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The goal of cellular respiration and metabolism in animals and plants is, ultimately, the conversion of one type of energy source to another. Presumably, the original energy source comes in a form that cannot be immediately used to support cellular activities. For humans, our external energy sources are the foods we eat. Once we ingest and digest the food, our cells metabolic processes convert the energy contained within the food into a form of energy that can function in our cells. These constant conversions are what allow us to perform our day-to-day activities.
Since energy is the ultimate goal of metabolism, it will be helpful to understand what these various external and internal energy sources really are. Different foods are composed primarily of one of the following three macromolecules: carbohydrates (breads and pastas), lipids (fats and oils), or proteins (meats and beans). During digestion of food, when the food is first broken down internally, these large molecules are broken into subunits. Depending on their type, subunits can be metabolized in different ways and then used as internal energy sources.
The distinct means of metabolizing specific subunits all have the same goal, the production of the primary cellular energy source: adenosine triphosphate.
Figure 3.3: Chemical structure of ATP
As you can see in the figure above, ATP contains three phosphate groups. These groups are primarily responsible for ATP's role as an energy source. During metabolic reactions, these phosphate groups can be transferred from ATP to yield either adenosine diphosphate (ADP) or adenosine monophosphate (AMP). \(ATP -> ADP + P + energy\), or \(ATP -> AMP + 2P + energy\). The release of one or more phosphate groups is energetically favorable: the reaction produces more energy than was needed to break off the phosphate group(s). ATP can also undergo a reaction with water to yield ADP or AMP to release energy. The cell can use the energy produced from the breakdown of ATP for whatever purpose is necessary such as moving substances against a concentration gradient or building new proteins. Often, the energetically favorable breakdown of ATP is coupled with another, energetically unfavorable reaction.
ATP synthesis is almost exactly opposite to the process by which ATP is broken down to produce energy. In ATP synthesis, phosphate groups are added on to either ADP or AMP. While this process is not as favorable, it is able to occur with the energy derived from metabolizing foods. In addition to ATP, there are a number of other reactive molecules that are involved in the production of cellular energy. These are called coenzymes, and their role is to help transfer other chemical groups like hydrogens. Coenzymes work in conjunction with metabolic enzymes to drive metabolic reactions. Among these are nicotinamide adenine dinucleotide (NADH) and acetyl coenzyme A (acetyl CoA). We will discuss the specific roles of both these molecules more in following sections.
Basics of Metabolism
Metabolism is a process of energy acquisition and conversion. It is necessary because organisms are constantly undergoing cellular changes – they are not in a state of equilibrium. Metabolism is an attempt to regulate cellular conditions by making internal changes to maintain a steady cellular state. As a general rule, nature's tendency is towards conditions of disorder. This means that disorderly conditions are energetically favorable--they release energy. This is also known as the second law of thermodynamics – the entropy (or disorder) of a system always increases or remains constant. Highly ordered and organized conditions are not energetically favorable and require energy to create. As a result, the thousands of reactions that constantly occur inside us to maintain cellular organization need energy. The body produces this needed energy by breaking down ATP, and then using this energy to promote energetically unfavorable, but biologically necessary reactions.
In order to begin any of these processes, cells need an external energy source. The breakdown of the external source can provide the energy that can couple to drive other reactions. Cells acquire this external energy in one of two ways. Phototrophs get their energy from the sun through photosynthesis. Plants are phototrophs. Plants use light energy to convert carbon dioxide and water into carbohydrates and oxygen. Chemotrophs, such as humans, derive energy from the breakdown of organic compounds such as carbohydrates, lipids, and proteins. Our focus in discussing cell respiration and metabolism will be on this second, chemical type of energy acquisition. The relationship between phototrophs and chemotrophs is complimentary: chemotrophs require oxygen and expire carbon dioxide while phototrophs require carbon dioxide and expire oxygen. Additionally, many of the carbohydrates ingested by chemotrophs derive from the metabolic carbohydrate products of phototrophs.
Among chemotrophs, there are two major categories of metabolic pathways. The distinction between the two is that one involves degradation reactions while the other involves synthesis reactions. Catabolic pathways involve the breakdown of ingested food molecules. Anabolic pathways involve the synthesis of essential biomolecules. Along each of these pathways, a number of enzymes work in combination to help drive the reactions. Catabolic pathways are involved in breaking down carbohydrates and proteins into their polysaccharide, or sugar, and amino acid subunits. These reactions release energy needed by the cell (this is why food, the source of carbohydrates and proteins, is essential for survival). Anabolic pathways take the simple products of catabolic degradation—ATP, for example—and use energy from their degradation to synthesize complex biomolecules.
As we have mentioned, the breakdown of ATP is an energetically favorable reaction. This is true because it involves splitting one larger, more organized molecule into two smaller ones. The energy that is released in this process can be used to drive other, less favorable reactions forward. In this way, ATP acts as a major energy source for cells.
As one can imagine, there are many different anabolic and catabolic reactions going on at any second in our bodies. As a result, metabolic pathways must be highly regulated as to ensure that the proper enzymes for synthesis and degradation are active at the appropriate times. Some of this regulation is made possible by different metabolic processes occurring in distinct parts of the cell.
Oxidation and Reduction Reactions
There are a number of different types of metabolic reactions that typically take place. One class of reactions are oxidation and reduction reactions. These reactions involve the gain and loss of electrons and often also involve the cleavage of carbon-hydrogen bonds. When they are favorable, such reactions yield a large amount of free energy. In order to understand the specifics of what occurs in these reactions, a strong chemistry background is necessary. Here, it will suffice to understand that an oxidation reaction involves the loss of electrons (which corresponds to the breaking of bonds) and that a reduction reaction involves a gain of electrons (corresponding to a making of bonds).
Again, these descriptions of metabolic reactions are just simple introductions so that we can use them in our discussion of glycolysis and the citric acid cycle. The specifics of these reactions use organic chemistry that is not covered here.
