L abbr. (1) liter; (2) leucine.
l- abbr. Levorotatory.
labiate /LAY-bee-ət, -ate/ (1) having lips; (2) like lips.
lacrimal /LACK-rə-məl/ Pertaining to lacrimation.
lacrimal fluid /LACK-rə-məl/ The lubricating liquid secreted by the lacrimal glands onto the surface of the eyeballs. When excessive secretion occurs the lacrimal fluid overflows the eyelids and pours out onto the face. The individual drops of this overflow are called tears.
lacrimal gland /LACK-rə-məl/ The gland that secretes tears (i.e., lacrimal fluid). The lacrimal gland lies in a depression at the outer angle of the orbit on the inner side of the external process of the frontal bone.
lacrimal sac /LACK-rə-məl/ A small expanded chamber at the upper end of the nasolacrimal duct. Lacrimal fluid flows from the eyes through the nasolacrimal duct into the nasal cavity. This flow can cause a runny nose during excessive tearing, as in states of high emotion or allergic reaction. PICTURE
lacrimation /LACK-rə-MAY-shən/ The production of lacrimal fluid.
lactate dehydrogenase (LDH) /LACK-tate dee-hī-DRAWJ-ə-naze/ An enzyme, present in both plants and animals, that catalyzes the interconversion of lactate and pyruvate (and, at the same time, the interconversion of NADH and NAD+). SEE DIAGRAM
lactation /lack-TAY-shən/ The production of milk.
lactic acid /LACK-tik/ An alpha-hydroxy acid CH3CH(OH)COOH that plays a variety of biological roles. It is known in three isomers: (1) a dextrorotatory L-form found in muscle tissue and in the blood; (2) a levorotatory D-form, a product of sucrose fermentation; and (3) a DL-form found foods that have undergone fermentation (such as beer, pickles, and sour milk).
lactogenic hormone /lack-tə-JEN-ik/ (also luteotropin /lood-ee-ə-TRŌ-pə/ or luteotrophin /lood-ee-ə-TRŌ-feen/) A hormone, produced by the pituitary gland, promoting lactation in mammals (and brooding in birds).
lactose (4-(β-D-galactoside)-D-glucose) /LACK-tīs/ A disaccharide composed of two hexoses linked by a β-galactoside. The enzyme β-galactosidase cleaves lactose into glucose and galactose. Lactose is abundant in milk.
lacuna (pl lacunae) /lah-KOON-ə, pl: lah-KOON-ī/ A small open space or gap, as within a cell or bony tissue.
lageniform /lə-JEN-ə-form/ Bottle-shaped.
Lagomorpha /lawg-ə-MORF-ə/ The mammalian order comprised of the rabbits, hares, and pikas.
Lake Agassiz An immense prehistoric lake that once existed in northern North America. MORE INFORMATION
Lamarkian inheritance /lə-MARK-ee-ən/ Name given to the idea that an alteration in the body, caused use or disuse of the affected part, will tend to be passed on to the offspring of the affected individual. This idea is widely attributed to Jean-Baptiste Lamarck, but he did not originate the idea. It was accepted by all of his contemporaries and thereafter, by most naturalists, up to about the year 1900.
lambda (λ) /LAM-də/ One microliter (μl).
lambda phage /LAM-də/ See: bacteriophage lambda.
lamina /LAM-ə-nə/ (pl laminae /LAM-ə-nī/) (1) any thin, flat layer; (2) either of two flattened portions of a vertebral arch, each of which lie between the spinous process and one of the pedicles (PICTURE).
laminectomy /lam-ə-NECK-tə-mee/ Surgical excision of a vertebral lamina.
langurs Any of various Asiatic slender, long-tailed monkeys (family Colobidae).
larva /LARV-ə/ (pl larvae /LARV-ee, -eye/) A free-living, sexually immature form that occurs in the life cycle of many animals; a larval form is often quite distinct from the adult (e.g., maggot and fly, tadpole and frog) and the two usually differ in diet.
larval /LARV-əl/ Of or pertaining to larvae.
larviparous /lar-VIP-er-əs/ Laying larvae, as opposed to eggs.
laryngoscope /lair-IN-gə-skope/ An instrument for examining the larynx.
larynx /LAIR-ənks/ The organ of speech (voice-box) that makes up the upper end of the trachea.
lateral /LAT-er-əl/ Toward, at, or pertaining to the side.
laterodorsal /LAT-er-ō-DORE-səl/ Denoting a position in the region where back and side meet.
latifoliate /LAT-ə-FOAL-ee-ət/ Broad-leaved.
