Bioenergetics

It is stored in most cells, particularly in muscle cells. Other forms of chemical energy, such as those available from food, must be transformed into ATP before they can be utilized by the muscle cells. Since energy is released when ATP is broken down, energy is required to rebuild or resynthesize it. The energy for ATP resynthesis comes from three different series of chemical reactions that take place within the body.

Two of the three depend upon the type of food eaten, whereas the other depends upon a chemical compound called phosphocreatine.


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The energy released from any of these three series of reactions is coupled with the energy needs of the reaction that resynthesizes ATP. The separate reactions are functionally linked together in such a way that the energy released by the one is always used by the other. Aerobic and anaerobic systems usually work concurrently.

When describing activity, it is not a question of which energy system is working, but which predominates. The term metabolism refers to the various series of chemical reactions that take place within the body.

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Aerobic refers to the presence of oxygen, whereas anaerobic means with series of chemical reactions that does not require the presence of oxygen. When it is broken down, a large amount of energy is released. The energy released is coupled to the energy requirement necessary for the resynthesis of ATP. Thus, the amount of energy obtainable through this system is limited. The phosphogen stored in the working muscles is typically exhausted in seconds of vigorous activity.

However, the usefulness of the ATP-CP system lies in the rapid availability of energy rather than quantity.

This is important with respect to the kinds of physical activities that humans are capable of performing. This system is known as anaerobic glycolysis.

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In this system, the breakdown of sugar supplies the necessary energy from which ATP is manufactured. When sugar is metabolized anaerobically, it is only partially broken down and one of the byproducts is lactic acid. This process creates enough energy to couple with the energy requirements to resynthesize ATP. Another limitation of the lactic acid system that relates to its anaerobic quality is that only a few moles of ATP can be resynthesized from the breakdown of sugar as compared to the yield possible when oxygen is present. This system cannot be relied on for extended periods of time.

Oxidative Medicine and Cellular Longevity

For example, exercises that are performed at maximum rates for between 1 and 3 minutes depend heavily upon the lactic acid system for ATP energy. In activities such as running meters or a mile, the lactic acid system is used predominately for the "kick" at the end of a race. This stage of the aerobic system occurs on the cristae infoldings on the membrane of the mitochondria. I mentioned earlier that this process is celled cellular respiration, and cellular respiration is very similar to photosynthesis, just in a backwards direction.

Now the first step, we have to break glucose down into a molecule known as pyruvate. And this step gives off ATP. And ATP is the usable form of energy, but we're not giving off a whole lot yet.

Bioenergetics - Wikipedia

We still have a lot of energy forms contained in the bonds of pyruvate. So pyruvate is then broken down into acetyl-CoA. And TCA stands for tricarboxylic acid. And this cycle is also known by a couple other names like the Kreb cycle and the citric acid cycle. Once again, there's a series of reactions that occur in this cycle, but the specifics of these reactions are less important than the outcome, which is production of C02 as well as NADH and FADH2.

Cellular Respiration Part 1: Glycolysis

Now these molecules are similar to the NADPH in photosynthesis, in that they're high-energy election carriers. Now they enter a series of reactions known as the electron transport chain. And in this reaction, the hydrogen from these high-energy election carriers is bumped off. And we have oxygen over here, which is combined with the hydrogen to form water, or H And these hydrogens here drive an enzymatic pump that produces ATP.

Bioenergetics: The transformation of free energy in living systems

And you can see here that photosynthesis and cellular respiration are very similar reactions, just in the opposite direction. And although they may have different intermediates, actually the products of one are the reactants of the other, and vice versa, and so let me demonstrate that. In photosynthesis, our reactants are H20 and C02, and our products are oxygen and glucose, whereas in cellular respiration we have glucose as a reactant, as well as oxygen. We're now producing C02 and water, and you can actually see this if I write out the equations for photosynthesis and for cellular respiration.

Now there's an important reactant in product that isn't added in these chemical equations, and that is energy. And in photosynthesis, the energy is a reactant, putting this energy into the chemical bonds of glucose. Whereas in cellular respiration, the energy is a product.

And we're taking that energy from the chemical bonds of glucose. So let me just show you one more way to demonstrate how energy changes in these two reactions. Now, to do this I'm gonna draw a reaction diagram. And on the x-axis, here, we have the reaction progress, and on the y-axis we have free energy.


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Which is also known as G. I'm gonna just dim down the reaction a little bit so that we can work over the top of it. So if you look at the free energy level for where the reactants of photosynthesis start, to where they end with glucose, you're adding energy to this. So you have a low free energy level and you're going to a higher free energy level. So energy is added, and that's from the sunlight, whereas in cellular respiration, you go from the reactants of glucose with lots of free energy, to low free energy with ATP, you are releasing energy.

And it's this release in free energy that allows our body to do work, like pump the heart.