Cellular Respiration Explained
Cellular respiration is a process that is undergone in cells to break down molecules and produce ATP. The energy released from the broken down molecules are a result of spontaneous catabolic reactions. The most basic 3 metabolic stages within an animal cell are separated as followed: glycolysis, the Krebs Cycle, and the Electron Transport chain. Glycolysis and Krebs Cycle both synthesize ATP through substrate level phosphorylation which is ineffective through net ATP yield while the Electron Transport chain uses oxidative phosphorylation which has a highly effective ATP net yield.
The first step of cellular respiration in animal cells is anarobic, meaning it does not require oxygen. This step of cellular respiration is glycolysis and in the end yields only a net gain of 2 ATP molecules. Glycolysis occurs in the cytosol of the cell and is divided into two phases. The first phase is the investment phase in which 2 ATP is utilized though as series of catalyzed reactions to break down glucose into two glyceraldehydes phosphates. The two glyceraldehydes are then used in turn in the next phase of glycolysis: the pay-off phase. In the pay-off phase, another series of catalyzed reactions take place to produce 4 ATP and two pyruvate. In the end, the positive net gain of glycolysis is a mere 2 ATP. The pyruvate product of glycolysis is then sent to the Krebs Cycle if oxygen is present. If oxygen is not present, fermentation occurs.
Before the pyruvate can be used in the Krebs Cycle, the next step of cellular respiration (also anareobic), they need to be converted into Acetyl CoA. This process begins as the pyruvate leave the cytosol and enter the mitochondrial matrix through a transport protein. Soon after the pyruvate undergoes several oxidation reduction reactions they result in Acetyl CoA.
The Acetyl CoA then enters the Krebs Cycle in the mitochondrial matrix where it undergoes 7 catalyzed reactions twice yielding in the end 6 NADH, 2 FADH2, 4CO2, and 2 ATP molecules. The Krebs Cycle cycles twice because in the beginning two pyruvate were available to make two Acetyl CoA. The end yields of one cycle also allows for another cycle to go again with the second Acetyl CoA. In the end the positive net yield of ATP from the Krebs Cycle is 2 ATP. Again because this is anaerobic cellular respiration and does not involve oxidative phosphorylation there is not a large yielding of ATP molecules.
The NADH and FADH2 of the Krebs Cycle then play a role in the 3rd stage of cellular respiration: the Electron Transport Chain. Here, the NADH and FADH2 carry electrons from the Krebs Cycle to be used in the Electron Transport Chain. The electrons are then transferred through a series of protein acceptors. This process however does not make ATP directly. The Electron Transport Chain works with Chemiosmosis as a team. Chemiosmosis utilizes oxidative phosphorylization and creates large yields of ATP. This is the most productive and efficient production of ATP in animal cell cellular respiration. This process is aerobic and requires an oxygen molecule to recieve H+ protons at the end of the ETC (Electron Transport Chain).
Chemiosmosis works by first having a NADH shuttle an electron to the chain of protein acceptors. In the process the NADH loses a H+ to change into NAD+. As the electron moves through the chain of proteins, H+ is pumped out of the cell into the intermembrane space outside of the mitochondrial matrix by proteins. The electron at the end of the chain finds its way to an oxygen to form water. The concentration of H+ outside of the mitochondrial matrix eventually builds up to a high concentration in so that there are more H+ outside of the mitochondrial matrix than there are inside. Following the natural tendency to strive for equilibrium the H+ find their way to return to the inside of the mitochondrial matrix through the ATP Synthase protein. Theory is that the urge for the H+ to go to the lower gradient inside the cell is harnessed by the ATP Synthase protein which generates ATP by phosphorylating ADP. Chemiosmosis transfers energy from oxidation reduction reactions to ATP. The end result with the cooperation of both the Electron Transport Chain and Chemiosmosis is about 34 ATP.
In the end, the total ATP generated by cellular respiration for one molecule of glucose has a maximum positive yield of about 38 ATP.
