Skip to main content
Biology LibreTexts

25.19: Glossary: S

  • Page ID
  • saliva

    fluid secreted by salivary glands that keeps the mouth moist and contains the digestive enzymes amylase and lipase

    salivary gland

    one of many exocrine glands in the mouth that secrete saliva into the mouth through ducts


    individuals or events that are actually selected for study in an experiment or other scientific investigation and generally are a subset of all possible cases that could be selected for study


    basic functional unit of skeletal and cardiac muscles, containing actin and myosin protein filaments that slide over one another to produce a shortening of the sarcomere and a muscle contraction


    gradual decrease in the ability to maintain skeletal muscle mass that occurs in later adulthood

    saturated fatty acid

    simple lipid molecule in which a chain of carbon atoms is bonded to as many hydrogen atoms as possible because adjacent carbon atoms in the chain share only single bonds

    savanna hypothesis

    hypothesis that many human traits such as upright bipedalism evolved as adaptations to a savanna habitat


    decomposer that consumes the soft tissues of dead animals such as the remains of a predator’s kill


    distinctive way of gaining knowledge about the natural world that tries to answer questions with evidence and logic

    scientific investigation

    plan for asking questions and testing possible answers to them using evidence and logic

    scientific law

    statement describing what always happens under certain conditions in nature

    scientific method

    process typically followed in a scientific investigation that includes such steps as making observations, asking a question, forming a hypothesis, testing the hypothesis, drawing conclusions, and communicating results

    scientific racism

    ideology popular until the early 20th century that race is a valid biological concept and that human behavior is partly determined by race

    scientific theory

    broad explanation that is widely accepted because it is strongly supported by a great deal of evidence


    pouch-like external structure of the male reproductive system, located behind the penis, that contains the testes, epididymes, and part of the vas deferens

    sebaceous gland

    gland in the dermis of the skin that produces sebum, an oily substance that waterproofs the skin and hair

    secondary pollutant

    contaminant in the environment that forms when primary pollutants react after being released into the environment

    secondary sex characteristic

    trait that is different in males and females but is not directly involved in reproduction, such as male facial hair and female breasts

    secondary succession

    change over time in the numbers and types of species that live in an area that was previously colonized by living things but has been disturbed

    secondhand smoke

    smoke that enters the air from burning cigarettes or from the lungs of smokers


    fluid containing sperm and glandular secretions, which nourishes sperm and carries them through the urethra and out of the body

    seminal vesicle

    one of a pair of glands of the male reproductive system that secretes fluid into semen

    seminiferous tubule

    one of the many tiny tubes contained within the testes where sperm are produced


    component of a homeostatic mechanism that senses the value of a variable and sends data on it to the control center

    sensory nerve

    nerve of the peripheral nervous system that transmits information from sensory receptors in the body to the central nervous system

    sensory neuron

    type of neuron that carries nerve impulses from sensory receptors in tissues and organs to the central nervous system; also called afferent neuron

    sensory receptor

    specialized nerve cell that responds to a particular type of stimulus such as light or chemicals by generating a nerve impulse

    Sertoli cell

    type of cell that lines the seminiferous tubules in the testes and plays several roles in sperm production

    set point

    physiologically optimum value for a given biological variable such as body temperature

    sex chromosome

    X or Y chromosome

    sex hormone

    endocrine hormone secreted mainly by gonads that controls sexual development and reproduction

    sex-linked gene

    gene located on a sex chromosome

    sex-linked trait

    trait controlled by a gene located on a sex chromosome

    sex ratio

    number of males per 100 females in a population

    sexual dimorphism

    differences between the phenotypes of males and females of the same species

    sexually transmitted infection (STI)

    infection caused by a pathogen that spreads mainly through sexual contact; also called sexually transmitted disease (STD)

    sexual reproduction

    type of reproduction that involves the fertilization of gametes produced by two parents and produces genetically variable offspring

    single nucleotide polymorphism (SNP)

    variation in just one nucleotide in alleles that are present in a population at frequencies greater than 1 percent

    sinus rhythm

    normal, rhythmical beating of the heart

    sixth mass extinction

    current mass extinction caused primarily by habitat loss due to human actions

    skeletal muscle

    voluntary, striated muscle that is attached to bones of the skeleton and helps the body move

    skeletal system

    human body system that consists of all the bones of the body as well as cartilage and ligaments and provides an internal framework for the body


    major organ of the integumentary system that covers and protects the body and helps maintain homeostasis, for example, by regulating body temperature


    part of the human skeleton that provides a bony framework for the head and includes bones of the cranium and face

    sleep apnea

    disorder characterized by pauses in breathing during sleep, usually because of physical blockage of air flow

    sliding filament theory

    theory that explains muscle contraction by the sliding of myosin filaments over actin filaments within muscle fibers

