Everyone knows we breathe in Oxygen and exhale Carbon Dioxide that was released from our cells. But most people don't understand just why we need that Oxygen for life. It's primary role can be found deep inside the Mitochondria, in one of the critical stages of Cellular Respiration.
Glucose produced from the process of Photosynthesis will be come an essential reactant to accompany Oxygen in the energy-releasing phase of the our metabolism. Remember, it's not energy creation - it's energy release from a chemical storage unit constructed by the autotrophs. The beginning stage of cellular respiration is Glycolysis, and it tells an ancient tale of the struggle for energy. Since Glycolysis occurs in the cytoplasm, a region of the cell found in all living things, this represents the most likely source of energy for even the most primitive of organisms billions of years ago. Glycolysis is the breakdown of a 6-Carbon Glucose molecule into two 3-Carbon Pyruvate molecules. Some energy is given off during this breakdown, and a very small contribution is made to the energy fund for the body.
Following this process, the Pyruvate will either move to the Krebs Cycle (aerobic respiration) or down the anaerobic pathway that leads to fermentation and the buildup of lactic acid or alcohols. If we're lucky and have done our breathing exercises, the Krebs Cycle will happily accept the Pyruvate, which have conveniently been converted to Acetyl Co-A right before they enter the cycle. The Krebs Cycle itself does not generate much energy in the form of ATP, however it creates some wonderfully powerful molecules of NADH+ and FADH2+ which will slide on down to the Electron Transport Chain.
Here's where the big money can be found. The Electron Transport Chain functions much like that seen in Photosynthesis (hmmmmmmm, verrryyy interesting! I wonder why...). Electrons are passed along the chain, creating a strong negative force which helps to pull Protons to one side of the membrane. Because the protons create an imbalanced gradient, they will move passively down the gradient through a fantastic protein structure known as an ATP synthase. This little molecule spins like a water wheel, happily slapping Phosphate groups onto ADP molecules, and in the process, making a total of about 32 ATP per molecule of Glucose. What a bargain!
So why do we need Oxygen again? It turns out that as all of those protons (H+) are being shuttled around, some of them must come out of the system to keep the gradient effective. These H+ bind to the Oxygen we breathe in and make water, which will then exit our cells. So, the water we drink comes from the air we breathe, and the air we breathe comes from the water we drink. Cool, huh?
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