ileum: last part of the small intestine; connects the small intestine to the large intestine; important for absorption of B-12
immune tolerance: acquired ability to prevent an unnecessary or harmful immune response to a detected foreign body known not to cause disease or to self-antigens
immunodeficiency: failure, insufficiency, or delay at any level of the immune system, which may be acquired or inherited
imprinting: identification of parents by newborns as the first organism they see after birth
inbreeding: mating of closely related individuals
inbreeding depression: increase in abnormalities and disease in inbreeding populations
incomplete dominance: in a heterozygote, expression of two contrasting alleles such that the individual displays an intermediate phenotype
incus: (also, anvil) second of the three bones of the middle ear
indeterminate cleavage: cleavage pattern in which individual blastomeres have the character of "stem cells," and are not yet predetermined to develop into specific cell types
induced fit: dynamic fit between the enzyme and its substrate, in which both components modify their structures to allow for ideal binding
induced mutation: mutation that results from exposure to chemicals or environmental agents
inducible operon: operon that can be activated or repressed depending on cellular needs and the surrounding environment
inductive reasoning: form of logical thinking that uses related observations to arrive at a general conclusion
inert gas: (also, noble gas) element with filled outer electron shell that is unreactive with other atoms
inferior vena cava: drains blood from the veins that come from the lower organs and the legs
inferior vena cava: one of the main veins in the human body
infertility: inability to conceive, carry, and deliver children
inflammation: localized redness, swelling, heat, and pain that results from the movement of leukocytes and fluid through opened capillaries to a site of infection
ingestion: act of taking in food
inhibin: hormone made by Sertoli cells; provides negative feedback to hypothalamus in control of FSH and GnRH release
inhibitor: molecule that binds to a protein (usually an enzyme) and keeps it from functioning
inhibitory postsynaptic potential (IPSP): hyperpolarization of a postsynaptic membrane caused by neurotransmitter molecules released from a presynaptic cell
initiation complex: protein complex containing eIF-2 that starts translation
initiation site: nucleotide from which mRNA synthesis proceeds in the 5' to 3' direction; denoted with a “+1”
initiator tRNA: in prokaryotes, called tRNAMetf; in eukaryotes, called tRNAi; a tRNA that interacts with a start codon, binds directly to the ribosome P site, and links to a special methionine to begin a polypeptide chain
innate behavior: instinctual behavior that is not altered by changes in the environment
innate immunity: immunity that occurs naturally because of genetic factors or physiology, and is not induced by infection or vaccination
inner cell mass: inner layer of cells in the blastocyst
inner ear: innermost part of the ear; consists of the cochlea and the vestibular system
inorganic compound: chemical compound that does not contain carbon; it is not part of or produced by a living organism
inositol phospholipid: lipid present at small concentrations in the plasma membrane that is converted into a second messenger; it has inositol (a carbohydrate) as its hydrophilic head group
inositol triphosphate (IP3): cleavage product of PIP2 that is used for signaling within the cell
insectivorous plant: plant that has specialized leaves to attract and digest insects
inspiratory capacity (IC): tidal volume plus inspiratory reserve volume
inspiratory reserve volume (IRV): amount of additional air that can be inspired after a normal inhalation
insulin: hormone produced by the beta cells of the pancreas in response to high blood glucose levels; functions to lower blood glucose levels
insulin-like growth factor (IGF): growth-promoting protein produced by the liver
integral protein: protein integrated into the membrane structure that interacts extensively with the membrane lipids' hydrocarbon chains and often spans the membrane
integument: layer of sporophyte tissue that surrounds the megasporangium, and later, the embryo
intercalary meristem: meristematic tissue located at nodes and the bases of leaf blades; found only in monocots
intercellular signaling: communication between a cell
intercostal muscle: muscle connected to the rib cage that contracts upon inspiration
interferon: cytokine that inhibits viral replication and modulates the immune response
interkinesis: (also, interphase II) brief period of rest between meiosis I and meiosis II
interlobar artery: artery that branches from the segmental artery and travels in between the renal lobes
intermediate filament: cytoskeletal component, comprised of several fibrous protein intertwined strands, that bears tension, supports cell-cell junctions, and anchors cells to extracellular structures
intermittent symptom: symptom that occurs periodically
internal fertilization: fertilization of egg by sperm inside the body of the female
internal receptor: (also, intracellular receptor) receptor protein that is located in the cytosol of a cell and binds to ligands that pass through the plasma membrane
internode: region between nodes on the stem
interphase: period of the cell cycle leading up to mitosis; includes G1, S, and G2 phases (the interim period between two consecutive cell divisions)
intersexual selection: selection of a desirable mate of the opposite sex
interspecific competition: competition between species for resources in a shared habitat or environment
interstitial cell of Leydig: cell in seminiferous tubules that makes testosterone
interstitial fluid: fluid between cells
intertidal zone: part of the ocean that is closest to land; parts extend above the water at low tide
