Lactic acid fermentation • Fermentation whose end product is lactic acid (lactate).

Lagging strand • In DNA replication, the daughter strand that is synthesized in discontinuous stretches. (See Okazaki fragments.)

Lamella (la mell´ ah) (L. lamina: thin sheet] • Layer.

Larva (plural: larvae) [L. lares: guiding spirits] • An immature stage of any invertebrate animal that differs dramatically in appearance from the adult.

Larynx (lar´ inks) [Gk. larynx: voice box] • A structure between the pharynx and the trachea that includes the vocal cords.

Lateral • Pertaining to the side.

Lateral bud • Located above the point of attachment of leaf to stem, an axillary meristem, short stem, immature leaves, and covering scales.

Lateral gene transfer • The transfer of genes from one prokaryotic species to another.

Lateral meristems • The vascular cambium and cork cambium, which give rise to secondary tissue in plants.

Laticifers (luh tiss´ uh furs) • In some plants, elongated cells containing secondary plant products such as latex.

Leader sequence • A sequence of amino acids at the amino-terminal end of a newly synthesized protein; determines where the protein will be placed in the cell.

Leading strand • In DNA replication, the daughter strand that is synthesized continuously. (Contrast with lagging strand.)

Lenticel (len´ ti sill) • Spongy region in a plant's periderm, allowing gas exchange.

Leukocyte (loo´ ko sight) [Gr. leukos: clear + kytos: container] • A white blood cell.

Lichen (lie´ kun) • 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) • Any molecule that binds to a receptor site of another (usually larger) 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.

Linked (also called sex-linked) • A character that is coded for by a gene on the X chromosome.

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. (Compare with phospholipids.)

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.)

Long-day plants • A plant that requires long day to flower.

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. lumen: light] • The cavity inside any tubular organ or structure, such as the gut or a kidney tubule.

Luteinizing hormone • A gonadotropin produced by the anterior pituitary. It stimulates the gonads to produce sex hormones.

Lymph [L. lympha: liquid] • 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.

Lymphoid tissue • Tissues of the immune defense system dispersed throughout the body and consisting of: thymus, spleen, bone marrow, lymph nodes, blood, and lymph.

Lysis (lie´ sis) [Gr. lysis: break apart] • Bursting of a cell.

Lysogenic cycle • A form of viral replication in which the virus becomes incorporated into the bacterial chromosome and the host cell is not killed. (Contrast with lytic cycle.)

Lysosome (lie´ so soam) [Gr. lysis: break away + soma: body] • A membrane-enclosed organelle 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 cycle • A form of viral reproduction that lyses the host bacterium releasing the new viruses. (Contrast with lysogenic cycle.)