Laurasia /lore-AYZH-ə/ One of the two supercontinents produced by the initial splitting of Pangea (the other was Gondwana). It existed from the Jurassic to the Paleogene and was composed of the older, once separate continents of Avalonia, Baltica, and Laurentia. With the opening up of the Atlantic Ocean (beginning in the late Mesozoic), Laurasia broke up into North America, Greenland, and western Europe). MAP
Laurentia /lore-ENT-ee-yə/ A continental plate that was in existence from the Late Precambrian to the Silurian. It later became part of Euramerica, then part of Pangea, and still later, part of Laurasia. Today, the corresponding deposits underlie Greenland, as well as the northern parts of North America, the U.K. and Ireland.
lawn (usually in the phrase bacterial lawn) A film of bacteria forming a continuous surface.
LCAT abbr. Lecithin-cholesterol acyltransferase.
LDH abbr. Lactate dehydrogenase.
LDL abbr. Low-density lipoprotein.
lecithin-cholesterol acyltransferase (LCAT), also phosphatidylcholine-sterol O-acyltransferase An enzyme that converts free cholesterol into cholesteryl ester (CE). Since the latter is more hydrophobic than free cholesterol, it tends to move toward the core of lipoprotein particles. This causes the reaction to become unidirectional, since any newly synthesized CE is sequestered within particles. In the blood plasma, LCAT is bound to HDLs and LDLs.
legume A member of an economically important family, Fabaceae, of pod-bearing plants that includes peas, beans, lentils, soy, alfalfa, clovers, peas, beans, lupins, mesquite, carob, peanuts, wisteria, locust tree, and Kentucky coffeetree. Legumes are notable for their ability to carry out nitrogen fixation, due to the presence on their roots of nodules containing nitrogen-fixing bacteria, which can carry out this process only in association with a legume.
lek A site where males, especially avian males, congregate to display to females.
leiomyoma /LIE-ō-my-Ō-mə/ A benign tumor of smooth muscle tissue.
leiomyosarcoma /LIE-ō-MY-ō-sar-KŌ-mə/ A sarcoma arising from smooth muscle cells. Such tumors are most common in the abdomen, uterus, and pelvis, but can occur almost anywhere in the body.
lenticels /LENT-ə-sellz/ On the stems of woody plants, localized regions that lack suberin. Gas exchange occurs through the lenticels (photosynthesis takes place even in leafless trees in winter if the temperature rises above about 40 degrees F).
lentiform /LENT-ə-form/ Lens-shaped.
Lepidoptera /LEP-əd-DAWP-ter-ə/ The order of insects including moths and butterflies (lepidopterans).
leprology /lep-PRAWL-ə-jee/ The scientific study of leprosy.
leprosy /LEP-rə-see/ A chronic disease caused by the bacillus Mycobacterium leprae. Leprosy is now curable, although for many centuries it was not. Untreated it causes progressive, permanent, and disfiguring damage to the skin, nerves, eyes, and appendages. It primarily affects the peripheral nerves and upper respiratory tract. PICTURE 1 | PICTURE 2 | MAP OF WORLD LEPROSY DISTRIBUTION (2003)
leptospirosis (also Weil's disease) /LEP-tə-spə-RŌ-səs/ A disease caused by spirochetes of the genus Leptospira. These microorganisms have an exceptionally broad host range (Leptospira causes disease not only in humans and other mammals, but also in birds, amphibians, and reptiles. The disease is transmitted when infected urine or semen comes in contact with the eyes, mucous membranes, or broken skin. Although many different animals are carriers of this disease, it is most commonly spread to humans via the urine of infected rats and mice, either in contaminated foods or directly by skin contact. Leptospirosis begins with flu-like symptoms (vomiting, fever, myalgia, intense headache). The patient seems to recover, then a more serious phase sets in which can lead to meningitis, liver damage, kidney failure, and death.