intervertebral disc: composed of fibrous cartilage; lies between adjacent vertebrae from the second cervical vertebra to the sacrum
intestinal phase: third digestive phase; begins when chyme enters the small intestine triggering digestive secretions and controlling the rate of gastric emptying
intine: inner lining of the pollen
intracellular hormone receptor: a hormone receptor in the cytoplasm or nucleus of a cell
intracellular mediator: (also, second messenger) small molecule that transmits signals within a cell
intracellular signaling: communication within cells
intramembranous ossification: process of bone development from fibrous membranes
intrapleural space: space between the layers of pleura
intrasexual selection: competition between members of the same sex for a mate
intraspecific competition: competition between members of the same species
introduction: opening section of a scientific paper, which provides background information about what was known in the field prior to the research reported in the paper
intron: non–protein-coding intervening sequences that are spliced from mRNA during processing
inversion: soles of the feet moving inward, toward the midline of the body
invertebrata: (also, invertebrates) category of animals that do not possess a cranium or vertebral column
ion: atom or chemical group that does not contain equal numbers of protons and electrons
ion channel-linked receptor: cell-surface receptor that forms a plasma membrane channel, which opens when a ligand binds to the extracellular domain (ligand-gated channels)
ionic bond: chemical bond that forms between ions with opposite charges (cations and anions)
iris: pigmented, circular muscle at the front of the eye that regulates the amount of light entering the eye
irregular bone: bone with complex shapes; examples include vertebrae and hip bones
irreversible chemical reaction: chemical reaction where reactants proceed unidirectionally to form products
island biogeography: study of life on island chains and how their geography interacts with the diversity of species found there
islets of Langerhans (pancreatic islets): endocrine cells of the pancreas
isomerase: enzyme that converts a molecule into its isomer
isomers: molecules that differ from one another even though they share the same chemical formula
isotonic: situation in which the extracellular fluid has the same osmolarity as the fluid inside the cell, resulting in no net water movement into or out of the cell
isotope: one or more forms of an element that have different numbers of neutrons
isthmus: tissue mass that connects the two lobes of the thyroid gland
iteroparity: life history strategy characterized by multiple reproductive events during the lifetime of a species
J-shaped growth curve: shape of an exponential growth curve
jasmonates: small family of compounds derived from the fatty acid linoleic acid
jejunum: second part of the small intestine
joint: point at which two or more bones meet
juxtaglomerular cell: cell in the afferent and efferent arterioles that responds to stimuli from the macula densa
juxtamedullary nephron: nephron that lies in the cortex but close to the renal medulla
K-selected species: species suited to stable environments that produce a few, relatively large offspring and provide parental care
karyogamy: fusion of nuclei
karyogram: a karyotype's photographic image
karyokinesis: mitotic nuclear division
karyotype: an individual's chromosome number and appearance; includes the size, banding patterns, and centromere position
keystone species: species whose presence is key to maintaining biodiversity in an ecosystem and to upholding an ecological community’s structure
kidney: organ that performs excretory and osmoregulatory functions
kin selection: sacrificing one’s own life so that one’s genes will be passed on to future generations by relatives
kinase: enzyme that catalyzes the transfer of a phosphate group from ATP to another molecule
kinesis: undirected movement of an organism in response to a stimulus
kinesthesia: sense of body movement
kinetic energy: energy type that takes place with objects or particles in motion
kinetochore: protein structure associated with the centromere of each sister chromatid that attracts and binds spindle microtubules during prometaphase
kinetoplast: mass of DNA carried within the single, oversized mitochondrion, characteristic of kinetoplastids (phylum: Euglenozoa)
kingdom: domain division in the taxonomic classification system
Kozak’s rules: determines the correct initiation AUG in a eukaryotic mRNA; the following consensus sequence must appear around the AUG: 5’-GCC(purine)CCAUGG-3’; the bolded bases are most important
Krebs cycle: (also citric acid cycle) alternate name for the citric acid cycle, named after Hans Krebs, who first identified the steps in the pathway in the 1930s in pigeon flight muscles; see citric acid cycle
labia majora: large folds of tissue covering the inguinal area
labia minora: smaller folds of tissue within the labia majora
labyrinth: bony, hollow structure that is the most internal part of the ear; contains the sites of transduction of auditory and vestibular information
lac operon: operon in prokaryotic cells that encodes genes required for processing and intake of lactose
lactase: enzyme that breaks down lactose into glucose and galactose
lacuna: space in cartilage and bone that contains living cells
lagging strand: during replication, the strand that is replicated in short fragments and away from the replication fork
lamella: layer of compact tissue that surrounds a central canal called the Haversian canal
lamina: leaf blade
lamprey: jawless fish characterized by a toothed, funnel-like, sucking mouth
lancelet: member of Cephalochordata; named for its blade-like shape
large 60S ribosomal subunit: second, larger ribosomal subunit that binds to the RNA to translate it into protein
large intestine: digestive system organ that reabsorbs water from undigested material and processes waste matter
