biological species concept A definition of species that relies on the breeding behavior of populations in nature. Accordingly, groups of populations that actually or potentially interbreed in nature and are reproductively isolated from other such groups constitute a species.
biology The study of living things.
biomass Living material, generally measured by weight.
biome An ecosystem dominated by a large vegetation formation, whose boundaries are largely determined by climate. The same biome type can occur on two different continents and have different species, but the two regions will bear striking similarities.
biosphere The interactive collection of all the world's ecosystems. Also thought of as that portion of the Earth that supports life.
biotechnology The use of living organisms to create products or facilitate processes.
biotic Pertaining to living things.
birth control pill A pill that prevents pregnancy by disrupting the normal hormonal cycle in a way that suppresses development of the follicle and also prevents ovulation. Without ovulation of a mature oocyte, there is no egg in a position to be fertilized.
bivalves A class of molluscs that includes mussels, clams, and oysters, among other organisms.
blade In plants, the major, broad part of a leaf.
blastocoel The fluid-filled cavity in a blastula.
blastocyst Hollow, fluid-filled ball of cells that is formed in the early stages of the embryonic development of humans and other mammals. In non-mamallian animals, the blastocyst is known as the blastula.
blastula A ball of cells with an interior, fluid-filled cavity (the blastocoel), that is produced during animal development by cell rearrangement at the end of the cleavage phase.
blood-brain barrier The covering over tiny blood vessels in the brain, made of neuroglia cells, that prevents some substances from passing from the general circulation into the brain. This barrier helps protect the brain from potentially damaging substances.
B-lymphocyte (B cell) A type of lymphocyte that is central to antibody-mediated immunity. B cells bearing a specific antibody reproduce rapidly, then both produce additional antibody to fight the infection and also retain some cells that remember the antigen so a reinfection can be prevented.
body segmentation A repetition of body parts in an animal. An example can be found in the vertebrae that make up the human vertebral column or backbone.
bone A type of supportive connective tissue that contains mineral deposits, primarily calcium, which, along with flexible collagen fibers, make the tissue both strong and resistant to shattering.
bottleneck effect A change in allele frequencies in a population due to chance following a sharp reduction in the population's size.
Bowman¡¯s capsule A hollow structure in the vertebrate kidney that receives the filtrate from the blood and passes it into the proximal tubule of the nephron. This structure is the point where wastes are transferred from the blood into the kidney.
bryophyte A plant that lacks a vascular (fluid transport) structure. Bryophytes are the most primitive of the four principal varieties of plants. Mosses are the most familiar example.
bud An undeveloped plant shoot, composed mostly of meristematic tissue.
buffering system A solution that helps to keep pH at optimal levels. These solutions are generally weak acids or bases that can work to neutralize sudden infusions of acid or base by accepting or donating hydrogen ions. Buffering systems help living beings function by keeping pH at whatever level is best in different parts of an organism.
bulbourethral gland A gland that, in human males, contributes fluids to the semen as it proceeds down the urethra in the process of ejaculation.
bundle-sheath cells Special cells, wrapped around leaf veins, that receive CO2 from a four-carbon molecule in the C4 pathway. In C4 plants, the CO2 concentration is high and the Calvin cycle takes place in these cells.
