M–P
11.1 A–D
 
11.2 E–H
 
11.3 I–L
 
 
11.4 M–P
 
11.5 Q–T
 
11.6 U–Z
 
M–P
M
Malpighian tubules
The organ of blood filtration in arthropods.
medulla oblongata
Part of the brain responsible for the control of involuntary functions such as breathing, cardiovascular regulation, and swallowing.
meiosis
A type of cellular reproduction that results in the formation of four haploid cells from one diploid cell. Contains two cellular divisions, meiosis I and meiosis II, that follow only one round of DNA replication. Meiosis produces germ cells.
Mendel, Gregor
Gregor Mendel (1822–1884) was an Austrian monk and scientist. Through a series of experiments with pea plants, he discovered the basic laws of heredity, including dominance, segregation, and independent assortment.
menstrual cycle
A 28-day hormone sequence that defines the production, ovulation, and menstruation of eggs in the female reproductive system. If fertilization of the egg occurs, the menstrual cycle stops.
mesophyll
The internal tissue of a leaf between the epidermal cells; specialized for photosynthesis. Contains the palisade and spongy layer.
messenger RNA (mRNA)
An RNA molecule that specifies the amino acid sequence of a protein. In transcription, messenger RNA molecules copy the genetic information stored in DNA. The mRNA then bring the recipes for proteins from the nucleus to ribosomes in the cytoplasm.
metaphase
The second stage of mitosis in which microtubules align the chromosomes in the center of the cell along the metaphase plate; the stage of meiosis I and II during which the chromosomes align at the center of the cell.
minerals
Inorganic molecules required by the body to carry out life processes. Important minerals are iron, a necessary component of hemoglobin; iodine, which is essential for making thyroid hormone; and calcium, which is required by the bones and for many cellular processes.
mitochondria
Double membrane-bound organelles that produce most of the energy in eukaryotic cells through the process of aerobic (cellular) respiration, which generates ATP.
mitosis
The phase of the eukaryotic cell cycle in which the cell divides. The four steps of mitosis are prophase, metaphase, anaphase, and telophase.
mitotic spindle
A complex of microtubules that forms between opposite poles of a cell during mitosis. The mitotic spindle is formed by the centrioles and serves to separate and move chromosomes to opposite ends of the cell for division.
molecular clock
A molecule or gene sequence that has a constant rate of change through accumulation of neutral substitutions and is therefore a good measuring stick for the relatedness of different species.
monocot
A flowering plant (angiosperm) that possesses one cotyledon during embryonic development. Usually has fibrous roots, flower parts in threes, and parallel veins in leaves.
monomer
Each of the repeating units that make up a polymer.
monosaccharide
A carbohydrate monomer. Glucose and fructose are common examples.
muscle
Structures that create movement in an organism by contracting under a stimulus from a neuron. There are three types of muscle: skeletal, which is responsible for voluntary movement; smooth, which is responsible for involuntary movement; and cardiac, which makes up the heart.
mutation
An error in the sequence of nucleotides in DNA or RNA that in turn affects the production of proteins. There are two main types of mutations: substitution mutations and frameshift mutations. A substitution mutation occurs when one nucleotide is replaced by another; these mutations can range from ineffectual to drastic, depending on how the new nucleotide changes the protein coded for. Frameshift mutations occur when a nucleotide is either inserted or deleted into the code; these mutations are always drastic and often fatal, since an insertion or deletion will affect every codon in a particular genetic sequence by throwing the entire three-by-three codon frame out of whack.
myelin sheath
A structure that speeds the movement of action potentials along the axon of a neuron. The sheath is built of Schwann cells, which wrap themselves around the axon of the neuron, leaving small gaps in between known as the nodes of Ranvier.
myosin
Protein filaments that, along with actin, allow muscles to contract.
N
NADH
An energy-carrying coenzyme produced by glycolysis and the Krebs cycle. NADH carries energy to the electron transport chain, where it is stored in ATP.
natural selection
The theory, first proposed by Darwin, which holds that organisms produce as many offspring as possible, which compete for limited resources. Organisms’ characteristics vary, and certain characteristics will allow organisms to survive and reproduce more effectively. These adaptive characteristics will be more prevalent in subsequent generations. Natural selection is the engine of evolution, choosing the most fit genes to pass from one generation to the next.