larynx: voice box, a short passageway connecting the pharynx and the trachea
latency: virus that remains in the body for a long period of time but only causes intermittent symptoms
lateral line: sense organ that runs the length of a fish’s body; used to detect vibration in the water
lateral meristem: meristematic tissue that enables a plant to increase in thickness or girth
lateral rotation: rotation away from the midline of the body
law of dominance: in a heterozygote, one trait will conceal the presence of another trait for the same characteristic
law of independent assortment: genes do not influence each other with regard to sorting of alleles into gametes; every possible combination of alleles is equally likely to occur
law of mass action: chemical law stating that the rate of a reaction is proportional to the concentration of the reacting substances
law of segregation: paired unit factors (i.e., genes) segregate equally into gametes such that offspring have an equal likelihood of inheriting any combination of factors
layering: method of propagating plants by bending a stem under the soil
leading strand: strand that is synthesized continuously in the 5'-3' direction, which is synthesized in the direction of the replication fork
learned behavior: behavior that responds to changes in the environment
lens: transparent, convex structure behind the cornea that helps focus light waves on the retina
lenticel: opening on the surface of mature woody stems that facilitates gas exchange
lepidosaur: modern lizards, snakes, and tuataras
leptin: hormone produced by adipose tissue that promotes feelings of satiety and reduces hunger
lichen: close association of a fungus with a photosynthetic alga or bacterium that benefits both partners
life cycle: the sequence of events in the development of an organism and the production of cells that produce offspring
life history: inherited pattern of resource allocation under the influence of natural selection and other evolutionary forces
life science: field of science, such as biology, that studies living things
life table: table showing the life expectancy of a population member based on its age
ligand: molecule produced by a signaling cell that binds with a specific receptor, delivering a signal in the process
ligase: enzyme that catalyzes the formation of a phosphodiester linkage between the 3' OH and 5' phosphate ends of the DNA
light harvesting complex: complex that passes energy from sunlight to the reaction center in each photosystem; it consists of multiple antenna proteins that contain a mixture of 300 to 400 chlorophyll a and b molecules as well as other pigments like carotenoids
light microscope: an instrument that magnifies an object using a beam of visible light that passes and bends through a lens system to visualize a specimen
light-dependent reaction: first stage of photosynthesis where certain wavelengths of the visible light are absorbed to form two energy-carrying molecules (ATP and NADPH)
light-independent reaction: second stage of photosynthesis, through which carbon dioxide is used to build carbohydrate molecules using energy from ATP and NADPH
lignin: complex polymer impermeable to water
limbic system: connected brain areas that process emotion and motivation
linkage: phenomenon in which alleles that are located in close proximity to each other on the same chromosome are more likely to be inherited together
linkage analysis: procedure that analyzes recombining genes to determine if they are linked
lipase: enzyme that chemically breaks down lipids
lipid: macromolecule that is nonpolar and insoluble in water
lipid-derived hormone: hormone derived mostly from cholesterol
litmus paper: (also, pH paper) filter paper treated with a natural water-soluble dye that changes its color as the pH of the environment changes in order to use it as a pH indicator
liver: organ that produces bile for digestion and processes vitamins and lipids
liverworts: most primitive group of the non-vascular plants
loam: soil that has no dominant particle size
lobes of the kidney: renal pyramid along with the adjoining cortical region
locus: position of a gene on a chromosome
logistic growth: leveling off of exponential growth due to limiting resources
long bone: bone that is longer than wide, and has a shaft and two ends
long-term depression (LTD): prolonged decrease in synaptic coupling between a pre- and postsynaptic cell
long-term potentiation (LTP): prolonged increase in synaptic coupling between a pre-and postsynaptic cell
loop of Henle: part of the renal tubule that loops into the renal medulla
loose (areolar) connective tissue: type of connective tissue with small amounts of cells, matrix, and fibers; found around blood vessels
Lophotrochozoa: clade of protostomes that exhibit a trochophore larvae stage or a lophophore feeding structure
lower limb: consists of the thigh, the leg, and the foot
lung capacity: measurement of two or more lung volumes (how much air can be inhaled from the end of an expiration to maximal capacity)
lung volume: measurement of air for one lung function (normal inhalation or exhalation)
luteinizing hormone (LH): reproductive hormone in both males and females, causes testosterone production in males and ovulation and lactation in females
lycophyte: club moss
lymph: watery fluid that bathes tissues and organs with protective white blood cells and does not contain erythrocytes
lymph node: specialized organ that contains a large number of macrophages that clean the lymph before the fluid is returned to the heart
lymphocyte: leukocyte that is histologically identifiable by its large nuclei; it is a small cell with very little cytoplasm
lysis: bursting of a cell
lysis buffer: solution to break the cell membrane and release cell contents
lysogenic cycle: type of virus replication in which the viral genome is incorporated into the genome of the host cell
