Overview of Metabolism


Metabolism is the sum total of all chemical reactions involved in maintaining the living state of the cells, and thus the organism. In general metabolism may be divided into two categories: catabolism or the break down of molecules to obtain energy; and anabolism or the synthesis of all compounds needed by the cells (examples are DNA, RNA, an protein synthesis). The diagram on the left contains a summary of all the types of metabolism that will be examined. In this module, the electron transport chain is examined.

Bioenergetics is a term which describes the biochemical or metabolic pathways by which the cell ultimately obtains energy.

Nutrition is a science that deals with the relation of food substance to living things. In the study of nutrition, the following items must be considered: a) bodily requirement for various substances; b) function in body; c) amount needed; d) level below which poor health results. Essential foods supply energy (calories) and supply the necessary chemicals which the body itself cannot synthesize. Food provides a variety of substances that are essential for the building, upkeep, and repair of body tissues, and for the efficient functioning of the body.

A complete diet must supply the elements; carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, and at least 18 other inorganic elements. The major elements are supplied in carbohydrates, lipids, and protein. In addition, at least 17 vitamins and water are necessary. If an essential nutrient is omitted from the diet, certain deficiency symptoms appear.


Foods supply carbohydrates in three forms: starch, sugar, and cellulose (fiber). Starch and sugar are major and essential sources of energy for humans. A lack of carbohydrates in the diet would probably result in an insufficient number of calories in the diet. Cellulose furnishes bulk in the diet.

Since the tissues of the body need glucose at all times, the diet must contain substances such as carbohydrates or substances which will yield glucose by digestion or metabolism. For the majority of the people in the world, more than half of the diet consists of carbohydrates from rice, wheat, bread, potatoes, macaroni.


All life requires protein since it is the chief tissue builder and part of every cell in the body. Among other functions, proteins help to: make hemoglobin in the blood that carries oxygen to the cells; form anti-bodies that fight infection; supply nitrogen for DNA and RNA genetic material; and supply energy.

Proteins are necessary for nutrition because they contain amino acids. Among the 20 or more amino acids, the human body is unable to synthesize 8, therefore, these amino acids are called essential amino acids. A food containing protein may be of poor biological value if it is deficient in one or more of the 8 essential amino acids: lysine, tryptophan, methionine, leucine, isoleucine, phenylalanine, valine, and threonine. Proteins of animal origin have the highest biological value because they contain a greater amount of the essential amino acids. Foods with the best quality protein are listed in diminishing quality order: whole eggs, milk, soybeans, meats, vegetables, and grains.

Fats and Lipids:

Fats are concentrated sources of energy because they give twice as much energy as either carbohydrates or protein on a weight basis. The functions of fats are to: make up part of the structure of cells, form a protective cushion and heat insulation around vital organs, carry fat soluble vitamins, and provide a reserve storage for energy.

Three unsaturated fatty acids which are essential include: linoleic, linolinic, and arachidonic and have 2, 3, and 4 double bonds respectively. Saturated fats, along with cholesterol, have been implicated in arteriosclerosis, “hardening of the arteries”. For this reason, the diet should be decreased in saturated fats (animal) and increased in unsaturated fat (vegetable).
a) MH + NAD+ —> NADH + H+ + M + energy

b) ADP + P + energy —> ATP + H2O


The minerals in foods do not contribute directly to energy needs but are important as body regulators and as essential constituents in many vital substances within the body. A MINERAL is rather loosely defined as any element not normally a part of the structures of carbohydrates, proteins, and fats. More than 50 elements are found in the human body.

About 25 elements have been found to be essential, since a deficiency produces specific deficiency symptoms. All of the minerals required by the human body are probably not known at this time. Although minerals may not be part of the structures of carbohydrates, proteins, and fats, they are mixed in the foods in trace amounts during the growing process by uptake from the soil.

Major Minerals Include: calcium, phosphorus, iron, sodium, potassium, and chloride ions.

Other Essential Minerals Include: copper, cobalt, manganese, zinc, magnesium, fluorine, and iodine.


Vitamins are essential organic compounds that the human body cannot synthesize by itself and must therefore, be present in the diet. The term vitamin (vital amines) was coined by Casmir Funk from the Latin vita meaning “life” (essential for life) and amine because he thought that all of these compounds contained an amine functional group.

Vitamins particularly important in metabolism include:

Vitamin A: The yellow and green pigments found in vegetables are called carotenes which are pro vitamins and are converted into Vitamin A. The role of vitamin A in Vision has already been discussed in a previous page.

Vitamin B2 is better known as riboflavin and is widely distributed in many foods. Riboflavin is used to form a coenzyme FAD important in the utilization of oxygen in the cells.

Niacin, also known as nicotinic acid, is also in the B complex of vitamins. Nicotinic acid was first obtained from the alkaloid nicotine in tobacco and was later found in many plant and animal tissues as niacin