3) Biology

Abductor

A muscle that moves a limb away from the center of the body.

Absorption

The process by which substances are taken up into or across tissues.

Acetylcholine

A neurotransmitter found throughout the nervous system (somatic motor neurons, preganglionic parasympathetic and sympathetic nerves, and postganglionic parasympathetic neurons); metabolized by acetylcholinesterase.

Acrosome

The large vesicle at the head of a sperm cell containing enzymes that degrade the ovum cell membrane to allow fertilization.

Actin

A protein found in the cytoskeleton and muscle cells; it is the principal constituent of the thin filaments and microfilaments.

Action potential

An abrupt change in the membrane potential of a nerve or muscle caused by changes in membrane ionic permeability; results in conduction of an impulse in nerves or contraction in muscles.

Active immunity

An immune response (antibody production or cellular immunity) acquired in response to exposure to an antigen.

Active site

Substrate-binding region of an enzyme.

Adaptation

The development of characteristics that enable an organism to survive and reproduce in its habitat.

Adaptive immunity

Highly specific form of immunity that retains chemical memory of each invader encountered and is able to tailor the immune response to the specific pathogen.

Adaptive radiation

The evolutionary process by which one species gives rise to several species, each specialized for different niches.

Adductor

A muscle that moves a limb toward the center of the body.

Adenine

A purine base present in DNA and RNA; it forms hydrogen bonds with thymine and uracil.

Adenosine triphosphate (ATP)

A nucleotide molecule consisting of adenine, ribose, and three phosphate moieties; the outer two phosphates are bound by high-energy bonds.

Adipose

Refers to fatty tissue, fat-storing tissue, or fat within cells.

Aerobic

Refers to a biological process that occurs in the presence of molecular oxygen (O2) or to organisms that cannot live without molecular oxygen.

Afferent (sensory) neuron

A neuron that picks up impulses from sensory receptors and transmits them toward the central nervous system.

Agranulocyte

Type of leukocyte that does not contain cytoplasmic granules, including lymphocytes and monocytes.

Albumin

Protein synthesized in the liver that maintains the oncotic pressure of the blood and serves as a carrier for many drugs and hormones.

Allantois

One of four embryonic membranes; it contains the growing embryo's waste products.

Allele

Alternative forms of the same gene coding for a particular trait; alleles segregate during meiosis.

Allergy

A type of autoimmunity in which a person's immune system becomes over-activated by common substances in the environment.

Alveolus

Basic functional unit of the lung; a tiny sac specialized for passive gas exchange between the lungs and the blood.

Amino acids

The building blocks of proteins, each containing an amino group, a carboxylic acid group, and a side chain (or R group) attached to the α-carbon.

Amnion

The innermost fluid-filled embryonic membrane; it forms a protective sac surrounding the embryos of birds, reptiles, and mammals.

Amplification

Characteristic of a signaling cascade, in which the binding of a single peptide hormone to a membrane-bound receptor results in a signal that increases in strength through the signaling cascade.

Anaerobic

Refers to a biological process that can occur without oxygen or to organisms that can live without molecular oxygen.

Anaphase

The stage of mitosis or meiosis characterized by the migration of chromatids or homologous chromosomes to opposite poles of the dividing cell.

Androgen

Any male sex hormone, such as testosterone.

Antibiotic

Substance that kills or inhibits the growth of bacteria or fungi (usually by disrupting cell wall assembly or by binding to ribosomes, thus inhibiting protein synthesis).

Antibody

Immune or protective protein whose synthesis is induced by the presence of foreign substances (antigens) in the body; each antibody binds to a specific antigen in an immune response; also called immunoglobulin.

Antigen

A substance that binds to an antibody; may be foreign or a self-antigen.

Antigen-binding region

Portion of an antibody that is specific for a particular antigen; the area of the antibody to which the antigen binds.

Aortic valve

One of the semilunar valves, separating the left ventricle from the aorta.

Apoptosis

Process by which a cell undergoes programmed cell death in a highly organized manner in response to either external or internal signals.

Appendicular skeleton

Peripheral portion of the skeleton consisting of arms, legs, and pelvic and pectoral girdles.

Archenteron

The central cavity in the gastrula stage of embryological development; it is lined by endoderm and ultimately gives rise to the adult digestive tract.

Arterioles

Small arterial structures that link the arteries to the capillaries.

Artery

Thick-walled, muscular blood vessel that generally carries blood away from the heart.

Articular cartilage

Cartilaginous coating at the ends of bones that provides a smooth surface for articulation of bones within a joint.

Asexual reproduction

Any reproductive process that does not involve the fusion of gametes (such as budding).

Asters

Star-shaped structures that form around the centrosome during mitosis.

Atrium

One of two paired structures on either side of the heart, into which blood returning from either the body (right atrium) or the lungs (left atrium) flows.

Autocrine

Form of cell-cell communication in which a cell releases a substance that then binds to the membrane of the releasing cell to either inhibit or activate a cellular activity.

Autoimmunity

Inappropriate immune response that targets self-antigens.

Autonomic nervous system

Subdivision of the peripheral nervous system responsible for involuntary activities, which is further subdivided into the parasympathetic and sympathetic nervous systems.

Autosome

Any chromosome other than a sex chromosome.

Axial skeleton

Midline structures of the skeleton including the skull, vertebral column, and rib cage; provides the central framework of the body.

Axon

The long fiber of a neuron; it conducts impulses away from the cell body toward the synapse.

Axon hillock

Transition point between the cell body (soma) and the axon of a neuron; the site of action potential initiation.

Bacillus

Rod-shaped bacterium.

Bacteriophage

A virus that invades bacteria and sometimes uses bacterial RNA and ribosomes to self-replicate.

