Science / Biology Glossary

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.

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 . . . View Full Definition

Macroevolution: The combination of events associated with the origin, diversification, extinction, and interactions of organisms which produced the species that currently inhabit the Earth. Large scale evol . . . View Full Definition

Macromolecules: Large molecules made up of many small organic molecules that are often referred to as monomers; e.g., carbohydrates, lipids, proteins, and nucleic acids. Macromolecules are polymers of monomers.

Macronucleus: In ciliates, the large nucleus that carries up to several hundred copies of the genome and controls metabolism and asexual reproduction.

Macronutrients: 1. Elements needed by plants in relatively large (primary) or smaller (secondary) quantities. 2. Foods needed by animals daily or on a fairly regular basis.

Macrophages: A type of white blood cell derived from monocytes that engulf invading antigenic molecules, viruses, and microorganisms and then display fragments of the antigen to activate helper T cells; . . . View Full Definition

Malleus: One of the bones comprising the middle ear of mammals.

Malpighian Tubules: The excretory organs of insects; a set of long tubules that open into the gut.

Mammal-Like Reptiles: Group of Permian-Triassic reptiles having some possible mammalian features, notably a more prominent dentary (tooth-bearing) bone and reduction of the incus and malleus (which are part of th . . . View Full Definition

Mantle: In mollusks, a membranous or muscular structure that surrounds the visceral mass and secretes a shell if one is present.

Marine Biome: The aquatic biome consisting of waters containing 3.5% salt on average; includes the oceans and covers more than 70% of the Earths surface; divided into benthic and pelagic zones.

Marsupials: Pouched mammals. The young develop internally, but are born while in an embryonic state and remain in a pouch on the mothers abdomen until development is complete; this group includes kangar . . . View Full Definition

Mass Extinction: A time during which extinction rates are generally accelerated so that more than 50% of all species then living become extinct; results in a marked decrease in the diversity of organisms. Ma . . . View Full Definition

Mast Cells: Cells that synthesize and release histamine, as during an allergic response; found most often in connective tissue surrounding blood vessels.

Matter: Anything that has mass and occupies space.

Matter Cycling: The fiow of matter through various organisms and the physical environment of an ecosystem.

Maximum Sustainable Yield (MSy): The maximum number of a food or game population that can be harvested without harming the populations ability to grow back.

Medulla: 1. A term referring to the central portion of certain organs; e.g., the medulla oblongata of the brain and the adrenal medulla, which synthesizes epinephrine and norepinephrine. 2. In more c . . . View Full Definition

Medulla Oblongata: The region of the brain that, with the pons, makes up the hindbrain; controls heart rate, constriction and dilation of blood vessels, respiration, and digestion.

Medusa: The motile bell-shaped form of body plan in cnidarians; e.g., jellyfish.

Megakarocytes: Cells found in the bone marrow that produce platelets.

Megaspore Mother Cell: Cells that undergo meiosis to produce megaspores.

Megaspores: Four haploid cells produced by meiosis in the ovule of a fiower. Usually, three of these cells degenerate, with the remaining cell becoming the female gametophyte phase of the plants life cy . . . View Full Definition

Meiosis: Cell division in which the chromosomes replicate, followed by two nuclear divisions. Each of the resulting gametes (in animals, spores in plants) receives a haploid set of chromosomes. Reduc . . . View Full Definition

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