Lipids refer to a broad group of biomolecules that are not soluble in water. The group includes fats, waxes, sterols, phospholipids, lipoproteins and the fat-soluble vitamins, A, D, E and K. The general characteristics of lipids are best described by defining their different functions in the human body.
The foremost lipids that provide energy for cells in the human body are triglycerides. Triglycerides are composed of a glycerol molecule. Triglycerides may be saturated or unsaturated, which refers to the amount of double bonds between carbon atoms in the fatty acids. The fatty acids in triglycerides provide over twice the amount of energy as carbohydrates and proteins during cellular metabolism. If the body does not require energy, the excess triglycerides are stored in fat tissue in cells called adipocytes.
The main transportation fluid in the body is the blood, which the "United States Geological Survey" reported is greater than 80 percent water. The transport of water-insoluble lipids in the body is accomplished by chylomicrons, HDLs, LDLs, and VLDLs, which are part lipid and part protein. These molecules surround lipids with their fat-soluble portions and project outward their protein subunits, which are water-soluble.
All cells in the body and also those of all other organisms are separated from their environment by biological membranes. The main components of biological membranes are phospholipids. Phospholipids consist of fatty acid chains with phosphate head groups, which are water soluble. They form bilayers with the water-insoluble fatty acids on the inside and the phosphate heads on the outside. Cholesterol is a waxy sterol lipid that also makes up a large portion of biological membranes. It can be manufactured in the body entirely from fatty acids.
The steroid hormones are all synthesized from cholesterol mainly in the gonads and adrenal glands. The steroid hormones include estrogen, progesterone, testosterone, cortisol, and aldosterone. The steroid hormones act on many different cells in the body and are transported to their targets in the blood by binding to proteins. "The Medical Biochemistry Page" reported steroid hormones regulate many functions of the body, such as reproduction, bone remodeling, blood volume and pressure and immune system activity.
Like other lipids, the fat-soluble vitamins are transported to where they are utilized by lipoproteins. A review article by "Colorado State University" stated that the fat-soluble vitamins are involved in bone growth, skin growth, calcium homeostasis, preventing cell damage and blood clotting, among many other functions. When fat-soluble vitamins are plentiful they are stored in the liver or fatty tissue until needed.