The Rehash of Cellular Respiration

This week in AP Biology we expanded even more on the catabolic process that is cellular respiration. Cellular respiration can be broken down into three smaller processes. These processes are glycolysis, Citric Acid or Krebs’s Cycle, and Oxidative phosphorylation which is made up of the electron transport chain and chemiosmosis.

The first reaction that occurs is glycolysis in the cytoplasm in this reaction glucose is broken down from a six carbon molecule into two three carbon molecules called pyruvate if oxygen is present. The result of this is four molecules of ATP although we had to use two molecules of ATP to start giving us a net profit of two ATPs and two molecules NADH or FADH₂. If the process has to occur anaerobically rather than aerobically the glucose molecule is still broken down and the ATP is till produced, but lactic acid fermentation and alcohol fermentation may occur. In muscle cells when the muscle needs more energy than cellular respiration is giving out it performs lactic acid fermentation. When the glucose is broken down into pyruvate the pyruvate is then reduced by NADH and lactate is formed as a waste product.  The lactic acid builds up and can only be removed by exposure to oxygen. In certain prokaryotes and other anaerobic organisms alcohol fermentation is the source of energy. The pyruvate produced by glycolysis is changed into ethanol in alcohol fermentation.

After the glucose is broken down into two pyruvates the pyruvates must then be converted to acetyl CoA and diffuse across the mitochondria membrane before it can enter the Citric Acid Cycle. After the pyruvates are changed into acetyl CoA they enter the Citric Acid Cycle. The Citric Acid Cycle ends up reeasing the original six carbon atoms that were part of glycolysis at the begging as well as 3 NADH, 1 FADH₂, and 1 ATP for every turn. Since there are 2 acetyl CoAs the cycle has two turns giving it at the end 4NADH, 2 FADH₂, and 2 ATPs.

The energy held in the NADH and FADH₂ electron carriers is then used by the electron transport system. Unlike glycolysis and the Citric Acid Cycle which produces ATP through substrate level phosphorylation the electron transport chain and chemiosmosis produce ATP through oxidative phosphorylation. The electron carriers then deposit electrons to the electron transport chain. The loss of energy from the electrons is used to pump protons across the mitochondria’s inner membrane. Once the electrons are done they then combine with two hydrogen ions and oxygen to form water. The large concentration of protons or H⁺ ions then power the enzyme ATP synthase which starts pumping out ATP. This part of the process is called chemiosmosis. The end result of this can produce up to 26 or 28 ATPs.