Slide 1 Recall that at the end of glycolysis, there are two molecules of pyruvic acid. These pyruvic acid molecules have a lot of free energy and, if oxygen is present, can be further metabolized by the respiring cell. If no oxygen is present - the pyruvate is reduced during fermentation. Also recall that glycolysis has produced two molecules of NADH. We have already learned that if there is no oxygen present - the cell will use fermentation to regenerate the NAD+ needed by glycolysis. Now we will learn what happens to both pyruvate and NADH when oxygen is present.
Slide 2 If oxygen is present pyruvate enters the Krebs cycle (in the form of acetyle CoA) and electrons go to the electron transport chain. NADH is oxidized by the electron transport chain to regenerate NAD+ (oxygen is the terminal e- acceptor).
Slide 3 Glycolysis takes place in the cytoplasm of the cell while the Krebs cycle enzymes are located in the mitochondria. In eukaryotes, pyruvate is transported across the mitochondrial membrane and then converted to acetyl CoA (with the production of NADH and carbon dioxide). Acetyl CoA is a 2-carbon compound. Acetyl CoA then enters the Krebs cycle. During the Krebs cycle, electrons are removed from acetyl CoA and these electrons reduce more NAD+, along with another electron carrier, FAD. The ATP that is produced is generated via substrate-level phosphorylation, just as in glycolysis. Each NADH and FADH2 molecule that is formed is an important source of energy that will be used to generate more ATP.
Slide 4 So, where's the energy at this point? Starting with a single glucose molecule, a respiring cell will produce a net of 4 ATP at the end of the krebs cycle. All of the carbon has been lost as CO2 there is no more energy in the bonds of an organic molecule. So - where is the rest of the energy?
Slide 5 To answer this question let's look at this view of the Krebs cycle. It is not necessary to know this level of detail but you should know the main inputs and outputs. Acetyl CoA enters the cycle and the electrons removed from this molecule are used to reduce NAD+ and FAD. After one turn of the cycle, 3 molecules of NADH are produced and one molecule of FADH2. A single molecule of ATP is also produced. For each glucose molecule - the cycle turns twice - because there were two pyruvate molecules produced during glycolysis. This means that per glucose - the Krebs cycle produces 2 ATP, 6NADH, and 2 FADH2.
Slide 6 We can now answer this question. The rest of the energy that was once in the glucose molecule is now in the high energy electron carrier molecules - NADH and FADH2. Remember, these are high energy electron carrier molecules - there is free energy in the electrons these molecules carry and the next stage of respiration - the electron transport chain - will be able to convert that energy to energy if the form of more ATP molecules.