lysosome: organelle in an animal cell that functions as the cell’s digestive component; it breaks down proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles
lytic cycle: type of virus replication in which virions are released through lysis, or bursting, of the cell
macroevolution: broader scale evolutionary changes that scientists see over paleontological time
macromolecule: large molecule, typically formed by the joining of smaller molecules
macronutrient: nutrient that is required in large amounts for plant growth; carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur
macrophage: large phagocytic cell that engulfs foreign particles and pathogens
macula densa: group of cells that senses changes in sodium ion concentration; present in parts of the renal tubule and collecting ducts
madreporite: pore for regulating entry and exit of water into the water vascular system
major depression: mental illness characterized by prolonged periods of sadness
major histocompatibility complex (MHC) I/II molecule: protein found on the surface of all nucleated cells (I) or specifically on antigen-presenting cells (II) that signals to immune cells whether the cell is healthy/normal or is infected/cancerous; it provides the appropriate template into which antigens can be loaded for recognition by lymphocytes
malleus: (also, hammer) first of the three bones of the middle ear
Malpighian tubule: excretory tubules found in arthropods
maltase: enzyme that breaks down maltose into glucose
mammal: one of the groups of endothermic vertebrates that possesses hair and mammary glands
mammary gland: in female mammals, a gland that produces milk for newborns
mantle: (also, pallium) specialized epidermis that encloses all visceral organs and secretes shells
mark and recapture: technique used to determine population size in mobile organisms
marsupial: one of the groups of mammals that includes the kangaroo, koala, bandicoot, Tasmanian devil, and several other species; young develop within a pouch
mass extinction: event or environmental condition that wipes out the majority of species within a relatively short geological time period
mass number: total number of protons and neutrons in an atom
mast cell: leukocyte that produces inflammatory molecules, such as histamine, in response to large pathogens and allergens
mastax: jawed pharynx unique to the rotifers
materials and methods: section of a scientific paper that includes a complete description of the substances, methods, and techniques that the researchers used to gather data
mating factor: signaling molecule secreted by yeast cells to communicate to nearby yeast cells that they are available to mate
matrix: component of connective tissue made of both living and nonliving (ground substances) cells
matrix protein: envelope protein that stabilizes the envelope and often plays a role in the assembly of progeny virions
matter: anything that has mass and occupies space
maximum parsimony: applying the simplest, most obvious way with the least number of steps
mechanoreceptor: sensory receptor modified to respond to mechanical disturbance such as being bent, touch, pressure, motion, and sound
medial rotation: rotation toward the midline of the body
medulla: middle layer of an organ like the kidney or adrenal gland
medusa: free-floating cnidarian body plan with mouth on underside and tentacles hanging down from a bell
megafauna: large animals
megagametogenesis: second phase of female gametophyte development, during which the surviving haploid megaspore undergoes mitosis to produce an eight-nucleate, seven-cell female gametophyte, also known as the megagametophyte or embryo sac.
megapascal (MPa): pressure units that measure water potential
megaphyll: larger leaves with a pattern of branching veins
megasporangium: tissue found in the ovary that gives rise to the female gamete or egg
megaspore: female spore
megasporocyte: megaspore mother cell; larger spore that germinates into a female gametophyte in a heterosporous plant
megasporogenesis: first phase of female gametophyte development, during which a single cell in the diploid megasporangium undergoes meiosis to produce four megaspores, only one of which survives
megasporophyll: bract (a type of modified leaf) on the central axis of a female gametophyte
meiosis: a nuclear division process that results in four haploid cells
meiosis I: first round of meiotic cell division; referred to as reduction division because the ploidy level is reduced from diploid to haploid
meiosis II: second round of meiotic cell division following meiosis I; sister chromatids are separated into individual chromosomes, and the result is four unique haploid cells
Meissner’s corpuscle: (also, tactile corpuscle) encapsulated, rapidly-adapting mechanoreceptor in the skin that responds to light touch
membrane potential: difference in electrical potential between the inside and outside of a cell
memory cell: antigen-specific B or T lymphocyte that does not differentiate into effector cells during the primary immune response but that can immediately become an effector cell upon reexposure to the same pathogen
meninge: membrane that covers and protects the central nervous system
menopause: loss of reproductive capacity due to decreased sensitivity of the ovaries to FSH and LH
menstrual cycle: cycle of the degradation and regrowth of the endometrium
meristem: plant region of continuous growth
meristematic tissue: tissue containing cells that constantly divide; contributes to plant growth
Merkel's disk: unencapsulated, slowly-adapting mechanoreceptor in the skin that responds to touch
meroblastic: partial cleavage; takes place in cells with a large amount of yolk
mesocarp: middle part of a fruit
mesocosm: portion of a natural ecosystem to be used for experiments
mesoglea: non-living, gel-like matrix present between ectoderm and endoderm in cnidarians
mesohyl: collagen-like gel containing suspended cells that perform various functions in the sponge
