Cellular Respiration: Harvesting Chemical Energy 9.1 Catabolic pathways yield energy by oxidizing organic fuels 9.2 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate 9.3 The citric acid cycle completes the energy yielding oxidation of organic molecules 9.4 During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis 9.5 Fermentation enables some cells to produce ATP without the use of oxygen 9.6 Glycolysis and the citric acid cycle connect to many other metabolic pathways - The energy stored in the organic molecules of food ultimately comes from the sun. - Energy flows into an ecosystem as sunlight and leaves as heat. The chemical elements essential to life are recycled 1- Photosynthesis generates oxygen and organic molecules used by the mitochondria of eukaryotes (including plants and algae) as fuel for cellular respiration. 2- Respiration breaks this fuel down, generating ATP. 3- The waste products of respiration, carbon dioxide and water, are the raw materials for photosynthesis. -(Figure 9.2) Objectives: How cells harvest the chemical energy stored in organic molecules and use it to generate ATP, the molecule that drives most cellular work. - Presenting some basics about respiration - Focusing on the three key pathways of respiration: glycolysis, the citric acid cycle, and oxidative phosphorylation. 1
1- Catabolic pathways yield energy by oxidizing organic fuels Catabolic Pathways and Production of ATP - One catabolic process, fermentation, is a partial degradation of sugars that occurs without the use of oxygen. - The most prevalent and efficient catabolic pathway is cellular respiration, in which oxygen is consumed as a reactant along with the organic fuel. - In eukaryotic cells, mitochondria house most of the metabolic equipment for cellular respiration. - The overall process of respiration can be summarized as follows: ([198]G = 686 kcal/mol) - Catabolism is linked to work by a chemical drive shaft ATP. - To keep working, the cell must regenerate its supply of ATP from ADP and Pi. - To understand how cellular respiration accomplishes this, let s examine the fundamental chemical processes known as oxidation and reduction. Redox Reactions: Oxidation and Reduction - Why do the catabolic pathways that decompose glucose and other organic fuels yield energy? - The answer is based on the transfer of electrons during the chemical reactions. - The relocation of electrons releases energy stored in organic molecules, and this energy ultimately is used to synthesize ATP. The Principle of Redox - Electron transfers during chemical reactions are called oxidation reduction reactions, or redox reactions. - In a redox reaction, the loss of electrons from one substance is called oxidation, and - the addition of electrons to another substance is known as reduction. (Note that adding electrons is called reduction; negatively charged electrons added to an atom reduce the amount of positive charge of that atom.) 2
- An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one, just as a ball loses potential energy when it rolls downhill. - A redox reaction that relocates electrons closer to oxygen releases chemical energy that can be put to work. Oxidation of Organic Fuel Molecules During Cellular Respiration - Organic molecules that have an abundance of hydrogen are excellent fuels because their bonds are a source of hilltop electrons, whose energy may be released as these electrons fall down an energy gradient when they are transferred to oxygen. - The important point, not visible in the respiration equation, is that the status of electrons changes as hydrogen is transferred to oxygen. - The main energy foods, carbohydrates and fats, are reservoirs of electrons associated with hydrogen. - Only the barrier of activation energy holds back the flood of electrons to a lower energy state. - Body temperature is not high enough to initiate burning, of course. - Instead, enzymes in your cells will lower the barrier of activation energy, allowing the sugar to be oxidized in a series of steps. Stepwise Energy Harvest via NAD + and the Electron Transport Chain - In cellular respiration, glucose and other organic fuels are broken down in a series of steps, each one catalyzed by an enzyme. - At key steps, electrons are stripped from the glucose. - Each electron travels with a proton thus, as a hydrogen atom. - The hydrogen atoms are not transferred directly to oxygen, but instead are usually passed first to a coenzyme called NAD+ (nicotinamide adenine dinucleotide). - As an electron acceptor, NAD + functions as an oxidizing agent during respiration. How does NAD + trap electrons from glucose and other organic molecules? - Enzymes called dehydrogenases remove a pair of hydrogen atoms (two electrons and two protons) from the substrate (a sugar, for example), thereby oxidizing it. - The enzyme delivers the two electrons along with one proton to its coenzyme, NAD +. 3
