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M

Machine - A device that changes the size and direction of a force.
Macromolecule - A crystal with covalent bonds; network crystals.
Macronutrients - Elements organisms require in relatively large amounts.
Magnetic field - The area around a magnet where magnetic forces act.
Magnetic induction - The process by which a material is made into a magnet.
Magnetic pole - The concentration of force at the end of a magnet.
Magnetism - A force of attraction or repulsion due to an arrangement of electrons.
Magnetosphere - The Earth's magnetic field.
Malignant - Refers to a tumor whose cells break away from the parent tumor.
Mandible - The lower jaw in vertebrates; a chewing mouth part in arthropods.
Marine ecosystems - Because of the ocean's size, marine ecosystems consist of zones marked by differences in depth and light penetration.
Marsupials - Pouched mammals.
Mass - The amount of matter in an object.
Mass number - Represents the sum of the protons and neutrons in an atom.
Mass percent - A solution concentration describing the percent of a solution's total mass that is solute.
Matter - Anything that has inertia.
Maxilla - The upper jaw in vertebrates.
Mechanical advantage - The number of times a machine multiplies an effort force.
Medulla oblongata - The part of the brain stem that controls heart rate, breathing rate, and flow of blood through the blood vessels.
Medusa - A bell-shaped body form, specialized for swimming
Mega - The Metric system prefix for 1 million.
Meiosis - The process of nuclear division in gametes that reduces the number of chromosomes by half.
Melting - Changing a solid into a liquid.
Meninges - Refers to the three protective layers covering the brain and spinal cord.
Meniscus - The curve of the surface of a liquid confined in a cylindar.
Menstrual cycle - The 28 day hormone cycle of human females.
Meristems - Growing regions where cells divide in plants.
Mesoderm - The middle layer of cells of a gastrula that forms the muscles and interior organs.
Mesons - Hadrons composed of a quark and an antiquark.
Messenger RNA - mRNA, a single uncoiled strand that transmits information from DNA to the ribosomes during protein synthesis.
Metallic bond - Electrons are distributed equally through a metallic crystal.
Metallic conduction - The movement of electrons in metals from an area of high potential to an area of low potential energy.
Metalloids - Elements touching either side of the metal-nonmetal line on the table having properties of both metals and nonmetals.
Metals - Elements that normally lose electrons to comply with the octet rule; elements on the left side of the periodic table.
Metaphase - The stage of mitosis when chromosomes line up across the middle of the cell.
Metamerism - Refers to the segmentation of the body.
Metamorphosis - A series of developmental changes.
Metastasize - The breaking away of cells from a tumor, spreading tumors to other parts of the body.
Methanogens - Bacteria to which oxygen is a poison, so they must live in anaerobic conditions.
Micronutrients - Trace elements needed only in minute amounts.
Micropyle - The small opening in a plant ovule through which the sperm reaches the egg cell.
Microscope - An optical device that enlarges images of very small objects.
Midbrain - Part of the brain stem that controls reflexes.
Middle ear - Contains three bones, hammer, anvil, and stirrup, connecting the eardrum of the outer ear to the cochlea of the inner ear.
Migration - The movement of animals to escape poor weather and food supply.
Milky Way - The name of our galaxy.
Milli - The SI prefix for ¹/₁₀₀₀.
Millibar - A unit of pressure equal to 100 Newtons/m².
Milt - The fuid containing the sperm in fish.
Minerals - Naturally occurring inorganic solids with a more or less definite chemical composition and an orderly structure; pure elements needed in small amounts by the body.
Mirror - A surface that reflects light, forming an image.
Miscible - Refers to two liquids that will dissolve in each other.
Mitochondria - Respiration centers of a cell.
Mitosis - The process by which chromosomal DNA is duplicated and divided into two identical sets during cell division.
Mixture - Two or more substances mixed together but not chemically combined.
Molality - A concentration in moles of solute per kilogram of solvent.
Molarity - A concentration in moles of solute per dm³ of solution.
Molars - Flattened teeth for grinding and crushing.
Molar volume - At STP, 22.4 dm³ of any gas contains one mole of molecules.
Mole - 6.02 X 10²³ of anything; the formula mass of a substance in grams.
Molecule - The smallest particle of a compound.
Mole fraction - A solution concentration that compares the number of moles of one substance to the total number of moles in the solution.
Molting - The shedding of an exoskeleton.
Momentum - The product of the mass of an object and its velocity.
Monadelphous - A flower whose stamens are attached by their filaments into a single group.
