B) In the second part (shaded), the high-energy derivatives of glucose are oxidized and split into pyruvic acid. The second part yields re­duced NAD and a net gain in ATP.

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In glycolysis, the breakdown of the diphosphorylated sugar is coupled to ATP synthesis. The combined reaction still releases energy, but at the reduced level of about 4500 cal/mol:

Fig.7-4The reactions and enzymes of glycolysis.

The reactions attaching two phosphates to a glucose molecule in the initial steps require two molecules of ATP. Therefore, there is a net gain of two ATP, as well as two mole­cules of reduced NAD, for every glucose entering glycolysis. The overall reactants and products of glycolysis are therefore:

Fig.7-5NAD (nicotinamide adenine dinucleotide), an elec­tron carrier reduced in glycolysis and other cellular reactions.

Fig.7-6A summary of the glycolytic sequence

The overall reactants and products of glycolysis are therefore:

At the opposite end of the sequence, pyruvic acid may be modified to yield other products. In one of the most important of these modifications, pyruvic acid is converted into lactic acid after accepting electrons from the NAD reduced earlier in the seauence:

Another glycolytic variation occurs in organisms such as yeasts. In this modification, pyruvic acid accepts electrons from reduced NAD and is converted by additional reactions into ethyl alcohol (two carbons) and CO₂: