labia majora The outer folds of skin that cover and protect the genital region in women. PICTURE
labia minora Thin membranous folds of skin outside the vaginal opening. PICTURE
lactose intolerance A genetic trait characterized by the absence of the enzyme lactase, which breaks down lactose, the main sugar in milk and other dairy products.
Langerhans' cells Epidermal cells that participate in the inþammatory response by engulfing microorganisms and releasing chemicals that mobilize immune system cells.
large intestine Consists of the cecum, appendix, colon, and rectum; absorbs some nutrients, but mainly prepares feces for elimination.
larva A stage in the development of many insects and other organisms including sea urchins and sponges. In sponges, sexual reproduction results in the production of motile ciliated larvae.
larynx A hollow structure at the beginning of the trachea. The vocal cords extend across the opening of the larynx.
lateral roots Roots extending away from the main (or taproot) root.
latitudinal diversity gradient The decrease in species richness that occurs as one moves away from the equator.
latitudinal gradient As latitude increases, a gradient of cooler, drier conditions occurs.
Laurasia The northern part of the supercontinent of Pangaea, composed of the present-day North America, Europe, and Asia.
Laurentia Name applied to the "core" of North America in the times from the breakup of the precambrian supercontinent Rodinia to the formation of Pangaea.
law of the minimum Holds that population growth is limited by the resource in shortest supply.
L-dopa A chemical related to dopamine that is used in the treatment of Parkinson's disease.
leaf primordia Young leaves, recently formed by the shoot apical meristem, located at the tip of a shoot.
leaf veins Vascular tissue in leaves, arranged in a net-like network (reticulate vennation) in dicots, and running parallel (parallel vennation) to each other in monocots.
leaves The site of photosynthesis; one of the three major organs in plants.
leukocytes White blood cells; primarily engaged in fighting infection.
lichens Autotrophic organisms composed of a fungus (sac or club fungus) and a photosynthetic unicellular organism (e.g., a cyanobacterium or alga) in a symbiotic relationship; are resistant to extremes of cold and drought and can grow in marginal areas such as Arctic tundra.
life history The age at sexual maturity, age at death, and age at other events in an individual's lifetime that inþuence reproductive traits.
ligaments Dense parallel bundles of connective tissue that strengthen joints and hold the bones in place.
light reactions The photosynthetic process in which solar energy is harvested and transferred into the chemical bonds of ATP; can occur only in light. PICTURE
lignin A polymer in the secondary cell wall of woody plant cells that helps to strengthen and stiffen the wall; related term lignified.
linkage The condition in which the inheritance of a specific chromosome is coupled with that of a given gene. The genes stay together during meiosis and end up in the same gamete.
lipids One of the four classes of organic macromolecules. Lipids function in the long-term storage of biochemical energy, insulation, structure and control. Examples of lipids include the fats, waxes, oils and steroids (e.g. testosterone, cholesterol).
lipases Enzymes secreted by the pancreas that are active in the digestion of fats.
lithosphere The solid outer layer of the Earth; includes both the land area and the land beneath the oceans and other water bodies.
lobe-finned Fish with muscular fins containing large jointed bones that attach to the body; one of the two main types of bony fish.
logistic growth model A model of population growth in which the population initially grows at an exponential rate until it is limited by some factor; then, the population enters a slower growth phase and eventually stabilizes.
long-day plants Plants that þower in the summer when nights are short and days are long; e.g., spinach and wheat.
loop of Henle A U-shaped loop between the proximal and distal tubules in the kidney.
lungfish A type of lobe-finned fish that breathe by a modified swim bladder (or lung) as well as by gills.
lungs Sac-like structures of varying complexity where blood and air exchange oxygen and carbon dioxide; connected to the outside by a series of tubes and a small opening. In humans, the lungs are situated in the thoracic cavity and consist of the internal airways, the alveoli, the pulmonary circulatory vessels, and elastic connective tissues.
luteal phase The second half of the ovarian cycle when the corpus luteum is formed; occurs after ovulation. PICTURE
luteinizing hormone (LH) A hormone secreted by the anterior pituitary gland that stimulates the secretion of testosterone in men and estrogen in women.
lymph Interstitial þuid in the lymphatic system.
lymphatic circulation A secondary circulatory system that collects þuids from between the cells and returns it to the main circulatory system; the circulation of the lymphatic system, which is part of the immune system.
lymphatic system A network of glands and vessels that drain interstitial þuid from body tissues and return it to the circulatory system.
lymph hearts Contractile enlargements of vessels that pump lymph back into the veins; found in fish, amphibians, and reptiles.
lymphocytes White blood cells that arise in the bone marrow and mediate the immune response; include T cells and B cells.
Lyon hypothesis Idea proposed by Mary Lyon that mammalian females inactivate one or the other X-chromosome during early embryogenesis. This deactivated chromosome forms the Barr body. PICTURE 1 | PICTURE 2
lysosomes Membrane-enclosed organelles containing digestive enzymes. The lysosomes fuse with food vacuoles and enzymes contained within the lysosome chemically breakdown and/or digest the food vacuole's contents.
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