Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Keratin, highly fibrous and resistant protein that makes up most of the material in the cells forming the epidermis, hair, nails, scales, feathers, beaks, horns, and hooves of animals. These cells originate from permanent populations of germinal cells, and as they migrate outward they undergo specific patterns of differentiation in a process called keratinization. That is, the cells become increasingly filled with microfibrils of keratin, and the nuclei and organelles of the cells are reabsorbed. Little is understood, however, about how the cells differentiate to form such diverse and efficient structures as the elastic outer layer of the skin of mammals or the stiff scales of fish. An important quality of keratin is its ability to extend and contract reversibly.