Draft Chapter on why eating and breathing are connected

This is the long-awaited and much-anticipated explanation (ok, maybe not) of how all that food you are going to eat will be converted to cellular gasoline. Get yourself a snack, settle down in a comfortable chair, and let’s begin. First, consider just what it is that you eat. In essence, all food contains various amounts of three basic types of nutrients: carbohydrates, proteins, and fats. Of course, there are vitamins, minerals and other minor components, but let’s stick to the major players, and while we’re at it, let’s also ignore fat. We shouldn’t be eating much of that anyway!

So, carbohydrates and proteins. Both of these nutrients are shaped like chemical trains. Carbohydrates are just sugar “boxcars” connected in a long string; proteins are amino acid or AA “boxcars” connected end-to-end to make the “train” we call a protein. This is what we put in our mouths.

The next place that the food hits is our digestive tract: mouth, stomach, intestines. Here the trains are dismantled to their individual boxcars. The single sugars and AAs are then absorbed into the blood stream and eventually end up in individual cells. (Remember that the basic functional units of the body are microscopic membrane-enclosed cells.) We previously compared cells to cities, making our organs countries and our bodies a planet!  Probably the blood vessels would be equivalent to the interstate system.

Now the cell cities have a problem. Their machines and factories are set up to use one type of energy only, cellular energy, which is called ATP (adenosine triphosphate). But, sugar is different type and AAs still another type of energy. This is analogous to asking your toaster to run on wind power without first converting that power to electricity. Not going to happen. No matter how much you blow on your toaster, it is not going to heat up your Poptart.

In the cell, there is an amazing and highly efficient process to take all the different types of energy (various types of sugar, AAs, and even fats) and convert them to ATP. Why would the cells do this? Well, think about how when you run out of gas in your car, you can’t keep going. But, because our cells can convert so many things to their “gasoline,” when they run out of gas they can in effect just switch to burning coal or even, if needed, remove a seat and burn that!

Back to cells. To keep things a little simpler, let’s just consider the conversion of the sugar glucose (what you find in energy drinks) into ATP. Glucose is a sugar that contains six carbon atoms; the energy of the glucose is held in the connections that hold the carbon atoms together. In order to release the energy, the carbon atoms are taken apart. The now energy-depleted carbons are released as CO₂. Actually, this is much like the burning of gasoline, which is made of long strings of carbon atoms, and produces CO₂ as a byproduct. But, just as it would be dangerous to release all the energy in your car’s gas tank at once (putting a match to it) and would not get you very far down the road, it is also important for the cell to take the glucose apart in a slow and controlled fashion. The process is fascinating and is illustrated in the figure below and in the Youtube at http://www.qcc.cuny.edu/BiologicalSciences/Faculty/DMeyer/respiration.html.

First, in a series of ten carefully controlled steps, the glucose is broken in half. Remember that the process needs to be gradual to prevent excessive loss of heat energy. The energy that was trapped in the bond between the central carbons is converted to ATP energy  and energy carrying molecules. The energy carriers (ECs) are made from vitamin B3 and will be used later in the ATP-generating process.

Next, the three carbon fragments are fed into the cell’s powerplant. As they enter, the “door” shaves off one more carbon, provides the two-carbon fragment with an escort, and the energy from the connection is harvested in an EC. The one-carbon byproduct is released as CO₂.

Once inside the powerplant, the now two-carbon fragment is taken around an eight-step process and broken down completely. This yields CO₂ and more ECs (Step 5). Now we have some cellular energy (ATP) and lots of ECs. The CO₂ is loaded into the blood interstate system to be taken to the lungs, where we breathe it out.

The next step requires me to take those of you who have forgotten back to some basic chemistry. Remember atoms? These most basic of chemical units are made of a nucleus of protons and neutrons surrounded by a cloud of spinning electrons. Atomic hydrogen is even simpler, being made of one proton (positively charged) and one energetic electron (negatively charged).

Back to our powerplant. First notice that the powerplant has two walls. There is a reason for that: between the outer and inner wall is comparable to the lake at the top of a waterfall, inside the lower one is like the bottom of the waterfall. The water in this analogy is made of protons. The job of the ECs is to give energy packets (energized electrons) to a machine inserted in the inner wall. This machine uses some of the energy to pump protons to the space between the walls and passes the rest of the energy to the next machine. That one does the same thing and then passes the remaining energy on. The last machine uses up the remaining energy to pump a proton and then puts the now completely depleted electron onto oxygen. A couple of protons are added to the oxygen with depleted electrons and we have H₂O or waste water. Now you know why you breathe; the oxygen is a “trashcan” for depleted electrons!

Of course, you may have noticed that the powerplant still has not generated much ATP. An amazing and complex machine called ATP synthase (blue) does this. ATP synthase is very much like a hydroelectric plant. Instead of the energy of running water being used to make electricity, the energy of running protons (from the intermembrane space to the inner space of the powerplant) is used to make ATP. Basically, ATP synthase is made of a channel, rotors, a shaft and ATP-making machines. When the protons move through the channel, their energy makes the rotors turn, which in turn rotates the shaft. This activates the ATP-making machines to make ATP. You can find a nice animation of the process at http://www.youtube.com/watch?v=uOoHKCMAUMc.

About 40% of the energy in glucose is converted to ATP energy. This is what is used to fuel all the cell city’s machines and factories. What about the other 60%? Well, if the body is analogous to a planet (see above), it is used to prevent global climate change! That is, the “waste” energy from converting food energy to ATP energy is what keeps us warm. Of course, there are other “planetary” mechanisms such as shivering and sweating, but the cellular powerplants are a major player.

Obviously, this is an incredibly important process, but unlike human powerplants, there is nobody to run (or build) the machines. So, how is it regulated? Why don’t we get a fever every time we eat and suffer from hypothermia every morning? There is way too much to explain here, but I will mention two mechanisms of control. First, some of the machines involved in breaking the original glucose in half are turned off by excess amounts of their products. They are turned on by hormones released when you need more energy. If the machines are turned off, then you store the glucose for later. If you store too much glucose, then it is converted to fat. Not good news.

Another important place where regulation of this process occurs is the door into the powerplant. Again, if the cell needs more ATP energy, then the door is open. If not, the door is closed. This door is actually a highly complex machine in itself, which I would love to tell you all about. But, you’ve probably had more than enough biology for now! However, I do just have to tell you that some parts of the door-machine into the powerplant are made of vitamins B1 and B2. Now you know why energy supplements frequently contain B vitamins. We need three of the B vitamins to run our cellular powerplants!

I hope you enjoyed this peek into what happens to your food and, incidentally, into why you breathe!