What products of the Krebs cycle will be used in the next stage of aerobic respiration?

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trophyhunter1 eNotes educator| Certified Educator

Aerobic respiration has three stages--glycolysis, citric acid cycle (Krebs Cycle) and oxidative phosphorylation.

This process requires an organic molecule like glucose to be used as a fuel or energy reserve to produce ATP, which can be used for cellular work.

In aerobic respiration, oxygen is a reactant which combines with glucose in a catabolic or energy releasing process. The wastes generated at the end of this process are carbon dioxide and water which can be released by exhaling.

Glucose and oxygen form carbon dioxide, water, and ATP --usually 32 or 34 molecules.

Glucose is oxidized in a series of steps using different enzymes. As electrons are stripped from the glucose, they are transferred to a coenzyme called NAD+. This is an oxidizing agent. When NAD+ accepts two electrons and a proton, it becomes NADH. NADH has stored energy and it is used to make ATP as these electrons fall down their energy gradient to oxygen, the final electron acceptor.

NADH brings the electrons that came from glucose to the top of the electron transport chain which is high energy and they "fall" to the lower energy end where oxygen captures electrons and H+ to form water (H2O). This is an exergonic reaction.

The path that electrons travel is therefore from glucose to NADH to the electron transport chain to oxygen. 

Glycolysis breaks down glucose into two molecules of pyruvate which occurs in the cytosol. 

Pyruvate enters the mitochondrion and is oxidized to acetyl CoA which enters the citric acid or Krebs Cycle. During the citric acid cycle, NAD+ is reduced to NADH when it accepts two electrons and a H+ ion. This substance transports the electrons  originally from the glucose to the third stage of respiration. Also, some ATP is generated during the Krebs cycle which can be used for cellular work. Yet other energy can be captured as FADH2.

In the electron transport chain, the electrons move from molecule to molecule until they reach the final electron acceptor which is oxygen to form water. Also, ATP is generated by oxidative phosphorylation. The inner membrane of the mitochondria is the site for electron transport and the production of ATP.


gsenviro eNotes educator| Certified Educator

Respiration is the process by which glucose (obtained from the food we eat) is broken down and useful energy is released in the form of ATP molecules. This process can take place either in the presence of oxygen (called aerobic respiration) or absence of oxygen (called anaerobic respiration). Depending on the presence/absence of oxygen, it can generate different numbers of ATP molecules per amount of glucose processed. The aerobic respiration process has 3 parts: glycolysis, Krebs cycle (or citric acid cycle) and electron transport chain or oxidative phosphorylation.

In the process of glycolysis (which takes place in the cytoplasm), the glucose molecule is broken into 2 pyruvate molecules and this process releases a net total of 2 ATP molecules. Apart from this, 2 NADH molecules are also generated in glycolysis. The pyruvate molecules get converted to acetyl coenzyme A molecules, along with the production of carbon dioxide (which is exhaled) and NADH molecules. These 2 acetyl coenzyme A molecules then enter the Krebs cycle (which takes place in the mitochondria) and get converted to molecules of NADH, FADH2, ATP, and carbon dioxide. Among all these products, NADH and FADH2 molecules are the ones that undergo the next steps in the oxidative phosphorylation process. It is the last step of aerobic respiration, the oxidative phosphorylation, which produces the maximum number of ATP molecules.

Overall, the process of aerobic respiration produces 36 molecules of ATP for every molecule of glucose.  

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