Biology 101: Cellular Respiration

Take a deep breath. Now what just happened?

Okay, you took in air and the oxygen in it was absorbed by your lungs and travelled through your bloodstream to your cells. Now what? What do your cells want oxygen for?

Cells (as I put in my last post) are complex organisms, always in motion, always working. That work is powered by the mitochondia (also called 'the powerhouses of the cell'). The process of energy production that they do is called cellular respiration.

Cellular respiration is a process that converts a molecule of sugar (glucose)--or some other energy source: carbohydrate, protein, or fat--and six molecules of oxygen into six molecules of carbon dioxide and six molecules of water. (The chemical formula is C6H12O6 + 6 O2 → 6 CO2 + 6 H2O.) It actually consists of three processes: glycolysis, the citric acid cycle (aka the Krebs cycle), and something called oxidative phosphorylation (aka the electron transport chain).
Glycolysis is the process where glucose (or some other carbohydrate/protein/fat) is broken down into the chemical pyruvate. It takes place in the fluid of the cell (which is called the cytosol). This is done as a first step and is in itself a complex process that creates a small amount of energy in the form of molecules of ATP and NADH. Basically the cell uses these molecules as ways to store energy, sort of like little batteries that can be plugged in and used when energy is needed. Once glycolysis is complete a couple of things can happen.

The most likely (in our bodies, anyway) thing to happen next is that the pyruvate enters the citric acid cycle. This is a really complex circle of reactions that take the pyruvate and break it down into carbon dioxide and water. It takes place in the mitochondria in our cells and whether it happens or not is decided by whether there is oxygen available or not.

If there isn't oxygen available (either because this is happening in a muscle that can't get oxygen quickly enough or because we're talking about yeast or bacteria), alternatively cells can use fermentation, which creates a lot of waste products--lactic acid in the case of your muscles, and which is why they become sore after hard work, as well as what happens from the bacteria in yogurt, and alcohol in the case of yeast, and people drink the waste products.

Assuming that the citric acid cycle happens, a few more energy molecules (ATP, NADH, and FADH2) are created. But the real energy pay-off is from the third part of the respiration process. This is called oxidative phosphorylation which breaks down the hydrogen from the glucose (or whatever) into electrons and protons (which is all hydrogen is, an electron and a proton) and sends the electrons along an electron transport chain in the membrane of the mitochondrion (the singular of mitochonria) and pumps the protons back and forth through the membrane. The whole process of the electrons travelling along the transport system reminds me of electricity (ie, electrons flowing through a wire). And the process creates a whole lot of ATP, which is what powers all the work your cells do.

Now here's what keeps it going. At the end of that transport chain is a molecule of oxygen. Oxygen is, in this case, the electron acceptor--it's what attracts the electrons and keeps them flowing through the transport chain. I think of it almost like a magnet--it strongly attracts the electrons and keeps the whole thing running. When the electrons and protons arrive, they combine back to hydrogen and then combine with the oxygen to form water (H2O). Then you pee out the extra water (and breathe out the carbon dioxide created in the citric acid cycle).

I've quoted a couple of times the line that "you can only live 3 minutes without air, you can live 3 days without water, and you can live 3 weeks without food." (See Air, 5/7/09, and Water, 5/10/09.) We need water to keep everything fluid in our bodies. Here is why we need food and air. We need food for those molecules of glucose (etc) to start the process of cellular respiration. And we need air to supply the oxygen to finish the process of cellular respiration. And, as you can tell by the fact that we can make it three weeks without food, but only three minutes without air, we really need that oxygen.

So, now that you know why we need oxygen, another question is, where does the oxygen come from? That's the topic of my next post.

Quote of the Day: "...When your muscles are doing lots of work, they need lots of ATP. Your cells make ATP by doing cellular respiration. In order to make ATP, you need oxygen to accept electrons at your electron transport chain. So, as you use up your ATP in your muscles, you breathe faster to bring in more oxygen, so you can have more oxygen in you mitochondia to accept more electrons, to make more ATP. This is why you breathe.
"Everything you already knew about breathing, such as bringing oxygen to your lungs and having your red blood cells carry it around your body, is all true, but that's really more about how you get oxygen to your cells, not why your cells need it. The why is all about electron transport chains. Really. And if you're denied oxygen for some reason, you die because no oxygen = no final electron acceptor = no ATP = no cellular work = cells cease to function = death." - René Fester Kratz