This pathway traps the glucose molecule in the cell and uses energy to modify it so that the six-carbon sugar molecule can be split evenly into the two three-carbon molecules. Figure 2. The first half of glycolysis uses two ATP molecules in the phosphorylation of glucose, which is then split into two three-carbon molecules. Step 1. The first step in glycolysis is catalyzed by hexokinase, an enzyme with broad specificity that catalyzes the phosphorylation of six-carbon sugars.
Hexokinase phosphorylates glucose using ATP as the source of the phosphate, producing glucosephosphate, a more reactive form of glucose. This reaction prevents the phosphorylated glucose molecule from continuing to interact with the GLUT proteins, and it can no longer leave the cell because the negatively charged phosphate will not allow it to cross the hydrophobic interior of the plasma membrane.
Step 2. In the second step of glycolysis, an isomerase converts glucosephosphate into one of its isomers, fructosephosphate. An isomerase is an enzyme that catalyzes the conversion of a molecule into one of its isomers. This change from phosphoglucose to phosphofructose allows the eventual split of the sugar into two three-carbon molecules. Step 3. The third step is the phosphorylation of fructosephosphate, catalyzed by the enzyme phosphofructokinase.
A second ATP molecule donates a high-energy phosphate to fructosephosphate, producing fructose-1,6-bisphosphate. In this pathway, phosphofructokinase is a rate-limiting enzyme.
This is a type of end product inhibition, since ATP is the end product of glucose catabolism. Step 4. The newly added high-energy phosphates further destabilize fructose-1,6-bisphosphate.
The fourth step in glycolysis employs an enzyme, aldolase, to cleave 1,6-bisphosphate into two three-carbon isomers: dihydroxyacetone-phosphate and glyceraldehydephosphate.
Step 5. In the fifth step, an isomerase transforms the dihydroxyacetone-phosphate into its isomer, glyceraldehydephosphate. Thus, the pathway will continue with two molecules of a single isomer. At this point in the pathway, there is a net investment of energy from two ATP molecules in the breakdown of one glucose molecule. So far, glycolysis has cost the cell two ATP molecules and produced two small, three-carbon sugar molecules.
Both of these molecules will proceed through the second half of the pathway, and sufficient energy will be extracted to pay back the two ATP molecules used as an initial investment and produce a profit for the cell of two additional ATP molecules and two even higher-energy NADH molecules.
Figure 3. Step 6. The sugar is then phosphorylated by the addition of a second phosphate group, producing 1,3-bisphosphoglycerate. Note that the second phosphate group does not require another ATP molecule.
Here again is a potential limiting factor for this pathway. If oxygen is available in the system, the NADH will be oxidized readily, though indirectly, and the high-energy electrons from the hydrogen released in this process will be used to produce ATP. Step 7. In the seventh step, catalyzed by phosphoglycerate kinase an enzyme named for the reverse reaction , 1,3-bisphosphoglycerate donates a high-energy phosphate to ADP, forming one molecule of ATP.
This is an example of substrate-level phosphorylation. A carbonyl group on the 1,3-bisphosphoglycerate is oxidized to a carboxyl group, and 3-phosphoglycerate is formed. Step 8. In the eighth step, the remaining phosphate group in 3-phosphoglycerate moves from the third carbon to the second carbon, producing 2-phosphoglycerate an isomer of 3-phosphoglycerate.
The enzyme catalyzing this step is a mutase a type of isomerase. Step 9. Enolase catalyzes the ninth step. This enzyme causes 2-phosphoglycerate to lose water from its structure; this is a dehydration reaction, resulting in the formation of a double bond that increases the potential energy in the remaining phosphate bond and produces phosphoenolpyruvate PEP.
Step Many enzymes in enzymatic pathways are named for the reverse reactions, since the enzyme can catalyze both forward and reverse reactions. Two ATP molecules were used in the first half of the pathway to prepare the six-carbon ring for cleavage, so the cell has a net gain of two ATP molecules and two NADH molecules for its use. If the cell cannot catabolize the pyruvate molecules further, it will harvest only two ATP molecules from one molecule of glucose.
Mature mammalian red blood cells are not capable of aerobic respiration —the process in which organisms convert energy in the presence of oxygen—and glycolysis is their sole source of ATP.
If glycolysis is interrupted, these cells lose their ability to maintain their sodium-potassium pumps, and eventually, they die. The last step in glycolysis will not occur if pyruvate kinase, the enzyme that catalyzes the formation of pyruvate, is not available in sufficient quantities. In this situation, the entire glycolysis pathway will proceed, but only two ATP molecules will be made in the second half.
Thus, pyruvate kinase is a rate-limiting enzyme for glycolysis. Glycolysis is the first pathway used in the breakdown of glucose to extract energy. It was probably one of the earliest metabolic pathways to evolve and is used by nearly all of the organisms on earth. Glycolysis consists of two parts: The first part prepares the six-carbon ring of glucose for cleavage into two three-carbon sugars.
ATP is invested in the process during this half to energize the separation. Two ATP molecules are invested in the first half and four ATP molecules are formed by substrate phosphorylation during the second half.
If oxygen is available, aerobic respiration will go forward. In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into mitochondria, which are the sites of cellular respiration. There, pyruvate will be transformed into an acetyl group that will be picked up and activated by a carrier compound called coenzyme A CoA.
The resulting compound is called acetyl CoA. CoA is made from vitamin B5, pantothenic acid. Acetyl CoA can be used in a variety of ways by the cell, but its major function is to deliver the acetyl group derived from pyruvate to the next stage of the pathway in glucose catabolism.
