lactose intolerance A genetic trait characterized by the absence of the enzyme lactase, which breaks down lactose, the main sugar in milk and other dairy products.

Langerhans' cells Epidermal cells that participate in the inþammatory response by engulfing microorganisms and releasing chemicals that mobilize immune system cells.

large intestine Consists of the cecum, appendix, colon, and rectum; absorbs some nutrients, but mainly prepares feces for elimination.

larva A stage in the development of many insects and other organisms including sea urchins and sponges. In sponges, sexual reproduction results in the production of motile ciliated larvae.

larynx A hollow structure at the beginning of the trachea. The vocal cords extend across the opening of the larynx.

lateral roots Roots extending away from the main (or taproot) root.

latitudinal diversity gradient The decrease in species richness that occurs as one moves away from the equator.

latitudinal gradient As latitude increases, a gradient of cooler, drier conditions occurs.

Laurasia The northern part of the supercontinent of Pangaea, composed of the present-day North America, Europe, and Asia.

Laurentia Name applied to the "core" of North America in the times from the breakup of the precambrian supercontinent Rodinia to the formation of Pangaea.

law of the minimum Holds that population growth is limited by the resource in shortest supply.

L-dopa A chemical related to dopamine that is used in the treatment of Parkinson's disease.

leaf primordia Young leaves, recently formed by the shoot apical meristem, located at the tip of a shoot.

leaf veins Vascular tissue in leaves, arranged in a net-like network (reticulate vennation) in dicots, and running parallel (parallel vennation) to each other in monocots.

leaves The site of photosynthesis; one of the three major organs in plants.

leukocytes White blood cells; primarily engaged in fighting infection.

lichens Autotrophic organisms composed of a fungus (sac or club fungus) and a photosynthetic unicellular organism (e.g., a cyanobacterium or alga) in a symbiotic relationship; are resistant to extremes of cold and drought and can grow in marginal areas such as Arctic tundra.

life history The age at sexual maturity, age at death, and age at other events in an individual's lifetime that inþuence reproductive traits.

ligaments Dense parallel bundles of connective tissue that strengthen joints and hold the bones in place.

light reactions The photosynthetic process in which solar energy is harvested and transferred into the chemical bonds of ATP; can occur only in light.

lignin A polymer in the secondary cell wall of woody plant cells that helps to strengthen and stiffen the wall; related term lignified.

linkage The condition in which the inheritance of a specific chromosome is coupled with that of a given gene. The genes stay together during meiosis and end up in the same gamete.

lipids One of the four classes of organic macromolecules. Lipids function in the long-term storage of biochemical energy, insulation, structure and control. Examples of lipids include the fats, waxes, oils and steroids (e.g. testosterone, cholesterol).

lipases Enzymes secreted by the pancreas that are active in the digestion of fats.

lithosphere The solid outer layer of the Earth; includes both the land area and the land beneath the oceans and other water bodies.

lobe-finned Fish with muscular fins containing large jointed bones that attach to the body; one of the two main types of bony fish.

logistic growth model A model of population growth in which the population initially grows at an exponential rate until it is limited by some factor; then, the population enters a slower growth phase and eventually stabilizes.

long-day plants Plants that þower in the summer when nights are short and days are long; e.g., spinach and wheat.

loop of Henle A U-shaped loop between the proximal and distal tubules in the kidney.

lungfish A type of lobe-finned fish that breathe by a modified swim bladder (or lung) as well as by gills.

lungs Sac-like structures of varying complexity where blood and air exchange oxygen and carbon dioxide; connected to the outside by a series of tubes and a small opening. In humans, the lungs are situated in the thoracic cavity and consist of the internal airways, the alveoli, the pulmonary circulatory vessels, and elastic connective tissues.

luteal phase is formed; occurs after The second half of the ovarian cycle when the corpus luteumovulation.

luteinizing hormone (LH) A hormone secreted by the anterior pituitary gland that stimulates the secretion of testosterone in men and estrogen in women.

lymph Interstitial þuid in the lymphatic system.

lymphatic circulation A secondary circulatory system that collects þuids from between the cells and returns it to the main circulatory system; the circulation of the lymphatic system, which is part of the immune system.

lymphatic system A network of glands and vessels that drain interstitial þuid from body tissues and return it to the circulatory system.

lymph hearts Contractile enlargements of vessels that pump lymph back into the veins; found in fish, amphibians, and reptiles.

lymphocytes White blood cells that arise in the bone marrow and mediate the immune response; include T cells and B cells.

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's contents.

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.



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.



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 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.

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.

megaspore mother cell Cells that undergo meiosis to produce megaspores.

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).

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.

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.



mesentaryEpithelial 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".



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.



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.

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.



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.



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.

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.

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.



midbrain A network of neurons that connects with the forebrain and relays sensory signals to other integrating centers.

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 steroidhormones 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.

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.

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.

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.

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.

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.

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.

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.

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.

myosin Thick protein filaments in the center sections of sarcomeres.

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nares Nostrils; the openings in the nose through which air enters.

nastic movement A plant's response to a stimulus in which the direction of the response is independent of the direction of the stimulus. Non-directional plant movements.

natural selection The process of differential survival and reproduction of Þtter genotypes; can be stabilizing, directional, or disruptive. Better adapted individuals are more likely to survive to reproductive age and thus leave more offspring and make a larger contribution to the gene pool than do less Þt individuals. The differential survival and reproductive successes of individuals in a variable population that powers the evolutionary process. When all individuals survive and reproduce (except for chance occurrences) natural selection works at a lower rate, if at all.

nectaries Nectar-secreting organs in þowering plants that serve as insect feeding stations and thus attract insects, which then assist in the transfer of pollen.

negative feedback The stopping of the synthesis of an enzyme by the accumulation of the products of the enzyme-mediated reaction.

negative feedback control Occurs when information produced by the feedback reverses the direction of the response; regulates the secretion of most hormones.

negative feedback loop A biochemical pathway where the products of the reaction inhibit production of the enzyme that controlled their formation.

nektonic organisms "Swimmers"; one of the two main types of organisms in the pelagic zone of the marine biome.

nephridium The excretory organ in þatworms and other invertebrates; a blind-ended tubule that expels waste through an excretory pore.

nephron A tubular structure that is the Þltering unit of the kidney; consists of a glomerulus and renal tubule.



nerve cord A dorsal tubular cord of nervous tissue above the notochord of a chordate.

nerve net An interconnected mesh of neurons that sends signals in all directions; found in radially symmetrical marine invertebrates, such as jellyÞsh and sea anemones, that have no head region or brain.



nerves Bundles of neuronal processes enclosed in connective tissue that carry signals to and from the central nervous system.



nervous system One of eleven major body organ systems in animals; coordinates and controls actions of internal organs and body systems, receives and processes sensory information from the external environment, and coordinates short-term reactions to these stimuli.

net primary productivity (NPP) The rate at which producer (usually plants) biomass is created in a community.

net secondary productivity (NSP) The rate at which consumer and decomposer biomass is produced in a community.

neural tube A tube of ectoderm in the embryo that will form the spinal cord.

neuromuscular junction The point where a motor neuron attaches to a muscle cell.

neurons Highly specialized cells that generate and transmit bioelectric impulses from one part of the body to another; the functional unit of the nervous system.A cell of the nerve tissue having a cell body input zone of dendrites and an output zone of an axon (of varying length). The electrochemical nerve impulse/message is transmitted by neurons.

neurotoxin Chemical that paralyzes nerves. Neurotoxins are produced by a variety of organisms, most notably some of the heterotrophicdinoflagellates.

neurotransmitters Chemicals released from the tip of an axon into the synaptic cleft when a nerve impulse arrives; may stimulate or inhibit the next neuron. The chemical that crosses the synaptic cleft and causes the transmission of the nerve message in an adjacent neuron or the stimulation of an effector cell (muscle or gland).



neutron An uncharged subatomic particle in the nucleus of an atom. The large (mass approximately equal to 1 atomic mass unit), electrically neutral particle that may occur in the atomic nucleus.

