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 evolutionary change such as the evolution of new species (or even higher taxa) and extinction of species.
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. PICTURE
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; ultimately stimulating the production of antibodies against the antigen. PICTURE
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. PICTURE
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 the reptilian jaw along with the dentary). The mammal-like reptiles are thought to have been the reptile group from which the mammals later evolved.
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 Earth's 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 mother's abdomen until development is complete; this group includes kangaroos, koalas, and opossums. One of the three reproductive "strategies" of living mammals g-laying and placental being the other two), marsupials finish development in a pouch or under hairy coverings attached to the mother.
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. Mass extinctions are thought to have occurred numerous times in Earth history, often from a variety of reasons: impacts, tectonism, changes in primary productivity of the seas, etc.
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 þow 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 population's 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 common usage, the area in the brain that regulates breathing, heartbeat, blood pressure and similar activities.
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. PICTURE
medusa The motile bell-shaped form of body plan in cnidarians; e.g., jellyfish.
megakarocytes Cells found in the bone marrow that produce platelets.
megaspores Four haploid cells produced by meiosis in the ovule of a þower. Usually, three of these cells degenerate, with the remaining cell becoming the female gametophyte phase of the plant's life cycle. Large (palynologists consider the megaspores to generally be above 200 micrometers in diameter) spores that develop into the megagametophyte, which in turn produces eggs. PICTURE
megaspore mother cell Cells that undergo meiosis to produce megaspores. PICTURE
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. Reduction/division by which ploidy, the number of sets of homologous chromosomes, is reduced in the formation of haploid cells that become gametes (or gametophytes in plants). PICTURE
Meissner's corpuscles Sensory receptors concentrated in the epidermis of the fingers and lips that make these areas very sensitive to touch.
melanin A pigment that gives the skin color and protects the underlying layers against damage by ultraviolet light; produced by melanocytes in the inner layer of the epidermis.
melanocytes The cells in the inner layer of the epidermis that produce melanin.
membrane-attack complex (MAC) A large cylindrical multiprotein complex formed by the complement system; kills invading microorganisms by embedding in their plasma membrane, creating a pore through which þuid þows, ultimately causing the cell to burst. PICTURE
menstrual cycle The recurring secretion of hormones and associated uterine tissue changes; typically 28 days in length. PICTURE
menstruation The process in which the uterine endometrium breaks down and sheds cells, resulting in bleeding; occurs approximately once a month. The first day marks the beginning of the menstrual and ovarian cycles. PICTURE
meristematic tissue Embryonic tissue located at the tips of stems and roots and occasionally along their entire length; can divide to produce new cells; one of the four main tissue systems in plants. PICTURE
mesentary Epithelial cells supporting the digestive organs.
mesoderm The middle layer of cells in embryonic development; gives rise to muscles, bones, and structures associated with reproduction. The middle embryonic tissue layer. Cells and structures arising from the mesoderm include the bone, blood, muscle, skin, and reproductive organs.
mesoglea A gel-like matrix that occurs between the outer and inner epithelial layers in cnidarians.
mesophyll Layer of leaf tissue between the epidermis layers; literally meaning "middle of the leaf". PICTURE
mesophytic leaves The leaves of plants that grow under moderately humid conditions with abundant soil and water.
Mesozoic Era The period of geologic time beginning 245 million years ago and ending 65 million years ago; the age of the dinosaurs and cycads, the Mesozoic falls between the Paleozoic and Cenozoic Eras and includes the Triassic, Jurassic, and Cretaceous Periods.
messenger RNA (mRNA) "Blueprint" for protein synthesis that is transcribed from one strand of the DNA (gene) and which is translated at the ribosome into a polypeptide sequence. PICTURE
metabolic pathway A series of individual chemical reactions in a living system that combine to perform one or more important functions. The product of one reaction in a pathway serves as the substrate for the following reaction. Examples include glycolysis and Kreb's cycle.
metabolism The sum of all chemical reactions (energy exchanges) in cells.
metamorphosis The process of changing from one form to another; e.g., in insects, from the larval stage to the pupal stage to the reproductive adult stage.
metaphase The stage of eukaryotic cell division (mitosis or meiosis) in which the chromosomes line up at the equator of the cell. PICTURE
metastasis The process in which cancer cells break away from the original tumor mass and establish new tumor sites elsewhere in the body.
