NAD+. Glycolysis is just going to stop at some point. So, the purpose of fermentation is to regenerate NAD+ to allow glycolysis to continue.

Looking at question 23, which of the following reactions requires ATP as a substrate? And looking at these, the answer is hexokinase. The first reaction of glycolysis which is going to take glucose and turn it into glucose-6-phosphate or phosphate if we're being Dutch, and it's going to spend an ATP in the process to phosphorylate that glucose molecule. So, looking at the other answer choices, protein kinase A is not part of glycolysis, right? That's a trick question. Protein kinase A does use ATP as a substrate to phosphorylate whatever it's working on. But it's not part of glycolysis. So trick answer there. Aldolase doesn't use ATP at all. Aldolase is the enzyme that takes Fructose 1,6-bisphosphate and chops it in half and yields DHAP and G3P. There's no ATP involved there whatsoever. Pyruvate kinase actually produces ATP. ATP is a product of pyruvate kinase, not a substrate. And glyceraldehyde-3-phosphate dehydrogenase is the enzyme that takes G3P and phosphorylates it. So this is also kind of a tricky answer choice because, you do wind up with 1,3-bisphosphoglycerate. So you are adding a phosphate group in the course of this reaction, but it's not coming from ATP. It's actually coming from inorganic phosphate, being added on here. So, trick question and this is also just a side note, this is the reaction that produces NADH from NAD+. So again, not the right answer choice though. D is tricky because that reaction does result in but it's coming from inorganic phosphate not ATP. Okay. Moving on to 24. 24 is the opposite of 23. Which of the following reactions produces ATP? And we just said that ATP is a product of pyruvate kinase, right? We just went through all these reactions, so don't need to do it again. The answer here is pyruvate kinase. Which of the following is a ketose to aldose isomerization? The answer is triose phosphate isomerase, and you might recall that triose phosphate isomerase is the enzyme that converts DHAP into G3P. And it does this by basically transferring the phosphate group around on the molecule. Alright. Let's oh and sorry and of course in the process, the sugar goes from being a ketose to an aldose. Alright, let's turn the page.

Here’s what students ask on this topic:

What is the net ATP yield from glycolysis starting with one molecule of glucose?

The net ATP yield from glycolysis starting with one molecule of glucose is 2 ATP. During glycolysis, 4 ATP molecules are produced, but 2 ATP molecules are consumed in the initial steps, resulting in a net gain of 2 ATP. Additionally, glycolysis produces 2 NADH molecules per glucose molecule.

How does starting with fructose 1,6-bisphosphate affect ATP yield in glycolysis?

Starting with fructose 1,6-bisphosphate in glycolysis increases the ATP yield to 4 ATP. This is because the initial steps that consume 2 ATP (hexokinase and phosphofructokinase reactions) are bypassed. Therefore, the net result is 4 ATP and 2 NADH per molecule of fructose 1,6-bisphosphate.

What is the role of NAD+ in glycolysis and fermentation?

NAD+ plays a crucial role in glycolysis by accepting electrons during the conversion of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate, forming NADH. In fermentation, NAD+ is regenerated from NADH, allowing glycolysis to continue under anaerobic conditions. This regeneration is essential because glycolysis would halt if NAD+ were depleted.

Which enzyme in glycolysis uses ATP as a substrate?

Hexokinase is the enzyme in glycolysis that uses ATP as a substrate. It catalyzes the phosphorylation of glucose to form glucose 6-phosphate, consuming one ATP molecule in the process. This is the first step of glycolysis.

Which enzyme in glycolysis produces ATP?

Pyruvate kinase is the enzyme in glycolysis that produces ATP. It catalyzes the conversion of phosphoenolpyruvate (PEP) to pyruvate, generating one ATP molecule per PEP molecule. This is one of the final steps in glycolysis.

What is the purpose of fermentation in cells?

The purpose of fermentation in cells is to regenerate NAD+ from NADH, allowing glycolysis to continue under anaerobic conditions. Without this regeneration, glycolysis would stop due to the depletion of NAD+, halting ATP production. Fermentation ensures a continuous supply of NAD+ for glycolysis.