Adp

Figure 4.8

ATP provides energy for anabolic reactions and is regenerated by catabolic reactions.

Energy released and utilized by metabolic reactions

is sometimes referred to as the anaerobic phase of cellular respiration.

Glycolysis can be summarized by three main events (fig. 4.9):

1. First, glucose is phosphorylated by the addition of two phosphate groups, one at each end of the molecule. Although this step requires ATP, it "primes" the molecule for some of the energy-releasing reactions that occur later on.

2. Second, the 6-carbon glucose molecule is split into two 3-carbon molecules.

3. Third, NADH is produced, ATP is synthesized, and two 3-carbon pyruvic acid molecules result. Note that some of the reactions of glycolysis release hydrogen atoms. The electrons of these hydrogen atoms contain much of the energy associated with the chemical bonds of the original glucose molecule. To keep this energy in a form the cell can use, these hydrogen atoms are passed in pairs to molecules of the hydrogen carrier NAD+ (nicotinamide adenine dinucleotide). In this reaction, two of the electrons and one hydrogen nucleus bind to NAD+ to form NADH. The remaining hydrogen nucleus (a hydrogen ion) is released as follows:

NADH delivers these high-energy electrons to the electron transport chain elsewhere in the mitochondria, where most of the ATP will be synthesized.

ATP is also synthesized directly in glycolysis. After subtracting the two ATP used in the priming step, this gives a net yield of two ATP per molecule of glucose.

Disruptions in glycolysis or the reactions that follow it can devastate health. Clinical Application 4.1 il-

Key:

P = Phosphate

Glucose

Phase 1

Priming

Essentials of Human Physiology

Essentials of Human Physiology

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