methanogens A group of archaebacteria that produce methane as a by product of their metabolism.
methionine The amino acid coded for by the initiation codon; all polypeptides begin with methionine, although post-translational reactions may remove it.
micelles Structures formed when bile salts surround digested fats in order to enable the water-insoluble fats to be absorbed by the epithelial cells lining the small intestine. PICTURE
microevolution A small-scale evolutionary event such as the formation of a species from a preexisting one or the divergence of reproductively isolated populations into new species.
microfilaments Rods composed of actin that are found in the cytoskeleton and are involved in cell division and movement. PICTURE 1 | PICTURE 2
microgametophyte Stage of the plant life cycle that develops from or within a microspore. The microgametophyte produces sperm in specialized structures known as antheridia.
micronucleus In the protistan group known as the ciliates, the small nucleus containing a single copy of the genome that is used for sexual reproduction.
micronutrients Elements that are required by plants in very small quantities, but are toxic in large quantities: iron, manganese, molybdenum, copper, boron, zinc, and chloride.
micropyle The end of the embryo sac where the egg cell and synergids are located.
microsporangia Structures of the sporophyte in which microspores are produced by meiosis. In flowering plants the microsporangia are known as anther sacs. PICTURE
microspore mother cell Cells in the microsporangium that undergo meiosis to produce microspores. In flowering plants the microspore is known as the pollen grain, and contains a three-celled male.
microspores Four haploid cells produced by the meiotic division in the pollen sacs of þowers or microsporangia of gymnosperms. Microspores undergo mitotic division and become encased in a thick protective wall to form pollen grains. Small, size usually less than 200 micrometers, spores produced by meiosis. Microspores either germinate into the male gametophyte or have the male gametophyte develop inside the microspore wall.
microtubules Filaments about 25 nanometers in diameter found in cilia, þagella, and the cytoskeleton. PICTURE
microvilli Hair-like projections on the surface of the epithelial cells of the villi in the small intestine; increase the surface area of the intestine to improve absorption of digested nutrients. PICTURE
midbrain A network of neurons that connects with the forebrain and relays sensory signals to other integrating centers. PICTURE
middle lamella A layer composed of pectin that cements two adjoining plant cells together.
migration Movement of organisms either permanently (as in the migration of humans to the Americas) or temporarily (migratory birds such as Canadian geese).
mineralocorticoids A group of steroid hormones produced by the adrenal cortex that are important in maintaining electrolyte balance.
minerals Trace elements required for normal metabolism, as components of cells and tissues, and in nerve conduction and muscle contraction.
minimum viable population (MVP) The smallest population size that can avoid extinction due to breeding problems or random environmental þuctuations.
mitochondria Self-replicating membrane-bound cytoplasmic organelles in most eukaryotic cells that complete the breakdown of glucose, producing NADH and ATP (singular term: mitochondrion). The powerhouse of the cell. Organelles within eukaryotes that generate (by chemiosmosis) most of the ATP the cell needs to function and stay alive. PICTURE
mitosis The division of the cell's nucleus and nuclear material of a cell; consists of four stages: prophase, metaphase, anaphase, and telophase. Cell xeroxing. Mitosis occurs only in eukaryotes. The DNA of the cell is replicated during interphase of the cell cycle and then segregated during the four phases of mitosis.
mitotic spindle A network of microtubules formed during prophase. Some microtubules attach to the centromeres of the chromosomes and help draw the chromosomes apart during anaphase. PICTURE
mold Type of fossil preservation where the original material of the fossil has decayed but has left an impression in the surrounding sediments. Molds are often filled with a different material, producing strikingly beautiful fossils.
mole Avogadro's number (6.02 X 1023 atoms) of a substance.
molecules Units of two or more atoms held together by chemical bonds. The combination of atoms by chemical bonds with the component atoms in definite porportions, such as water (two H to one O).
molecular biology Field of biology that studies the molecular level of organization.