niche The biological role played by a species.

niche overlap The extent to which two species require similar resources; speciÞes the strength of the competition between the two species.

nicotine adenine dinucleotide phosphate (NADP+) A substance to which electrons are transferred from photosystem I during photosynthesis; the addition of the electrons reduces NADP, which acquires a hydrogen ion to form NADPH, which is a storage form of energy that can be transferred to the Calvin Cycle for the production of carbohydrate.

node The stem region of a plant where one or more leaves attach. Where leaves are attached to stems.

node of Ranvier A gap between two of the Schwann cells that make up an axon's myelin sheath; serves as a point for generating a nerve impulse.

nondisjunction The failure of chromosomes to separate properly during cell division. The unequal segregation of chromosomes during meiosis. This forms cells with either too many (possibly one or more single or sets of chromosomes too many) or too few chromosomes. Thought to be a common cause for Down Syndrome, where sufferers often have an extra copy of chromosome 21.

nonvascular plants Plants lacking lignified vascular tissue (xylem), vascularized leaves, and having a free-living, photosynthetic gametophyte stage that dominates the life cycle. Common examples are the mosses and liverworts.

norepinephrine A hormone produced in the adrenal medulla and secreted under stress; contributes to the "Þght or þight" response.

notochord that will become the In chordates, a cellular rod that runs the length of the body and provides dorsal support. Also, a structure of mesoderm in the embryovertebrae of the spinal column. The stiff rod-like structure that all members of the Phylum Chordata develop at some stage during their life.

nuclear area In prokaryotic cells, a region containing the cell's genetic information. Unlike the nucleus in eukaryotic cells, it is not surrounded by a membrane.

nuclear pores Openings in the membrane of a cell's nuclear envelope that allow the exchange of materials between the nucleus and the cytoplasm.

nucleic acids Polymers composed of nucleotides; e.g., DNA and RNA.

nucleoid The area of the prokaryotic cytoplasm where the chromatin is localized.

nucleolus A round or oval body in the nucleus of a eukaryotic cell; consists of DNA and RNA and produces ribosomal RNA (pl.: nucleoli).

nucleosomes Spherical bodies formed by coils of chromatin. The nucleosomes in turn are coiled to form the Þbers that make up the chromosomes.

nucleotide sequences The genetic code encrypted in the sequence of bases along a nucleic acid.

nucleotides The subunits of nucleic acids; composed of a phosphate, a sugar, and a nitrogen-containing base.The fundamental structural unit of the nucleic acid group of organic macromolecules. Some nucleotides are involved in information storage (as nucleotides in DNA), protein synthesis (as nucleotides in RNA), and energy transfers (as single nucleotide ATP, GTP, and double nucleotide NADH and NADPH).

nucleus (atom) An atom's core; contains protons and one or more neutrons (except hydrogen, which has no neutrons).

nucleus (cell) The largest, most prominent organelle in eukaryotic cells; a round or oval body that is surrounded by the nuclear envelope and contains the genetic information necessary for control of cell structure and function.



nyctinasty A nastic movement in a plant that is caused by light and dark.

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occipital lobe The lobe of the cerebral cortex located at the rear of the head; is responsible for receiving and processing visual information.

oils Triglycerides that are liquid at room temperature.

oncogenes Genes that can activate cell division in cells that normally do not divide or do so only slowly. A gene that when over-expressed leads to cancer, but which normally functions in cell division.

"one gene, one enzyme hypothesis Holds that a single gene controls the production, specificity, and activity of each enzyme in a metabolic pathway. Thus, mutation of such a gene changes the ability of the cell to carry out a particular reaction and disrupts the entire pathway.

"one gene one polypeptide hypothesis" A revision of the one gene, one enzyme hypothesis. Some proteins are composed of different polypeptide chains encoded by separate genes, so the hypothesis now holds that mutation in a gene encoding a specifc polypeptide can alter the ability of the encoded protein to function and thus produce an altered phenotype.

oocyte A cell that will/is undergo/ing development into a female gamete.

oogenesis The production of ova.The development of a diploid cell into a haploid ovum or egg cell.

open community A community in which the populations have different density peaks and range boundaries and are distributed more or less randomly.

opposable The capability of being placed against the remaining digits of a hand or foot; e.g., the ability of the thumb to touch the tips of the fingers on that hand.

opsins Molecules in cone cells that bind to pigments, creating a complex that is sensitive to light of a given wavelength.

orders Taxonomic subcategories of classes.

Ordovician extinction Paleozoic-aged mass extinction possibly related to glaciation in the southern-hemisphere supercontinent Gondwana.

Ordovician Period Geologic period of the Paleozoic Era after the Cambrian Period between 500 and 435 million years ago. Major advances during this period include the bony fish and possibly land plants (during the late Ordovician).

organelles Cell components that carry out individual functions; e.g., the cell nucleus and the endoplasmic reticulum.Subcellular structures (usually membrane-bound and unique to eukaryotes) that perform some function, e.g. chloroplast, mitochondrion, nucleus.

organism An individual, composed of organ systems (if multicellular). Multiple organisms make up a population.

organs Differentiated structures consisting of tissues and performing some specific function in an organism.Structures made of two or more tissues which function as an integrated unit. e.g. the heart, kidneys, liver, stomach.

organ systems Groups of organs that perform related functions.

osmoconformers Marine organisms that have no system of osmoregulation and must change the composition of their body þuids as the composition of the water changes; include invertebrates such as jellyÞsh, scallops, and crabs.

osmoregulation The regulation of the movement of water by osmosis into and out of cells; the maintenance of water balance within the body.

osmoregulators Marine vertebrates whose body þuids have about one-third the solute concentration of seawater; must therefore undergo osmoregulation.

osmosis Diffusion of water molecules across a membrane in response to differences in solute concentration. Water moves from areas of high-water/low-solute concentration to areas of low-water/high-solute concentration.Diffusion of water across a semi-permeable barrier such as a cell membrane, from high water potential (concentration) to lower water potential (concentration).

osmotic pressure Pressure generated by water moving by osmosis into or out of a cell.

ossification The process by which embryonic cartilage is replaced with bone.



osteoarthritis A degenerative condition associated with the wearing away of the protective cap of cartilage at the ends of bones. Bone growths or spurs develop, restricting movement and causing pain.

osteoblasts Bone-forming cells.



osteoclasts Cells that remove material to form the central cavity in a long bone.

osteocytes Bone cells that lay down new bone; found in the concentric layers of compact bone. Bone cell, a type of connective tissue.

osteoporosis A disorder in which the mineral portion of bone is lost, making the bone weak and brittle; occurs most commonly in postmenopausal women.

out of Africa hypothesis Holds that modern human populations (Homo sapiens) are all derived from a single speciation event that took place in a restricted region in Africa.

ovaries 1) In animals, the female gonads, which produce eggs (ova) and female sex hormones. 2) In þowers, part of the female reproductive structure in the carpel; contain the ovules, where egg development occurs. The lower part of the carpel that contains the ovules within which the female gametophyte develops.

overkill The shooting, trapping, or poisoning of certain populations, usually for sport or economic reasons.

oviducts Tubes that connect the ovaries and the uterus; transport sperm to the ova, transport the fertilized ova to the uterus, and serve as the site of fertilization; also called the fallopian tubes or uterine tubes.

ovule In seed plants, a protective structure in which the female gametophyte develops, fertilization occurs, and seeds develop; contained within the ovary.Structures inside the ovary of the flower within which the female gametophyte develops after megasporogenesis has produced a megaspore inside each ovule.



ovum The female gamete, egg.

oxidation The loss of electrons from the outer shell of an atom; often accompanied by the transfer of a proton and thus involves the loss of a hydrogen ion. The loss of electrons or hydrogens in a chemical reaction.



oxytocin A peptide hormone secreted by the posterior pituitary that stimulates the contraction of the uterus during childbirth.

ozone A triatomic (O3) form of oxygen that is formed in the stratosphere when sunlight strikes oxygen atoms. This atmospheric ozone helps Þlter radiation from the sun.

