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Chapter 21: 14-21

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You may want to review enzyme activity in the weblecture on how cells use energy.

Nutrients: digestion and purpose

Remember the organic molecules we discussed in chapter 3? These included

Some of the different types of molecules can be manufactured by the cell. It can, for example, make twelve of the twenty amino acids, but not the other eight. It can make glycerol, but not linoleic acid. When the human cell can't make a particular basic molecula, that molecule is "essential" -- it must be part of our diet and obtained by eating it. And certain basic elements and minerals are necessary to construct those molecules we can manufacture ourselves -- so these are the essential minerals and vitamins. Of course, we may take in molecules we could otherwise make; that reduces the work our bodies must do in order to stay healthy.

Each type of nutrient requires a specific process for digestion. Carbohydrates are broken down into simple sugars by specialized enzymes in the mouth and the intestines. Proteins are broken into amino acids by enzymes in the stomach, which are activated by the hydrochloric acid secreted by the stomach lining. Fats are broken down into fatty acids and simple glycerol molecules by lipase from the pancrease. Lack of any specific digestive enzyme will cause nutritional deficiencies in the organism even if the organism is eating a healthy diet.

Each type of nutrient contributes to specific metabolic processes. Carbohydrates are used as energy stores; lipids (fats) as both energy sources and to make certain kinds of molecules (remember that lipids form a major component of all cell membranes). Proteins are necessary to make enzymes, which facilitate metabolic processes, and various cellular structures. Other organic compounds (vitamins) and inorganic compounds (minerals) are required by the body for cell growth and to sustain and control metabolic processes.

A body's intake of nutrients must be balanced with its requirements for the nutrient. Lack of nutrients can cause serious problems. A deficiency of iron in the diet will result in a lack of hemoglobin in the red blood cells, reducing the body's ability to absorb oxygen and causing the disease anemia, which is marked by constant fatigue. A surplus of fat in the diet will be stored--at the expense of other metabolic processes, putting a strain on the heart.

Check out the information at the National Institute of Health Digestive Diseases Information site. This site explains how normal systems work, before discussing the various problems the digestive system can run into as a result of disease.