Proteins make up about 42% of the dry weight of our bodies. The protein collagen—which holds our skin, tendons, muscles, and bones together—makes up about a quarter of the body’s total protein. All of our cells and even blood are packed with protein molecules. This watercolor painting by David S. Goodsell shows part of a red blood cell, at the bottom left, filled with hemoglobin molecules. The upper half of the painting shows blood serum, containing yellow, Y-shaped antibodies and other proteins. In purple, proteins poke through the blood cell’s membrane.