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Pacinian corpuscles Sensory receptors located deep in the epidermis that detect pressure and vibration.

paleontology The study of ancient life by collection and analysis of fossils.

Paleozoic Era The period of time beginning 570 million years ago ending 245 million years ago; falls between the Proterozoic and Mesozoic Eras and is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods.

palindrome A sequence that reads the same in either direction; in genetics, refers to an enzyme recognition sequence that reads the same on both strands of DNA.

palisade Layer of mesophyll cells in leaves that are closely placed together under the epidermal layer of the leaf.Palisade parenchyma: Columnar cells located just below the upper epidermis in leaves the cells where most of the light absorbtion in photosynthesis occurs.

palynology The study of palynomorphs and other acid-resistant microfossils usually produced by plants, protists, and fungi.

palynomorph Generic term for any object a palynologist studies.

pancreas A gland in the abdominal cavity that secretes digestive enzymes into the small intestine and also secretes the hormonesinsulin and glucagon into the blood, where they regulate blood glucose levels.A digestive organ that produces trypsin, chymotrypsin and other enzymes as a pancreatic juice, but which also has endocrine functions in the production of the hormones somatostatin, insulin, and glucagon.

pancreatic islets Clusters of endocrine cells in the pancreas that secrete insulin and glucagon; also known as islets of Langerhans.

Pangaea The name proposed by German meteorologist Alfred Wegener for a supercontinent that existed at the end of the Paleozoic Era and consisted of all the Earth's landmasses.

parallel evolution The development of similar characteristics in organisms that are not closely related (not part of a monophyletic group) due to adaptation to similar environments and/or strategies of life.

parasites Organisms that live in, with, or on another organism. The parasites beneÞt from the association without contributing to the host, usually they cause some harm to the host.

parasitism A form of symbiosis in which the population of one species beneÞts at the expense of the population of another species; similar to predation, but differs in that parasites act more slowly than predators and do not always kill the host.A type of symbiosis in which one organism benefits at the expense of the other, for example the influenza virus is a parasite on its human host. Viruses, are obligate intracellular parasites.

parasympathetic system The subdivision of the autonomic nervous system that reverses the effects of the sympathetic nervous system. Part of the autonomic nervous system that controls heartbeat, respiration and other vital functions.

parenchyma One of the three major cell types in plants. Parenchyma cells have thin, usually multisided walls, are unspecialized but carry on photosynthesis and cellular respiration and can store food; form the bulk of the plant body; found in the þeshy tissue of fruits and seeds, photosynthetic cells of leaves, and the vascular system. Generalized plant cells whose numerous functions include photosynthesis, storage, bulk of herbaceous stem tissues, lateral transport in woody stems. Parenchyma are variously shaped but are characterized by thin walls and remain alive at functional maturity.



parietal lobe The lobe of the cerebral cortex that lies at the top of the brain; processes information about touch, taste, pressure, pain, and heat and cold.

passive transport Diffusion across a plasma membrane in which the cell expends no energy.

pectin A substance in the middle lamella that cements adjoining plant cells together.

pectoral girdle In humans, the bony arch by which the arms are attached to the rest of the skeleton; composed of the clavicle and scapula.



pedigree analysis A type of genetic analysis in which a trait is traced through several generations of a family to determine how the trait is inherited. The information is displayed in a pedigree chart using standard symbols.

pelagic zone One of the two basic subdivisions of the marine biome; consists of the water above the sea þoor and its organisms.

pelvic girdle In humans, the bony arch by which the legs are attached to the rest of the skeleton; composed of the two hipbones.

pelvis The hollow cavity formed by the two hipbones.

penicillin The first of the so-called wonder drugs; discovered by Sir Alexander Fleming.

pepsin An enzyme produced from pepsinogen that initiates protein digestion by breaking down protein into large peptide fragments. An enzyme, produced by the stomach, that chemically breaks down peptide bonds in polypeptides and proteins.

pepsinogen An inactive form of pepsin; synthesized and stored in cells lining the gastric pits of the stomach.

peptic ulcer Damage to the epithelial layer of the stomach lining; generally caused by bacterial infection.

peptide bond A covalent bond that links two amino acids together to form a polypeptide chain. A covalent bond between the amine end of one amino acid and the acid end of another amino acid.



peptides Short chains of amino acids.

perichondrium A layer of connective tissue that forms around the cartilage during bone formation. Cells in the perichondrium lay down a peripheral layer that develops into compact bone.

perennials Plants that persist in the environment for more than one year (as in the case of annuals).

period The fundamental unit in the hierarchy of time units; a part of geologic time during which a particular sequence of rocks designated as a system was deposited. Units of geological time that are the major subdivisions of Eras.

periosteum A Þbrous membrane that covers bones and serves as the site of attachment for skeletal muscles; contains nerves, blood vessels, and lymphatic vessels.

peripheral nervous system The division of the nervous system that connects the central nervous system to other parts of the body. Components of the nervous system that transmit messages to the central nervous system.

peristalsis Involuntary contractions of the smooth muscles in the walls of the esophagus, stomach, and intestines that propel food along the digestive tract. Waves of muscle contraction in the esophagus that propel food from the oral cavity to the stomach.



Permian Period The last geologic time period of the Paleozoic Era, noted for the greatest mass extinction in earth history, when nearly 96% of species died out.

peroxisomes Membrane-bound vesicles in eukaryotic cells that contain oxidative enzymes.

pesticides Chemicals that are applied to agricultural crops or domesticated plants and which kill or inhibit growth of insects.

petals Usually brightly colored elements of a þower that may produce fragrant oils; nonreproductive structures that attract pollinators. Sterile leaf-like (white, colorless, but usually colored) structures in flowers that serve to attract pollinators.

petiole The generally non-leafy part of the leaf that attaches the leaf blade to the stem; celery and rhubarb are examples of a leaf petiole that we use as food. The stalk connecting the leaf blade to the stem.

petrifaction Mode of fossilization where f organic matter is replaced with silica.

PGA (phosphoglycerate) A three-carbon molecule formed when carbon dioxide is added to ribulose biphosphate (RuBP) during the dark reaction of photosynthesis (Calvin, or Calvin-Benson Cycle). PGA is converted to PGAL, using ATP and NADPH.

PGAL (phosphoglyceraldehyde) A substance formed from PGA during the dark reaction of photosynthesis. Some PGAL leaves the cycle and can be converted to glucose, while other PGAL molecules are used to reform ribulose biphosphate (RuBP) to continue the dark reaction.

pH The negative logarithm of the H+ ion concentration. The pH is a measure of the acidity or basic character of a solution. Since it measures a fraction, the larger the pH number, the less H ions are present in a solution.



phagocytes White blood cells that can engulf (by phagocytosis) and destroy microorganisms including viruses and bacteria; cells in this category include neutrophils and monocytes.

phagocytosis A form of endocytosis in which white blood cells surround and engulf invading bacteria or viruses.



pharynx The passageway between the mouth and the esophagus and trachea. Food passes from the pharynx to the esophagus, and air passes from the pharynx to the trachea.

phenotype The observed properties or outward appearance of a trait. The physical expression of the alleles posessed by an organism.

pheromones Chemical signals that travel between organisms rather than between cells within an organism; serve as a form of communication between animals.

phloem Tissue in the vascular system of plants that moves dissolved sugars and other products of photosynthesis from the leaves to other regions of the plant. Phloem tissue consists of cells called sieve tubes and companion cells. Cells of the vascular system in plants that transport food from leaves to other areas of the plant.

phosphate group A chemical group composed of a central phosphorous bonded to three or four oxygens. The net charge on the group is negative.



phospholipids Asymmetrical lipid molecules with a hydrophilic head and a hydrophobic tail. Lipids with a phosphate group in place of one of the three fatty acid chains. Phospholipids are the building blocks of cellular membranes. Phospholipids have hydrophilic heads (glycerol and phosphate) and hydrophobic tails (the non-polar fatty acids).