nephridium
A blood filtration and excretory organ characteristic of segmented worms.
nephron
Tiny, tubule structures responsible for the filtering of blood in the kidneys of vertebrates.
neritic zone
The medium depth zone of the marine biome. Extends to 600 feet beneath the water’s surface and sits on the continental shelf, hundreds of miles from any shore. Algae, crustaceans, and fish inhabit this region.
nervous system
Control system of the body that functions by sending impulses through neurons to receive information and spur muscles to action.
neuron
The functional unit of the nervous system. A neuron is a specialized cell able to carry an action potential and made up of dendrites, a cell body, and an axon. There are three types of neurons: sensory, motor, and interneurons.
niche
The unique role a population plays in a community. A niche includes all characteristics that define the way a population exists in a community, from where the members live to what they eat, when they sleep, and how they reproduce.
nitrogenous base
One of the nitrogen-containing bases in DNA and RNA nucleotides. There are five nitrogenous bases in living organisms. DNA is composed of adenine, thymine, guanine, and cytosine. RNA is composed of adenine, uracil, guanine, and cytosine.
nondisjunction
Occurs when a pair of homologous chromosomes fails to separate during gamete formation. The offspring produced from these gametes have either one too many or one too few of a particular chromosome. Nondisjunction is the cause of genetic disorders such as Down syndrome.
notochord
A longitudinal rod of cells that forms in the least developed chordates and in embryonic stages of more developed chordates.
nucleolus
A dense, spherical body inside the nucleus of a cell. The nucleolus makes the RNA that is a structural component of the ribosomes.
nucleotide
The monomer of a nucleic acid. Nucleotides consist of (1) a phosphate group, a group of atoms containing phosphorus; (2) a sugar; and (3) a nitrogenous base, a compound containing nitrogen that removes H+ ions from solution. Nucleotides are grouped into two general classes, depending on the sugar group that they carry: deoxyribonucleic acids (DNA) contain the sugar deoxyribose and ribonucleic acids (RNA) contain the sugar ribose. Nucleotides are further divided by the type of nitrogenous base that they carry. DNA is composed of adenine, thymine, guanine, and cytosine. RNA is composed of adenine, uracil, guanine, and cytosine.
nucleus
The large, central organelle of eukaryotes. The nucleus contains the genetic material of the cell and controls cellular activities.
O
olfactory epithelium
Region near the top of the nasal cavity with chemoreceptors and neurons that inform the sense of smell.
oogenesis
The process in which haploid egg cells (ova) form through meiotic division.
organ
A discrete unit of tissues that work together to perform specific functions within the body.
organelle
Specialized membrane-bound structure in a cell that performs a specific function. Examples of organelles include the endoplasmic reticulum and the mitochondria.
osmosis
Process by which water naturally travels from an area of high water concentration to low water concentration.
ovary
In animals, the female gonad that produces ova and sex hormones. In plants, the ovaries are the structure at the base of the pistil that contains the eggs.
ovule
Structure that contains the female gametophyte and gametes; after fertilization, develops into a seed.
oxidative phosphorylation
Part of the electron transport chain. A process occurring in the mitochondria that results in the formation of ATP from the flow of electrons across the inner membrane to bind with oxygen.
P
pancreas
A digestive organ that releases enzymes into the small intestine. Also an endocrine gland that regulates glucose levels in the blood by the release of insulin or glucagon from specialized cells called islets of Langerhans.
parathyroid
Four small glands embedded on the posterior surface of the thyroid gland. The parathyroid glands produce a hormone that regulates the level of calcium in the bloodstream.
parthenogenesis
Asexual reproduction in which an unfertilized gamete (usually female) produces female offspring. Parthenogenesis vastly increases the speed at which a population can grow, though it results in a loss of genetic diversity among members of the population.
pelagic zone
The open-ocean zone at the greatest depth in a marine habitat. This zone is divided into a photic (down to 600 feet below the water’s surface) and aphotic zone.
peptide bond
The bond between the amino acids in a protein. Formed by dehydration synthesis.
peripheral nervous system (PNS)
The pathways by which the central nervous system receives sensory information from the body and sends commands to muscles. The peripheral nervous system is divided into two halves. The sensory system brings information in from the body, while the motor system sends commands out to muscles.