Basophil

Type of granulocytic leukocyte that largely participates in allergic reactions and local inflammation.

Bile

A solution of salts, pigments, and cholesterol produced by the liver and stored in the gallbladder; it emulsifies large fat droplets when secreted into the small intestine via the bile duct.

Bilirubin

Product of the breakdown of hemoglobin that is modified to a more soluble form in the liver.

Binary fission

A type of asexual reproduction characteristic of prokaryotes in which there is equal nuclear and cytoplasmic division.

Blastocoel

The fluid-filled central cavity of the blastula.

Blastocyst

A mammalian blastula, consisting of the trophoblastic cells and an inner cell mass.

Blastopore

Opening of the archenteron to the external environment in the gastrula stage of embryonic development.

Blastula

The early embryonic stage during which the embryo is a hollow, fluid-filled sphere of undifferentiated cells.

Blastulation

Process by which a solid mass of early embryonic cells, known as the morula, becomes the blastula, a hollow fluid-filled sphere of undifferentiated cells.

Bohr effect

Changes in the affinity of hemoglobin for oxygen caused by changes in the environment; when pH is low (increased concentration of hydrogen ions), the oxyhemoglobin dissociation curve shifts right, indicating a decreased affinity of hemoglobin for oxygen and more efficient off-loading of oxygen from hemoglobin.

Bolus

An initial dose of medication; in the digestive system, chewed food leaving the mouth, traveling through the esophagus, and entering the stomach.

Bone marrow

Central portion of bones, especially long bones, that contains fat and developing blood cells, including erythrocytes, leukocytes, and megakaryocytes.

Bone matrix

Organic and inorganic minerals that provide strength to compact bone; organic components include collagen, glycoproteins, and other peptides; inorganic components include calcium, phosphate, and hydroxide ions (in hydroxyapatite).

Bowman's capsule

The cup-like structure of the nephron; it collects the glomerular filtrate and channels it into the proximal convoluted tubule.

Bronchi

Tube-like passages for air that connect the trachea to the bronchioles.

Bronchioles

Passageways for air that start at the bronchi, dividing into continuously smaller passageways that eventually lead to the alveoli, where gas exchange occurs.

Brush-border enzymes

Group of enzymes present on the luminal surface of cells lining the duodenum that break down larger biomolecules into monomers that are able to be absorbed.

Bundle of His

Part of the conduction system of the heart; it carries impulses from the AV node to the ventricles.

Callus

Area of excessive deposition of keratin in response to repeated strain due to friction.

Canaliculi

Small canals connecting lacunae within the bone matrix with Haversian canals, allowing for the flow of nutrients and wastes.

Capillary

Small, thin-walled blood vessel where gas, nutrient, and waste exchange occurs between blood and tissues.

Capsid

Protein coat surrounding a virus.

Cardiac output

Total blood volume pumped by the left ventricle in one minute, found by multiplying the heart rate by the stroke volume.

Cartilage

A firm, elastic, translucent connective tissue produced by cells called chondrocytes.

Catabolism

The chemical breakdown of complex substances (macromolecules) to yield simpler substances and energy.

Cecum

The first part of the large intestine; accepts material flowing through the ileocecal valve and is the point of attachment of the appendix.

Cell body

Portion of a neuron where the nucleus, endoplasmic reticulum, and ribosomes are located; also known as the soma.

Cell theory

A foundational belief in modern biology that all living things are composed of cells, that the cell is the basic functional unit of life, that all cells arise from preexisting cells, and that DNA is the genetic material.

Cell-mediated immunity

Type of immunity that uses cytotoxic chemicals released from cells to cause death of cells that have been infected by viruses.

Central nervous system (CNS)

The brain and spinal cord.

Centriole

A small organelle in the cytoplasm of animal cells; it organizes the spindle apparatus during mitosis or meiosis.

Centromere

The area of a chromosome where sister chromatids are joined; it is also the point of attachment to the spindle fiber during mitosis and meiosis.

Centrosomes

Paired cylindrical organelles, located in the cytoplasm, that contain the centrioles.

Cerebellum

The section of the mammalian hindbrain that controls muscle coordination and equilibrium.

Cerebral cortex

The outer layer of the fore-brain, consisting of grey matter; it is the site of higher cognitive functions in humans.

Cervix

Lower end of the uterus that marks the transition between the vagina and the uterus.

Chemical digestion

Enzymatic cleavage of chemical bonds within foodstuffs, resulting in smaller molecules.

Chemotaxis

Movement of cells toward or away from a chemical within the environment.

Chiasmata

Sites where crossing over occurs between homologous chromosomes during meiosis.

Chief cells

Cells within the stomach that secrete pepsinogen, a zymogen that is converted to its active form, pepsin, by the acidic environment of the stomach.

Chondrin

Elastic cartilage matrix substance secreted by chondrocytes.

Chondrocyte

A differentiated cartilage cell that synthesizes the cartilaginous matrix.

Chromatid

Each of the two chromosomal strands formed by DNA replication in the S phase of the cell cycle; held together by the centromere.

Chromosome

A filamentous body found within the nucleus of a eukaryotic cell or nucleoid region of a prokaryotic cell, composed of DNA.

Chylomicron

Soluble lipid molecule that consists of triglycerides and esterified cholesterol molecules; absorbed into lacteals from the digestive tract.

Chyme

Aqueous mixture of food and secretions that leaves the stomach to enter the duodenum.

Cilia

Projection from a cell involved in movement of materials on the outside of the cell.

Circadian rhythm

A behavioral pattern based on a 24-hour cycle, related to cycling of hormones such as cortisol and melatonin.