phosphorylation The chemical attachment of phosphorous to a molecule, usually associated with the storage of energy in the covalent bond that is also formed. Example: attachment of the third phosphate group to ADP in the formation of the higher energy form, ATP. Photophosphorylation is a type of phosphorylation associated with the formation of ATP in the photosynthesis process.

photic zone The layer of the ocean that is penetrated by sunlight; extends to a depth of about 200 meters.

photoperiodism The ability of certain plants to sense the relative amounts of light and dark in a 24-hour period; controls the onset of þowering in many plants.

photosynthesis The process by which plant cells use solar energy to produce ATP. The conversion of unusable sunlight energy into usable chemical energy, associated with the actions of chlorophyll.



photosystems Clusters of several hundred molecules of chlorophyll in a thylakoid in which photosynthesis takes place. Eukaryotes have two types of photosystems: I and II. The series of green photoreceptive pigments involved in the light reactions, which occur in the thylakoids of the chloroplast (in eukaryotes). Energy from light is passed to the electrons as they move through the photosystem pigments.

phototrophs Organisms that use sunlight to synthesize organic nutrients as their energy source; e.g., cyanobacteria, algae, and plants.

phototropism The reaction of plants to light in which the plants bend toward the light. Plant response to light by unequal growth caused by concentration of the plant hormone Indole Acetic Acid (IAA, an auxin) on the darker side of the plant shoot.

phycocyanin An accessory pigment found in cyanobacteria and the chloroplasts of red algae.

phycoerythrin An accessory pigment found in cyanobacteria and the chloroplasts of red algae.

phylogeny 1) the study of evolutionary relationships within a monophyletic group. 2) evolutionary hypotheses represented as a dendrogram or branching diagram.



phylogenetic Pertaining to a phylogeny.

phylum The broadest taxonomic category within kingdoms (pl.: phyla).

phytochrome A pigment in plant leaves that detects day length and generates a response; partly responsible for photoperiodism.

phytoplankton A þoating layer of photosynthetic organisms, including algae, that are an important source of atmospheric oxygen and form the base of the aquatic food chain.

pilus Projection from surface of a bacterial cell (F+) that can donate genetic material to another (F-).

pineal gland A small gland located between the cerebral hemispheres of the brain that secretes melatonin.

pioneer community The initial community of colonizing species.

pistil Female reproductive structures in flowers, consisting of the stigma, style, and ovary. Also known as a carpel in some books.

pith Central area in plant stems, largely composed of parenchyma tissue modified for storage.



pituitary gland A small gland located at the base of the brain; consists of an anterior and a posterior lobe and produces numerous hormones. The master gland of the endocrine system, the pituitary releases hormones that have specific targets as well as those that stimulate other glands to secrete hormones. Part of the pituitary is nerve tissue, the rest is glandular epithelium.

placenta An organ produced from interlocking maternal and embryonic tissue in placental mammals; supplies nutrients to the embryo and fetus and removes wastes.

placental mammals One of three groups of mammals that carry their young in the mother's body for long periods during which the fetus is nourished by the placenta. Humans are placental mammals.

planaria Small free-living þatworms (Phylum Platyhelminthes) with bilateral symmetry and cephalization. The freshwater type is often used as an experimental organism.

planktonic organisms "Floaters"; one of the two main types of organisms in the pelagic zone of the marine biome.

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Plantae The plant kingdom; nonmobile, autotrophic, multicellular eukaryotes. Kingdom of the plants, autotrophic eukaryotes with cellulose in their cell walls and starch as a carbohydrate storage product, with chlorophylls a and b as photosynthesis pigments.

plasma The liquid portion of the blood. Along with the extracellular þuid, it makes up the internal environment of multicellular organisms.

plasma cells Cells produced from B cells that synthesize and release antibodies.



plasmids Self-replicating, circular DNA molecules found in bacterial cells; often used as vectors in recombinant DNA technology. Small circles of double-stranded DNA found in some bacteria. Plasmids can carry from four to 20 genes. Plasmids are a commonly used vector in recombinant DNA studies.

plasmodesmata Junctions in plants that penetrate cell walls and plasma membranes, allowing direct communication between the cytoplasm of adjacent cells (sing.: plasmodesma).

plasmolysis Osmotic condition in which a cell loses water to its outside environment.

plastids Membrane-bound organelles in plant cells that function in storage (of food or pigments) or food production. Term for any double membrane-bound organelle. Chloroplasts contain the chemicals for photosynthesis, amyloplasts (also known as leukoplasts) store starch, chromoplasts contain colorful pigments such as in the petals of a flower or epidermis of a fruit.

platelets In vertebrates, cell fragments that bud off from the megakaryocytes in the bone marrow; carry chemicals needed for blood clotting. Cell fragment functioning in blood clotting.

plate tectonics The movement of the plates that make up the surface of the Earth. The revolutionary paradigm in geology that the earth's crust is composed of rigid segments (plates) in constant (although considered slow in a human-scale time frame) motion (tectonics) relative to each other.

pleiotropic A term describing a genotype with multiple phenotypic effects. For example: sickle-cell anemia produces a multitude of consequences in those it affects, such as heart disease, jidney problem, etc.

Pleistocene The first geologic epoch of the Quaternary Period of the Cenozoic Era that ended 10,000 years ago with the retreat of the last glaciers.

pleura A thin sheet of epithelium that covers the inside of the thoracic cavity and the outer surface of the lungs.

pleural cavity The space between the sheets of pleura (one covering the inside of the thoracic cavity, the other covering the outside of the lungs).

polar covalent bond A covalent bond in which atoms share electrons in an unequal fashion. The resulting molecule has regions with positive and negative charges. The presence of polar covalent bonds allows other polar molecules to surround molecule: example: glucose sugar in water.

pollen grains The containers for male gametophytes of seed plants produced in a microsporangium by meiosis. Microspores produced by seed plants that contain the male gametophyte.

pollen tube Structure produced by the tube nucleus in the pollen grain through which the sperm nucleus (or nuclei in angiosperms) proceed to travel through to reach the egg.

pollination The transfer of pollen from the anthers to the stigma by a pollinating agent such as wind, insects, birds, bats, or in a few cases the opening of the flower itself.

polygenic inheritance Occurs when a trait is controlled by several gene pairs; usually results in continuous variation.

polymer Organic molecule composed of smaller units known as monomers. A large molecule composed of smaller subunits, for example starch is a polymer of glucose, proteins are polymers of amino acids.

polymerase chain reaction (PCR) A method of amplifying or copying DNA fragments that is faster than cloning. The fragments are combined with DNA polymerase, nucleotides, and other components to form a mixture in which the DNA is cyclically amplified.

polynucleotides Long chains of nucleotides formed by chemical links between the sugar and phosphate groups.

polyp The sessile form of life history in cnidarians; e.g., the freshwater hydra.

polyploidy Abnormal variation in the number of chromosome sets. The condition when a cell or organism has more than the customary two sets of chromosomes. This is an especially effective speciation mechanism in plants since the extra chromosomes will establish reproductive isolation with the parental population(s), an essential for speciation.

polysaccharides Long chains of monosaccharide units bonded together; e.g., glycogen, starch, and cellulose.

pons The region that, with the medulla oblongata, makes up the hindbrain, which controls heart rate, constriction and dilation of blood vessels, respiration, and digestion.

population A group of individuals of the same species living in the same area at the same time and sharing a common gene pool. A group of potentially interbreeding organisms in a geographic area.

population dynamics The study of the factors that affect the growth, stability, and decline of populations, as well as the interactions of those factors.

portal system An arrangement in which capillaries drain into a vein that opens into another capillary network.

positive feedback Biochemical control where the accumulation of the product stimulates production of an enzyme responsible for that product's production.

positive feedback control Occurs when information produced by the feedback increases and accelerates the response.

