Cellular respiration can be divided into two stages:
Organic compounds are converted into three-carbon molecules of pyruvic acid, producing a small amount of ATP and NADH (an electron carrier molecule). Glycolysis is an anaerobic process because it does not require the presence of oxygen.
2. Aerobic Respiration
If oxygen is present in the cell’s environment, pyruvic acid is broken down and NADH is used to make a large amount of ATP through the process known as aerobic respiration (covered later). Pyruvic acid can enter otherpathways if there is no oxygen present in the cell’s environment. The combination of glycolysis and these anaerobic pathways is called fermentation.
Many of the reactions in cellular respiration are redox reactions. Recall that in a redox reaction, one reactant is oxidized (loses electrons) while another is reduced (gains electrons). Although many kinds of organic compounds can be oxidized in cellular respiration, it is customary to focus on the simple sugar called glucose (C6H12O6). The following equation summarizes cellular respiration: C6H12O6 + 6O2 —–> 6CO2 + 6H2O + energy (ATP)
This equation, however, does not explain how cellular respiration occurs. It is useful to examine each of the two stages. The first stage of cellular respiration is glycolysis.
Glycolysis is a biochemical pathway in which one six-carbon molecule of glucose is oxidized to produce two three-carbon molecules of pyruvic acid. Like other biochemical pathways, glycolysis is a series of chemical reactions catalyzed by specific enzymes. All of the reactions of glycolysis take place in the cytosol and occur in four main steps.
In step 1, two phosphate groups are attached to one molecule of glucose, forming a new six-carbon compound that has two phosphate groups. The phosphate groups are supplied by two molecules of ATP, which are converted into two molecules of ADP in the process.
In step 2 , the six-carbon compound formed in step 1 is split into two three-carbon molecules of glyceraldehyde 3-phosphate (G3P). Recall that G3P is also produced by the Calvin cycle in photosynthesis.
In step 3 ,the two G3P molecules are oxidized, and each receives a phosphate group. The product of this step is two molecules of a new three-carbon compound. The oxidation of G3P is accompanied by the reduction of two molecules of nicotinamide adenine dinucleotide (NAD+) to NADH. NAD+ is similar to NADP+, a compound involved in the light reactions of photosynthesis. Like NADP+, NAD+ is an organic molecule that accepts electrons during redox reactions.
In step 4, the phosphate groups added in step 1 and step 3 are removed from the three-carbon compounds formed in step 3 . This reaction produces two molecules of pyruvic acid. Each phosphate group is combined with a molecule of ADP to make a molecule of ATP. Because a total of four phosphate groups were added in step 1 and step 3 , four molecules of ATP are produced. Notice that two ATP molecules were used in step 1, but four were produced in step 4. Therefore, glycolysis has a net yield of two ATP molecules for every molecule of glucose that is converted into pyruvic acid. What happens to the pyruvic acid depends on the type of cell and on whether oxygen is present.
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