mesophyll: middle layer of chlorophyll-rich cells in a leaf
messenger RNA (mRNA): RNA that carries information from DNA to ribosomes during protein synthesis
metabolism: all the chemical reactions that take place inside cells, including anabolism and catabolism
metabolome: complete set of metabolites which are related to an organism's genetic makeup
metabolomics: study of small molecule metabolites in an organism
metacarpus: five bones that comprise the palm
metagenomics: study of multiple species' collective genomes that grow and interact in an environmental niche
metamerism: series of body structures that are similar internally and externally, such as segments
metaphase: stage of mitosis during which chromosomes are aligned at the metaphase plate
metaphase plate: equatorial plane midway between the two poles of a cell where the chromosomes align during metaphase
metatarsal: one of the five bones of the foot
Metazoa: group containing all animals
microbial mat: multi-layered sheet of prokaryotes that may include bacteria and archaea
microbiology: study of the structure and function of microorganisms
microcosm: re-creation of natural ecosystems entirely in a laboratory environment to be used for experiments
microevolution: changes in a population’s genetic structure
microfilament: the cytoskeleton system's narrowest element; it provides rigidity and shape to the cell and enables cellular movements
microglia: glia that scavenge and degrade dead cells and protect the brain from invading microorganisms
micronutrient: nutrient required in small amounts; also called trace element
microphyll: small size and simple vascular system with a single unbranched vein
micropropagation: propagation of desirable plants from a plant part; carried out in a laboratory
micropyle: opening on the ovule sac through which the pollen tube can gain entry
microRNA (miRNA): small RNA molecules (approximately 21 nucleotides in length) that bind to RNA molecules to degrade them
microsatellite polymorphism: variation between individuals in the sequence and number of microsatellite DNA repeats
microscope: an instrument that magnifies an object
microsporangium: tissue that gives rise to the microspores or the pollen grain
microspore: male spore
microsporocyte: smaller spore that produces a male gametophyte in a heterosporous plant
microsporophyll: central axis of a male cone on which bracts (a type of modified leaf) are attached
microtubule: the cytoskeleton system's widest element; it helps the cell resist compression, provides a track along which vesicles move through the cell, pulls replicated chromosomes to opposite ends of a dividing cell, and is the structural element of centrioles, flagella, and cilia
microvilli: cellular processes that increase the surface area of cells
middle ear: part of the hearing apparatus that functions to transfer energy from the tympanum to the oval window of the inner ear
midsagittal plane: plane cutting through an animal separating the individual into even right and left sides
migration: long-range seasonal movement of animal species
Milankovitch cycles: cyclic changes in the Earth's orbit that may affect climate
mineral: inorganic, elemental molecule that carries out important roles in the body
mineral soil: type of soil that is formed from the weathering of rocks and inorganic material; composed primarily of sand, silt, and clay
mineralocorticoid: corticosteroid that affects ion and water balance
mismatch repair: type of repair mechanism in which mismatched bases are removed after replication
mitochondria: (singular = mitochondrion) cellular organelles responsible for carrying out cellular respiration, resulting in producing ATP, the cell’s main energy-carrying molecule
mitochondria-first hypothesis: proposal that prokaryotes acquired a mitochondrion first, followed by nuclear development
mitosis: (also, karyokinesis) period of the cell cycle during which the duplicated chromosomes are separated into identical nuclei; includes prophase, prometaphase, metaphase, anaphase, and telophase
mitosome: nonfunctional organelle carried in the cells of diplomonads (Excavata) that likely evolved from a mitochondrion
mitotic phase: period of the cell cycle during which duplicated chromosomes are distributed into two nuclei and cytoplasmic contents are divided; includes karyokinesis (mitosis) and cytokinesis
mitotic spindle: apparatus composed of microtubules that orchestrates the movement of chromosomes during mitosis
mixotroph: organism that can obtain nutrition by autotrophic or heterotrophic means, usually facultatively
model organism: species that researchers study and use as a model to understand the biological processes in other species represented by the model organism
model system: species or biological system used to study a specific biological phenomenon to be applied to other different species
modern synthesis: overarching evolutionary paradigm that took shape by the 1940s and scientists generally accept today
molality: number of moles of solute per kilogram of solvent
molarity: number of moles of solute per liter of solution
mold: tangle of visible mycelia with a fuzzy appearance
mole: gram equivalent of the molecular weight of a substance
molecular biology: study of biological processes and their regulation at the molecular level, including interactions among molecules such as DNA, RNA, and proteins
molecular cloning: cloning of DNA fragments
molecular systematics: technique using molecular evidence to identify phylogenetic relationships
molecule: chemical structure consisting of at least two atoms held together by one or more chemical bonds
molecule: two or more atoms chemically bonded together
Mollusca: phylum of protostomes with soft bodies and no segmentation
monocarpic: plants that flower once in their lifetime
monocot: related group of angiosperms that produce embryos with one cotyledon and pollen with a single ridge