precambrian Informal term describing 7/8 of geologic time from the beginning of the earth to the beginning of the Cambrian Period of the Paleozoic Era. During this time the atmosphere and oceans formed, life originated (or possibly "colonized" Earth), eukaryotes and simple animals evolved and by the end of the precambrian they began to accumulate hard preservable parts, the common occurrence of which marks the beginning of the Cambrian.

precipitation The part of the hydrologic cycle in which the water vapor in the atmosphere falls to Earth as rain or snow.

predation One of the biological interactions that can limit population growth; occurs when organisms kill and consume other living organisms.

predatory release Occurs when a predator species is removed from a prey species such as by great reduction in the predator's population size or by the migration of the prey species to an area without major predators. The removal of the predator releases the prey from one of the factors limiting its population size.

prehensile movement The ability to seize or grasp.

prenatal testing Testing to detect the presence of a genetic disorder in an embryo or fetus; commonly done by amniocentesis or chorionic villi sampling.

presymptomatic screening Testing to detect genetic disorders that only become apparent later in life. The tests are done before the condition actually appears, such as with Huntington disease.

prey switching The tendency of predators to switch to a more readily available prey when one prey species becomes rare; allows the Þrst prey population to rebound and helps prevent its extinction.

primary cell wall The cell wall outside the plasma membrane that surrounds plant cells; composed of the polysaccharide cellulose.

primary body Those parts of a plant produced by the shoot and root apical meristems.

primary compounds Chemicals made by plants and needed for the plant's own metabolism.

primary growth Cells produced by an apical meristem. The growth a plant by the actions of apical meristems on the shoot and root apices in producing plant primary tisues.

primary macronutrients Elements that plants require in relatively large quantities: nitrogen, phosphorus, and potassium.

primary meristems The apical meristems on the shoot and root apices in plants that produce plant primary tissues.

primary root The Þrst root formed by a plant.

primary structure The sequence of amino acids in a protein.

primates The taxonomicorder of mammals that includes prosimians (lemurs and tarsiers), monkeys, apes, and humans; characteristics include large brain, stereoscopic vision, and grasping hand.

principle of independent assortment Mendel's second law; holds that during gamete formation, alleles in one gene pair segregate into gametes independently of the alleles of other gene pairs. As a result, if enough gametes are produced, the collective group of gametes will contain all combinations of alleles possible for that organism.

principle of segregation Mendel's Þrst law; holds that each pair of factors of heredity separate during gamete formation so that each gamete receives one member of a pair.

prions Infectious agents composed only of one or more protein molecules without any accompanying genetic information.

producers The Þrst level in a food pyramid; consist of organisms that generate the food used by all other organisms in the ecosystem; usually consist of plants making food by photosynthesis.

progesterone One of the two female reproductive hormones secreted by the ovaries.

prokaryote Type of cell that lacks a membrane-bound nucleus and has no membrane organelles; a bacterium. Prokaryotes are more primitive than eukaryotes. Cells lacking membrane-bound organelles and having a single circular chromosome, and ribosomes surrounded by a cell membrane. Prokaryotes were the first forms of life on earth, evolving over 3.5 billion years ago.

prolactin A hormone produced by the anterior pituitary; secreted at the end of pregnancy when it activates milk production by the mammary glands.

promoter The speciÞc nucleotide sequence in DNA that marks the beginning of a gene.



prophase 1) The Þrst stage of mitosis during which chromosomes condense, the nuclear envelope disappears, and the centrioles divide and migrate to opposite ends of the cell. 2) The first stage of mitosis and meiosis (although in meiosis this phase is denoted with either a roman numeral I or II) where the chromatin condenses to form chromosomes, nucleolus dissolves, nuclear envelope dissolves, and the spindle begins to form.

prostaglandins A class of fatty acids that has many of the properties of hormones; synthesized and secreted by many body tissues and have a variety of effects on nearby cells.

prostate gland A gland that is located near and empties into the urethra; produces a secretion that enhances sperm viability. Gland involved in the reproductive system in males, the prostate secretes a sperm activating chemical into the semen during the arousal/ejaculation response.

proteinoids Polymers of amino acids formed spontaneously from inorganic molecules; have enzyme-like properties and can catalyze chemical reactions.

proteins Polymers made up of amino acids that perform a wide variety of cellular functions. One of the classes of organic macromolecules that function as structural and control elements in living systems. Proteins are polymers of amino acids linked together by peptide bonds.

prothallus In ferns, a small heart-shaped bisexual gametophyte.



Protista The taxonomicKingdom from which the other three eukaryotic kingdoms (Fungi, Animalia and Plantae) are thought to have evolved. The earliest eukaryotes were single-celled organisms that would today be placed in this admittedly not monophyletic group. The endosymbiosis theory suggests that eukaryotes may have evolved independently several times.

protists Single-celled organisms; a type of eukaryote.

proton A subatomic particle in the nucleus of an atom that carries a positive charge. The positively charged (+1) subatomic particle located in the atomic nucleus and having a mass slightly less than that of a neutron. Elements differ by the number of protons in their atoms.

protostomes Animals in which the Þrst opening that appears in the embryo becomes the mouth; e.g., mollusks, annelids, and arthropods.

protozoa Single-celled protists grouped by their method of locomotion. This group includes Paramecium, Amoeba, and many other commonly observed protists.

proximal tubule The winding section of the renal tubule where most reabsorption of water, sodium, amino acids, and sugar takes place.



pseudocoelom In nematodes, a closed þuid-containing cavity that acts as a hydrostatic skeleton to maintain body shape, circulate nutrients, and hold the major body organs.

pseudocoelomates Animals that have a body cavity that is in direct contact with the outer muscular layer of the body and does not arise by splitting of the mesoderm; e.g., roundworms.

pseudopodia Temporary cytoplasmic extensions from a cell that enables it to move (sing.: pseudopodium).



pulmonary artery The artery that carries blood from the right ventricle of the vertebrate heart to the lungs. Artery carrying oxygen-poor blood from the heart to the lungs.



pulmonary circuit The loop of the circulatory system that carries blood to and from the lungs.



pulmonary vein The vein that carries oxygenated blood from the lungs to the left atrium of the heart.Veins carrying oxygenated blood from the lungs to the heart.



punctuated equilibrium A model that holds that the evolutionary process is characterized by long periods with little or no change interspersed with short periods of rapid speciation.

purine One of the groups of nitrogenous bases that are part of a nucleotide. Purines are adenine and guanine, and are double-ring structures.

pyloric sphincter The ring of muscle at the junction of the stomach and small intestine that regulates the movement of food into the small intestine.

pyrimidine One of the groups of nitrogenous bases that are part of a nucleotide. Pyrimidines are single ringed, and consist of the bases thymine (in DNA), uracil (replacing thymine in RNA), and cytosine.

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quantum models of speciation Models of evolution that hold that speciation sometimes occurs rapidly as well as over long periods, as the classical theory proposed.

Quaternary Period The most recent geologic period of the Cenozoic Era, the Quaternary began 2 million years ago with the growth of northern hemisphere continental glaciers and the ice age.

quaternary structure In some proteins, a fourth structural level created by interactions with other proteins. Aspect of protein structure determined by the number and arrangement of polypeptides in a large protein such as hemoglobin.

