Nutrients and Biochemistry

An updated version of this lesson is available at Visionlearning: Carbohydrates & Fat&Protein

There are 3 major classes of organic chemicals that are essential to living organisms: carbohydrates, fats and proteins. In this lesson, we will discuss the basic structure and function of these macronutrients and look at some general information on nutrition and diet.

Carbohydrates
Carbohydrates serve as the main energy source for the human body. Chemically, carbohydrates are organic molecules in which carbon, hydrogen and oxygen bond together in the ratio: C_x(H₂O)_y where x and y are some whole numbers. Animals break down carbohydrates during metabolism to release energy. This reaction is shown below for the carbohydrate glucose:

C₆H₁₂O₆ + 6 O₂ 6 CO₂ + 6 H₂O + energy

Animals obtain carbohydrates by eating food that contains them, such as plant material. Plants manufacture carbohydrates by harvesting energy from sunlight to run the above reaction in reverse, as shown below:

6 CO₂ + 6 H₂O + energy (from sunlight) C₆H₁₂O₆ + 6 O₂

Carbohydrates come in 2 types: the simple sugars and the complex carbohydrates.

Simple Sugars: Simple sugars are sweet in taste and are broken down quickly in the body to release energy (as shown in the reaction at top). The basic unit in a carbohydrate is the sugar, or saccharide, unit. The simplest sugars are the monosaccharides, these are sugars that are composed of a single saccharide molecule. Three of the most common monosaccharides are glucose, fructose and galactose. Fructose is the main sugar found in many fruits. The structure of these monosaccharides are shown below:


glucose	fructose	galactose

Complex Carbohydrates: The complex carbohydrates are polymers of simple sugars. A polymer (simply translated as 'many parts') is a molecule that consists of repeating units of a smaller molecule. For example, a chain can be thought of as a polymer of the links in the chain. The complex carbohydrates are molecules in which many mono-, or di-, saccharides bond together in a chain. The three most common complex carbohydrates are: starch, glycogen and cellulose:

Starch is a straight chain polymer of glucose. Plants manufacture starch by bonding many glucose molecules end-to-end as a means of storing energy for future use. Animals can digest starch easily by breaking it down into glucose and using the sugar as an energy source. Some common sources of starch are potatoes, bread (made from grains), beans, corn, etc.
Glycogen is also a polymer of glucose. But rather than a straight chain of glucose units, glycogen is a more highly branched chain of glucose molecules. Glycogen is the main energy reserve in animals. When excess glucose is eaten by an animal, this glucose is bonded together to form glycogen molecules which the animal stores in the liver and muscle tissue as an "instant" source of energy.
Cellulose is yet a third polymer of the monosaccharide glucose. Cellulose differs from starch and glycogen because the glucose-to-glucose bonds in cellulose are different and are much more difficult to break apart. Most animals (with the exception of cockroaches, cows and some other creatures) cannot digest cellulose. Since plants do not have bones to provide support for the plant, cellulose is used as a structural molecule to support leaves and other plant parts. Despite the fact that it cannot be used as an energy source in most animals, cellulose in the diet is often referred to as dietary fiber. Fiber helps exercise the digestive track and keep it clean and healthy.

Fats
Fats are a sub-group of compounds found in living organisms that are insoluble in water known as lipids. Fats are triglycerides, molecules made from the reaction of one molecule of glycerol with three fatty acids, as depicted in the reaction below:

Fats serve several nutritional purposes. First, fats can be used as a reserve, or long-term, source of energy in animals. During periods of low food consumption, fat reserves in the body can be mobilized and broken down to release energy. Fats serve as an insulation material to allow body heat to be conserved and also lining and protecting delicate internal organs from physical damage. In addition, fats are essential to the proper maintenance of cell membranes in the body and are used in the production of certain hormones and steroids.
Fats can be classified as either saturated or unsaturated depending on the number of carbon-carbon double bonds in the Fatty Acid chains of the molecule. Fats that contain no or few double bonds are referred to as saturated fats. These fats tend to be solid at room temperature, such as butter or lard. The consumption of saturated fats carries some health risks in that they have been linked to arteriosclerosis (hardening of the arteries) and heart disease. Unsaturated fats contain more double bonds in their structure. These fats are generally liquids at room temperature (fats that are liquid at room temperature are referred to as oils). Monounsaturated fats, such as olive oil and canola oil, appear to be particularly beneficial in the fight against heart disease.

Proteins
Proteins are polymers of amino acids. While there are hundreds of thousands of different proteins that exist in nature, they are all made up of different combinations of just 20 amino acids. Proteins are large molecules that may consist of hundreds, or even thousands of amino acids. The 20 amino acids found in nature all have the general structure:

Where the R group at bottom varies depending on the specific amino acid in question. This R group can be simply an H atom, as in the amino acid glycine, or a more complex organic structure. When 2 amino acids bond together, the OH group highlighted in red above is released from one amino acid, and the H atom highlighted in red is released from a second amino acid. The nitrogen from the second amino acid then bonds to the carbon from the first to create a peptide bond, as illustrated below (the peptide bond is highlighted in red).

A Peptide Bond

In proteins, many peptide bonds form between many amino acids to create long chains (thus proteins are also called polypeptides because they contain many peptide bonds).
Proteins serve many purposes in the body. Structural proteins such as keratin and collagen are the main ingredients in your hair, muscles, tendons and skin and help give structure to the body. In addition to adding structure, other proteins perform a wide range of functions in the human body. Amylase is a protein that helps your body digest starch, the protein hemoglobin is responsible for transporting oxygen in the blood stream, insulin helps regulate the storage of glucose in the body and the list goes on and on. There are an estimated 100,000 different proteins in the human body alone. Each has a different structure and performs a different function in the body.
Because proteins perform such specific tasks in the body, each protein has to be manufactured in the body to suit the individual's specific needs. When an animal eats protein, that protein is broken down in the digestive tract into its individual amino acids. These amino acids are then recombined in the body in the specific sequence needed to form whichever protein the animal needs at that point in time. Thus protein in food is just a source of amino acids. Some good food sources of protein include beans, milk and cheese, fish and meats.

General Nutrition
In addition to the macronutrients described above, there are many micronutrients (substances needed in small amounts) essential to healthy living. These micronutrients include the vitamins and minerals. A healthy diet includes the proper ratio of macronutrients along with the essential quantities of micronutrients. The U.S. Department of Agriculture has put together a Food Pyramid to help educate the public on a proper diet.

The food groups at the bottom of the pyramid, including the grains and cereals, should make up the largest portion of an individual's diet. Those groups at the top of the pyramid, including the fats and oils, should make up the smallest portion of the diet. For further guidance on using this pyramid, visit the USDA Food Pyramid site.

For additional information regarding food and nutrition visit:

The USDA Dietary Guidelines web site
The USDA Food and Nutrition Center site which includes a cool, searchable Food Composition database
The Food and Drug Administration's Guidelines on Food Labeling
And finally, for those fast food junkies, Olen Publishing has put together a neat Food Finder that allows you to search the menus of the major fast-food restaurants for nutritional data.