V

Vaccine: A harmless variant or derivate of a pathogen that stimulates a host’s immune system to mount defenses against the pathogen.
Vacuolar membrane: A membrane that encloses the central vacuole in a plant cell, separating the cytosol from the the vacuolar contents, called cell sap, also known as tonoplast.
Valence: The bonding capacity of an atom, generally equal to the number of unpaired electrons in the atom’s outer most shell.
Valence electron: An electron in the outermost shell.
Valence shell: The outermost energy shell of an atom, containing  the valence electrons involved in the chemical reaction of that atom.
van der Waals interactions: Weak attractions between molecules or parts of molecules that are brought about by localized charge fluctuations.
Variance component: A contribution to the phenotypic variance due to a specific kind of variation. It includes environmental, additive, dominance, and interaction variances.
Variety: A number of different types of things.
Variant: A genetic concept indicating one type (genotype) among many.
Variation: Differences between members of the same species.
Variability: Having much variation or diversity.
vas deferens: The tube in the male reproductive tract where the sperm travels from the epididymis tot he urethra.
Vasa recta: The capillary system that serves the loop of Henle.
Vascular bundle: A strand of vascular tissues (both xylem and phloem) in a stem or leave.
Vascular cambium: A cylinder of meristematic tissue in woody plants that adds layers of secondary vascular tissue called secondary xylem (wood) and secondary phloem.
Vascular cylinder: The central cylinder of vascular tissue in a root.
Vascular plants: Division of plants with vascular tissues, which function in transporting fluids; including all living species except mosses, liverworts, and hornworts.
Vascular tissue: Plant tissue consisting of cells joined into tubes that transport water and nutrients throughout the plant body.
Vascular tissue system: A system formed by xylem and phloem throughout a vascular plant, serving as a transport system for water and nutrients, respectively.
Vasocongestion: The filling of a tissue with blood, caused by increased blood flow through the arteries of that organ.
Vasoconstriction: A decrease in the diameter of superficial blood vessels triggered by nerve signals that contract the muscles of the vessel walls.
Vasodilation: An increase in the diameter of superficial blood vessels triggered by nerve signals that relax the muscles of the vessel walls.
Vegetal pole: The portion of the egg where most yolk is concentrated; opposite to animal pole.
Vegetative reproduction: Cloning of plants by asexual means.
Vein: (1) In animals, a vessel that returns blood to the heart. (2) In plants, a vascular bundle of a leaf.
Ventilation: Any method of increasing contact between the respiratory medium and the respiratory surface.
Ventral: Pertaining to the underside, or bottom, of a bilaterally symmetrical animal.
Ventricle: (1) A heart chamber that pumps blood out of a heart. (2) A space of the vertebrate brain, filled with cerebrospinal fluid.
Venule: A vessel that coveys blood between a capillary bed and a vein.
Vernalization: The use of cold treatment to induce a plant to flower.
Vertebrate: A chordate animal with a backbone; include the mammals, reptiles (including birds), amphibians, sharks and rays, and ray-finned fishes and lobe fins.
Vertical descent: The evolution of species by a branching pattern. Also known as vertical evolution.
Vertical inheritance: The transmission of traits from parent to offspring. Also known as vertical transmission.
Vesicle: A sac made of membrane inside of cells.
Vessel element: A short wide water-conducting cell found in the xylem of most angiosperms and a few non-flowering vascular plants. Dead at maturity, vessel elements are aligned end to end to formed micropipes called vessels.
Vestigial organ:  A structure of marginal, if any, importance to an organism. They are historical remnants of structures that had important functions in ancestors.
Villus (plural villi): (1) A fingerlike projection in the inner surface of the small intestine. (2) A fingerlike projection of the chorion in the mammalian placenta. Large numbers of  villi increases the surface area of these organs.
Viral envelop:  A membrane that cloaks the capsid that in turn encloses  a viral genome.
Viroid: A plant pathogen composed of molecules of naked circular RNA only several hundred nucleotide long.
Virulence: The degree of pathogenicity of a parasite.
Virulent: A term describing a pathogen against which a plant has little specific defense.
Virulent phage: A phage that reproduces only by a lytic cycle.
Visceral mass: One of the three main parts of a mollusc, containing most of the internal organs.
Visible light: That portion of the electromagnetic spectrum detected as various colors by the human eye, ranging in wavelength from about 380 nm to about 750 nm.
Vital capacity: The maximum volume of air that a respiratory system can inhale and exhale.
Vitamin: An organic molecule required in the diet in very small amounts. Vitamins serve primarily as coenzymes or parts of coenzymes.
Vitamin D: One of the fat-soluble vitamins. The active form functions as a hormone, acting in concert with parathyroid hormone in bone and promoting the uptake of calcium from food within the intestines.
Vitreous humor: A jelly-like material that fills the posterior cavity of the vertebrate eye.
Viviparous: Referring to a type of development in which the young are born alive after having being nourished in the uterus by blood from the placenta.
Vocal cord: One of a pair of stringlike tissues in the larynx. Air rushing past the tense vocal cords makes them vibrate, producing sounds.
Voltage-gated ion channel:A specialized ion channel that opens or closes in response to changes in membrane potential.
Vulnerable: Capable or susceptible to being wounded or hurt. Open to assault.