Like petroleum, food is used for essentially two purposes. We use food as an energy source and a raw material to to build tissues.

There are essentially 4 groups of food molecules. They are:

1) Carbohydrates- "quick" energy source   i.e. sugar, starch, etc.

2) Fats- "slow" energy source and raw material for cell membranes    i.e. oils, fats

3) Proteins - raw material for building muscles and enzymes

4) Vitamins - used in very small quantities. Vitamins act as "helpers" in several important chemical reactions that occur within the body.

Food as Energy

Our bodies "burn" food just like we burn wood in our wood stoves and campfires. As in all combustion reaction reactions, oxygen gas (O2) is required and the products are carbon dioxide (CO2) and water (H2O) and energy. There are two main differences in how the body "burns" its fuel and how we burn wood or gasoline.

1) While combustion reactions take place in one step releasing lots of heat, our metabolism takes place in lots of small steps.

2) This allows manageable amounts of energy to be released so that our body can trap some of the energy in the form of ATP instead of releasing it all as heat. The energy that is trapped can be used to do work such as muscle contraction, active transport or other processes that require energy.

Our bodies require somewhere between 2,000 and 3,000 Calories of energy daily, depending on our age, gender and activity levels. If we ingest less calories than we need, our body will use up stored energy present in our muscles and fat. If we ingest more calories than we require, we will store the excess energy as fat.

Foods With Energy

Fats and carbohydrates are two major food groups that we use for energy.

Carbohydrates

It is recommended that carbohydrates make up about 60 % of our total calories. The word carbohydrate literally means carbon and water. The ratio of hydrogen in carbohydrates is always 2:1, just as it is in water. Some familiar carbohydrates are:

There are 3 classes of carbohydrates. They are:

1) Monosaccharides (simple sugars) - These include glucose, fructose and galactose.
2) Disaccharides (2 simple sugars bonded to each other)- These include sucrose and lactose.

• Sucrose is made up of a glucose and a fructose joined together.
• Lactose is made up of glucose and galactose joined together.

3) Polysaccharides (many simple sugars bonded in a row)- These include starch, cellulose and glycogen.

• Starch is the polysaccharide made by plants. This is how they store the glucose they make during photosynthesis. Animals can consume starch and break it into its component glucose molecules for energy.

• Cellulose is a structural polysaccharide made for plants' cell walls. Cellulose is not digestible by mammals. Ruminants and termites have intestinal bacteria that break cellulose down into its component glucose molecules.

• Glycogen is a polysaccharide found in animals only. This is how glucose is stored. Glycogen is found primarily in the muscle tissue and the liver. It is broken down into its component glucose molecules when energy is needed by the animal.