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.
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.
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