As previously discussed, this initial phase of all forms of respiration brings about the conversion of each 6-carbon glucose molecule to two 3-carbon pyruvic acid molecules via three main steps, each mediated by enzymes. The three main steps are as follows:
1. Phosphorylation, whereby the 6-carbon sugars receive phosphates
2. Sugar cleavage, which involves the splitting of 6-car-bon fructose into two 3-carbon sugar fragments
3. Pyruvic acid formation, which involves the oxidation of the sugar fragments
Energy needed to initiate the process is furnished by an ATP molecule, which also furnishes the phosphate group for the phosphorylation of the sugar glucose to yield glucose 6-phosphate. Another ATP, with the aid of the enzyme fructoki-nase, yields fructose bisphosphate (fructose 1,6-diphosphate). As a result of the cleavage of the fructose bisphosphate, two different 3-carbon sugars are produced, but ultimately, only two glyceraldehyde 3-phosphate (GA3P) molecules remain. These two 3-carbon sugars are oxidized to two 3-carbon acids, and, in the successive production of several of these acids, phosphate groups are removed from the acids. The phosphate groups combine with ADP, producing a net direct gain of two ATP molecules during glycolysis. In addition, hydrogen is removed as GA3P is oxidized. This hydrogen is picked up by the acceptor molecule, NAD, which becomes NADH. Glycolysis, which requires no oxygen gas, is summarized in Figure 10.14.
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