Lipids are fats, oils and waxes and are very significant to animal cells. They can be used as an energy reserve and for insulation under the skin, in the adipose tissue. The myelin sheath which surrounds the nerve cells contains lipids. Lipids are important for the developing nervous system of a fetus. Lipids can be converted into energy when needed and used in cellular respiration because they can be broken down into acetic acid and enter the Krebs cycle to release high energy A.T.P. molecules for cellular processes to proceed. Fat can be protective around certain areas of the body; for example, women have a higher fat to muscle ration than men probably to protect a developing fetus. Fats can store fat soluble vitamins, A, D, E, K in the body. Steroids in the body contain fats. Lipids are found in the double layered cell membrane since it is composed of two layers of lipids and protein in between.