When is glucose completely oxidized

We breathe in order to have oxygen at the bottom of the ETS. Without it, cellular respiration cannot occur. The enzymes of the electron transport chain are imbedded in the cristae of the mitochondria in a specific pattern. This allows the high energy electrons to "fall" from one carrier to the next like a ball rolling down a set of stairs.

The electron transport system requires oxygen in order to function properly. PO 4 is transferred from the phosphorylated sugars to ADP. Produces 2 ATP molecules from each glucose molecule. This replaces the 2 ATP used in the energy investment phase.

Step 3 : Prepares for the next reaction. Moves the phosphate from Carbon 3 to Carbon 2. Step 4 : Enzymes remove water. This makes the bond holding the phosphate to Carbon 2 weak and unstable. Step 5 : 2 ATP molecules are produced by substrate level phosphorylation.

Glucose is oxidized into 2 molecules of pyruvic acid in an exergonic reaction. The Krebs cycle completes the oxidation of organic molecules.

It releases the energy that is stored in the 2 molecules of pyruvate. Pyruvate can only be completely oxidized in the presence of oxygen.

Before we can enter the Krebs cycle, we must form acetyl Co-A. Once inside the mitochondrion, pyruvate is converted to acetyl Co-A in a reaction that uses multiple enzymes. CO 2 is released. Krebs cycle. For every glucose molecule that is split during glycolysis:. Steps of the Krebs cycle: each step is enzyme mediated. Acetyl Co-A breaks apart and the 2-carbon acetate bonds to a 4-carbon molecule of oxaloacetate a compound found naturally in the mitochondrial matrix and forms citric acid.

Citric acid is converted to its isomer, isocitric acid. Isocitric acid loses CO 2 leaving a 5 carbon molecule. The 5 carbon compound is oxidized and NAD is reduced.

Catalyzed by multiple enzymes:. CO 2 is removed from the 5 carbon molecule. Remaining 4 carbon molecule is oxidized and NAD is reduced. 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. In the citric acid cycle, the acetyl group from acetyl CoA is attached to a four-carbon oxaloacetate molecule to form a six-carbon citrate molecule. Through a series of steps, citrate is oxidized, releasing two carbon dioxide molecules for each acetyl group fed into the cycle.

Because the final product of the citric acid cycle is also the first reactant, the cycle runs continuously in the presence of sufficient reactants. Prior to the start of the first step, pyruvate oxidation must occur. Then, the first step of the cycle begins: This is a condensation step, combining the two-carbon acetyl group with a four-carbon oxaloacetate molecule to form a six-carbon molecule of citrate.

CoA is bound to a sulfhydryl group -SH and diffuses away to eventually combine with another acetyl group. This step is irreversible because it is highly exergonic. The rate of this reaction is controlled by negative feedback and the amount of ATP available.

If ATP levels increase, the rate of this reaction decreases. If ATP is in short supply, the rate increases. In step two, citrate loses one water molecule and gains another as citrate is converted into its isomer, isocitrate.

Steps 3 and 4. CoA binds the succinyl group to form succinyl CoA. In step five, a phosphate group is substituted for coenzyme A, and a high-energy bond is formed.

This energy is used in substrate-level phosphorylation during the conversion of the succinyl group to succinate to form either guanine triphosphate GTP or ATP. There are two forms of the enzyme, called isoenzymes, for this step, depending upon the type of animal tissue in which they are found.

One form is found in tissues that use large amounts of ATP, such as heart and skeletal muscle. This form produces ATP. The second form of the enzyme is found in tissues that have a high number of anabolic pathways, such as liver. This form produces GTP. In particular, protein synthesis primarily uses GTP. Step six is a dehydration process that converts succinate into fumarate.

Unlike NADH, this carrier remains attached to the enzyme and transfers the electrons to the electron transport chain directly. This process is made possible by the localization of the enzyme catalyzing this step inside the inner membrane of the mitochondrion. Water is added to fumarate during step seven, and malate is produced. The last step in the citric acid cycle regenerates oxaloacetate by oxidizing malate.

In eukaryotic cells, unlike prokaryotes, NADH generated in the cytoplasm during glycolysis must be transported across the mitochondrial membrane before it can transfer electrons to the electron transport chain.

For simplicity, however, we will look at the theoretical maximum yield of ATP per glucose molecule oxidized by aerobic respiration. Keep in mind, however, that less ATP may actually be generated.

The net energy yield from anaerobic glucose metabolism can readily be calculated in moles of ATP. In the initial phosphorylation of glucose step 1 , 1 mol of ATP is expended, along with another in the phosphorylation of fructose 6-phosphate step 3.