monocyte: type of white blood cell that circulates in the blood and lymph and differentiates into macrophages after it moves into infected tissue
monoecious: describes a species in which the male and female reproductive organs are on the same plant
monogamy: mating system whereby one male and one female remain coupled for at least one mating season
monogastric: digestive system that consists of a single-chambered stomach
monohybrid: result of a cross between two true-breeding parents that express different traits for only one characteristic
monomer: smallest unit of larger molecules that are polymers
monophyletic group: (also, clade) organisms that share a single ancestor
monosaccharide: single unit or monomer of carbohydrates
monosomy: otherwise diploid genotype in which one chromosome is missing
monotreme: egg-laying mammal
morning sickness: condition in the pregnant people during the first trimester; includes feelings of nausea
mortality rate: proportion of population surviving to the beginning of an age interval that die during the age interval
mosses: group of bryophytes in which a primitive conductive system appears
motor end plate: sarcolemma of the muscle fiber that interacts with the neuron
MRSA: (methicillin-resistant Staphylococcus aureus) very dangerous Staphylococcus aureus strain resistant to multiple antibiotics
mucin: complex glycoprotein found in mucus
mucosa-associated lymphoid tissue (MALT): collection of lymphatic tissue that combines with epithelial tissue lining the mucosa throughout the body
mucus: sticky protein-containing fluid secretion in the lung that traps particulate matter to be expelled from the body
Müllerian mimicry: type of mimicry where species share warning coloration and all are harmful to predators
multiple cloning site (MCS): site that multiple restriction endonucleases can recognize
multiple fruit: fruit that develops from multiple flowers on an inflorescence
muscle spindle: proprioceptive stretch receptor that lies within a muscle and that shortens the muscle to an optimal length for efficient contraction
mutation: variation in the nucleotide sequence of a genome
mutualism: symbiotic relationship between two species where both species benefit
myc: oncogene that causes cancer in many cancer cells
mycelium: mass of fungal hyphae
mycetismus: ingestion of toxins in poisonous mushrooms
mycology: scientific study of fungi
mycorrhiza: mutualistic association between fungi and vascular plant roots
mycorrhizae: a mutualistic relationship between a plant and a fungus. Mycorrhizae are connections between fungal hyphae, which provide soil minerals to the plant, and plant roots, which provide carbohydrates to the fungus
mycosis: fungal infection
mycotoxicosis: poisoning by a fungal toxin released in food
myelin: fatty substance produced by glia that insulates axons
myocardial infarction: (also, heart attack) complete blockage of the coronary arteries and death of the cardiac muscle tissue
myocardium: heart muscle cells that make up the middle layer and the bulk of the heart wall
myofibril: long cylindrical structures that lie parallel to the muscle fiber
myofilament: small structures that make up myofibrils
myopia: (also, nearsightedness) visual defect in which the image focus falls in front of the retina, thereby making images in the distance blurry, but close-up images clear
myosin: contractile protein that interacts with actin for muscle contraction
Myxini: hagfishes
nacre: calcareous secretion produced by bivalves to line the inner side of shells as well as to coat intruding particulate matter
nasal cavity: opening of the respiratory system to the outside environment
natural killer (NK) cell: lymphocyte that can kill cells infected with viruses or tumor cells
natural science: field of science that is related to the physical world and its phenomena and processes
natural selection: reproduction of individuals with favorable genetic traits that survive environmental change because of those traits, leading to evolutionary change
nauplius: larval stage in the early development of crustaceans
nectar: liquid rich in sugars produced by flowers to attract animal pollinators
nectar guide: pigment pattern on a flower that guides an insect to the nectaries
negative feedback loop: feedback to a control mechanism that increases or decreases a stimulus instead of maintaining it
negative gravitropism: growth away from Earth’s gravity
negative polarity: ssRNA viruses with genomes complementary to their mRNA
negative regulator: protein that prevents transcription
nematocyst: harpoon-like organelle within cnidocyte with pointed projectile and poison to stun and entangle prey
Nematoda: phylum of worm-like animals that are triploblastic, pseudocoelomates that can be free-living or parasitic
Nemertea: phylum of dorsoventrally flattened protostomes known as ribbon worms
Neognathae: birds other than the Paleognathae
Neornithes: modern birds
nephridia: excretory structures found in annelids
nephridiopore: pore found at the end of nephridia
nephron: functional unit of the kidney
neritic zone: part of the ocean that extends from low tide to the edge of the continental shelf
net consumer productivity: energy content available to the organisms of the next trophic level
net primary productivity: measurement of the energy accumulation within an ecosystem, calculated as the total amount of carbon fixed per year minus the amount that is oxidized during cellular respiration
net primary productivity: energy that remains in the primary producers after accounting for the organisms’ respiration and heat loss
net production efficiency (NPE): measure of the ability of a trophic level to convert the energy it receives from the previous trophic level into biomass
neural stimuli: stimulation of endocrine glands by the nervous system
neural tube: tube-like structure that forms from the ectoderm and gives rise to the brain and spinal cord
neurobiology: study of the biology of the nervous system
