Audio Glossary

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  1. lake: An aquatic biome, the communities of which live in standing bodies of fresh water of variable size, ranging from a few square meters to thousands of square kilometers.
  2. land transformation: Changes made by humans to the land surface of Earth that alter the physical or biological characteristics of the affected regions. Compare water transformation.
  3. large intestine: See colon.
  4. lateral meristem: Perpetually young, undifferentiated cells that cause a plant to increase in thickness when they divide. There are two types of lateral meristem tissue: vascular cambium and cork cambium.
  5. law of independent assortment: Mendel's second law, which states that when gametes form, the separation of alleles for one gene is indepen dent of the separation of alleles for other genes. We now know that this law does not apply to genes that are linked.
  6. law of segregation: Mendel's first law, which states that the two copies of a gene separate during meiosis and end up in different gametes.
  7. leaf: A plant organ whose function is to produce food by photosynthesis.
  8. learned behavior: A behavior acquired by trial and error or by watching others. Compare fixed behavior.
  9. lens: A structure that concentrates light on photoreceptors.
  10. lever system: A system consisting of a rigid structure that pivots around a fulcrum; it can either increase the force generated at the expense of speed or increase the speed generated at the expense of force.
  11. lichen: A symbiosis of an alga (kingdom Protista) and a fungus (kingdom Fungi).
  12. life cycle engineering: An approach in which a business seeks to document (and reduce, as necessary) the total environmental impact of its products, from manufacture to disposal.
  13. ligament: A collagen-rich connective structure that attaches bone to bone in vertebrate skeletons. Compare tendon.
  14. ligase: An enzyme that can connect two DNA fragments to each other; used in DNA technology when a gene from one species is inserted into the DNA of another species.
  15. light reactions: A series of chemical reactions that harvest energy from sunlight and use it to produce energy-rich compounds such as ATP and NADPH. The light reactions occur at the thylakoid membranes of chloroplasts and produce O2 as a waste product. Compare dark reactions.
  16. lineage: A group of closely related individuals, species, genera, or the like, depicted as a branch on an evolutionary tree.
  17. linked genes: See genetic linkage.
  18. Linnaean hierarchy: The classification scheme used by biologists to organize and name organisms. Its seven levels are species, genus, family, order, class, phylum, and kingdom.
  19. lipid: A hydrophobic molecule that contains fatty acids; a key component of cell membranes. See also phospholipid bilayer.
  20. liver: An organ associated with the upper small intestine and with the circulatory system. The liver produces bile and plays an important role in carbohydrate and lipid metabolism.
  21. load arm: The section of a lever that extends from the fulcrum to the point at which work is done. Compare power arm.
  22. locus (pl. loci): The physical location of a gene on a chromosome.
  23. low-density lipoprotein (LDL): A molecule that carries cholesterol through the circulatory system to cells that need it. Compare high-density lipoprotein.
  24. lower respiratory system: A collective term for the trachea, bronchi, and lungs. Compare upper respiratory system.
  25. lumen: The space enclosed by the membrane of an organelle.
  26. lumen: The gas exchange structure of a terrestrial animal, formed by an infolding of epidermal tissue. Compare gill.
  27. luteinizing hormone (LH): A hormone produced by the pituitary gland that works with follicle-stimulating hormone to regulate sperm development in males and to regulate the menstrual cycle in females.
  28. lymph node: A pocket of tissue lying along a lymphatic duct that contains large numbers of white blood cells and traps pathogens.
  29. lymphatic duct: One of a network of tubes that return interstitial fluid to the circulatory system.
  30. lymphatic system: The network of vessels that returns interstitial fluid from the body to the circulatory system.
  31. lymphocyte: Any of several types of white blood cells that bind to specific antigens and then contribute in various ways to the destruction of the pathogens that bear those antigens.
  32. lysosome: A specialized vesicle with an acidic lumen, containing enzymes that break down macromolecules.