- The other proton is released as a hydrogen ion (H + ) into the surrounding solution. - By receiving two negatively charged electrons but only one positively charged proton, NAD + has its charge neutralized when it is reduced to NADH. - NAD + is the most versatile electron acceptor in cellular respiration and functions in several of the redox steps during the breakdown of sugar. How do electrons that are extracted from food and stored by NADH finally reach oxygen? - Cellular respiration brings hydrogen and oxygen together to form water. - Respiration uses an electron transport chain to break the fall of electrons to oxygen into several energy releasing steps instead of one explosive reaction. - Electron transfer from NADH to oxygen is an exergonic reaction with a free energy change of 53 kcal/mol ( 222 kj/mol). - Instead of this energy being released and wasted in a single explosive step, electrons cascade down the chain from one carrier molecule to the next, losing a small amount of energy with each step until they finally reach oxygen, the terminal electron acceptor, which has a very great affinity for electrons. - Each downhill carrier is more electronegative than its uphill neighbor, with oxygen at the bottom of the chain. - Thus, the electrons removed from food by NAD + fall down an energy gradient in the electron transport chain to a far more stable location in the electronegative oxygen atom. The Stages of Cellular Respiration: A Preview Respiration is a cumulative function of three metabolic stages: - Glycolysis (does not require O 2 ) - The citric acid cycle (requires O 2) - Oxidative phosphorylation: electron transport and chemiosmosis (requires O 2 ) 4
- Glycolysis, which occurs in the cytosol, begins the degradation process by breaking glucose into two molecules of a compound called pyruvate. - The citric acid cycle, which takes place within the mitochondrial matrix, completes the breakdown of glucose by oxidizing a derivative of pyruvate to carbon dioxide. - During steps of glycolysis and citric acid cycle, dehydrogenase enzymes transfer electrons from substrates to NAD +, forming NADH. - Oxidative phosphorylation is the mode of ATP synthesis in which: - The electron transport chain accepts electrons from the breakdown products of the first two stages (most often via NADH) and passes these electrons from one molecule to another. - At the end of the chain, the electrons are combined with molecular oxygen and hydrogen ions (H + ), forming water. - The energy released at each step of the chain is stored in a form the mitochondrion can use to make ATP. Substrate level phosphorylation: -Some ATP is made by direct enzymatic transfer of a phosphate group from an organic substrate to ADP. 5
Cellular Respiration The most prevalent and efficient catabolic pathway for the production of ATP, in which oxygen is consumed as a reactant along with the organic fuel. Overview of Cellular Respiration Organic compounds such as glucose store energy in their arrangements of atoms. These molecules are broken down and their energy extracted in cellular respiration. The first stage of cellular respiration occurs in the cytosol, while the second and third stages occur in mitochondria. In cellular respiration, electrons are transferred from glucose to coenzymes such as NAD + and finally to oxygen; the energy released by this relocation of electrons is used to make ATP. Carbon dioxide and water are given off as byproducts. Cellular respiration occurs in most cells of both plants and animals. It takes place in the mitochondria, where energy from nutrients converts ADP to ATP. ATP is used for all cellular activities that require energy. Brief description for each stage of cellular respiration Glycolysis is a series of steps in which a glucose molecule is broken down into two molecules of pyruvate. As the chemical bonds in glucose are broken, electrons (and hydrogen ions) are picked up by NAD +, forming NADH. Glucose is oxidized and NAD + is reduced. A net output of two ATP molecules are also produced in glycolysis for every glucose molecule processed. But most of the energy released by the breakdown of glucose is carried by the electrons attached to NADH. Glycolysis The splitting of glucose into pyruvate. Glycolysis is the one metabolic pathway that occurs in all living cells, serving as the starting point for fermentation or aerobic respiration. The pyruvate molecules are modified as they enter the mitochondrion, releasing carbon dioxide. The altered molecules enter a series of reactions 6
called the citric acid cycle. More carbon dioxide is released as the citric acid cycle completes the oxidation of glucose. Two ATPs are formed per glucose, but most of the energy released by the oxidation of glucose is carried by NADH and FADH 2. Citric acid cycle A chemical cycle involving eight steps that completes the metabolic breakdown of glucose molecules to carbon dioxide; occurs within the mitochondrion; the second major stage in cellular respiration. Oxidative phosphorylation The production of ATP using energy derived from the redox reactions of an electron transport chain. Almost all of the ATP produced by cellular respiration is banked in the final phase oxidative phosphorylation. The NADH and FADH 2 molecules produced in glycolysis and the citric acid cycle donate their electrons to the