Online Biology Dictionary - L

L (1) liter; (2) leucine.

label, radioactive A radioactive isotope artificially introduced into a biological molecule for the purpose of detecting the molecule's presence.

labiate /LAY-bee-uht, -ate/ (1) having lips; (2) like lips.

lacuna (pl lacunae) /lah-KOON-uh, pl: lah-KOON-eye/ A small open space or gap, as within a cell or bony tissue.

lactate /LACK-tate/ The salt or ester of lactic acid.

lactate dehydrogenase (LDH) /LACK-tate dee-high-DRAWJ-uh-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-shun/ The production of milk.

lacteals /LACK-tee-uhls/ lymphatic vessels connected to the intestines and taking up some of the products of digestion.

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-tuh-JEN-ik/ (also luteotropin
/lood-ee-uh-TROH-puh/ or luteotrophin /lood-ee-uh-TROH-feen/) A hormone, produced by the pituitary gland, promoting lactation in mammals (and brooding in birds).

lacuna (pl lacunae) A space within a cell or other biological structure.

lageniform /luh-JEN-uh-form/ Bottle-shaped.

lagging strand The strand of the DNA double helix that is synthesized discontinuously and that contains Okazaki fragments, as opposed to the leading strand. SEE ANIMATION

lag growth phase The period (preceding the logarithmic growth phase) during which there is little increase in the number of organisms present in a population. See: logarithmic growth phase.

lagomorph /LAWG-uh-morf/ A member of the order Lagomorpha.

Lagomorpha /lawg-uh-MORF-uh/ The mammalian order comprised of the rabbits, hares, and pikas.

Lake Agassiz (click to enlarge)

Lake Agassiz /AWG-uh-see/ An immense prehistoric lake that once existed in north-central North America. It is named for Louis Agassiz, the first scientist to realize it had been created by glaciers that acted as dams. Far larger than any modern lake, its waters were fresh, not salt. At its greatest extent it covered an area larger than California (see map). It has been estimated that it held more water than is today contained in all the freshwater lakes of the world. Lake Agassiz drained at the end of the last glaciation, about 10,000 years ago.

Lamarkian inheritance /luh-MARK-ee-uhn/ 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-duh/ One microliter (μl).

lambda phage /LAM-duh/ See: bacteriophage lambda.

larva (pl larvae) /LARV-uh, pl: LARV-ee or LARV-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) and the two usually differ in diet.

larviparous /lar-VIP-er-uhs/ Laying larvae, as opposed to eggs.

lateral /LAT-er-uhl/ Toward, at, or pertaining to the side.

later-generation hybrids /HIGH-brids/ Hybrids produced by breeding of hybrids in generations subsequent to the original (F₁) cross. MORE INFORMATION

latifoliate /LAT-uh-FOAL-ee-uht/ Broad-leaved.

laterodorsal /LAT-er-oh-DORE-suhl/ Denoting a position in the region where back and side meet.

Laurasia /lore-AYZH-uh/ One of the two supercontinents produced by the initial splitting of Pangea (the other was Gondwana). It existed from the Jurassic to the Early Tertiary and was composed of the older, once separate continents of Avalonia, Baltica, and Laurentia. SEE MAP

Laurentia /lore-ENT-ee-yuh/ 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. SEE MAP

lawn A film of bacteria forming a continuous surface.

LCAT Lecithin-cholesterol acyltransferase.

LDH Lactate dehydrogenase.

LDL Low-density lipoprotein.

leading strand The strand of the DNA double helix that is synthesized without interruption, as opposed to the lagging strand. SEE ANIMATION

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.

lek A site where males, especially avian males, congregate to display to females.

leiomyoma /LIE-oh-my-OH-muh/ A benign tumor of smooth muscle tissue.

leiomyosarcoma /LIE-oh-MY-oh-sar-KOH-muh/ 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.

lentiform /LENT-uh-form/ Lens-shaped.

leopon /LEP-on, -uhn/ The hybrid produced by crossing a lioness with a leopard. The reciprocal cross produced with a lion father is called a lipard. MORE ABOUT LEOPONS AND LIPARDS

Lepidoptera /LEP-uhd-DAWP-ter-uh/ The order of insects including moths and butterflies (lepidopterans).

leprology /lep-PRAWL-uh-jee/ The scientific study of leprosy.

leprosy /LEP-ruh-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. MAP OF WORLD LEPROSY DISTRIBUTION

Leptospira /LEP-toe-SPIGH-ruh/ A genus of spirochetes containing the causative agents of leptospirosis.

leptospirosis (also Weil's disease) /LEP-toe-SPIGH-roe-suhs/ 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-tuh-teen/ A stage of meiotic prophase I. MORE INFORMATION.

leucine (Leu or L) /LOO-seen/ Leucine, one of the essential amino acids. Due to its hydrocarbon side chain, leucine is hydrophobic. CODONS | MOLECULAR STRUCTURE

leucoplasts (also leukoplasts) /LUKE-uh-PLASTS/ Colorless plastids found in endosperm, tubers, and cotyledons. They often serve in the storage of starch, proteins, or lipids, although they may also perform biosynthetic functions, such as the synthesis of fatty acids, many amino acids, and various other compounds. Although they have a variable ("amoeboid") appearance leucoplasts are usually much smaller than chloroplasts.

