Nutritions - Proteins Proteins are the main structural components of cell structures like cell membrane and nucleus, and especially the energy-transformers mitochondria cells. Proteins may also be used as a fuel, or even energy reactors (enzymes) in our bodies. Therefore they can cause chemical reactions in the cells, and can assist to form hormones. Proteins are powerful compounds that build and repair body tissues, from hair and fingernails to muscles. In addition to maintaining the body's structure, proteins speed up chemical reactions in the body, serve as chemical messengers, fight infection, and transport oxygen from the lungs to the body's tissues. Although protein provides 4 calories of energy per gram, the body uses protein for energy only if carbohydrate and fat intake is insufficient. When used as an energy source, protein is diverted from the many critical functions it performs for our bodies. Protein takes up 10% of our daily calories intake.