leptotene /LEP-tə-teen/ A stage of meiotic prophase I. MORE INFORMATION.
lesion /LEEZH-ən/ (1) A local change in tissue due to disease; (2) a wound; (3) a patch of infection on the skin.
leukocytes or leucocytes (also white cells or white blood corpuscles) /LUKE-ə-SIGHTS/ Cells of the immune system that protect the body against contagious disease and rid it of useless or toxic debris. MORE INFORMATION
leukotrichia /LOO-kō-TRICK-ee-e/ The condition of having white hair.
levorotatory /LEV-ō-ROTE-tə-tore-ee/ Rotating a plane of polarized light counterclockwise (said of a chemical compound). Compare: dextrorotatory
levoversion /lev-ō-VER-shən/ (also levorotation /LEV-ō-rō-TAY-shən/ or levotorsion /LEV-ō-TORE-shən/) A turning to the left. Compare: dextroversion
levulose /LEV-yə-lōse, -lōze/ An alternative name for fructose.
lichens /LIKE-əns/ Organisms composed of a fungus and an alga (usually a green, or blue-green, alga), growing in an intimate, closely entangled, symbiotic relationship. Lichens are most commonly seen on rock outcrops and tree bark. As a lichen grows and respires it gives off carbon dioxide and water, which combine to form carbonic acid. This acid decomposes rocks and releases nutrients essential for the growth of other living things. Lichens do not injure trees. PICTURE
lienitis /lie-en-NIGHT-əs/ Inflammation of the spleen.
lienocele /lie-EN-ō-seel/ Pertaining to the spleen and pancreas.
lienopancreatic /LIE-ə-no-pan-kree-AT-ick/ Pertaining to the spleen and pancreas.
lienopathy /lie-en-OP-ə-thee/ Any disease of the spleen.
ligament /LIG-ə-mehnt/ A fibrous band of connective tissue serving to bind bones together at joints.
ligase (also DNA ligase) /LIE-gaze/ An enzyme catalyzing a condensation reaction that links ("ligates") two DNA molecules through the formation of a phosphodiester bond between the 3' hydroxyl and 5' phosphate of adjacent nucleotides.
ligature /LIG-ə-cher/ (1) the act of binding or tying; (2) a thread, cord, or wire for tying blood vessels.
lignification /LIG-nə-fə-KAY-shən/ The process of becoming woody.
lignite /LIG-nite/ A usually brownish-black coal, in which the texture of the original wood can often be seen; intermediate between peat and bituminous coal.
ligulate /LIG-yool-ət/ Like a spoon, small tongue or strap.
limbus /LIM-bus/ The border of an organ or other body part.
limnal /LIM-null/ Of or pertaining to lakes and ponds.
limnology /lim-NALL-ə-jee/ The study of the physical and biological conditions of freshwater, particularly of lakes and ponds — limnologist /lim-NALL-ə-jəst, -jist/
limnoplankton /LIM-no-PLANK-tən/ Freshwater plankton.
lineage /LIN-əj/ A descent through a single line from a progenitor.
lingual /LING-yoo-əl/ Pertaining to the tongue.
linguiform /LING-wə-form/ Tongue-shaped.
linkage disequilibrium /LINK-əj dis-eek-wə-LIB-ree-əm/ Nonrandom association of non-allelic genes.
linked genes/markers Genes/markers that, due to their occurrence on the same chromosome, are inherited together at rates above random expectation. See: linkage.
lipard /LIP-urd/ See: leopon.
lipase /LIP-aze, LIPE-aze/ An enzyme that breaks down lipids.
lipids /LIP-ədz/ Any of a wide variety of biological molecules, which are only sparingly soluble in water, including fats, fat-soluble vitamins, monoglycerides, diglycerides, phospholipids, waxes and sterols. Lipids are generally soluble in nonpolar solvents such as ether or chloroform. They are important structural components of plasma membranes. They also serve in energy storage and as signaling molecules.
