What Are Nutrient Molecules?

What Are Nutrient Molecules?

A wide variety of molecules serve as nutrients for animals, including water, proteins and amino acids, carbohydrates, fats and lipids, nucleic acids, inorganic salts, and vitamins. This article discusses briefly each of them.

Water

Of all the constituents of animal tissues, none is more pervasively important to living tissue than water. This unique and marvelous substance can constitute 95% or more of the weight of some animal tissues. It is replenished in most animals by drinking and by ingestion as a constituent of food. Some marine and desert animals depend almost entirely on metabolic water '" water produced during the oxidation of fats and carbohydrates '" to replace water lost by evaporation, defecation, and urination.

Proteins and Amino Acids

Proteins are used as structural components of tissues, as channels, transporters, and regulatory molecules, and as enzymes. They can also be utilized as energy sources if first broken down into amino acids. The proteins of animal tissues are composed of about 20 different amino acids. The capability to manufacture these amino acids differs among species. Essential amino acids are those that cannot be synthesized by an animal, but that are required for synthesis of essential proteins.

Carbohydrates

Carbohydrates are used primarily as sources of chemical energy to be either metabolized immediately as glucose or stored as glycogen. However, they may also be converted to metabolic intermediates for the formation of other biomolecules, including amino acids and fats. Conversely, proteins can be converted by most animals into carbohydrates. The major food sources of carbohydrates are the sugars, starches, and cellulose found in plants and the glycogen stored in animal tissues.

Lipids

Lipid (fat) molecules are especially suitable as concentrated energy reserves. Each gram of fat provides over 20% more caloric energy than a gram of carbohydrate, and nonpolar nature of fat molecules allows them to be stored in large quantities without binding water, whereas stored carbohydrates are richly hydrated, binding about twice their dry weight in water molecules. Far more energy can therefore be stored in the form of fat per unit volume of tissue. Fat is commonly stored by animals to be used during periods of caloric deficit, such as hibernation, when energy expenditure exceeds energy intake. Lipids are also important in certain tissue components, such as plasma membranes and other membrane-based organelles of the cell.

Nucleic Acids

Although nucleic acids are essential for the genetic machinery of the cell, all animal cells appear to be capable of synthesizing them from simple precursors. Thus, the intake of intact nucleic acids is not necessary from a nutritional perspective.

Inorganic Salts

Some chlorine, sulfate, phosphate, and carbamate salts of the metals calcium, potassium, sodium, and magnesium are important constituents of intracellular and extracellular fluids. Calcium phosphate occurs as hydroxyapatite, a crystalline material that lends hardness and rigidity to the bones of vertebrates. Iron, copper, and other metals are required for redox reactions (as cofactors) and for oxygen transport and binding (in myoglobin and hemoglobin). Many enzymes require specific metal atoms to complete their catalytic functions.

Vitamins

Vitamins are diverse and chemically unrelated group of organic substances that generally are required in small quantities, primarily to act as cofactors for enzymes. Detailed vitamin requirements are known primarily for humans and for domesticated animals grown for their meat, eggs, or other products. Very little is known about the vitamins involved in the metabolism of lower vertebrates and especially invertebrates.