Metabolism Chemistry

From Chempedia

Will Jarvis, Dan Clark, Bryan Jake, and Dave Heineger
Chemistry 1022
Prof. Christy Haynes

Metabolism

For most, metabolism is the process by which the body breaks down food during digestion into usable energy. While this is not incorrect, it is only part of the process. The formal definition of metabolism is any biochemical modification of chemical compounds in cells and all living organisms.^[1] There are literally hundreds of chemical pathways in complex organisms, but they may all be classified into one of two processes: catabolic or anabolic. Catabolism is the first part of metabolism, in which complex molecules are broken down into simpler compounds known as fuel molecules. Anabolism is the conversion of these fuel molecules into energy usable by the organism. Anabolic processes are involved in organism growth, building, and activity. The most important catabolic pathway (and the crux of most metabolic processes) is that of cellular respiration. As such, it will be the focal point of this article. It is defined as, "an oxidation-driven flow of electrons, through or within a membrane, from reduced coenzymes to an electron acceptor, usually accompanied by the generation of ATP."^[2] Figure 1, below, is an overview of cellular respiration.

Figure 1^[3]

Cellular respiration is the process by which cells convert ingested materials (i.e. food) into ATP molecules for energy use. There are four distinct parts to this reaction: glycolysis, the TCA Cycle, Electron Transport/Proton Pumping, and ATP Synthesis. After ingestion, food is broken down by enzymes into carbohydrates, fatty acids, and proteins. The fatty acids and proteins are oxidized and eventually form other amino acids. The carbohydrates are further refined into glucose, C₆H₁₂O₆. This glucose is used in the first step of cellular respiration, glycolysis. Glycolysis takes place in the cytoplasm of cells.

[4]

During the process, glucose is oxidized to form pyruvate, C₃H₄O₃. A small amount of useable ATP is generated, as well as the electron-carrier molecule NADH. The fate of these chemicals depends on the availability of oxygen. In anaerobic (non-oxygen) using organisms, the process can be carried no further and these two chemicals are converted into ADP + P_iand NAD+ in the process of fermentation. This allows glycolysis to continue, but generates a toxic product (lactic acid or ethanol) that the body must dispose of. Aerobic (oxygen using) organisms transport the NADH and Pyruvate into mitochondria in the cell to begin the second phase of cellular respiration, the TCA Cycle. The TCA Cycle, also known as the Krebs Cycle, is a series of reactions that further oxidize the pyruvate from glycolysis (see figure 2, below). After being taken into a mitochondrion, pyruvate reacts with pyruvate dehydrogenase (PDH) to form acetyl coenzyme A (acetyl CoA). From this, several tri-carboxyl (three carbons) intermediate acids are formed, giving the cycle its name. At the end of the cycle, four molecules of NADH and one FADH₂, another electron carrier, are formed. Three molecules of CO₂waste are also formed, but these are dealt with by the organisms respiration.

Figure 2 .^[5]

Now, the NADH and FADH₂molecules are transported to the inner membrane of the mitochondrion. Recall that the membrane of a mitochondrion is made up of an outer membrane, an inter-membrane space, and a folded inner membrane. Embedded on the surface of this membrane are several proteins that form an electron transport chain.^[6] The NADH and FADH₂molecules release their electrons to these proteins, which are eventually used to create water. Releasing these electrons also causes the proteins of the inner membrane to transfer an H+ proton to the inter-membrane space. This sequestering of H+ protons causes a concentration gradient to form between the inter-membrane space and the inside of the mitochondria and outside of the cell. These protons are trapped until the concentration gradient reaches a critical value. When this value is reached, proton pumps (also embedded in the inner-membrane) are used to force H+ ions back into the mitochondrion. Release of these protons is the catalyst for ATP synthesis. Attached to these proton pumps are molecules responsible for synthesizing ATP. When three protons are released, ADP is phosphorylated, resulting in ATP.^[7] These ATP molecules are then used throughout the body to drive anabolic processes. For example, muscle flexion and extension happens through a process driven by ATP. The rate of metabolism is driven by the rates at which these reactions occur. Most of these rates depend on the nature of the chemicals being used, as well as availability. Most bio-regulation of metabolism is done by the hormone thyroxine. Thyroxine is secreted by the thyroid gland to regulate the rate of oxidation in cells.^[8] Since most metabolic reactions are oxidation reactions, this is the most effective means for the brain to control the rate of individual cell respiration. Controlling cell respiration effectively controls the most important catabolic process in the body. By limiting or providing an excess of ATP, the body is able to vicariously regulate anabolic processes through thyroxine.

Footnotes

1. ^ Chemistry Daily, The Chemistry Encyclopedia. http://www.chemistrydaily.com/chemistry/metabolism (accessed Sept. 2005).
2. ^ Becker, W; Kleinsmith, L; Hardin, J. The World of the Cell, 5th Edition; Benjamin Cummings: New York, NY, 2003; p 398.
3. ^ Ibid, 399.
4. ^ The Biology Project, Metabolism. http://www.biology.arizona.edu/biochemistry/problem_sets/metabolism/01t.html, (accessed Sept. 2005).
5. ^ Becker, W; Kleinsmith, L; Hardin, J. The World of the Cell, 5th Edition; Benjamin Cummings: New York, NY, 2003; p 408.
6. ^ The Biology Project, Metabolism. http://www.biology.arizona.edu/biochemistry/problem_sets/metabolism/01t.html, (accessed Sept. 2005).
7. ^ Becker, W; Kleinsmith, L; Hardin, J. The World of the Cell, 5th Edition; Benjamin Cummings: New York, NY, 2003; p 435.
8. ^ The Free Dictionary. http://www.thefreedictionary.com/thyroxine (accessed Sept. 2005).