lipid bilayer /LIP-əd BĪ-lay-er/ An enclosing layer, composed primarily of phospholipids, that forms the boundaries of cells and cellular vesicles.
lipoblast /LIP-ə-blast/ An immature fat cell.
lipocyte /LIP-ə-sight/ A fat cell.
liposarcoma /LIP-ō-sar-KŌ-mə/ A sarcoma arising from fat cells.
liposome /LIP-pə-zome/ A vesicle, composed of phospholipids like those found in the lipid bilayer of a cell's plasma membrane. Liposomes occur naturally, but can also be artificially constructed for various purposes, such as to deliver drugs to target cells. A liposome usually has an aqueous core.
listeriosis /lis-tir-ee-Ō-səs/ A serious human disease characterized by sepsis and meningitis. It mainly affects newborns, pregnant women, the elderly, and adults with compromised immune systems. Infection results from consumption of food contaminated with the bacteria.
littoral /LID-ə-rəl, LIT-/ Of, near, on, or pertaining to the shore, especially of a sea or ocean.
living adj. Alive; being in the state of life, as opposed to dead.
living thing n. An organism.
living fossils Organisms (such as the horseshoe crab, dawn redwood, crocodilians, the coelocanth Latimeria chalumnae or the inarticulate brachiopod Lingula) known from past geological eras that survive today and that have remained to all appearances unchanged.
Lk abbr. Linking number.
lobe /lōb/ (1) Any well-defined region of an organ, marked off from other such regions by a fissure; (2) a rounded projection (earlobe); (3) a projecting division, as of a leaf.
locus (pl loci) /LŌ-kəs, pl: LŌ-sī/The position on a chromosome occupied by a gene.
loess /ləs/ A deposit composed of loose material derived from the sedimentation of wind-blown dust.
Locusta migratoria /LOW-kəst-ə mee-grə-TORE-ee-yə or mī-grə-/ The Old-world migratory, or "plague" locust.
logarithmic growth phase (also log or exponential growth phase) /low-gə-RITH-mick/ The phase of vigorous growth during which cell number doubles every 20-30 minutes. Compare: lag growth phase
loins The region of the back and sides between the ribs and pelvis.
lorica /lə-RIKE-ə/ (pl loricae /lə-RICE-ee/) A hard protective shell present in certain cells and microorganisms.
Loxosceles /lock-SAW-sə-leez/ The genus comprised of the recluse spiders (also known as brown or fiddle-back spiders). The venomous bite of these shy spiders, which often infest homes, causes local necrosis and sometimes death. MORE INFORMATION
luciferin /loo-SIF-ə-rən/ A compound found in bioluminescent organisms. It emits a nearly heatless light when it undergoes oxidation.
lumen (1) The space enclosed within a living cell or organelle; (2) the space within a tubular organ or other tube-like living structure; (3) the bore, as of a needle.
lump To assign organisms (formerly assigned to separate taxonomic categories) to the same category.
lunate /LOON-ate/ Cresent-shaped.
luteotrophin See: lactogenic hormone.
luteotropin See: lactogenic hormone.
lymphatic system (also lymphatics) /lim-FAT-ik/ A system of vessels and nodes containing lymph, which returns protein and fluid from the tissues to the blood. It connects with the blood circulatory system in the large veins near the heart.
lymphectasia /limf-ek-TAY-zhə/ Dilation of lymphatic vessels.
lymphocyte A leukocyte present in lymph. There are two main types, the large granular lymphocytes and the small lymphocytes. Most large granular lymphocytes are natural killer (NK) cells that protect the body from tumor, and virus-infected, cells. Small lymphocytes are of two types, T cells and B cells.
lymphoid /LIM-foid/ Producing, or pertaining to lymph.
lymphoidectomy /LIM-foid-ECK-tə-mee/ Surgical excision of lymphoid tissue.
lyriform Shaped like a lyre.
lyse /LĪS/ v. (1) intransitive: to rupture (said of individual cells); (2) transitive: to cause a cell to rupture. See also: lysis.
lysis /LĪ-sis/ (1) Rupture of a cell wall resulting in the dissolution of the cell; (2) In a medical context: the subsidence of a fever or disease.
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