Muscles are made of overlapping protein fibers called actin and myosin, which shorten (contract) the muscle by a oar-like mechanism—the myosin "heads" grab the actin fibers and pull them towards each other, increasing the overlapping and shortening the muscle. The muscle relaxes when the myosin head releases the actin, a process that uses energy in the form of ATP. Ever wondered why dead people are called "stiffs"?!
1st Video on Muscle Structure
2nd, more detailed Video on Actin/Myosin interaction
The immune system is a complicated system of several trillion cells (out of 100 trillion in our whole body) that help protect us from foreign invaders. The molecules on the surface of invading bacteria and viruses are called antigens, which are a kind of protein that helps the invader enter our cells, where it uses our cellular machinery to copy its DNA to make more viruses. Some of our immune defenders, like antibodies, are also proteins; these help to mark the foreign cells for destruction.
The Flu Virus Video
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