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Take a Deep Breath: Cellular Respiration

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Editor’s note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that’s because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News & Views is delighted to present this series, “The Designed Body.” For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.

So far in this series we seen that we live in a world made from matter and that matter is made up of atoms and molecules that follow the laws of nature. Our body has trillions of cells which are made up of atoms and molecules that must follow the laws of nature as well. These laws demand that to produce, control, or move anything require energy. To produce DNA, RNA, and all of the proteins the cell needs to live, and to transport them where they are needed, entails expending energy. So does moving certain chemicals across the plasma membrane, as the sodium-potassium pump does to let the cell control its chemical content and volume.

the-designed-body4.jpgBut how does the cell get this energy? It does so through through cellular respiration. Here’s how.

Cellular respiration is the process in the cell by which the chemical energy within the glucose molecule is released by breaking the bonds between its atoms. In contrast to a car engine, which uses a spark and molecular oxygen to quickly release the energy within gasoline to produce a small explosion, cellular respiration uses molecular oxygen and a series of specific enzymes to release the energy from within the glucose molecule in a much more controlled fashion. During this chemical reaction one glucose molecule (C6H12O6) reacts with six oxygen molecules (6 O2) to release the energy the cell needs, while at the same time producing six carbon dioxide molecules (6 CO2) and six water molecules (6 H2O).

The first phase of cellular respiration is called glycolysis. Glycolysis takes place inside the cytosol (cellular fluid). This process doesn’t use molecular oxygen and is therefore anaerobic. Glycolysis uses ten specific enzymes in a chain reaction to help breakdown glucose into two molecules of pyruvate (C3H6O3). Along the way a small amount of energy is released. Pyruvate then moves into the mitochondria where the second (citric acid cycle) and third (electron transport chain) parts of cellular respiration take place. Both of these processes require molecular oxygen and are therefore aerobic.

In the mitochondria, pyruvate enters the citric acid cycle, which uses eight specific enzymes and molecular oxygen in a chain reaction, to break it down into carbon dioxide (CO2). The hydrogen released is picked up by carrier molecules. In the mitochondria these hydrogen-carrying molecules enter the electron transport chain, which uses a series of specific proteins and molecular oxygen to form water (H2O). Along the way more energy is released. The addition of molecular oxygen in aerobic metabolism yields about 15 times more energy than what comes from anaerobic metabolism (glycolysis) alone.

Experience teaches that the body’s need for molecular oxygen is so acute that without it we can only live about four minutes. This means that the cells in our body need the large amount of energy released from glucose in the presence of molecular oxygen to survive. In other words, we must have enough molecular oxygen to drive the citric acid cycle and the electron transport chain in the mitochondria of our cells because the small amount of energy provided by anaerobic glycolysis alone is not enough to keep our cells living for very long. There are five important points to remember about cellular respiration.

First, if you put glucose in coffee or tea it could sit there for an eternity and it would never turn into two molecules of pyruvate by the laws of nature alone. Glycolysis can’t take place without the ten specific enzymes needed to facilitate each of the ten different chemical reactions required to convert one molecule of glucose (C6H12O6) into two molecules of pyruvate (C3H6O3) while releasing some energy.

Second, bubbling molecular oxygen into a solution containing pyruvate would never, on its own, naturally bring about carbon dioxide and water. The citric acid cycle and the electron transport chain, present in the mitochondria, cannot work properly without the eight specific enzymes and the series of specific proteins they need to release the required amount of energy from the glucose molecule.

Third, each of the twenty or more enzymes and carrier molecules involved in cellular respiration is made up of about three hundred or more amino acids lined up in a specific order which gives each of them the ability to perform their function. Since there are twenty different amino acids that can be used by the cell to produce a given protein, the chances of any one of these molecules coming into being at random is at least one chance in 20300. Clearly, this is impossible and it is why our cells, rather than relying on chance and the laws of nature alone, use the instructions contained in the DNA in their nuclei to produce enough of these types of molecules to keep us alive. How much is enough and how the production of these molecules is controlled is unknown.

Fourth, the specific enzymes and series of proteins work in a specific order (pathway), like in a chain reaction, to produce the energy the body needs from cellular respiration. If any one part is missing or not working properly then the whole system can malfunction, resulting in death. For example, the poison cyanide blocks the function of just one of the enzymes in the electron transport chain. And arsenic blocks one of the enzymes in the citric acid cycle. Ingesting enough of either of these poisons can quickly lead to death. Although there is enough glucose and molecular oxygen in the cells, they can’t get the energy they need to stay alive because each of these poisons has blocked just one component of the cellular respiration pathway.

Finally, the energy released from the glucose molecule by cellular respiration is not in an immediately useable form. In a car, the semi-explosive release of energy from the combustion of gasoline in the presence of molecular oxygen in its engine is immediately used to power it down the road. This takes place by way of the actions of the pistons through the transmission and the drive train. However, the cell takes the energy it gets from the breakdown of glucose and stores it, like a battery, in certain molecules.

The commonest energy-storage molecule in the cell is ATP (adenosine triphosphate). It is known as the energy currency of the cell and has three high energy phosphate bonds. An enzyme called ATP synthase takes the energy from cellular respiration and uses it to produce ATP. The cell also has enzymes called ATPases which are attached to its micro-machines (like the sodium-potassium pump) that can use the energy stored in ATP (like a battery powering an appliance) for the work they need to do to keep the cell functioning properly. Our cells are continuously releasing energy from glucose and storing it as ATP while at the same time releasing the stored energy in ATP.

Because it must follow the rules, the cell needs many different innovations to be able to take control. When it comes to the laws of nature, and life or death, real numbers have real consequences. Having reminded ourselves of these facts, let’s move on and consider the human body as a whole. For, as opposed to single-celled organisms, which can live independently, the same cannot be said for our cells.

Image: � rudisetiawan / Dollar Photo Club.