There are three kinds of nutrients that provide your cells with energy, and that you require in large quantities. These are called macronutrients and include carbohydrates -- of which polysaccharides are a subset -- lipids or fats, and proteins. Your cells use the macronutrients both for energy generation and to provide building blocks for cellular molecules.
While you need vitamins and minerals, called micronutrients, to stay healthy and functional, it's the macronutrients that you rely upon for cellular energy. Carbohydrates, fats, and proteins contain chemical energy in their bonds. When you consume foods, you break down the macronutrient constituents into smaller molecules that you absorb into the bloodstream. You then burn these smaller molecules at the cellular level to provide energy needs, or store them for later energy use.
Polysaccharides are one class of carbohydrates. The digestible polysaccharides are more commonly called starch. Fiber is also a polysaccharide, but it isn't digestible and doesn't provide any energy, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." Polysaccharides are made up of long chains of building blocks called monosaccharides, which are sugars. When you consume starch, you break it down into its constituent monosaccharides and absorb them into the bloodstream.
Lipids are fats. There are many different kinds of lipids, including cholesterol and the phospholipids that compose your cell membranes, but the lipids you consume in food and use for energy are triglycerides. Triglycerides contain much more energy per gram than carbohydrates and proteins, so they're a rich and valuable source of cellular energy, explains Dr. Lauralee Sherwood in her book "Human Physiology." You can burn triglycerides for energy immediately, or store them in fatty tissue.
When you consume proteins, you break them into their constituent molecules, which are called amino acids. Amino acids serve several purposes in the body: Like monosaccharides and triglycerides, they're a source of energy for the cells. They're also the precursor molecules for certain neurotransmitters and other communication chemicals. Finally, they're the building blocks for the proteins your cells make. For instance, the cells of your digestive tract make digestive enzymes, which are proteins, by assembling amino acids.
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004