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A&P1_Cell_Structure_and_Physiology Also see Cellular Respiration
(updated 9/24/05)
In order to understand how muscles and other cells use energy, it is necessary to understand cellular respiration, which involves converting the energy in food into a form which the body can use. We can use carbohydrates, lipids, or proteins as fuel, but the most common fuel for the body is the use of the simple, monosaccharide sugar, glucose. We first must establish some properties of chemical reactions in the body, as illustrated below. If you eat plant starch (amylose), digestion begins in the mouth, where the salivary glands produce a small amount of amylase, the enzyme that digests amylose from a polysaccharide into a disaccharide (the double sugar, maltose) shown at right, below. Digestion involves hydrolysis (breakdown using water), by injecting H+ onto the oxygen linking the two sugars together, and OH- onto carbon (#4)(Carbon #1 is the one on the right point of the molecule). This breaks the molecule into two glucose molecules. The body usually maintains a blood glucose level of approximately 70-110 mg/dl. The liver may rearrange many glucose molecules into animal starch (glycogen) for storage in the liver or within muscles by dehydration (removing water) synthesis. Similar reactions are used on lipids and proteins.
A typical animal fat is a triglyceride (glycerol and three fatty acids). Mono- and diglycerides contain one and two fatty acids, respectively. Glycerol is essentially 1/2 a glucose molecule. Enzymes that digest fats are known collectively as lipases.
Note the terminology used when joining amino acids together. Enzymes that digest proteins are known collectively as peptidases.
One role of proteins in the body is to form enzymes, which catalyze all reactions. They do this by temporarily combining with one or two other molecules (the substrate) to either form a product or break it down into two products. All reactions require an enzyme and a cofactor (a metallic ion) or a coenzyme (a vitamin).
Metabolic pathways involve many enzymes to change a substrate into a final product. Our DNA governs the production of these enzymes at the right place and time to allow us to function properly and remain alive. Gene mutations alter enzyme structure and function, and produce variability within species.
Within our cells, cellular respiration uses food that we have consumed to produce usable energy in the form of adenosine triphosphate (ATP). This process involves approximately 25 steps, some of which are shown below. The first diagram is a summary of respiration. Glycolysis is the first process, involving 10 reactions to break glucose into pyruvic acid (pyruvate). It occurs in the cytoplasm of the cell and is anaerobic (without oxygen). If no oxygen becomes available to the cell, pyruvate is converted to lactic acid through fermentation. Yeasts carry this process one step farther and produce ethyl alcohol. Whenever oxygen is available, the pyruvate diffuses into the mitochondria (brown structure), where the remaining enzymes occur to carry out aerobic respiration, which produces much more ATP.
Phases of glycolysis.
The Krebs Citric Acid Cycle (KCAC), within the mitochondria.
The enzymes within the inner mitochondrial membrane power the electron transport chain, and produce most of the ATP (see summary of ATP production on next figure).
Summary of aerobic respiration. Oxygen is only used in the last step, as seen above, to produce water. The enzyme that performs this step, cytochrome oxidase, is blocked by cyanide.
This diagram illustrates how different organic nutrients are used in respiration.
Back to Env Sci Return to Intro to Biology Return to Science Standards
A&P1_Cell_Structure_and_Physiology Also see Cellular Respiration