Dr. Paul's Virtually Biology Show.

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L

Lactate(lactic acid). An organic acid produced from fermentation by a wide range of bacteria and the skeletal muscles of our bodies under anaerobic conditions.
Lactose. A disaccharide made from glucose and galactose. Lactose is commonly known as milk sugar because of it's abundance in milk. To taste lactose, try condensed milk which you can buy in the store as a canned product.
Latent Heat. Heat that is stored by a particular substance. We are interested in latent heat because water stores more heat per gram than any common substance. The fact that water stores so much heat helps to moderate the earth's climate.
Larva. An immature form of an animal. Insects, the larvae typically will be quite different in appearance and feeding preferences from the adult. Example: caterpillars are the larvae of butterflies and moths.
Lenticel. An opening in the surface of a twig though which gas exchange takes place.

learning. Learning is the ability of an organism to modify its behavior based on past experience. All animals appear to be capable of at least some simple learning as are some single celled organisms. Whether certain animals take advantage of their ability to learn is another matter.

Leucoplast(amyloplast). An organelle related to chloroplasts and that stores starch. The while appearance in the ground tissue of potato is due to leucoplasts. In actuality the leucoplast starch grains are typically clear, not white as implied by the name.
Lichen. A symbiotic association between fungi and one or more types of photosynthetic organism. The latter night be an algae or a cyanobacteria.
Ligases. A series of enzymes that tie or connect pieces of DNA together based on complementary ends of the DNA strands. These are important enzymes for genetic engineering because they allow scientists to splice genes into other sections of DNA, for example into opened bacterial plasmids.
Light. Electromagnetic radiation, typically that which is visible to either our eyes or the eyes of other animals. Light is not matter but pure kinetic energy.
Light dependent reactions. The first steps in photosynthesis in which light energy is captured and then converted to ATP and NADPH. These in turn supply energy for the rest of photosynthesis. Oxygen is produced as a waste material by the light dependent reactions, at least under current conditions.
Light independent Reactions. The Calvin cycle and other related reactions in photosynthesis where by carbon dioxide is fixed from the atmosphere and hydrogen ions and electrons are added to produce sugars, specifically PGAL.
Linkage. The tendency of genes located at different loci on the same chromosome to stay together during independent assortment more often than would be expected if the genes were on separate chromosomes.
Lipid. A lipid is any number of diverse organic compounds all of which are characterized by having long hydrocarbon skeletons and by being generally nonpolar. This means that lipids do not usually dissolve in water and are thus considered hydrophobic. However, the interactions between lipids and water are important in the organization of plasma membranes. Lipids do dissolve in non-polar solvents such as ether or benzene. Examples of lipids include steroids, fats and oils and phospholipids.
Local signaling molecules. Molecules a released by cells not in a gland that generally travel through the interstitial fluid and have their effect locally. Contrast hormones are released into the blood stream. Examples of local signaling molecules include histamine and prostaglandin's, as well as neurotransmitters.
Locus. The specific place or position of a gene on a chromosome.
Logistic growth. A type of population growth in which the population starts out growing exponentially but as resources become limiting, growth rate becomes zero and the population is at a long term equilibrium. Of course in real populations there's no guarantee of a painless approach to any sort of equilibrium.
Loop of Henle. A hairpin shaped region of the kidney's nephron that is involved with the resorption of salt and water. Longer loops of Henle are associated with more efficient water conservation. Hence desert animals have longer loops of Henle than do their non desert counterparts.
Lymphocyte. An of a number of cells associated with either specific or non specific aspects of the body's defense against foreign invaders.
Lysogenic pathway. A complex pattern of virus reproduction in which viral genetic material inserts itself into the host cell's genetic material. The host cell then reproduces, reproducing virus material along with its own genetic material. The result is a number of generations of host cells with the viral materal incorporated into their own genetic material. At some point an environmental trigger(perhaps stress) causes the viral genetic material to instruct the cells to produce new viral particles as in the lytic pathway.

Lysosome. An organelle containing digestive enzymes for intracellular digestion. In multicellular animals, lysosomes are important for programmed cell death(apopsis) that is a part of organismal development. Without cell death during development we would, at the very least, have webbed fingers.

Lytic pathway. A type simple pattern of virus reproduction in which a virus invades a cell, causes the cell to produce more viruses. When the cell is full of viruses it breaks open or lyses, releasing the new viruses.