 This video will cover glucose oxidation. The major parts are glycolysis and the citric acid cycle. This material comes from chapter 6 and 7 of OpenStack's biology. After watching this video, you should be able to answer the following study objectives. Define oxidation and reduction, and discuss the importance of electrons in the transfer of energy. Explain how ATP is used by the cell as an energy source. Describe the overall result in terms of molecules produced in the breakdown of glucose by glycolysis and the citric acid cycle. Describe how pyruvate the product of glycolysis is prepared for entry into the citric acid cycle. Oxidation is a chemical reaction where the reactant loses electrons. Whenever a chemical is oxidized, another chemical called the oxidizing agent must accept electrons or be reduced. Reduction is a chemical reaction where the reactant gains electrons. And whenever a chemical is reduced, another chemical called the reducing agent must provide the electrons. The example shown here is the oxidation of glucose. And so glucose is the reducing agent that gets oxidized, and oxygen is the oxidizing agent that gets reduced. In this process, glucose and oxygen are converted into the lower energy molecules, carbon dioxide, and water. Energy is never created or destroyed, so the chemical energy stored in glucose is released during this reaction. It is possible to perform this reaction by lighting glucose on fire in the presence of oxygen. This is a combustion reaction that will convert glucose and oxygen to carbon dioxide and water, releasing energy as heat and light. Most organisms oxidize glucose slowly in many small steps, storing some of the energy that is released by producing another chemical called adenosine triphosphate, or ATP. You'll see up here that I've written oil under loss of electrons and rig under gain of electrons. To help you remember that oxidation is loss of electrons and reduction is gaining electrons. So oil rig is a mnemonic device that may help you remember that oxidation is loss of electrons and reduction is gain of electrons. Here we see a figure of ATP, an illustration showing the chemical structure of ATP or adenosine triphosphate. ATP functions as an energy currency in the cell. It's a storage form of energy as we break down glucose as glucose becomes oxidized. Cells will synthesize ATP and then ATP can be used by the cell in order to perform work. As ATP is used it will be broken down to release a phosphate group. So we can see there are three phosphates in adenosine triphosphate. The sodium potassium pump is an excellent example of a protein that uses the energy stored in ATP to perform cellular work. Most cells have a high concentration of potassium and a low concentration of sodium in the cytoplasm compared to the extracellular fluid. These concentration gradients are created and maintained by this protein in the plasma membrane that uses the energy released when ATP is broken down in order to pump sodium out of the cytoplasm and potassium into the cytoplasm. So as you can see here each time ATP is broken down it releases an inorganic phosphate shown with P subscript I and ADP which is adenosine diphosphate. And every time ATP is broken down into ADP and inorganic phosphate the sodium potassium pump will pump three sodium ions out of the cell and two potassium ions into the cell. Glycolysis the first step in the oxidation of glucose within cells occurs in the cytoplasm. Proteins called enzymes function as catalysts that speed up the chemical reactions of glycolysis. There are 10 steps that gradually break down and oxidize glucose producing two smaller pyruvate molecules and in the process some energy released from the breakdown of glucose will be stored in other chemicals. Although the end goal of glucose oxidation is to synthesize ATP the first step of glycolysis involves using a molecule of ATP to activate glucose. Hexokinase is the enzyme that catalyzes a chemical reaction called phosphorylation where a phosphate group is transferred from ATP to glucose producing glucose 6-phosphate. The second step is an isomerization reaction catalyzed by phosphoglycosisomerase. This reaction reorganizes the atoms of glucose 6-phosphate producing fructose 6-phosphate. The third step catalyzed by phosphofructokinase also uses an ATP to activate fructose 6-phosphate by phosphorylation producing fructose 1-6-bisphosphate. In the fourth step fructose bisphosphate aldolase produces two three carbon molecules one of them called dihydroxyacetone diphosphate or DHAP. Dihydroxyacetone diphosphate is just commonly abbreviated DHAP and the other three carbon molecule is glyceraldehyde 3-phosphate or it's commonly just abbreviated G3P. The fifth step of glycolysis is the conversion of dihydroxyacetone diphosphate into another glyceraldehyde 3-phosphate. The enzyme triose phosphate isomerase converts our DHAP into G3P and so at this point in glycolysis one molecule of glucose that had six carbons has been converted into two molecules of glyceraldehyde 3-phosphate that each have three carbons. Now the sixth step of glycolysis is the first step that involves oxidation and reduction. Glyceraldehyde 3-phosphate dehydrogenase catalyzes the oxidation of glyceraldehyde 3-phosphate and reduction of NAD to produce 1-3-bisphosphoglycerate and NADH. The NADH produced by this reaction can be used in the mitochondria to produce ATP in a process called oxidative phosphorylation that we'll study later. Step 7 of glycolysis is the first reaction to produce ATP. Phosphoglycerate kinase transfers a phosphate from 1-3-bisphosphoglycerate to ADP producing 1 ATP and 3-phosphoglycerate. In step 8 of glycolysis, phosphoglycerate mutase catalyzes the isomerization reaction that reorganizes 3-phosphoglycerate into 2-phosphoglycerate. Then in step 9 of glycolysis, the enzyme enolase converts 2-phosphoglycerate into phosphoenolpyruvate, releasing a molecule of water. Finally, in the last step of glycolysis, pyruvate kinase transfers a phosphate group from phosphoenolpyruvate to ADP producing a molecule of ATP and pyruvate. As we've gone through glycolysis, the one molecule of glucose produced two pyruvate molecules and we got two molecules of NADH as well as a net two