smog  A local alteration in the atmosphere caused by human activity; mainly an urban problem that is often due to pollutants produced by fuel combustion.

smooth muscle  Muscle that lacks striations; found around circulatory system vessels and in the walls of such organs as the stomach, intestines, and bladder. Involuntary, not striated cells that control autonomic functions such as digestion and artery contraction. PICTURE

social behavior  Behavior that takes place in a social context and results from the interaction between and among individuals.

societies  The most highly organized type of social organization; consist of individuals that show varying degrees of cooperation and communication with one another; often have a rigid division of labor.

sodium-potassium pump The mechanism that uses ATP energy to reset the sodium and potassium ions after transmission of a nerve impulse.

soil Weathered rocks and minerals combined with air, water and organic matter that can support plants.

somatic  Relating to the non-gonadal tissues and organs of an organism's body.

somatic cell A cell that is not or will not become a gamete; the cells of the body.

somatic senses  All senses except vision, hearing, taste, and smell; include pain, temperature, and pressure.

somatic nervous system  The portion of the peripheral nervous system consisting of the motor neuron pathways that innervate skeletal muscles.

somatostatin Pancreatic hormone that controls the rate of nutrient absorption into the bloodstream.

somites  Mesodermal structures formed during embryonic development that give rise to segmented body parts such as the muscles of the body wall.

special senses  Vision, hearing, taste, and smell.

species  One or more populations of interbreeding or potentially interbreeding organisms that are reproductively isolated in nature from all other organisms. Populations of individuals capable of interbreeding and producing viable, fertile offspring. The least inclusive taxonomic category commonly used. PICTURE

species diversity  The number of living species on Earth.

species packing  The phenomenon in which present-day communities generally contain more species than earlier communities because organisms have evolved more adaptations over time.

species richness  The number of species present in a community.

gamete. PICTURE

spermatogenesis