peristalsis
The rolling motion of smooth muscle that moves food along the alimentary canal. Includes the passage from the esophagus to the stomach, the churning action of the stomach, and the passage through the small intestine.
pH
A scale for measuring the presence or absence of hydrogen ions in solution. Values between 0 and 7 indicate an excess of hydrogen ions. Such solutions are called acids. Values between 7 and 14 indicate the presence of compounds that counteract the effects of hydrogen ions. Such solutions are called bases. At pH 7, solutions are neutral.
phagocyte
A type of white blood cell that kills invading cells by ingesting them.
phagocytosis
A form of endocytosis in which a cell ingests a solid particle.
phenotype
The entire set of observable characteristics of an organism or cell; the physical traits of an organism. The genotype, together with environmental factors, defines the phenotype.
phloem
Vascular tissue composed of cells that are living at maturity; transports the products of photosynthesis throughout the plant body.
phospholipid
Type of lipid found in cell membranes. Phospholipids are made up of a single hydrophilic phosphate head and two nonpolar hydrophobic lipid tails.
phospholipid bilayer
A double layer of phospholipid molecules that provides the structure of the cell membrane. Formed naturally from the alignment of the two layers of lipids such that their hydrophobic tails point inward toward each other and their hydrophilic phosphate heads point outward into the watery environments inside and outside of the cell.
photic zone
Literally, zone with light. The photic zone is part of the marine pelagic zone and extends to 600 feet below the surface of the ocean. Photosynthetic plankton as well as bony fish, sharks, and whales inhabit this zone.
photoperiodism
An organism’s response to the length of day and night within a 24-hour period (photoperiod); in many plants, this phenomenon determines when flowering occurs.
photosynthesis
The process by which plants and other autotrophic organisms convert light energy into organic materials, such as glucose.
phylogeny
The evolutionary relationships of a genetically similar group of organisms.
pinocytosis
Form of endocytosis during which liquids are taken into the cell through the invagination of the cell membrane.
pioneer population
The first population to move into a geographic location and begin the process of ecological succession.
pistil
The female reproductive organ of the flower, composed of a stigma, style, and ovary; sometimes called the carpel.
pituitary
The “master” gland of the endocrine system. The pituitary releases hormones that control the other major glands of the endocrine system. Made up of the anterior and posterior pituitary. Controlled by the hypothalamus.
placenta
A structure that develops in the uterus during pregnancy; filter through which the embryo gains nutrition from the mother.
plasmids
Circular DNA molecules found in prokaryotes.
pollen
The male gametophyte of gymnosperms and angiosperms.
polymer
A large molecule consisting of the same or similar units attached in a series, forming a chain.
population
A group of interbreeding organisms in a particular locale exhibiting a unique set of characteristics such as patterns of growth and reproductive strategies.
predation
Term that refers to one organism eating another. Predation covers both carnivorous and herbivorous consumption.
producers
Autotrophic organisms such as plants, plankton, and chemosynthetic bacteria that are able to synthesize organic compounds using energy from the sun or chemical reactions. Producers do not have to consume other organisms to attain energy and are the foundation of every food web.
progesterone
Hormone that prepares the uterus for embryo implantation and helps to maintain pregnancy.
prokaryote
A single-celled organism that completely lacks membrane-bound intracellular organelles such as a nucleus or mitochondria; prokaryotes possess only a single circular strand of DNA. Prokaryotes are simpler than eukaryotes and arose earlier in evolutionary history. All bacteria are prokaryotes. Taxonomists group all prokaryotes into the kingdom Monera.
prophase
The first stage of mitosis, meiosis I, and meiosis II, during which the chromosomes become visible and the centrioles move to opposite ends of the cell and begin to form the spindle.
pseudopods
Temporary cytoplasmic protrusions of ameboid cells that function in movement and food uptake by phagocytosis.
Punnett square
A pictorial method of showing the gene combinations (genotypes) of offspring that might result from an experimental genetic cross of two parents.
pyruvate
The three-carbon end product of glycolysis. Pyruvate is the raw material of the Krebs cycle.
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