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race A subdivision of a species that is capable of interbreeding with other members of the species.

radially symmetrical In animals, refers to organisms with their body parts arranged around a central axis. Such animals tend to be circular or cylindrical in shape.

radiation Energy emitted from the unstable nuclei of radioactive isotopes.

radioactive decay The spontaneous decay of an atom to an atom of a different element by emission of a particle from its nucleus (alpha and beta decay) or by electron capture.

radioisotope Term applied to a radioactive isotope, such as carbon-14 or uranium 238. Radioisotope nuclei are unstable and spontaneously breakdown and emit one of a number of types of radiation.

radiometric time Type of absolute time determined by the relative porportions of radioisotopes to stable daughter isotopes.

ray-finned Taxonomic group of fish, such as trout, tuna, salmon, and bass, that have thin, bony supports holding the Þns away from the body and an internal swim bladder that changes the buoyancy of the body; one of the two main types of bony Þshes.

reabsorption The return to the blood of most of the water, sodium, amino acids, and sugar that were removed during Þltration; occurs mainly in the proximal tubule of the nephron.

receptacle The base that attaches a þower to the stem.

receptor Protein on or protruding from the cell surface to which select chemicals can bind. The opiate receptor in brain cells allows both the natural chemical as well as foreign (opiate) chemicals to bind.

recessive Refers to an allele of a gene that is expressed when the dominant allele is not present. An allele expressed only in homozygous form, when the dominant allele is absent.

recombinant DNA molecules New combinations of DNA fragments formed by cutting DNA segments from two sources with restriction enzyme and then joining the fragments together with DNA ligase. Interspecies transfer of genes usually through a vector such as a virus or plasmid.

recombinant DNA technology A series of techniques in which DNA fragments are linked to self-replicating forms of DNA to create recombinant DNA molecules. These molecules in turn are replicated in a host cell to create clones of the inserted segments.

recombination A way in which meiosis produces new combinations of genetic information. During synapsis, chromatids may exchange parts with other chromatids, leading to a physical exchange of chromosome parts; thus, genes from both parents may be combined on the same chromosome, creating a new combination.

red algae Common name for the algae placed in the division Rhodophyta.

red blood cell Component of the blood that transports oxygen with the hemoglobin molecule.

red tides Phenomenon associated with population explosions (blooms) of certain types of dinoflagellates; red structures inside the dinoflagellates cause the water to have a reddish color.



reduction The gain of an electron or a hydrogen atom. The gain of electrons or hydrogens in a chemical reaction.

reductional division The Þrst division in meiosis; results in each daughter cell receiving one member of each pair of chromosomes.

reflex A response to a stimulus that occurs without conscious effort; one of the simplest forms of behavior.

reflex arc Pathway of neurons, effector(s) and sensory receptors that participate in a reflex.

region of division The area of cell division in the tip of a plant root.

region of elongation The area in the tip of a plant root where cells grow by elongating, thereby increasing the length of the root.



region of maturation (differentiation) The area where primary tissues and root hairs develop in the tip of a plant root.

relative time Type of geologic time (absolute time being the other) that places events in a sequence relative to each other.

renal tubule The portion of the nephron where urine is produced.



renin An enzyme secreted by the kidneys that converts angiotensinogen into angiotensin II.

replication Process by which DNA is duplicated prior to cell division.



reproductive isolating mechanism Biological or behavioral characteristics that reduce or prevent interbreeding with other populations; e.g., the production of sterile hybrids. Establishment of reproductive isolation is considered essential for development of a new species.

reproductive system One of eleven major body organ systems in animals; is responsible for reproduction and thus the survival of the species.

reptiles Taxonomic class of vertebrates characterized by scales and amniotic eggs; the first truly terrestrial vertebrate group.

resolution In relation to microscopes, the ability to view adjacent objects as distinct structures.

resource partitioning The division of resources such that a few dominant species exploit most of the available resources while other species divide the remainder; helps explain why a few species are abundant in a community while others are represented by only a few individuals.

respiration 1) breathing as part of gas exchange; or 2) cellular metabolism.

respiratory surface A thin, moist, epithelial surface that oxygen can cross to move into the body and carbon dioxide can cross to move out of the body.

respiratory system One of eleven major body organ systems in animals; moves oxygen from the external environment into the internal environment and removes carbon dioxide from the body.

resting potential The difference in electrical charge across the plasma membrane of a neuron.

restriction enzymes A series of enzymes that attach to DNA molecules at speciÞc nucleotide sequences and cut both strands of DNA at those sites. A bacterial enzyme that cuts DNA at a specific recognition sequence. This is a bacterial defense against viral DNA and plasmid DNA and is now used as an important tool in biotechnology.

restriction fragment length polymorphism (RFLP) A heritable difference in DNA fragment length and fragment number; passed from generation to generation in a codominant way.

retina The inner, light-sensitive layer of the eye; includes the rods and cones.

retroviruses Viruses that contain a single strand of RNA as their genetic material and reproduce by copying the RNA into a complementary DNA strand using the enzyme reverse transcriptase. The single-stranded DNA is then copied, and the resulting double-stranded DNA is inserted into a chromosome of the host cell.



reverse transcriptase An enzyme used in the replication of retroviruses; aids in copying the retrovirus's RNA into a complementary strand of DNA once inside the host cell.

reverse transcription Process of transcribing a single-stranded DNA from a single-stranded RNA (the reverse of transcription); used by retroviruses as well as in biotechnology.

rheumatoid arthritis A crippling form of arthritis that begins with inþammation and thickening of the synovial membrane, followed by bone degeneration and disÞgurement.

rhizoids Filamentous structures in the plants group known as bryophytes that attach to a substrate and absorb moisture. The term is also applied to similar structures found outside the bryophytes.

rhizome In ferns, a horizontal stem with upright leaves containing vascular tissue.

rhodopsin A visual pigment contained in the rods of the retina in the eye..

ribonucleic acid (RNA) Nucleic acid containing ribose sugar and the base Uracil; RNA functions in protein synthesis. The single starnded molecule transcribed from one strand of the DNA. There are three types of RNA, each is involved in protein synthesis. RNA is made up nucleotides containing the sugar ribose, a phosphate group, and one of four nitrogenous bases (adenine, uracil, cytosine or guanine).



ribose Sugar found in nucleotides of RNA and in ATP.

ribosomal RNA One of the three types of RNA; rRNA is a structural component in ribosomes.

ribosomal subunits Two units that combine with mRNA to form the ribosomal-mRNA complex at which protein synthesis occurs.

ribosomes Small organelles made of rRNA and protein in the cytoplasm of prokaryotic and eukaryotic cells; aid in the production of proteins on the rough endoplasmic reticulum and ribosome complexes. The site of protein synthesis. The ribosome is composed of two subunits that attach to the mRNA at the beginning of protein synthesis and detach when the polypeptide has been translated.

RNA polymerase During transcription, an enzyme that attaches to the promoter region of the DNA template, joins nucleotides to form the synthesized strand of RNA and detaches from the template when it reaches the terminator region.

RNA transcript Term applied to RNA transcribed in the nucleus.



Rodinia Name applied to the precambrian supercontinent.



rods Light receptors in primates' eyes that provide vision in dim light.

root cap Structure that covers and protects the apical meristem in plant roots. Cells forming a protective series of layers over the root meristem.

root hairs Extensions of the root epidermis that increase the root's ability to absorb water.

root-leaf-vascular system axis Refers to the arrangement in vascular plants in which the roots anchor the plant and absorb water and nutrients, the leaves carry out photosynthesis, and the vascular system connects the roots and leaves, carrying water and nutrients to the leaves and carrying sugars and other products of photosynthesis from the leaves to other regions of the plant.



roots Organs, usually occurring underground, that absorb nutrients and water and anchor the plant; one of the three major plant organ systems.

root system Plant organ systems that anchors the plant in place, stores excess sugars, and absorbs water and mineral nutrients. That part of the plant below ground level.

RuBP Ribulose biphosphate; the 5-carbon chemical that combines with carbon dioxide at the beginning of the Calvin Cycle.

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S phase That period of interphase when new DNA is synthesized as part of replication of the chromatin.



salivary amylase An enzyme secreted by the salivary glands that begins the breakdown of complex sugars and starches.

salivary glands Glands that secrete salvia into the mouth.

saprophytes Organisms that obtain their nutrients from decaying plants and animals. Saprophytes are important in recycling organic material.

sapwood Layers of secondary xylem that are still functional in older woody plants; visible as the outer lighter areas in the cross section of a tree trunk.

sarcomeres The functional units of skeletal muscle; consist of Þlaments of myosin and actin.



saturated fat A fat with single covalent bonds between the carbons of its fatty acids.

