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What is the cellular fate of glucose?
How is glucose broken down?
How does glucose affect the body?
Which of the following statements concerning the fates of blood glucose is FALSE? A. Blood glucose may be used as energy B. Liver glycogen may be reconverted directly to blood glucose C. Muscle glycogen may be reconverted directly to blood glucose D. Blood glucose may be converted to and stored as fat
- Overview
- Introduction
- What is glycolysis?
- Highlights of glycolysis
- Detailed steps: Energy-requiring phase
- Detailed steps: Energy-releasing phase
- What happens to pyruvate and NADH ?
Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. Glycolysis consists of an energy-requiring phase followed by an energy-releasing phase.
Suppose that we gave one molecule of glucose to you and one molecule of glucose to Lactobacillus acidophilus—the friendly bacterium that turns milk into yogurt. What would you and the bacterium do with your respective glucose molecules?
Overall, the metabolism of glucose in one of your cells would be pretty different from its metabolism in Lactobacillus—check out the fermentation article for more details. Yet, the first steps would be the same in both cases: both you and the bacterium would need to split the glucose molecule in two by putting it through glycolysis1 .
Glycolysis is a series of reactions that extract energy from glucose by splitting it into two three-carbon molecules called pyruvates. Glycolysis is an ancient metabolic pathway, meaning that it evolved long ago, and it is found in the great majority of organisms alive today2,3 .
In organisms that perform cellular respiration, glycolysis is the first stage of this process. However, glycolysis doesn’t require oxygen, and many anaerobic organisms—organisms that do not use oxygen—also have this pathway.
Glycolysis has ten steps, and depending on your interests—and the classes you’re taking—you may want to know the details of all of them. However, you may also be looking for a greatest hits version of glycolysis, something that highlights the key steps and principles without tracing the fate of every single atom. Let’s start with a simplified version of the pathway that does just that.
Glycolysis takes place in the cytosol of a cell, and it can be broken down into two main phases: the energy-requiring phase, above the dotted line in the image below, and the energy-releasing phase, below the dotted line.
•Energy-requiring phase. In this phase, the starting molecule of glucose gets rearranged, and two phosphate groups are attached to it. The phosphate groups make the modified sugar—now called fructose-1,6-bisphosphate—unstable, allowing it to split in half and form two phosphate-bearing three-carbon sugars. Because the phosphates used in these steps come from ATP , two ATP molecules get used up.
The three-carbon sugars formed when the unstable sugar breaks down are different from each other. Only one—glyceraldehyde-3-phosphate—can enter the following step. However, the unfavorable sugar, DHAP , can be easily converted into the favorable one, so both finish the pathway in the end
•Energy-releasing phase. In this phase, each three-carbon sugar is converted into another three-carbon molecule, pyruvate, through a series of reactions. In these reactions, two ATP molecules and one NADH molecule are made. Because this phase takes place twice, once for each of the two three-carbon sugars, it makes four ATP and two NADH overall.
Each reaction in glycolysis is catalyzed by its own enzyme. The most important enzyme for regulation of glycolysis is phosphofructokinase, which catalyzes formation of the unstable, two-phosphate sugar molecule, fructose-1,6-bisphosphate4 . Phosphofructokinase speeds up or slows down glycolysis in response to the energy needs of the cell.
We’ve already seen what happens on a broad level during the energy-requiring phase of glycolysis. Two ATP s are spent to form an unstable sugar with two phosphate groups, which then splits to form two three-carbon molecules that are isomers of each other.
Next, we’ll look at the individual steps in greater detail. Each step is catalyzed by its own specific enzyme, whose name is indicated below the reaction arrow in the diagram below.
Step 1. A phosphate group is transferred from ATP to glucose, making glucose-6-phosphate. Glucose-6-phosphate is more reactive than glucose, and the addition of the phosphate also traps glucose inside the cell since glucose with a phosphate can’t readily cross the membrane.
Step 2. Glucose-6-phosphate is converted into its isomer, fructose-6-phosphate.
Step 3. A phosphate group is transferred from ATP to fructose-6-phosphate, producing fructose-1,6-bisphosphate. This step is catalyzed by the enzyme phosphofructokinase, which can be regulated to speed up or slow down the glycolysis pathway.
Step 4. Fructose-1,6-bisphosphate splits to form two three-carbon sugars: dihydroxyacetone phosphate (DHAP ) and glyceraldehyde-3-phosphate. They are isomers of each other, but only one—glyceraldehyde-3-phosphate—can directly continue through the next steps of glycolysis.
