Cellular Respiration To obtain energy to move and grow it is necessary for organisms to break down their food. Biologically this breakdown is known as Cellular Respiration and is accomplished by breaking down Glucose into CO2 in a series of chemical reactions.

This occurs in the Mitochondria of all living cells. There are 4 major steps to Cellular Respiration: Glycolysis, Krebs Cycle Preparation, Krebs Cycle and Electron Transport Chain.

Step 1: Glycolysis Occurs just outside the mitochondria in the cytoplasm. Is the first step in both aerobic and anaerobic respiration The word glycolysis refers to the splitting of glucose.

2 phosphate groups are bonded to a glucose molecule 2 phosphate groups are bonded to a glucose molecule. This uses 2 molecules of ATP. A group of chemical reactions break down the glucose molecule into 2 molecules of pyruvate (pyruvic acid). This produces 4 molecules of ATP. The net result is thus 2 molecules of ATP for every glucose. Also, 2 molecules of NADH are formed.

At this point Pyruvate can follow 1 of 2 pathways At this point Pyruvate can follow 1 of 2 pathways. One is aerobic and one is anaerobic. The anaerobic pathway is called fermentation and there are 2 types. Lactic acid fermentation and alcoholic fermentation.

Aerobic and Anaerobic Exercise When you exercise at a slow enough pace that your blood can supply your muscles with adequate oxygen, aerobic respiration takes place normally. This is aerobic exercise. During prolonged or strenuous activity, the cells cannot get sufficient oxygen. This is anaerobic exercise. In this case, only glycolysis can take place. This results in a buildup of lactic acid, which makes the muscle feel tired. After a period of rest, the muscle can get enough oxygen to convert lactic acid to pyruvic acid and begin aerobic respiration again. This repays the oxygen debt.

Anaerobic Respiration Video https://www.youtube.com/watch?v=cDC29iBxb3w

Step 2: Krebs Cycle Prep Once pyruvate is in the matrix of the mitochondrion, a step called Krebs Cycle Prep (Pyruvate Breakdown) takes place Here pyruvate breaks down into CO2, NADH, and a 2-carbon compound. This compound, combined with coenzyme A, forms acetyl CoA.

Step 3: Krebs Cycle Cycle is aerobic and occurs in the matrix of the mitochondria

Named for Sir Hans Krebs The Acetyl CoA molecules enter the Krebs cycle and combine with molecules to form Citric Acid. Through a series of reactions the Citric Acid is broken down into 4 CO2 and 16 H+ ions. Each turn of the Krebs Cycle takes one molecule of Acetyl CoA and converts it to 2 molecules of CO2, 3 molecules of NADH, 1 molecule of FADH2 and 1 molecule of ATP Because 2 Acetyl CoA go through the cycle the end yield is 4 CO2, 6 NADH, 2 FADH2 and 2 ATP.

Step 4: ETC Most of the energy released by breaking down glucose comes through the electron transport chain. Occurs in the inner mitochondrial membranes This is a highly organized system of enzymes and coenzymes on the inner membrane of the mitochondrion. NADH and FADH2 carry electrons into the chain and pass them from one compound to another. Eventually, oxygen is the final acceptor. At various points, energy is given up and used to pump hydrogen ions across the mitochondrial membrane. This creates an electrical gradient, which provides the energy to convert ADP to ATP

At the end of the chain the electrons are accepted by O2 which combines with the H+ ions to form water. Overall 32 ATP are made in this step which added to the 2 ATP made in Glycolysis and the 2 made in Krebs Cycle nets a total of 36 ATP molecules made.

Working out the Numbers In glycolysis, one molecule of glucose forms 2 molecules of ATP. In the Krebs cycle, 2 more molecules of ATP are formed (1 for each pyruvic acid) In the Electron Transport Chain, 32 molecules of ATP are formed from each glucose molecule. Therefore, each glucose molecule yields 36 ATP molecules. The net reaction is: C6H12O6 + 6O2  6CO2 + 6H2O + Energy (36 ATP)