Glycolysis 1: Videos & Practice Problems

Glycolysis is the metabolic pathway that converts glucose into pyruvate, consisting of two phases: the energy investment phase and the energy payoff phase. Initially, two ATP molecules are consumed to phosphorylate glucose, forming glucose 6-phosphate. This six-carbon compound is then split into two three-carbon molecules. In the energy payoff phase, ATP is generated through substrate level phosphorylation, along with NADH, which is crucial for energy production in cellular respiration. Ultimately, glycolysis transforms one glucose molecule into two pyruvate molecules, setting the stage for further metabolic processes.

Glycolysis 1

Now, let's turn our attention to glycolysis which is the process by which glucose is broken down into pyruvate and is going to be a nice lead in to the process of cellular respiration which is what we're going to spend a lot of time focusing on in the next unit. Now, glucose has many uses in the cell. You know, we're going to be focusing on glycolysis, but don't forget that glucose is converted into glycogen in animals, starch in plants, and sucrose as well for energy storage. And, additionally, glucose can be used for structural features in the extracellular matrix and cell wall where it'll be, you know, converted into various polysaccharides. Now, it can also be used to produce ribose 5 phosphate via the pentose phosphate pathway, and we're going to take a look at that a little later. Additionally, you know, the main function we're going to focus on is how glucose is turned into pyruvate, which is going to be the material used in aerobic cellular respiration. All right. So, let's start by just kind of taking an overview of glycolysis. We're going to get down into the trenches and look at all the individual reactions, but before we get there, let's take a nice casual look at the whole process.

So glycolysis has 2 phases. There's the energy investment phase, and it's the energy investment phase because we're actually going to spend ATP. See, we're going to burn 2 ATPs for the 1 molecule of glucose. So we're going to invest that energy into it, and then we're going to have the energy payoff phase, and this is where we, basically get to collect on that energy investment we made initially. And you can see that we actually generate ATP here, here, here, and here. And, we're also going to in this process generate NADH. And this is going to be used to produce energy in cellular respiration. So we're not actually going to get to what NADH does in this unit. We're going to talk about it next time, but it does lead to energy production. So, we're directly generating ATP here in the energy payoff phase, and we're also generating NADH, which will down the line produce some energy, via electron transport and oxidative phosphorylation topics for next next time.

So, before we move on, I also just want to kind of take an overview of what's happening in glycolysis. So we start with glucose, which is a 6 carbon molecule. See we have 6 carbons there. And then in the energy investment phase, we're going to add 2 phosphate groups to this molecule. So actually, the first phosphate group gets added immediately as glucose enters the cell. So as soon as glucose enters the cell, it is phosphorylated. And then if, from here, glucose can actually or the, well, it's no longer glucose. Now, now, it's glucose 6 phosphate. But glucose 6 phosphate can be used for a variety of different things. It doesn't necessarily, it's not necessarily committed to glycolysis. But if it does commit to glycolysis, then what's going to happen is we're going to actually add another phosphate group onto it. So that's those are the 2 ATP we're going to spend right there. Now, at the end of our energy investment phase, we're going to have this 6 carbon molecule with 2 phosphate groups and then we're going to split it, right? And we're going to split it into 2, 3-carbon molecules that each has a phosphate group, attached to them. Then we enter the energy payoff phase. And here, we're not going to break down these molecules, any further in terms of losing carbons, right? We're going to, or these molecules are going to stay as 3 carbon molecules 16 carbon molecule and we wind up with 2, 3-carbon molecules. Now, we are going to see some changes with those phosphate groups. So, you'll notice that we actually phosphorylate each 3-carbon molecule again so that we wind up with, 2, 3-carbon molecules that have 2 phosphate groups attached to them. And then we're actually going to use both of those phosphate groups to generate ATP via a process called substrate level phosphorylation. And, we're not really going to talk about, you know, the specifics of substrate level phosphorylation. That's the name of the process. And the significance of that is going to come up later when we compare it to oxidative phosphorylation, which is another process that generates ATP. But again, that's going to be next unit. So that's the overview of glycolysis. We start with a 6 carbon molecule, add 2 phosphates to it, break it in half, then, phosphorylate each of those 3-carbon molecules so that we have have 2, 3-carbon molecules with 2 phosphate groups. And then we're going to use both of those phosphate groups on that 3-carbon molecule to generate ATP, and what we're going to be left with is this, these 3-carbon molecules called pyruvate. So start with 1 glucose and we end with 2 pyruvates.

Alright. Let's flip the page and take a look at some of the specific reactions involved.

Here’s what students ask on this topic:

What are the main phases of glycolysis and their significance?

Glycolysis consists of two main phases: the energy investment phase and the energy payoff phase. In the energy investment phase, two ATP molecules are consumed to phosphorylate glucose, forming glucose 6-phosphate. This phase prepares the glucose molecule for subsequent breakdown. The energy payoff phase generates ATP and NADH. Specifically, ATP is produced through substrate-level phosphorylation, and NADH is generated, which will be used in cellular respiration for further energy production. Ultimately, glycolysis transforms one glucose molecule into two pyruvate molecules, setting the stage for further metabolic processes like the citric acid cycle and oxidative phosphorylation.

How is ATP generated during glycolysis?

ATP is generated during glycolysis through a process called substrate-level phosphorylation. In the energy payoff phase, each of the two 3-carbon molecules (formed from the initial 6-carbon glucose) undergoes reactions that transfer phosphate groups to ADP, forming ATP. Specifically, two ATP molecules are produced per 3-carbon molecule, resulting in a net gain of two ATP molecules per glucose molecule, as two ATPs were initially consumed in the energy investment phase. This direct transfer of phosphate groups to ADP is distinct from oxidative phosphorylation, which occurs later in cellular respiration.

What is the role of NADH in glycolysis?

NADH plays a crucial role in glycolysis as an electron carrier. During the energy payoff phase, NAD⁺ is reduced to NADH when it accepts electrons. This occurs in the reaction where glyceraldehyde 3-phosphate is converted to 1,3-bisphosphoglycerate. The NADH produced in glycolysis is then used in cellular respiration, specifically in the electron transport chain, to generate additional ATP through oxidative phosphorylation. Although NADH itself does not produce energy directly in glycolysis, it is essential for the continuation of cellular respiration and overall energy production in the cell.

What happens to glucose when it enters the cell during glycolysis?

When glucose enters the cell, it is immediately phosphorylated to form glucose 6-phosphate. This phosphorylation is catalyzed by the enzyme hexokinase and consumes one ATP molecule. The addition of the phosphate group prevents glucose from diffusing out of the cell and marks the first step in the energy investment phase of glycolysis. Glucose 6-phosphate can then either continue through glycolysis or be diverted to other pathways, such as glycogen synthesis or the pentose phosphate pathway, depending on the cell's needs.

What are the end products of glycolysis?

The end products of glycolysis are two molecules of pyruvate, two molecules of ATP (net gain), and two molecules of NADH per molecule of glucose. Pyruvate can enter the mitochondria to be further oxidized in the citric acid cycle under aerobic conditions, or it can be converted to lactate in anaerobic conditions. The ATP produced provides immediate energy for cellular processes, while NADH will be used in the electron transport chain to generate more ATP during cellular respiration.