Cellular respiration can be divided into two stages:
1. Glycolysis
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.
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
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 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
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