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What would be the problem if respiration stopped after glycolysis?

The goal of respiration is to catabolize organic compounds and to capture the energy that they contain in the bonds of ATP. ATP is the energy currency of the cell. Whenever an anabolic process is needed, the energy to fuel it is provided by ATP. Aerobic respiration (using oxygen) consists of 3 main and 1 small step.


1. Glycolysis: Glucose (6 carbons) is broken down into 2 pyruvates (each 3 carbons). 2 NADH and 2 ATP are produced. If respiration continues, the NADH will eventually go to the mitochondria to the electron transport chain.


2. Transition step: The two pyruvates are oxidized to produce two acetylCoA molecules and 2 NADH. NADH eventually make their way to the electron transport chain.


3. TCA cycle: The two acetyl CoA molecules enter the TCA cycle where 6 NADH, 2 FADH2, 2 ATP and 4 CO2 are formed. The NADH and FADH2 make their way to the electron transport chain.


4.  Electron transport chain: The electrons carried by NADH and FADH2 are delivered to electron transporters which pass them down a chain of proteins and eventually to the final electron acceptor, oxygen. As the electrons are passed, the energy from this is used to pump H+ (protons) across the inner mitochondrial membrane to create a gradient. The only way these protons can cross the membrane and go down their concentration gradient is through ATP synthase, resulting in oxidative phosphorylation of ADP and the formation of many ATP.  


If all 4 of these steps are completed, the cell will produce a maximum of 36 ATP per glucose molecule. However, if oxygen is not available, the process will stop after glycolysis. Notice that glycolysis produces only 2 ATP compared to 36 if the entire set of steps is completed. This is typically not enough to support life. Also, a supply of NAD+ is required during glycolysis to oxidize the intermediate molecules (collect electrons from them). In humans, the pool is finite and NAD+ must be regenerated (usually occurs when they drop off electrons to the electron transport chain). In order to do this without oxygen, lactate dehydrogenase is used to reduce the pyruvates produced by glycolysis to lactic acid. This regenerates NAD+ and keeps glycolysis going. There are side effects of lactic acid. In humans, lactic acid accumulates in muscles when they are overworked and run short of oxygen. The lactic acid results in soreness.  

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