molecules of ATP. Here is a chemical reaction showing the overall process of glycolysis where glucose and 2ADP as well as four inorganic phosphates and 2NAD are converted into two pyruvate, 2ATP, 2NADH, and two hydrogen ions. So at this point you may be wondering what is NAD or what is NADH? NAD stands for nicotinamide adenine dinucleotide. This is a derivative of the B-vitamin niacin. NADH is the reduced form of NAD when NAD accepts two electrons and one hydrogen ion, it becomes NADH. And so the primary function of NAD is to serve as an oxidizing agent when another chemical becomes oxidized, NAD will be reduced to form NADH. As we just saw when glyceraldehyde 3-phosphate was oxidized to form 1,3-bisphosphoglycerate, NAD was reduced to form NADH. Then NADH can be used by the mitochondria in order to produce ATP. And so here's an illustration of a mitochondrion. We can see that the mitochondrion has an outer membrane and an inner membrane and the inner membrane is highly folded forming structures called chrystae. Embedded in the inner mitochondrial membrane are lots of proteins forming the electron transport chain and another protein called ATP synthase that will use together the electron transport chain and ATP synthase will perform oxidative phosphorylation in order to synthesize a large amount of ATP from the NADH that is produced as glucose is oxidized. When pyruvate enters the mitochondria a large structure formed from multiple proteins working together known as the pyruvate dehydrogenase complex performs an oxidation reaction that produces one molecule of acetyl-CoA, one molecule of NADH and one molecule of carbon dioxide. Acetyl-CoA is formed from a large chemical called coenzyme A with a small two-carbon group called an acetyl group that is derived from pyruvate that becomes attached onto the sulfur atom of coenzyme A. The molecule of pyruvate becomes oxidized and two carbons are attached to the coenzyme A forming acetyl-CoA while one carbon and two oxygens are released from pyruvate as a carbon dioxide. And as this oxidation reaction proceeds NAD serves as the oxidizing agent that it becomes reduced forming NADH. Acetyl-CoA produced by the pyruvate dehydrogenase complex next enters the citric acid cycle inside of the mitochondria in the central compartment of the mitochondria known as the mitochondrial matrix. By combining with a four-carbon chemical known as oxaloacetate, acetyl-CoA is converted into a six-carbon chemical known as citrate. The citric acid cycle gets its name from citrate which is the conjugate base form of citric acid. In the next step citric acid or citrate is converted into isocitrate through an isomerization reaction. Then isocitrate is oxidized forming alpha-ketoglutarate. In the process one carbon is released with two oxygens as carbon dioxide. And as isocitrate is oxidized the oxidizing agent NAD becomes reduced to form NADH. In the next step alpha-ketoglutarate will be oxidized forming succinyl-CoA and another carbon with two oxygens will be released as carbon dioxide. In this process the oxidizing agent NAD will be reduced to form NADH and a coenzyme A will become bound to the four carbons that are left from the oxidation of alpha-ketoglutarate as a four-carbon group known as the succinyl group of succinyl-CoA. Then the next step succinyl-CoA will be converted to succinate and in this process a GDP will become phosphorylated by an inorganic phosphate to produce GTP and coenzyme A will be released as succinate is formed. GDP and GTP are very similar to ADP and ATP and we can convert GTP into ATP so for all intensive purposes we will just count GTP as being the same as ATP. Now the next step will convert succinate which has four carbons into fumarate that also has four carbons in this process succinate will be oxidized to produce fumarate and a oxidizing agent known as FAD will be reduced to form FADH2. Then FADH2 can be used in order to reduce another chemical called ubiquinone which is represented with a Q here into ubiquinol or QH2. Electrons are transferred first from succinate onto FAD forming FADH and then later as we study the electron transport chain we'll see how FADH becomes oxidized back to form FAD forming a ubiquinol in the process. In the next step fumarate reacts with a water molecule to form malate another four carbon molecule and in our last step malate will be converted into oxaloacetate in an oxidation reaction where the oxidating agent NAD becomes reduced to form NADH. Now as we go through this cycle with one pyruvate we got three NADH molecules and one GTP which can be converted to ATP as well as one FADH2. However for each glucose molecule that gets fully oxidized we will have two pyruvate that enter the citric acid cycle and a total of six NADH will be produced in the citric acid cycle. Two FADH2s will be produced in the citric acid cycle and two of these GTP molecules that can be converted to ATP will be produced in the citric acid cycle. This equation shows us the overall balance for glycolysis, the pyruvate dehydrogenase complex and the citric acid cycle for the oxidation of one molecule of glucose to six molecules of carbon dioxide, four ADP and inorganic phosphates will be converted to ATP and ten molecules of NAD will be reduced to NADH and two molecules of FAD will be reduced to FADH2. The energy that's released from glucose oxidation is stored in the chemical bonds of ATP, NADH and FADH2 and we'll see as we study oxidative phosphorylation will produce the majority of ATP that comes from breaking down glucose. We get four ATP produced by substrate level phosphorylation, the steps in glycolysis and the citric acid cycle where ATP is produced are substrate level phosphorylation because the enzyme directly produces ATP by phosphorylation. In the electron transport chain we'll see the oxidation of NADH and FADH2 is used to create a gradient of hydrogen ions across the inner mitochondrial membrane and then chemiosmosis is the mechanism where that hydrogen ion gradient will be used in order to synthesize ATP. We will get a total of 28 molecules of ATP from oxidative phosphorylation giving us a net of approximately 32 ATP molecules synthesized per glucose that's oxidized to six molecules of carbon dioxide.