leukocytes or leucocytes (also white cells or white blood corpuscles) /LUKE-uh-SIGHTS/ Cells of the immune system that protect the body against contagious disease and rid it of useless or toxic debris. MORE INFORMATION

levorotatory /LEV-oh-ROTE-tuh-tore-ee/ Rotating a plane of polarized light counterclockwise (said of a chemical compound). Compare: dextrorotatory

levoversion /lev-oh-VER-shun/ (also levorotation /LEV-oh-roe-TAY-shun/ or levotorsion /LEV-oh-TORE-shun/) A turning to the left. Compare: dextroversion

library A collection of cloned DNA fragments representing the genome of a particular organism.

lienopancreatic /LIE-uh-no-pan-kree-AT-ick/ Pertaining to the spleen and pancreas.

ligament /LIG-uh-mehnt/ A fibrous band of connective tissue serving to bind bones together at joints.

lignification /LIG-nuh-fuh-KAY-shun/ The process of becoming woody.

lignin /LIG-nin/ A hard material that joins with cellulose to form stiff cell walls in vascular plants; it also cements cells together providing structural strength to the plant as a whole.

ligulate /LIG-yool-uht/ Like a spoon, small tongue or strap.

limnoplankton /LIM-no-PLANK-tuhn/ Freshwater plankton.

lingual /LING-yoo-uhl/ Pertaining to the tongue.

linguiform /LING-wuh-form/ Tongue-shaped.

linkage /LINK-uhj/ The increased tendency of two alleles at different loci to segregate together because their loci are on the same chromosome.

linkage disequilibrium /LINK-uhj dis-eek-wuh-LIB-ree-uhm/ Nonrandom association of non-allelic genes.

linkage map /LINK-uhj/ A map of the relative positions of genes on a chromosome.

lipard /LIP-ar/ See: leopon.

lipase /LIP-aze LIPE-aze/ An enzyme that breaks down lipids.

lipids /LIP-uhdz/ Any of a wide variety of biological molecules including fats, fat-soluble vitamins, monoglycerides, diglycerides, phospholipids, waxes and sterols. Lipids are important structural components of plasma membranes. They also serve in energy storage and as signaling molecules.

lipid bilayer /LIP-uhd BIGH-lay-er/ An enclosing layer, composed primarily of phospholipids, that forms the boundaries of cells and cellular vesicles.

liposarcoma /LIP-oh-sar-KOH-muh/ A sarcoma arising from fat cells.

Liposome for drug delivery (click to enlarge)

liposome /LIGH-puh-soam/ 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.

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.

locus /LOW-kuhs/ (pl loci /LOW-kuhs/) The position on a chromosome occupied by a gene.

Locusta migratoria /LOW-kuhst-uh mee-gruh-TORE-ee-yuh or migh-gruh-/ The Old-world migratory, or "plague" locust.

logarithmic growth phase (also logarithmic phase) /lawg-uh-RITH-mick/ The phase of population growth during which the number of organisms doubles each time a certain time period elapses. See also: lag growth phase.

luciferin /loo-SIF-uh-ruhn/ A compound found in bioluminescent organisms. It emits a nearly heatless light when it undergoes oxidation.

Lucy Nickname for the most famous specimen of the hominid Australopithecus afarensis.

lumbar /LUHM-bar, LUHM-ber/ (1) pertaining to the lower back, or loins; (2) with reference to vertebrae, those lying between the thoracic and sacral vertebrae; (3) pertaining to the lateral portions of the abdomen that lie at the same level as the lumbar vertebrae.

lump To assign organisms &— formerly assigned to separate taxonomic categories &— to the same category.

lunate /LOON-ate/ Cresent-shaped.

luteinizing hormone (LH also ICSH) /lood-ee-uh-NIZE-ing/ A glycoprotein hormone that stimulates ovulation and the secretion of estrogen.

luteotrophin See: lactogenic hormone.

luteotropin See: lactogenic hormone.

lymph /LIMF/ A liquid, circulating through the lymphatic system, similar to blood plasma containing leukocytes, but no erythrocytes.

lymphangiosarcoma /lim-FAN-jee-oh-sar-KOH-muh/ A sarcoma arising in the cells that line the vessels of the lymphatic system.

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-zhuh/ Dilation of lymphatic vessels.

Human Lymphocyte (click to enlarge)

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.

lymphoidectomy /LIM-foid-ECK-tuh-mee/ Surgical excision of lymphoid tissue.

lyriform Shaped like a lyre.

lysine (Lys or K) /LIE-seen/ One of the essential amino acids. CODONS | MOLECULAR STRUCTURE

lysis /LIE-sis/ (1) Rupture of a cell wall resulting in the dissolution of the cell; (2) In a medical context, refers to the subsidence of a fever or disease.

lysogeny /lie-SAWJ-uh-nee/ The integration of a virus into the genome of a host bacterium as a prophage.

lysosome /LIE-suh-SOAM/ A eukaryotic membrane-bounded vesicle containing hydrolytic enzymes, that engulfs and breaks down macromolecules in the cell that require digestion.

Lystrosaurus