Glycolysis
Glycolysis is the first stage of aerobic (and anaerobic) respiration. It takes place in the cytoplasm of the cell. In glycolysis (“glucose breaking”), ATP is used to split glucose molecules into a three-carbon compound called pyruvate. This splitting produces energy that is stored in ATP and a molecule called NADH. The chemical formula for glycolysis is:
C6H12O6 + 2ATP + 2NAD+
2pyruvate + 4ATP + 2NADH
As the formula indicates, the cell must invest 2 ATP molecules in order to get glycolysis going. But by the time glycolysis is complete, the cell has produced 4 new ATP, creating a net gain of 2 ATP. The 2 NADH molecules travel to the mitochondria, where, in the next two stages of aerobic respiration, the energy stored in them is converted to ATP.
The most important things to remember about glycolysis are:
- Glycolysis is part of both aerobic and anaerobic respiration.
- Glycolysis splits glucose, a six-carbon compound, into two pyruvate molecules, each of which has three carbons.
- In glycolysis, a 2 ATP investment results in a 4 ATP payoff.
- Unlike the rest of aerobic respiration, which takes place in the mitochondria, glycolysis takes place in the cytoplasm of the cell.
- Unlike the rest of aerobic respiration, glycolysis does not require oxygen.
The Krebs Cycle
After glycolysis, the pyruvate sugars are transported to the mitochondria. During this transport, the three-carbon pyruvate is converted into the two-carbon molecule called acetate. The extra carbon from the pyruvate is released as carbon dioxide, producing another NADH molecule that heads off to the electron transport chain to help create more ATP. The acetate attaches to a coenzyme called coenzyme A to form the compound acetyl-CoA. The acetyl-CoA then enters the Krebs cycle. The Krebs cycle is called a cycle because one of the molecules it starts with, the four-carbon oxaloacetate, is regenerated by the end of the cycle to start the cycle over again.
The Krebs cycle begins when acetyl-CoA and oxaloacetate interact to form the six-carbon compound citric acid. (The Krebs cycle is also sometimes called the citric acid cycle.) This citric acid molecule then undergoes a series of eight chemical reactions that strip carbons to produce a new oxaloacetate molecule. The extra carbon atoms are expelled as CO2 (the Krebs cycle is the source of the carbon dioxide you exhale). In the process of breaking up citric acid, energy is produced. It is stored in ATP, NADH, and FADH2. The NADH and FADH2 proceed on to the electron transport chain.
The entire Krebs cycle is shown in the figure below. For the SAT II Biology, you don’t have to know the intricacies of this figure, but you should be able to recognize that it shows the Krebs cycle.
It is also important to remember that each glucose molecule that enters glycolysis is split into two pyruvate molecules, which are then converted into the acetyl-CoA that moves through the Krebs cycle. This means that for every glucose molecule that enters glycolysis, the Krebs cycle runs twice. Therefore, for one glucose molecule running through aerobic cell respiration, the equation for the Krebs cycle is:
