BIO Lecture 7: Outline of Respiration

Here is a big picture summary of how all of the different stages of metabolism fit together. We'll start looking at the specifics in the next lecture. Keep in mind that when glucose is oxidized there is very little ATP generated directly. Instead, the oxidation of glucose is accompanied by the reduction of high-energy electron carriers, primarily the reduction of NAD+ to NADH (remember from last lecture that when something is oxidized something else must be reduced). The energy in reduced NADH is then used to pump protons out of the interior of the mitochondria and create a proton gradient--this is what finally drives the production of ATP. You should know that oxygen is the final electron acceptor of the electron transport chain, and that anaerobic respiration is insufficient to sustain human life. In addition, fermentation produces lactic acid as a byproduct in humans, and ethanol in yeast. Finally, you should know where in the cell each stage of respiration occurs.

Here are the stages that oxidize glucose to produce CO2 and ATP:

1. Glycolysis: "glucose splitting"; anaerobic (no oxygen required) and occurs in cytoplasm; glucose is partially oxidized while it is split in half into two pyruvate molecules
--2 net ATP (4 total made, but 2 needed in investment phase)
--2 NADH produced (3 ATP in ETC for eukaryotes and 5 ATP for prokaryotes)

2. Fermentation: In short, in anaerobic conditions (without oxygen), electron transport cannot function and the limited supply of NAD+ becomes entirely converted to NADH. Therefore, fermentation has evolved to regenerate NAD+ in anaerobic conditions thereby allowing glycolysis to continue in the absence of oxygen. This process also occurs in the cytoplasm.
--0 ATP; main purpose is to reoxidize the NADH produced in glycolysis (pyruvate is the electron acceptor)

3. Pyruvate Dehydrogenase complex (PDC): the pyruvate produced in glycolysis is decarboxylated to form an acetyl group which is then attached to coenzyme A (a carrier that transfers the acetyl group into the Krebs cycle). Only occurs when oxygen is available but doesn't use oxygen. Occurs in mitochondria matrix for eukaryotes and in the cytoplasm for prokaryotes. It is an aerobic process.
--0 ATP produced
--2 NADH produced (makes 5 ATP in ETC).

4. Krebs cycle (aerobic process): the acetyl group from PDC is added to oxaloacetate to form citric acid--the citric acid is then decarboxylated and isomerized to regenerate the original oxaloacetate. Only occurs when oxygen is available but doesn't use oxygen. Occurs in the mitochondria matrix of eukaryotes and in the cytoplasm for prokaryotes. To fully oxidize the equivalent of one glucose molecule, two acetyl-CoA must be metabolized by the cycle.
Per glucose molecule:
--2 GTP produced (equivalent of 2 ATP)
--6 NADH produced (making 15 ATP in ETC)
--2 FADH2 produced (making 3 ATP in ETC)

4. Electron transport/oxidative phosphorylation: NADH and FADH2 are oxidized by ETC in the inner mitochondrial membrane for eukaryotes and across the inner cell membrane for prokaryotes. In other words, oxidation of metabolic fuels such as glucose and the oxidation of acetyl carbons to CO2 via the citric acid cycle yields the reduced cofactors NADH and FADH2. These compounds are forms of energy currency because their reoxidation--ultimately by molecular oxygen--is an exergonic reaction. The free energy thereby released is harvested to synthesize ATP. It's an aerobic process as well.
--NADH oxidized back to NAD+ and FADH2 oxidized back to FAD occur along with ATP production allowing earlier stages to continue

Summary: 30 ATP (eukaryotes) and 32 ATP (prokaryotes)

I know this is a lot to comprehend right now but the key is to understand how all of the processes function together.