Aerobic Respiration, Part 1: Glycolysis
You have read that nearly all of the energy used by living things comes to them in the bonds of the sugar, glucose. Glycolysis is the first step in the breakdown of glucose to extract energy for cell metabolism. Many living organisms carry out glycolysis as part of their metabolism. Glycolysis takes place in the cytoplasm of most prokaryotic and all eukaryotic cells.
Glycolysis begins with a molecule of glucose (C6H12O6). Various enzymes are used to break glucose down into two molecules of pyruvate (C3H4O3, basically a glucose molecule broken in half) (Figure 1). This process releases a small amount of energy.
Glycolysis consists of two distinct phases: energy-requiring, and energy-producing.
Energy-Requiring Steps
The first part of the glycolysis pathway requires an input of energy to begin. The first step in glycolysis is catalyzed by hexokinase, an enzyme with broad specificity that catalyzes the phosphorylation of six-carbon sugars. Hexokinase phosphorylates (adds a phosphate to) glucose using ATP as the source of the phosphate (Figure 2). This produces glucose-6-phosphate, a more chemically reactive form of glucose. This phosphorylated glucose molecule can no longer leave the cell because the negatively charged phosphate will not allow it to cross the hydrophobic interior of the plasma membrane.
Several additional enzymatic reactions occur (Figure 2), one of which requires an additional ATP molecule. At the end of the energy-requiring steps, the original glucose has been split into two three-carbon molecules, and two ATPs have been used as sources of energy for this process.
Energy-Producing Steps
So far, glycolysis has cost the cell two ATP molecules and produced two small, three-carbon sugar molecules. Both of these molecules will proceed through the second half of the pathway, and sufficient energy will be extracted to pay back the two ATP molecules used as an initial investment and produce a profit for the cell of two additional ATP molecules and two even higher-energy NADH molecules (Figure 3).
During the energy-producing steps, additional enzymes continue to catalyze the breakdown of glucose (Figure 3). The end result of these reactions is two 3-carbon molecules of pyruvate.
An important rate-limiting step occurs at step 6 in glycolysis. If you look at Figure 3, you will notice that during step 6, NAD+ is converted into NADH. NADH contains more energy than NAD+, and is therefore a desired product from this reaction. However, the continuation of the reaction depends upon the availability NAD+. Thus, NADH must be continuously converted back into NAD+ in order to keep this step going. If NAD+ is not available, the second half of glycolysis slows down or stops.
If oxygen is available in the system, the NADH will be converted readily back into NAD+ by the later processes in aerobic cellular respiration. However, if there is no oxygen available, NADH is not converted back into NAD+. Without NAD+, the r