Schwann cells Specialized glial cells that form the myelin sheath that coats many axons. Cells surrounding the axons of some neurons, thus forming the myelin sheath.

scientific method Systematic apporach of observation, hypothesis formation, hypothesis testing and hypothesis evaluation that forms the basis for modern science.

sclereids Plant cells with thick secondary walls that provide the gritty textures in pears.



sclerenchyma One of the three major cell types in plants; have thickened, rigid, secondary walls that are hardened with lignin; provide support for the plant. Sclerenchyma cells include Þbers and sclereids. Plant tissue type consisting of elongated cells with thickened secondary walls for support of the plant.



scrotum In mammals, a pouch of skin located outside the body cavity into which the testes descend; provides proper temperature for the testes.

secondary cell wall In woody plants, a second wall inside the primary cell wall; contains alternating layers of cellulose and lignin.

secondary compounds Plant products that are not important in metabolism but serve other purposes, such as attracting animals for pollination or killing parasites.

secondary extinction The death of one population due to the extinction of another, often a food species.

secondary growth Cells in a plant that are produced by a cambium. Increase in girth of a plant due to the action of lateral meristems such as the vascular cambium. The main cell produced in secondary growth is secondary xylem, better known as wood.



secondary immunity Resistance to an antigen the second time it appears. Because of the presence of B and T memory cells produced during the Þrst exposure to the antigen, the second response is faster and more massive and lasts longer than the primary immune response.

secondary macronutrients Elements that plants require in relatively small quantities: calcium, magnesium, and sulfur.

secondary (lateral) meristems Plant meristems that produce secondary growth from a cambium.

secondary phloem Phloem produced by the vascular cambium in a woody plant stem or root.

secondary structure The structure of a protein created by the formation of hydrogen bonds between different amino acids; can be a pleated sheet, alpha helix, or random coil. Shape of a protein caused by attraction between R-groups of amino acids.

secondary xylem Xylem produced by the vascular cambium in a woody plant stem or root; wood.

second law of thermodynamics (entropy) The energy available after a chemical reaction is less than that at the beginning of a reaction; energy conversions are not 100% efficient.

second messenger The mechanism by which nonsteroid hormones work on target cells. A hormone binds to receptors on the cell's plasma membrane activating a molecule&emdash;the second messenger&emdash;that activates other intracellular molecules that elicit a response. The second messenger can be cyclic AMP, cyclic GMP, inositol triphosphate, diacrylglycerol, or calcium.



secretin A hormone produced in the duodenum that stimulates alkaline secretions by the pancreas and inhibits gastric emptying.

secretion The release of a substance in response to the presence of food or speciÞc neural or hormonal stimulation.

sediment Loose aggregate of solids derived from preexisting rocks, or solids precipitated from solution by inorganic chemical processes or extracted from solution by organisms.

sedimentary rock Any rock composed of sediment, i.e., solid particles and dissolved minerals. Examples include rocks that form from sand or mud in riverbeds or on the sea bottom.

seed Structure produced by some plants in which the next generation sporophyte is surrounded by gametophyte nutritive tissues. An immature sporophyte in an arrested state of development, surrounded by a protective seed coat.

seed coat The tough outer layer of the seed, derived from the outer layers of the ovule.

segments Repeating units in the body parts of some animals.

segregation Separation of replicated chromosomes to opposite sides of the cell. Distribution of alleles on chromosomes into gametes during meiosis.

selective breeding The selection of individuals with desirable traits for use in breeding. Over many generations, the practice leads to the development of strains with the desired characteristics.

selectively permeable Term describing a barrier that allows some chemicals to pass but not others. The cell membrane is such a barrier.
semen A mixture of sperm and various glandular secretions.

semiconservative replication Process of DNAreplication in which the DNA helix is unwound and each strand serves as a template for the synthesis of a new complementary strand, which is linked to the old strand. Thus, one old strand is retained in each new molecule.



semilunar valve A valve between each ventricle of the heart and the artery connected to that ventricle.



seminal vesicles Glands that contribute fructose to sperm. The fructose serves as an energy source. The structures that add fructose and hormones to semen.

seminiferous tubules Tubules on the interior of the testes where sperm are produced.

sensor In a closed system, the element that detects change and signals the effector to initiate a response.

sensory cortex A region of the brain associated with the parietal lobe.

sensory input Stimuli that the nervous system receives from the external or internal environment; includes pressure, taste, sound, light, and blood pH.

sensory neurons Neurons that carry signals from receptors and transmit information about the environment to processing centers in the brain and spinal cord. Neurons carrying messages from sensory receptors to the spinal cord. Sometimes referred to as an afferent neuron.

sensory (afferent) pathways The portion of the peripheral nervous system that carries information from the organs and tissues of the body to the central nervous system.

sepals Modified leaves that protect a flower's inner petals and reproductive structures. Small, leaf-like structures in flowers that enclose and protect the developing flower. These are often green, but in many monocots they are the same color as the petals (in which case the term tepal is applied since sepals and petals look so much alike).

separation Splitting of the cytoplasm by cytokinesis (= cytokinesis).

severe combined immunodeficiency (SCID) A genetic disorder in which afþicted individuals have no functional immune system and are prone to infections. Both the cell-mediated immune response and the antibody-mediated response are absent.

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shoot The plant stem; provides support for the leaves and þowers; one of the three major plant organs; also referred to as the shoot system.

short-day plants Plants that þower during early spring or fall when nights are relatively long and days are short; e.g., poinsettia and dandelions.

sickle cell anemia Human autosomalrecessive disease that causes production of abnormal red blood cells that collapse (or sickle) and cause circulatory problems.

sieve cells Conducting cells in the phloem of vascular plants. sieve elements Tubular, thin-walled cells that form a system of tubes extending from the roots to the leaves in the phloem of plants; lose their nuclei and organelles at maturity, but retain a functional plasma membrane.

sieve plates Pores in the end walls of sieve elements that connect the sieve elements together. The end walls of sieve tube cells that are perforated (sieves).

sieve tube membersPhloem cells that form long sieve tubes.

silica Silicon dioxide.

Silurian Period The geological time period of the Paleozoic Era following the Ordovician, between 435 and 395 million years ago, when plants colonized the land.

simple leaf A leaf in which the blade does not form leaflets.

sink A body or process that acts as a storage device or disposal mechanism; e.g., plants and the oceans act as sinks absorbing atmospheric carbon dioxide. Also, a location in a plant where sugar is being consumed, either in metabolism or by conversion to starch.

sinoatrial (SA) node A region of modiÞed muscle cells in the right atrium that sends timed impulses to the heart's other muscle cells, causing them to contract; the heart's pacemaker.



sister chromatids Chromatids joined by a common centromere and carrying identical genetic information (unless crossing-over has occurred).

sleep movement In legumes, the movement of the leaves in response to daily rhythms of dark and light. The leaves are horizontal in daylight and folded vertically at night.

skeletal muscle Muscle that is generally attached to the skeleton and causes body parts to move; consists of muscle Þbers. Voluntary muscle cells that have a striated appearance. These muscles control skeletal movements and are normally under conscious control.

skeletal system One of eleven major body organ systems in animals; supports the body, protects internal organs, and, with the muscular system, allows movement and locomotion.

skin One of eleven major body organ systems in animals; the outermost layer protecting multicellular animals from the loss or exchange of internal þuids and from invasion by foreign microorganisms; composed of two layers: the epidermis and dermis.

sliding filament model Model of muscular contraction in which the actin Þlaments in the sarcomere slide past the myosin Þlaments, shortening the sarcomere and therefore the muscle.

slime molds Protistans that may represent a transition between protistans and fungi.

small intestine A coiled tube in the abdominal cavity that is the major site of chemical digestion and absorption of nutrients; composed of the duodenum, jejunum, and ileum.

smog A local alteration in the atmosphere caused by human activity; mainly an urban problem that is often due to pollutants produced by fuel combustion.

smooth muscle Muscle that lacks striations; found around circulatory system vessels and in the walls of such organs as the stomach, intestines, and bladder. Involuntary, not striated cells that control autonomic functions such as digestion and artery contraction.