In order for pyruvate which is the product of glycolysis to enter the Citric Acid Cycle the next pathway in cellular respiration , it must undergo several changes. The conversion is a three-step process Figure 5. Figure 5. Upon entering the mitochondrial matrix, a multi-enzyme complex converts pyruvate into acetyl CoA.
In the process, carbon dioxide is released and one molecule of NADH is formed. A carboxyl group is removed from pyruvate, releasing a molecule of carbon dioxide into the surrounding medium. The result of this step is a two-carbon hydroxyethyl group bound to the enzyme pyruvate dehydrogenase.
This is the first of the six carbons from the original glucose molecule to be removed. This step proceeds twice remember: there are two pyruvate molecules produced at the end of glycolysis for every molecule of glucose metabolized; thus, two of the six carbons will have been removed at the end of both steps.
An acetyl group is transferred to conenzyme A, resulting in acetyl CoA. The enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl CoA. Note that during the second stage of glucose metabolism, whenever a carbon atom is removed, it is bound to two oxygen atoms, producing carbon dioxide, one of the major end products of cellular respiration.
In the presence of oxygen, acetyl CoA delivers its acetyl group to a four-carbon molecule, oxaloacetate, to form citrate, a six-carbon molecule with three carboxyl groups; this pathway will harvest the remainder of the extractable energy from what began as a glucose molecule. This single pathway is called by different names, but we will primarily call it the Citric Acid Cycle.
In the presence of oxygen, pyruvate is transformed into an acetyl group attached to a carrier molecule of coenzyme A. The resulting acetyl CoA can enter several pathways, but most often, the acetyl group is delivered to the citric acid cycle for further catabolism. During the conversion of pyruvate into the acetyl group, a molecule of carbon dioxide and two high-energy electrons are removed. The carbon dioxide accounts for two conversion of two pyruvate molecules of the six carbons of the original glucose molecule.
At this point, the glucose molecule that originally entered cellular respiration has been completely oxidized. Chemical potential energy stored within the glucose molecule has been transferred to electron carriers or has been used to synthesize a few ATPs.
Like the conversion of pyruvate to acetyl CoA, the citric acid cycle takes place in the matrix of mitochondria. This single pathway is called by different names: the citric acid cycle for the first intermediate formed—citric acid, or citrate—when acetate joins to the oxaloacetate , the TCA cycle since citric acid or citrate and isocitrate are tricarboxylic acids , and the Krebs cycle , after Hans Krebs, who first identified the steps in the pathway in the s in pigeon flight muscles.
Almost all of the enzymes of the citric acid cycle are soluble, with the single exception of the enzyme succinate dehydrogenase, which is embedded in the inner membrane of the mitochondrion. Unlike glycolysis, the citric acid cycle is a closed loop: The last part of the pathway regenerates the compound used in the first step.
This is considered an aerobic pathway because the NADH and FADH 2 produced must transfer their electrons to the next pathway in the system, which will use oxygen. If this transfer does not occur, the oxidation steps of the citric acid cycle also do not occur. Note that the citric acid cycle produces very little ATP directly and does not directly consume oxygen.
Figure 6. It takes place in the cytoplasm of both prokaryotic and eukaryotic cells. It was probably one of the earliest metabolic pathways to evolve since it is used by nearly all of the organisms on earth. The process does not use oxygen and is, therefore, anaerobic.
Glycolysis is the first of the main metabolic pathways of cellular respiration to produce energy in the form of ATP. Through two distinct phases, the six-carbon ring of glucose is cleaved into two three-carbon sugars of pyruvate through a series of enzymatic reactions.
The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD to produce two molecules of NADH, another energy-carrying molecule.
Before pyruvate can enter the next stage of cellular respiration it needs to be modified slightly. The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.
Before you read about the last two stages of cellular respiration, you need to know more about the mitochondrion , where these two stages take place. A diagram of a mitochondrion is shown in Figure 4. The structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration. As you can see from the figure, a mitochondrion has an inner and outer membrane.
The space between the inner and outer membrane is called the intermembrane space. The space enclosed by the inner membrane is called the matrix. The second stage of cellular respiration the Krebs cycle takes place in the matrix. The third stage electron transport happens on the inner membrane.
Recall that glycolysis produces two molecules of pyruvate pyruvic acid , which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the Krebs cycle also known as the Citric Acid Cycle. The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.
The reactions that occur next are shown in Figure 4. This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle. After citric acid forms, it goes through a series of reactions that release energy. Carbon dioxide is also released as a waste product of these reactions.
This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two pyruvic acid molecules when it splits glucose.
After glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its chemical bonds has been stored in a total of 16 energy-carrier molecules. These molecules are:. The events of cellular respiration up to this point are exergonic reactions — they are releasing energy that had been stored in the bonds of the glucose molecule.
This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an endergonic reaction. Using an exothermic reaction to power an endothermic reaction is known as energy coupling. The Electron Transport Chain is the final stage of cellular respiration. The Electron transport chain is the third stage of cellular respiration and is illustrated in Figure 4.
During this stage, high-energy electrons are released from NADH and FADH 2 , and they move along electron-transport chains on the inner membrane of the mitochondrion. An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions H across the inner membrane, from the matrix into the intermembrane space.
This ion transfer creates an electrochemical gradient that drives the synthesis of ATP. As shown in Figure 4. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation.
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