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.


Steroids, large group of naturally occurring and synthetic lipids, or fat-soluble chemicals, with a great diversity of physiological activity. Included among the steroids are certain alcohols (sterols), bile acids, many important hormones, some natural drugs, and the poisons found in the skin of some toads (see Digitalis; Hormone). Various sterols found in the skin of human beings are transformed into vitamin D when they are exposed to the ultraviolet rays of the sun (see Vitamin: Vitamin D). Cholesterol, a major contributor to arteriosclerosis, is a sterol. Steroid hormones, which are similar to but not identical with sterols, include the adrenal cortical steroids hydrocortisone, cortisone, aldosterone, and progesterone; and the female and male sex hormones (see Estrogen; Testosterone). Most oral contraceptives are synthetic steroids consisting of female sex hormones that inhibit ovulation (see Birth Control). Perhaps the most widely used steroids in medicine are cortisone and various synthetic derivatives of this substance. Such steroids are prescription drugs used for a variety of skin ailments, rheumatoid arthritis, asthma and allergies, and various eye diseases, and in cases of adrenal insufficiency, or the malfunctioning of the adrenal cortex (see Adrenal Gland; Endocrine System).