During aerobic respiration, cells obtain energy in the presence of oxygen through a series of reactions known as the citric acid cycle. Glucose provides a key reaction intermediate necessary for these reactions to occur. Glucose is a six-carbon sugar molecule that gets broken down into two three-carbon pyruvate molecules. These pyruvate molecules, in the presence of oxygen, can enter the citric acid cycle, producing a significant amount of energy for the cell.
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Glucose can be obtained directly from the diet or by the breaking down of glycogen, a polymer of glucose molecules. During glycolysis, glucose is metabolized by the cell to produce energy. Glycolysis is not very efficient in terms of energy production, but the process itself generates a series of intermediates that can be used for other processes. One such intermediate is pyruvate. In the absence of oxygen, pyruvate can be converted to lactic acid or alcohol through a process known as fermentation. However, in the presence of oxygen, during aerobic respiration, pyruvate can enter the citric acid cycle.
The Citric Acid Cycle
The citric acid cycle is a series of reactions that ultimately produce a significant amount of energy for the cell. This cycle can only occur under aerobic conditions -- that is, conditions in which sufficient oxygen is present.
In the presence of oxygen, the pyruvate molecules formed at the end of glycolysis can enter the citric acid cycle by reacting with a compound called Acetyl-CoA. During this reaction, carbon dioxide is released. In fact, carbon dioxide is released in a number of steps during the citric acid cycle. This is, in part, an explanation of why aerobic respiration involves breathing in oxygen and breathing out carbon dioxide.
Electron Transport Chain
By definition, aerobic respiration requires oxygen. Oxygen is required because it is needed in the electron transport chain.
The electron transport chain of a cell is a series of reactions that pair chemical reactions between electron donors and electron acceptors to the transfer of protons across a cellular membrane. In aerobic respiration, oxygen is the ultimate electron acceptor.
The transfer of electrons creates a proton gradient. When the protons travel back across the membrane, flowing down the gradient, energy in the form of molecules called ATP, or adenosine tri-phosphate, is created.
If there is no oxygen present, the gradient cannot be set up, and these reactions cannot occur.
While glucose can provide energy to the cell through glycolysis, this process is not very efficient. An input of two ATP energy molecules gets the reaction started, but in the end, only four ATP energy molecules are created.
Glucose provides a greater role for more efficient energy production by providing the pyruvate molecules for entrance into the citric acid cycle. At the end of the citric acid cycle, 36 ATP energy molecules are created for every one glucose molecule completely metabolized.
Sources of Glucose
Glucose can be obtained directly from the diet. Glucose is a six-carbon monosaccharide sugar molecule, also known as dextrose, or simple table sugar. It is also part of a long chain of energy storage molecules called glycogen. When cells need more glucose to produce more energy, glycogen can be broken down to release individual glucose monomers, which can then enter the glycolysis pathway. Eventually the resulting pyruvate molecules can enter the citric acid cycle, provided oxygen is present.