Unit two. The Living Cell
7. How Cells Harvest Energy from Food
7.2. Using Coupled Reactions to Make ATP
The first stage in cellular respiration, called glycolysis, is a series of sequential biochemical reactions, a biochemical pathway. In 10 enzyme-catalyzed reactions, the six-carbon sugar glucose is cleaved into two three-carbon molecules called pyruvate. The Key Biological Process illustration below presents a conceptual overview of the process, while figure 7.4 provides a more detailed look at the series of 10 biochemical reactions. Where is the energy extracted? In each of two “coupled” reactions (steps 7 and 10 in figure 7.4), the breaking of a chemical bond in an exergonic reaction releases enough energy to drive the formation of an ATP molecule from ADP (an endergonic reaction). This transfer of a high-energy phosphate group from a substrate to ADP is called substrate-level phosphorylation. In the process, electrons and hydrogen atoms are extracted and donated to a carrier molecule called NAD+. The NAD+ carries the electrons as NADH to join the other electrons extracted during oxidative respiration, discussed in the following section. Only a small number of ATP molecules are made in glycolysis itself, two for each molecule of glucose, but in the absence of oxygen this is the only way organisms can get energy from food.
Figure 7.4. The reactions of glycolysis.
The process of glycolysis involves 10 enzyme-catalyzed reactions.
Glycolysis is thought to have been one of the earliest of all biochemical processes to evolve. Every living creature is capable of carrying out glycolysis.
Key Learning Outcome 7.2. In the first stage of cellular respiration, called glycolysis, cells shuffle chemical bonds in glucose so that two coupled reactions can occur, producing ATP by substrate-level phosphorylation.