There are many types of protein molecules in the body, and each type is specific to its function. For example, proteins such as keratin and collagen give strength and elasticity to hair, as well as to skin and tendons; haemoglobin and myoglobin are the oxygen-binding proteins of the blood and the muscle respectively; and ovalbumin, the principal protein of egg white is responsible for setting and foaming properties of eggs. A particularly important group of proteins known as enzymes, direct all the body cells to produce chemical reactions. These reactions provide the basis of every type of cell activity, including growth, repair, the production of energy and the excretion of waste products.