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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