neurodegenerative disorder: nervous system disorder characterized by the progressive loss of neurological functioning, usually caused by neuron death
neuron: specialized cell that can receive and transmit electrical and chemical signals
neurotransmitter: chemical ligand that carries a signal from one nerve cell to the next
neutron: uncharged particle that resides in an atom's nucleus; has a mass of one amu
neutrophil: phagocytic leukocyte that engulfs and digests pathogens
next-generation sequencing: group of automated techniques for rapid DNA sequencing
nitrification: conversion of ammonium into nitrite and nitrate in soils
nitrogen fixation: process by which gaseous nitrogen is transformed, or “fixed” into more readily available forms such as ammonia
nitrogenase: enzyme that is responsible for the reduction of atmospheric nitrogen to ammonia
noble gas: see inert gas
nociception: neural processing of noxious (such as damaging) stimuli
node: point along the stem at which leaves, flowers, or aerial roots originate
nodes of Ranvier: gaps in the myelin sheath where the signal is recharged
nodule: novel structure on the roots of certain plants (legumes) that results from the symbiotic interaction between the plant and soil bacteria, and is the site of nitrogen fixation
nodules: specialized structures that contain Rhizobia bacteria where nitrogen fixation takes place
non-electrolyte: solute that does not break down into ions when dissolved in water
non-endospermic dicot: dicot that stores food reserves in the developing cotyledon
non-vascular plant: plant that lacks vascular tissue, which is formed of specialized cells for the transport of water and nutrients
nondisjunction: failure of synapsed homologs to completely separate and migrate to separate poles during the meiosis' first cell division
nonparental (recombinant) type: progeny resulting from homologous recombination that exhibits a different allele combination compared with its parents
nonpolar covalent bond: type of covalent bond that forms between atoms when electrons are shared equally between them
nonrandom mating: changes in a population’s gene pool due to mate choice or other forces that cause individuals to mate with certain phenotypes more than others
nonrenewable resource: resource, such as fossil fuel, that is either regenerated very slowly or not at all
nonsense codon: one of the three mRNA codons that specifies termination of translation
nontemplate strand: strand of DNA that is not used to transcribe mRNA; this strand is identical to the mRNA except that T nucleotides in the DNA are replaced by U nucleotides in the mRNA
norepinephrine: neurotransmitter and hormone released by activation of the sympathetic nervous system
norepinephrine: hormone released by the adrenal medulla in response to a short-term stress hormone production by the gonads
Northern blotting: transfer of RNA from a gel to a nylon membrane
notochord: flexible, rod-shaped support structure that is found in the embryonic stage of all chordates and in the adult stage of some chordates
nuclear envelope: double-membrane structure that constitutes the nucleus' outermost portion
nucleic acid: biological macromolecule that carries the cell's genetic blueprint and carries instructions for the cell's functioning
nucleoid: central part of a prokaryotic cell's central part where the chromosome is located
nucleolus: darkly staining body within the nucleus that is responsible for assembling ribosome subunits
nucleoplasm: semi-solid fluid inside the nucleus that contains the chromatin and nucleolus
nucleosome: subunit of chromatin composed of a short length of DNA wrapped around a core of histone proteins
nucleotide: monomer of nucleic acids; contains a pentose sugar, one or more phosphate groups, and a nitrogenous base
nucleotide excision repair: type of DNA repair mechanism in which the wrong base, along with a few nucleotides upstream or downstream, are removed
nucleus: core of an atom; contains protons and neutrons
nucleus: cell organelle that houses the cell’s DNA and directs ribosome and protein synthesis
nucleus-first hypothesis: proposal that prokaryotes acquired a nucleus first, and then the mitochondrion
nutrient: essential substances for growth, such as carbon and nitrogen
O horizon: layer of soil with humus at the surface and decomposed vegetation at the base
obligate aerobes: organisms, such as humans, that must perform aerobic respiration to survive
obligate anaerobes: organisms that only perform anaerobic respiration and often cannot survive in the presence of oxygen
obstructive disease: disease (such as emphysema and asthma) that arises from obstruction of the airways; compliance increases in these diseases
occipital lobe: part of the cerebral cortex that contains visual cortex and processes visual stimuli
ocean upwelling: rising of deep ocean waters that occurs when prevailing winds blow along surface waters near a coastline
oceanic zone: part of the ocean that begins offshore where the water measures 200 m deep or deeper
Octamer box: (ATTTGCAT) nonessential eukaryotic promoter sequence that binds cellular factors to increase the efficiency of transcription; may be present several times in a promoter
octet rule: rule that atoms are most stable when they hold eight electrons in their outermost shells
odorant: airborne molecule that stimulates an olfactory receptor
Okazaki fragment: DNA fragment that is synthesized in short stretches on the lagging strand
olfaction: sense of smell
olfactory bulb: neural structure in the vertebrate brain that receives signals from olfactory receptors
olfactory epithelium: specialized tissue in the nasal cavity where olfactory receptors are located
olfactory receptor: dendrite of a specialized neuron
oligodendrocyte: glial cell that myelinates central nervous system neuron axons
oligosaccharin: hormone important in plant defenses against bacterial and fungal infections