Monera Prokaryotic kingdom that includes (in the most widely accepted classification system) archaebacteria, eubacteria, and cyanobacteria. Members of this kingdom were among the first forms of life over 3.5 billion years ago.
monocots One of the two major types of þowering plants; characterized by having a single cotyledon, þoral organs arranged in threesd or multiples of three, and parallel-veined leaves; include grasses, cattails, lilies, and palm trees. One of the two major groups in the Angiosperms, monocots are characterized by having a single seed leaf (cotyledon), flower parts in 3's or multiples of 3, monoaperturate pollen (although some dicots also have this feature), parallel veins in their leaves, and scattered vascular bundles in their stems. PICTURE
monoculture The growth of only one species in a given area; such as a cornfield or other agricultural field.
monocytes White blood cells that clean up dead viruses, bacteria, and fungi and dispose of dead cells and debris at the end of the inþammatory response.
monohybrid cross In genetics, a cross that involves only one characteristic. PICTURE
monomer An organic chemical unit linked to other units (usually by a covalent bond formed by the removal of water) to produce a larger molecule (macromolecule) known as a polymer.
monophyletic group A group of organisms descended from a common ancestor. For example: your immediate family may be considered such a group, being descended from a common ancestral group (grandparents, etc.).
monosaccharides Simple carbohydrates, usually with a five- or six-carbon skeleton; e.g., glucose and fructose. A carbohydrate composed of a single sugar unit, such as glucose, ribose, deoxyribose, etc. PICTURE
monotremes Egg-laying mammals; e.g., the spiny anteater and the duck-billed platypus.
morph A distinct phenotypic variant within a population.
morphological convergence The evolution of basically dissimilar structures to serve a common function. For example: the wings of birds and insects.
morula The solid-ball stage of the pre-emplantation embryo. PICTURE
mosaic evolution A pattern of evolution where all features of an organism do not evolve at the same rate. Some characteristics are retained from the ancestral condition while others are more recently evolved.
motor neurons Neurons that receive signals from interneurons and transfer the signals to effector cells that produce a response. Nerve cells connected to a muscle or gland. Sometimes also known as effector neurons.
motor output A response to the stimuli received by the nervous system. A signal is transmitted to organs that can convert the signals into action, such as movement or a change in heart rate.
motor (efferent) pathways The portion of the peripheral nervous system that carries signals from the central nervous system to the muscles and glands.
motor units Consist of a motor neuron with a group of muscle fibers; form the units into which skeletal muscles are organized; enable muscles to contract on a graded basis.
mouth The oral cavity; the entrance to the digestive system where food is broken into pieces by the teeth and saliva begins the digestion process.
mucus A thick, lubricating fluid produced by the mucous membranes that line the respiratory, digestive, urinary, and reproductive tracts; serves as a barrier against infection and, in the digestive tract, moistens food, making it easier to swallow.
multicellular Organisms composed of multiple cells and exhibiting some division of labor and specialization of cell structure and function.
multinucleate Cells having more than one nucleus per cell.
muscle fibers Long, multinucleated cells found in skeletal muscles; made up of myofibrils. One of the four major groups of vertebrate cell/tissue types. Muscle cells contract/relax, allowing movement of and/or within the animal. PICTURE 1 | PICTURE 2 | PICTURE 3
muscular system One of eleven major body organ systems in animals; allows movement and locomotion, powers the circulatory, digestive, and respiratory systems, and plays a role in regulating temperature. PICTURE
mutation Any heritable change in the nucleotide sequence of DNA; can involve substitutions, insertions, or deletions of one or more nucleotides.
mutation rate The average occurrence of mutations in a species per a given unit of time.
mutualism A form of symbiosis in which both species benefit. A type of symbiosis where both organisms benefit. The classic example is lichens, which is a symbiosis between an alga and a fungus. The alga provides food and the fungus provides water and nutrients.
mycelium The mass of interwoven filaments of hyphae in a fungus.
mycorrhiza Occurs when a fungus (basidiomycete or zygomycete) weaves around or into a plant's roots and forms a symbiotic relationship. Fungal hyphae absorb minerals from the soil and pass them on to the plant roots while the fungus obtains carbohydrates from the plant (pl.: mycorrhizae).
myelin sheath Layers of specialized glial cells, called Schwann cells, that coat the axons of many neurons.
myofibrils Striated contractile microfilaments in skeletal muscle cells. PICTURE
myosin Thick protein filaments in the center sections of sarcomeres. PICTURE 1 | PICTURE 2
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