    slow-twitch muscle fiber

    type of skeletal muscle cell that is mainly responsible for aerobic activities such as long-distance running

    small intestine

    long, narrow, tube-like organ of the digestive system where most chemical digestion of food and virtually all absorption of nutrients take place

    smooth muscle

    involuntary, nonstriated muscle that is found in the walls of internal organs such as the stomach

    sodium-potassium pump

    active transport mechanism in which sodium ions are pumped out of a cell and potassium ions are pumped into the cell with the help of a carrier protein and energy from ATP


    mixture of eroded rock, minerals, organic matter, and other materials that is essential for plant growth and forms the foundation of terrestrial ecosystems

    soluble fiber

    nondigestible carbohydrates in food that dissolve in water and slow the absorption of nutrients from the GI tract, thereby reducing insulin spikes

    somatic mutation

    mutation that occurs in a cell of the body other than a gamete

    somatic nervous system

    division of the peripheral nervous system that controls voluntary activities


    endocrine hormone produced by the pancreas that inhibits the production of growth hormone by the pituitary and the secretion of insulin and glucagon by the pancreas


    evolution of different adaptations in competing species, which allows them to live in the same area without competing

    special sense

    sense such as vision or hearing that has special sense organs that gather sensory information and change it into nerve impulses


    process by which a new species evolves


    group of organisms that are similar enough to mate and produce fertile offspring together


    gamete produced by a male organism


    process of producing sperm in the testes

    spermatogonium (plural, spermatogonia)

    diploid stem cell in a testis that undergoes mitosis to begin the process of spermatogenesis


    ring of muscles that can contract to close off an opening between structures, such as between the esophagus and stomach

    spinal cavity

    long, narrow body cavity inside the vertebral column that runs the length of the trunk and contains the spinal cord

    spinal cord

    thin, tubular bundle of nervous tissue that extends from the brainstem down the back to the pelvis and connects the brain with the peripheral nervous system


    secondary organ of the lymphatic system where blood and lymph are filtered

    spongy bone

    light-weight, porous inner layer of bone that contains bone marrow

    squamous cell carcinoma

    common type of skin cancer that affects squamous cells in the epidermis and rarely metastasizes

    stabilizing selection

    type of natural selection for a polygenic trait in which phenotypes at both extremes of the phenotypic distribution are selected against, resulting in a narrowing of the range of phenotypic variation


    type of complex carbohydrate (polysaccharide) that plants use to store energy

    stem cell

    undifferentiated cell that can develop into specialized types of cells


    surgical procedure that is generally irreversible and that makes it impossible for a woman to become pregnant or for a man to ejaculate viable, motile sperm


    type of lipid with a ring structure, such as cholesterol or a sex hormone

    steroid hormone

    type of endocrine hormone that is made of lipids and crosses the plasma membrane to bind with a receptor inside a target cell


    type of psychoactive drug that stimulates the brain and increases alertness and wakefulness


    something that triggers a behavior or other response


    sac-like organ of the digestive system between the esophagus and small intestine in which both mechanical and chemical digestion take place


    cerebrovascular accident in which a broken artery or blood clot results in lack of blood flow to part of the brain, causing death of brain cells


    growth deficit diagnosed in children who are at least two standard deviations below the median height for their age in a reference population

    subsistence strategy

    general way in which a group of people obtain food and the types of food they rely on most, such as hunting and gathering, farming, or fishing


    specific substance acted upon by a given enzyme


    short-chain soluble carbohydrate that is found in many foods, tastes sweet, and provides quick energy


    reddening of the skin that occurs when the outer layer of the skin is damaged by UV light from the sun or tanning lamps

    surrogate mother

    woman who agrees to become pregnant using a man’s sperm and her own egg, carries the fetus to term, and then gives up the baby at birth for adoption by the sperm donor and his partner

    sustainable use

    use of a natural resource in a way that meets the needs of the present and also preserves the resource for use by future generations


    salty fluid secreted into ducts by sweat glands in the dermis that excretes wastes and helps cool the body; also called perspiration

    sweat gland

    exocrine gland in the dermis of the skin that produces the salty fluid called sweat through a duct to the skin surface


    close relationship between organisms in two different species in which at least one of the organisms benefits from the relationship; mutualism, commensalism, or parasitism

    sympathetic division

    division of the autonomic nervous system that controls the fight-or-flight response

    sympatric speciation

    evolution of a new species that occurs without geographic separation first occurring between members of an original species


    space between the axon terminal of a neuron and an adjoining cell

    synovial joint

    movable joint in which a fluid-filled synovial cavity separates bones at the joint


    sexually transmitted infection caused by the bacterium Treponema pallidum that may eventually be fatal if untreated

    systemic circulation

    part of the cardiovascular system that carries blood between the heart and body


    part of a heartbeat in which the atria relax and fill with blood from the lungs and body, while the ventricles contract and pump blood out of the heart