social behavior Behavior that takes place in a social context and results from the interaction between and among individuals.

societies The most highly organized type of social organization; consist of individuals that show varying degrees of cooperation and communication with one another; often have a rigid division of labor.

sodium-potassium pump The mechanism that uses ATP energy to reset the sodium and potassium ions after transmission of a nerve impulse.

soil Weathered rocks and minerals combined with air, water and organic matter that can support plants.

somatic Relating to the non-gonadal tissues and organs of an organism's body.

somatic cell A cell that is not or will not become a gamete; the cells of the body.

somatic senses All senses except vision, hearing, taste, and smell; include pain, temperature, and pressure.

somatic nervous system The portion of the peripheral nervous system consisting of the motor neuron pathways that innervate skeletal muscles.

somatostatin Pancreatic hormone that controls the rate of nutrient absorption into the bloodstream.

somites Mesodermal structures formed during embryonic development that give rise to segmented body parts such as the muscles of the body wall.

special senses Vision, hearing, taste, and smell.

species One or more populations of interbreeding or potentially interbreeding organisms that are reproductively isolated in nature from all other organisms. Populations of individuals capable of interbreeding and producing viable, fertile offspring. The least inclusive taxonomic category commonly used.

species diversity The number of living species on Earth.

species packing The phenomenon in which present-day communities generally contain more species than earlier communities because organisms have evolved more adaptations over time.

species richness The number of species present in a community.

spermatogenesis The development of sperm cells from spermatocytes to mature sperm, including meiosis.

spicules Needle-shaped skeletal elements in sponges that occur in the matrix between the epidermal and collar cells.

spinal cord A cylinder of nerve tissue extending from the brain stem; receives sensory information and sends output motor signals; with the brain, forms the central nervous system. Nerve cell collections extending from the base of the brain to just below the last rib vertebrae.

spindle apparatusMicrotubule construction that aligns and segregates chromosomes during eukaryotic cell division.

spleen An organ that produces lymphocytes and stores erythrocytes.

spongy bone The inner layer of bone; found at the ends of long bones and is less dense than compact bone. Some spongy bone contains red marrow.

spongy mesophyllParenchyma cells found in plant leaves that are irregularly shaped and have large intracellular spaces.

sporangia The structures in which spores are produced (sing.: sporangium).

spores Impervious structures formed by some cells that encapsulate the cells and protect them from the environment; haploid cells that can survive unfavorable conditions and germinate into new haploid individuals or act as gametes in fertilization.

sporophyte The diploid stage of a plant exhibiting alternation of generations. The diploid, spore producing phase of the plant life cycle.

Sporozoans Members of the protists that are referred to as slime molds; may include organisms resembling the ancestors of fungi.

stability One of the phases of a population's life cycle. The population's size remains roughly constant, þuctuating around some average density. Also, the ability of a community to persist unchanged.

stabilizing selection A process of natural selection that tends to favor genotypic combinations that produce an intermediate phenotype; selection against the extremes in variation.

stalk A leaf's petiole; the slender stem that supports the blade of a leaf and attaches it to a larger stem of the plant.

stamens The male reproductive structures of a þower; usually consist of slender, thread-like filaments topped by anthers. The male reproductive structures in the flower, composed of a filament and anther.
stapes One of the three bones that function in hearing.

start codon The codon (AUG) on a messenger RNA molecule where protein synthesis begins.

steinkerns Internal casts of a fossil. Steinkerns may reveal internal anatomy of an organism, such as muscle attachment, and other details of soft tissue structure.

stem cells Cells in bone marrow that produce lymphocytes by mitotic division.
sternum The breastbone.

steroids Compounds with a skeleton of four rings of carbon to which various side groups are attached; one of the three main classes of hormones.

sticky ends Term applied to DNA sequences cut with restriction enzymes where the cuts will bond with each other or with another sequence cut with the same enzyme.

stigma Part of the female reproductive structure of the carpel of a þower; the sticky surface at the tip of the style to which pollen grains attach. The receptive surface of the pistil (of the flower) on which pollen is placed by a pollinator.

stimulus A physical or chemical change in the environment that leads to a response controlled by the nervous system.

stolons Stems that grow along the surface of the ground; a method of plant vegetaive propagation.

stomach The muscular organ between the esophagus and small intestine that stores, mixes, and digests food and controls the passage of food into the small intestine.

stomata Pores on the underside of leaves that can be opened or closed to control gas exchange and water loss. Openings in the epidermis (usually of the leaf) that allow gas exchange.

stomatal apparatus The stomata and guard cells that control the size of the stoma.

stop codon The codon on a messenger RNA molecule where protein synthesis stops.

stratification The division of water in lakes and ponds into layers with different temperatures and oxygen content. Oxygen content declines with depth, while the uppermost layer is warmest in summer and coolest in winter.

stressed community A community that is disturbed by human activity, such as road building or pollution, and is inadvertently simpliÞed. Some species become superabundant while others disappear.

stroma The matrix surrounding the grana in the inner membrane of chloroplasts. The area between membranes (thylakoids, grana) inside the chloroplast.

stromatolite A sedimentological and biological "fossil" representinmg colonies of bacteria altenating with layers of sediments. Becoming more common during the Proterozoic, stromatolites persist today in marine environments where grazing by herbivorous organisms is limited.



style Part of the female reproductive structure in the carpel of a þower; formed from the ovary wall. The tip of the style carries the stigma to which pollen grains attach. Part of the pistil that separates the stigma from the ovary.

subatomic particles The three kinds of particles that make up atoms: protons, neutrons, and electrons.

suberin Waxy, waterproof chemical in some plant cells, notably cork (in stems) and endodermis cells (in roots).

subspecies A taxonomic subdivision of a species; a population of a particular region genetically distinguishable from other such populations and capable of interbreeding with them.

substitution A type of mutation in which one base is substituted for another.

substrate feeders Animals such as earthworms or termites that eat the soil or wood through which they burrow.

sudden infant death syndrome (SIDS) A disorder resulting in the unexpected death during sleep of infants, usually between the ages of two weeks and one year. The causes are not fully understood, but are believed to involve failure of automatic respiratory control.

superior vena cava Blood from the head returns to the heart through this main vein.

suppressor T cells T cells that slow down and stop the immune response of B cells and other T cells. Immune system cells that shut off the antibody production when an infection is under control.

suprachiasmic nucleus (SCN) A region of the hypothalamus that controls internal cycles of endocrine secretion.

symbiosis An interactive association between two or more species living together; may be parasitic, commensal, or mutualistic. The relationship between two organisms.

sympathetic system The subdivision of the autonomic nervous system that dominates in stressful or emergency situations and prepares the body for strenuous physical activity, e.g., causing the heart to beat faster.

synapse The junction between an axon and an adjacent neuron.



synapsis The alignment of chromosomes during meiosis I so that each chromosome is beside its homologue.

synaptic cleft The space between the end of a neuron and an adjacent cell.

synaptic vesicles Vesicles at the synapse end of an axon that contain the neurotransmitters.

synergid Cells in the embryo sac of angiosperms that flank the egg cell. The pollen tube grows through one (usually the smaller) of the synergids.

synovial joint The most movable type of joint. The bones are covered by connective tissue, the interior of which is Þlled with synovial þuid, and the ends of the bones are covered with cartilage.

syphilis A sexually transmitted disease caused by a bacterial infection that produces an ulcer on the genitals and can have potentially serious effects if untreated.

systematics The classiÞcation of organisms based on information from observations and experiments; includes the reconstruction of evolutionary relatedness among living organisms. Currently, a system that divides organisms into Þve kingdoms (Monera, Protista, Plantae, Fungi, Animalia) is widely used.

systemic circuit The loop of the circulatory system that carries blood through the body and back to the heart.

systole The contraction of the ventricles that opens the semilunar valve and forces blood into the arteries.



systolic pressure The peak blood pressure when ventricles contract.

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