4.3 Cellular Respiration The process by which glucose molecules are broken down to release energy is cellular respiration. Cellular respiration is a series of chemical reactions which produces ATP. Most cellular processes use ATP for energy. There are two types of cellular respiration. One is aerobic meaning that it requires oxygen. The other is anaerobic, meaning it can take place without oxygen. Aerobic respiration produces 36 ATP molecules from each glucose molecule. Anaerobic respiration produces only 2 ATP molecules per glucose. Some organisms are only capable of anaerobic respiration. Other organisms are capable of both types of cellular respiration. In most organisms, cellular respiration begins in the cytoplasm of the cell and continues in the mitochondria. 34 of the 36 ATP made from a glucose molecule are made in the mitochondria. The cells of all organisms, with the exception of bacteria, generate energy in “factories” called mitochondria. When you compare the reactants and products of photosynthesis and cellular respiration, cellular respiration appears to be the reverse of photosynthesis. The reactants of one process are the products of the other.

C 6 H 12 O 6 +6O 2  6C0 2 +6H 2 O+ENERGY Cellular Respiration 6CO 2 +6H 2 O+ENERGY  C 6 H 12 O 6 +6O 2 Photosynthesis Aerobic Respiration can be divided into three stages: glycolysis, the Krebs cycle, and electron transport. Glycolysis: Both aerobic and anaerobic respiration begin with a step called glycolysis. Glycolysis is the process by which glucose is converted to pyruvate and energy is released. Pyruvate is a three carbon molecule that enters the Krebs cycle. Glycolysis occurs in the cytoplasm of the cell. Glycolysis is a ten step process with intermediate products made at each step. To split glucose into pyruvate, a cell must first use the energy of two ATP molecules. A single glucose molecule that has undergone glycolysis will produce a net yield of two NADH, two ATP, and two pyruvate molecules.

The Krebs cycle-also called the citric acid cycle because the first reaction forms citric acid. Pyruvate is transformed into acetyl-CoA, losing a carbon in the form of CO 2. Acetyl-CoA enters the cycle. Acetyl-CoA goes through a series of reactions that produce a four carbon compound, ATP and NADH. The four carbon compound is recycled into a compound that can react with acetyl-CoA. FADH 2 and NADH are formed. Electron Transport The process by which energy is transferred from NADH and FADH 2 to ATP is the Electron transport chain. This phase of aerobic respiration requires oxygen. The movement of electrons from NADH and FADH 2 occurs along and electron transport chain in the inner membrane of the mitochondrian. The electron transport chain generates 32 of the 36 ATP molecules produced from each glucose molecule. Due to some energy loss in the form of heat, aerobic respiration is relatively inefficient.

Anaerobic Respiration Certain cells in your body, like muscle cells, can produce energy without oxygen. Some unicellular organisms such as yeast can also break down carbohydrates without oxygen. Anaerobic respiration is the process that releases energy from food molecules in the absence of oxygen. Through fermentation, anaerobic organisms and cells can exist without oxygen by using the small amount of energy supplied by glycolysis. Fermentation The extraction of energy from pyruvate in the absence of oxygen. When milk has gone sour, it is due to lactic acid produced by the fermenting action of bacteria on pyruvate in the milk. There are two types of fermentation: alcoholic fermentation and lactic acid fermentation. In both types of fermentation, only two ATP molecules are formed from each molecule of glucose. Yeasts are unicellular fungi that use aerobic respiration when oxygen is present. But yeasts switch to anaerobic respiration when oxygen is absent. Yeasts carry out alcoholic fermentation, the type of anaerobic respiration that converts pyruvate to carbon dioxide and ethanol. The process gets its name from the fact that one of its products, ethanol, is an alcohol. Pyruvate + NADH  Ethanol + NAD + + CO 2

Alcoholic fermentation is an important economic resource. Bakers use the alcoholic fermentation of yeast to make breads. As yeasts break down the carbohydrates in dough, CO 2 is produced and trapped in the dough. The bubbles of trapped CO 2 cause the dough to rise and are visible as the small holes in baked bread. As the dough bakes, the yeasts die, and the alcohol evaporates. Alcoholic fermentation is also used to make wine, beer, and the ethanol that is added to gasoline to make gasohol. Animal cells cannot perform alcoholic fermentation. However, some cells, such as your muscle cells, can convert pyruvate to lactic acid. Anaerobic respiration in which pyruvate is converted to lactic acid is called lactic acid fermentation. During strenuous exercise, your breathing cannot provide you with all of the oxygen that your cells need. When muscle cells run out of oxygen, they switch from aerobic respiration to lactic acid fermentation. Pyruvate + NADH  Lactic acid +NAD + Athletes may experience muscle fatigue and soreness caused by the buildup of lactic acid. Most of the lactic acid made in the muscles diffuses into the bloodstream and then into the liver, where it will be converted back to pyruvate.

ATP and calories The process of cellular respiration occurs in most living organisms. Using this process, all organisms break down glucose molecules and convert the stored energy into ATP. Glucose that is not used immediately can be stored by organisms in different substances. Plants store excess glucose in starch molecules. Animals store energy in glycogen or fat. When the organism needs energy, these storage molecules can be broken down to yield ATP. The energy contained in these substances can be measured and expressed in units called calories. A calorie is the amount of heat energy needed to raise the temperature of 1 gram of water 1 degree Celsius. Although a calorie is expressed as a unit of heat energy, the energy can be used for all of the countless functions of the cell. You may be familiar with the Calorie value on food items that you buy in the store. When Calorie is spelled with a capital “C”, it actually represents a kilocalorie (kcal), or 1000 calories. To measure the energy value of food, a small sample of food is burned. The amount of heat the burning food gives off is measured using an instrument called a calorimeter. This process is used to determine the number of Calories a particular kind of food contains. People “counting Calories” are actually keeping track of the amount of energy in the food they consume.

Murder mysteries: If you read mystery novels, you are probably aware of the use of arsenic as a poison. Arsenic accumulates in mitochondria, where it disables cellular respiration and paralyzes energy production. How would the action of arsenic in mitochondria harm an organism?