omega fat: type of polyunsaturated fat that the body requires; numbering the carbon omega starts from the methyl end or the end that is farthest from the carboxylic end
omnivore: animal that consumes both plants and animals
oncogene: mutated version of a normal gene involved in the positive regulation of the cell cycle
oncogenic virus: virus that has the ability to cause cancer
oncolytic virus: virus engineered to specifically infect and kill cancer cells
one-child policy: China’s policy to limit population growth by limiting urban couples to have only one child or face the penalty of a fine
oogenesis: process of producing haploid eggs
open circulatory system: system in which the blood is mixed with interstitial fluid and directly covers the organs
operant conditioning: learned behaviors in response to positive and/or negative reinforcement
operator: region of DNA outside of the promoter region that binds activators or repressors that control gene expression in prokaryotic cells
operon: collection of genes involved in a pathway that are transcribed together as a single mRNA in prokaryotic cells
opposition: movement of the thumb toward the fingers of the same hand, making it possible to grasp and hold objects
opsonization: process that enhances phagocytosis using proteins to indicate the presence of a pathogen to phagocytic cells
orbital: region surrounding the nucleus; contains electron(s)
order: class division in the taxonomic classification system
organ: collection of related tissues grouped together performing a common function
organ of Corti: in the basilar membrane, the site of the transduction of sound, a mechanical wave, to a neural signal
organ system: level of organization that consists of functionally related interacting organs
organelle: small structures that exist within cells and carry out cellular functions
organelle: compartment or sac within a cell
organic compound: chemical compound that contains carbon
organic molecule: any molecule containing carbon (except carbon dioxide)
organic soil: type of soil that is formed from sedimentation; composed primarily of organic material
organism: individual living entity
organogenesis: formation of organs in animal embryogenesis
organogenesis: process of organ formation
origin: (also, ORI) region of the prokaryotic chromosome where replication begins (origin of replication)
Ornithorhynchidae: clade that includes the duck-billed platypus
osculum: large opening in the sponge’s body through which water leaves
osmoconformer: organism that changes its tonicity based on its environment
osmolarity: total amount of solutes dissolved in a specific amount of solution
osmophile: organism that grows in a high sugar concentration
osmoreceptor: receptor in the hypothalamus that monitors the concentration of electrolytes in the blood
osmoregulation: mechanism by which water and solute concentrations are maintained at desired levels
osmoregulator: organism that maintains its tonicity irrespective of its environment
osmosis: transport of water through a semipermeable membrane according to the water's concentration gradient across the membrane that results from the presence of solute that cannot pass through the membrane
osmotic balance: balance of the amount of water and salt input and output to and from a biological system without disturbing the desired osmotic pressure and solute concentration in every compartment
osmotic pressure: pressure exerted on a membrane to equalize solute concentration on either side
osseous tissue: connective tissue that constitutes the endoskeleton
ossicle: one of the three bones of the middle ear
ossification: (also, osteogenesis) process of bone formation by osteoblasts
Osteichthyes: bony fish
osteoblast: bone cell responsible for bone formation
osteoclast: large bone cells with up to 50 nuclei, responsible for bone remodeling
osteocyte: mature bone cells and the main cell in bone tissue
osteon: subunit of compact bone
osteon: cylindrical structure aligned parallel to the long axis of the bone
ostium: pore present on the sponge’s body through which water enters
ostium: (plural: ostia) holes between blood vessels that allow the movement of hemolymph through the body of insects, arthropods, and mollusks with open circulatory systems
ostracoderm: one of the earliest jawless fish covered in bone
outer ear: part of the ear that consists of the auricle, ear canal, and tympanum and which conducts sound waves into the middle ear
oval window: thin diaphragm between the middle and inner ears that receives sound waves from contact with the stapes bone of the middle ear
ovarian cycle: cycle of preparation of egg for ovulation and the conversion of the follicle to the corpus luteum
ovary: chamber that contains and protects the ovule or female megasporangium
oviduct: (also, fallopian tube) muscular tube connecting the uterus with the ovary area
oviger: additional pair of appendages present on some arthropods between the chelicerae and pedipalps
oviparity: process by which fertilized eggs are laid outside the female’s body and develop there, receiving nourishment from the yolk that is a part of the egg
ovoviviparity: process by which fertilized eggs are retained within the female; the embryo obtains its nourishment from the egg’s yolk and the young are fully developed when they are hatched
ovulate cone: cone containing two ovules per scale
ovulation: release of the egg by the most mature follicle
ovule: female gametophyte
oxidative phosphorylation: production of ATP using the process of chemiosmosis in the presence of oxygen
oxygen dissociation curve: curve depicting the affinity of oxygen for hemoglobin
oxygen-carrying capacity: amount of oxygen that can be transported in the blood
oxytocin: hormone released by the posterior pituitary to stimulate uterine contractions during childbirth and milk let-down in the mammary glands
Biology 2e, March 28, 2018, OpenStax, https://openstax.org/books/biology-2e/pages/1-introduction