Monochasium - A peduncle bearing a terminal flower and , below it, one branch producing a single lateral flower.
Monocots - Plants with one seed leaf.
Monoecious - A plant with both male and female flowers.
Monohybrid cross - A genetic cross involving only one trait.
Monophyletic - A phylogenetic group of animals including an ancestral form and all of its recognized descendant species.
Monosaccharide - A simple sugar with the general formula CH₂O.
Monotremes - Egg-laying mammals.
Morphology - A comparison of internal and external structures of organisms.
Mortality - The ratio of the number of deaths in a given time to the total population.
Moss - A small terrestrial plant with no vascular tissues.
Motion - A change in position, measured by distance and time.
Mullerian mimicry - Several dangerous species having similar patterns of warning coloration.
Multiple fruit - Derived from several flowers, an inflorescence.
Muon - A type of "heavy" electron.
Music - Sound with a pleasing quality, an identifiable pitch, and a repeated rhythm.
Mutagen - Anything that causes a mutation.
Mutation - Any change in the DNA of an organism.
Mutualism - A symbiotic relationship in which both organisms benefit equally.
Mycelium - A mat of interwoven fungi hyphae.
Mycorrhiza - A symbiotic association between a fungus and plant roots.
Mysteries - Some unexplained fun things to think about.

Narcotics - Drugs that are derived from opium.
Natality - The ratio of the number of births in a given time to the total population.
Natural Selection Theory - Organisms with favorable variations survive and reproduce at the highest rates.
Nearsighted - The eyeball is too thick, causing the image to focus in front of the retina; a person can't see distant objects, but can see near objects well.
Nematode - A general term for roundworms, based on their phylum name Nematoda.
Nematocysts - Specialized stinging cells for defense and capturing food.
Neoteny - The prolonged retention of larval body features to the point where reproduction occurs in larva-like individuals.
Neotype - A specimen selected to serve as a substitute for the holotype when all material on which the name was based is missing.
Neritic zone - Highly productive ocean areas lying over continental shelves.
Nerves - Groups of related neurons.
Net ionic equation - An equation that attempts to show only the exact particles involved in the reaction.
Network crystal - A crystal with covalent bonds; macromolecules.
Neurons - Cells composing the nervous system.
Neurotoxin - Venom proteins that disrupt nerve transmission.
Neurotransmitters - Chemical messangers that carry nerve impulses across a synapse.
Neutralization - The reaction of an acid with a base to form a salt and water.
Neutrino - An elementary particle not composed of quarks.
Neutron - A subatomic particle with an AMU of one and no charge, found in the nucleus of an atom; discovered in 1935 by Sir James Chadwick.
Newton - The SI unit of weight.
Newtonian Physics - The physics of regular, visible objects traveling at normal speeds.
Newton-meter - A unit of work, the Joule.
Newton's Law of Universal Gravitation - All objects in the universe attract each other by the force of gravity.
Newton's Laws of Motion - Three laws explaining forces and the motion of matter.
NFPA - National Fire Protection Association.
Niche - The place that an organism occupies in an ecosystem.
Nictitating membrane - A clear membrane that protects the eye of most amphibians.
Nitrification - Chemosynthetic bacteria oxidize NH₄OH to produce nitrates and nitrites.
Nitrogen cycle - Converts atmospheric nitrogen, N₂ , into a form plants and animals can use.
Noble Gases - Elements in Group 18 on the periodic table.
Nomenclature - Applying names to organisms.
Nondisjunction - A pair of chromosomes fail to separate during cell division.
Nonmetals - Elements that normally gain electrons to comply with the octet rule; elements on the right side of the periodic table.
Normal boiling point - The temperature at which the vapor pressure of a liquid is equal to standard atmospheric pressure.
Normal melting point - The temperature at which the vapor pressure of a solid and the vapor pressure of its liquid phase are equal.
Normality - A concentration in equivalents of solute per dm³ of solution.
Notochord - A firm, flexible rod of specialized cells that becomes the endoskeleton in vertebrates.
Nuclear reactions - Reactions involving the atomic nucleus.
Nucleotide - One of the monomer molecules that link together to form DNA and RNA molecules.
Nucleus - The positively charged center of an atom, containing almost all the atom's mass; the control center of a cell where DNA is normally found.
Nut - A dry fruit with one seed and a thick hard wall, surrounded by a cup or husk.
Nutrition - Refers to the combination of nutrients and minerals in the diet.
Nymph - An immature form that looks similar to the adult but does not have wings or sexual parts.