Oxygen is highly electronegative; it tends to pull electrons toward itself and away from other molecules. Consequently it takes energy to keep electrons away from oxygen. As the electrons move closer to oxygen, they lose energy and the energy that is released can be used to do work. Cellular respiration is actually a series of reactions in which electrons are sequentially moved from glucose (and its catabolic products) to oxygen, or in some cases, to an alternative terminal electron acceptor. In the process, energy is released. In other words, cellular respiration is a series of redox reactions in which energy is gradually made available to do work. The work done is the synthesis (anabolism) of ATP.
The general equation for cellular respiration is:
Think of this as a redox process. In this case, glucose is oxidized and oxygen is reduced. Remember, oxygen is highly electronegative (electrons are drawn to it). Ultimately, oxygen is reduced to form water and glucose is oxidized to produce carbon dioxide.
Although this redox reaction is written as one equation, it really happens in a series of steps. The reason for this should be evident from the previous two questions. That is, a high amount of energy is released from the oxidation of glucose and unless the body controls this oxidation, much of the energy is wasted as heat. Therefore the cell gradually oxidizes glucose in a series of controlled steps, and electrons (and accompanying energy) are gradually released. Restated, in the oxidation of glucose energy is gradually liberated and becomes available to synthesize ATP.
The general equation for cellular respiration is quite simple, however, the overall process actually takes place in several stages in different parts of the cell. For example, hydrogen atoms (and accompanying electrons) are not directly transferred from glucose to oxygen. There are several intermediate steps, with intermediate oxidants (electron carriers). The most prevalent electron carrier is nicotinamide adenine dinucleotide. This electron carrier can exist in its reduced form (NADH) or as an oxidized positive ion (NAD+). NAD+ is free to pick up electrons, whereas NADH has two more electrons and an additional proton.
A molecule of NAD+ or NADH consists of two nucleotides (adenine, found in RNA and DNA, and nicotinamide) joined together. In the presence of the enzyme dehydrogenase and hydrogen, NAD+ can become reduced to NADH. The structure of the oxidized form is shown on the left side of the figure, and the right side shows that portion of the molecule where reduction occurs.
NAD+ functions as an oxidizing agent (electron acceptor) during cellular respiration, picking up electrons from the catabolic products of glucose (along with hydrogen atoms). Each NAD+ molecule can be reduced with two high-energy electrons and one hydrogen atom. Importantly, once the transfer is complete and the reduced NADH has deposited its electrons, the regenerated NAD+ can pick up more electrons and begin again. In other words, NAD+ acts as an energy shuttle. Think of this process in the following way:
In this example, "X" is any molecule that gives up an electron and two hydrogens. Note that there is a single proton, H+, which remains unpaired after this transaction. These protons (hydrogen ions) accumulate and are very useful in certain stages of respiration (to be discussed later).
Energy II Part 1 VoiceThread Transcript
You now have the background information to learn the steps of cellular respiration. You should know:
If you are comfortable with all of these concepts, it's time to move on. If not, review the concepts that do not make sense.