Lactic acid • The end product of fermentation in vertebrate muscle and some microorganisms.
Lagging strand • In DNA replication, the daughter strand that is synthesized discontinuously.
Lamella • Layer.
Larynx (lar´ inks) • A structure between the pharynx and the trachea that includes the vocal cords.
Larva (plural: larvae) [L.: ghost, early stage] • An immature stage of any invertebrate animal that differs dramatically in appearance from the adult.
Lateral • Pertaining to the side.
Lateral gene transfer • The movement of genes from one prokaryotic species to another.
Lateral meristems • The vascular cambium and cork cambium, which give rise to secondary tissue in plants.
Laterization (lat´ ur iz ay shun) • The formation of a nutrient-poor soil that is rich in insoluble iron and aluminum compounds.
Law of independent assortment • The random separation during meiosis of nonhomologous chromosomes and of genes carried on nonhomologous chromosomes. Mendel’s second law.
Law of segregation • Alleles segregate from one another during gamete formation, Mendel’s first law.
Leader sequence • A sequence of amino acids at the N-terminal end of a newly synthesized protein, determining where the protein will be placed in the cell.
Leading strand • In DNA replication, the daughter strand that is synthesized continuously.
Lenticel • Spongy region in a plant’s periderm, allowing gas exchange.
Leukocyte (loo´ ko sight) [Gr. leukos: clear + kutos: hollow vessel] • A white blood cell.
Lichen (lie´ kun) [Gr. leikhen: licker] • An organism resulting from the symbiotic association of a true fungus and either a cyanobacterium or a unicellular alga.
Life cycle • The entire span of the life of an organism from the moment of fertilization (or asexual generation) to the time it reproduces in turn.
Life history • The stages an individual goes through during its life.
Life table • A table showing, for a group of equal-aged individuals, the proportion still alive at different times in the future and the number of offspring they produce during each time interval.
Ligament • A band of connective tissue linking two bones in a joint.
Ligand (lig´ and) • A molecule that binds to a receptor site of another molecule.
Lignin • The principal noncarbohydrate component of wood, a polymer that binds together cellulose fibrils in some plant cell walls.
Limbic system • A group of primitive vertebrate forebrain nuclei that form a network and are involved in emotions, drives, instinctive behaviors, learning, and memory.
Limiting resource • The required resource whose supply most strongly influences the size of a population.
Linkage • Association between genetic markers on the same chromosome such that they do not show random assortment and seldom recombine; the closer the markers, the lower the frequency of recombination.
Lipase (lip´ ase; lye´ pase) • An enzyme that digests fats.
Lipids (lip´ ids) [Gr. lipos: fat] • Substances in a cell which are easily extracted by organic solvents; fats, oils, waxes, steroids, and other large organic molecules, including those which, with proteins, make up the cell membranes. (See phospholipids.)
Litter • The partly decomposed remains of plants on the surface and in the upper layers of the soil.
Littoral zone • The coastal zone from the upper limits of tidal action down to the depths where the water is thoroughly stirred by wave action.
Liver • A large digestive gland. In vertebrates, it secretes bile and is involved in the formation of blood.
Lobes • Regions of the human cerebral hemispheres; includes the temporal, frontal, parietal, and occipital lobes.
Locus • In genetics, a specific location on a chromosome. May be considered to be synonymous with "gene."
Logistic growth • Growth, especially in the size of an organism or in the number of organisms that constitute a population, which slows steadily as the entity approaches its maximum size. (Contrast with exponential growth.)
Loop of Henle (hen´ lee) • Long, hairpin loop of the mammalian renal tubule that runs from the cortex down into the medulla, and back to the cortex. Creates a concentration gradient in the interstitial fluids in the medulla.
Lophophore • A U-shaped fold of the body wall with hollow, ciliated tentacles that encircles the mouth of animals in several different phyla. Used for filtering prey from the surrounding water.
Lordosis (lor doe´ sis) [Gk. lordosis: curving forward] • A posture assumed by females of some mammalian species (especially rodents) to signal sexual receptivity.
Lumen (loo´ men) [L.: light] • The cavity inside any tubular part of an organ, such as a piece of gut or a kidney tubule.
Lungs • A pair of saclike chambers within the bodies of some animals, functioning in gas exchange.
Luteinizing hormone • A gonadotropin produced by the anterior pituitary. It stimulates the gonads to produce sex hormones.
Lymph [L. lympha: water] • A clear, watery fluid that is formed as a filtrate of blood; it contains white blood cells; it collects in a series of special vessels and is returned to the bloodstream.
Lymph nodes • Specialized tissue regions that act as filters for cells, bacteria and foreign matter.
Lymphocyte • A major class of white blood cells. Includes T cells, B cells, and other cell types important in the immune response.
Lysis (lie´ sis) [Gr.: a loosening] • Bursting of a cell.
Lysogenic • The condition of a bacterium that carries the genome of a virus in a relatively stable form. (Contrast with lytic.)
Lysosome (lie´ so soam) [Gr. lysis: a loosening + soma: body] • A membrane-bounded inclusion found in eukaryotic cells (other than plants). Lysosomes contain a mixture of enzymes that can digest most of the macromolecules found in the rest of the cell.
Lysozyme (lie´ so zyme) • An enzyme in saliva, tears, and nasal secretions that attacks bacterial cell walls, as one of the body’s nonspecific defense mechanisms.
Lytic • Condition in which a bacterium lyses shortly after infection by a virus; the viral genome does not become stabilized within the bacterial cell. (Contrast with lysogenic.)