electron transport chain. At the end of the chain, oxygen exerts a strong pull on the electrons, and combines with them and hydrogen ions (protons) to form water. The electron transport chain converts chemical energy of moving electrons to a form that can be used to drive oxidative phosphorylation, which produces about 34 ATP molecules for each glucose molecule consumed. Electron Transport Most of the energy harvested from organic molecules during glycolysis and the citric acid cycle is stored in NADH and FADH 2. These molecules give up their high-energy electrons in the third phase of cellular respiration oxidative phosphorylation where most of the cell's ATP fuel is produced The electron transport chain is an array of molecules mostly proteins built into the inner membrane of the mitochondrion. NADH gives up its high-energy electrons to the first complex in the electron transport chain. The electrons move from one member of the chain to the next, giving up their energy as they are pulled from NADH toward highly electronegative oxygen. The energy given up by the flow of electrons is used to pump 7
hydrogen ions from the mitochondrial matrix into the intermembrane space. Oxygen captures the electrons in the very last step in electron transport. The last complex adds a pair of electrons to an oxygen atom and two hydrogen ions, forming water. The electron transport chain has used the energy of moving electrons to pump hydrogen ions into the intermembrane space. This buildup of hydrogen ions like water behind a dam stores the potential energy that was originally in the bonds of glucose molecules. The backed-up hydrogen ions give up their energy when they diffuse through a special protein in the membrane called ATP synthase. As hydrogen ions flow down their concentration gradient, ATP synthase captures their energy to make ATP. This mode of ATP production is called oxidative phosphorylation because it is powered by the transfer of electrons to oxygen Under normal conditions, almost all the ATP produced in the process of cellular respiration is manufactured by electron transport and oxidative phosphorylation about 34 ATPs for every glucose consumed. Very abstracted summary of cellular respiration can be as follow: Glycolysis and the citric acid cycle produce a small amount of ATP via substrate-level phosphorylation, but most of the cell's ATP is made via oxidative phosphorylation, when NADH and FADH 2 produced in glycolysis and the citric acid cycle give up to oxygen the electrons obtained from organic molecules. Fermentation Without the electronegative oxygen to pull electrons down the transport chain, oxidative phosphorylation ceases. However, fermentation provides a mechanism by which some cells can oxidize organic fuel and generate ATP without the use of oxygen. All cells are able to synthesize ATP via the process of glycolysis. In many cells, if oxygen is not present, pyruvate (pyruvic acid) is metabolized in a process called fermentation. By oxidizing the NADH produced in glycolysis, fermentation regenerates NAD +, which can take part in glycolysis once 8
again to produce more ATP. The net energy gain in fermentation is 2 ATP molecules per molecule of glucose. Fermentation complements glycolysis and makes it possible for ATP to be continually produced in the absence of oxygen. There are two types of fermentation. Alcohol fermentation, which occurs in yeast, results in the production of ethanol and carbon dioxide. Lactic acid fermentation, which occurs in muscle, results in the production of lactate (lactic acid). Efficiency of respiration A rough estimate of the efficiency of respiration can be made that is, the percentage of chemical energy stored in glucose that has been restocked in ATP. Recall that the complete oxidation of a mole of glucose releases 686 kcal of energy (ΔG = 686 kcal/mol). Phosphorylation of ADP to form ATP stores at least 7.3 kcal per mole of ATP. Therefore, the efficiency of respiration is 7.3 kcal per mole of ATP times 38 moles of ATP per mole of glucose divided by 686 kcal per mole of glucose, which equals 0.4. Thus, about 40% of the energy stored in glucose has been transferred to storage in ATP. The rest of the stored energy is lost as heat. We use some of this heat to maintain our relatively high body temperature (37 C), and we dissipate the rest through sweating and other cooling mechanisms. Cellular respiration is remarkably efficient in its energy conversion. By comparison, the most efficient automobile converts only about 25% of the energy stored in gasoline to energy that moves the car. Facultative anaerobes On the cellular level, our muscle cells behave as facultative anaerobes. In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes. Under aerobic conditions, pyruvate can be converted to acetyl CoA, and oxidation continues in the citric acid cycle. Under anaerobic conditions, pyruvate is diverted from the citric acid cycle, serving instead as an electron acceptor to recycle NAD +. To make the same amount of ATP, a facultative anaerobe would have to consume sugar at a much faster rate when fermenting than when respiring. 9