In the second half of glycolysis, the three-carbon sugars formed in the first half of the process go through a series of additional transformations, ultimately turning into pyruvate. In the process, four ATP molecules are produced, along with two molecules of NADH .
Here, we’ll look in more detail at the reactions that lead to these products. The reactions shown below happen twice for each glucose molecule since a glucose splits into two three-carbon molecules, both of which will eventually proceed through the pathway.
Detailed steps of the second half of glycolysis. All of these reactions will happen twice for one molecule of glucose.
Step 6. Two half reactions occur simultaneously: 1) Glyceraldehyde-3-phosphate (one of the three-carbon sugars formed in the initial phase) is oxidized, and 2) NAD+ is reduced to NADH and H+ . The overall reaction is exergonic, releasing energy that is then used to phosphorylate the molecule, forming 1,3-bisphosphoglycerate.
Step 7. 1,3-bisphosphoglycerate donates one of its phosphate groups to ADP , making a molecule of ATP and turning into 3-phosphoglycerate in the process.
Step 8. 3-phosphoglycerate is converted into its isomer, 2-phosphoglycerate.
At the end of glycolysis, we’re left with two ATP , two NADH , and two pyruvate molecules. If oxygen is available, the pyruvate can be broken down (oxidized) all the way to carbon dioxide in cellular respiration, making many molecules of ATP . You can learn how this works in the videos and articles on pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.
What happens to the NADH ? It can't just sit around in the cell, piling up. That's because cells have only a certain number of NAD+ molecules, which cycle back and forth between oxidized (NAD+ ) and reduced (NADH ) states:
NAD+ + 2e− + 2H+ ⇌ NAD H + H+
Glycolysis needs NAD+ to accept electrons as part of a specific reaction. If there’s no NAD+ around (because it's all stuck in its NADH form), this reaction can’t happen and glycolysis will come to a halt. So, all cells need a way to turn NADH back into NAD+ to keep glycolysis going.
There are two basic ways of accomplishing this. When oxygen is present, NADH can pass its electrons into the electron transport chain, regenerating NAD+ for use in glycolysis. (Added bonus: some ATP gets made!)
When oxygen is absent, cells may use other, simpler pathways to regenerate NAD+ . In these pathways, NADH donates its electrons to an acceptor molecule in a reaction that doesn’t make ATP but does regenerate NAD+ so glycolysis can continue. This process is called fermentation, and you can learn more about it in the fermentation videos.
Jul 17, 2023 · Three components that are crucial to fetal glucose metabolism are maternal serum glucose concentration, maternal glucose transport to the placenta, which is impacted by the amount of glucose the fetus uses, and finally, fetal pancreas insulin production.
- Mihir N. Nakrani, Robert H. Wineland, Fatima Anjum
- 2023/07/17
- 2021
Oct 1, 2004 · There are three major pathways for the cellular fate of glucose, including: 1) oxidation to pyruvate, which may undergo further oxidation in the citric acid cycle; 2) storage as the polysaccharide glycogen for rapid utilization at a later time; and 3) conversion to other sugars and intermediates essential for other important biosynthetic and/or ...
- Clara Bouche, Shanti Serdy, Shanti Serdy, Shanti Serdy, C. Ronald Kahn, C. Ronald Kahn, C. Ronald Ka...
- 2004
- Glycolysis. For bacteria, eukaryotes, and most archaea, glycolysis is the most common pathway for the catabolism of glucose; it produces energy, reduced electron carriers, and precursor molecules for cellular metabolism.
- Other Glycolytic Pathways. When we refer to glycolysis, unless otherwise indicated, we are referring to the EMP pathway used by animals and many bacteria.
- Transition Reaction, Coenzyme A, and the Krebs Cycle. Glycolysis produces pyruvate, which can be further oxidized to capture more energy. For pyruvate to enter the next oxidative pathway, it must first be decarboxylated by the enzyme complex pyruvate dehydrogenase to a two-carbon acetyl group in the transition reaction, also called the bridge reaction (see Appendix C and Figure \(\PageIndex{3}\)).
Google Classroom. Overview and steps of the citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle. Introduction. How important is the citric acid cycle? So important that it has not one, not two, but three different names in common usage today!
Apr 30, 2024 · Glucose often enters the body in isometric forms such as galactose and fructose (monosaccharides), lactose and sucrose (disaccharides), or starch (polysaccharides). Excess glucose is stored in the body as glycogen, a glucose polymer, utilized during fasting.
