obligate aerobe

(ob-lig-it air-obe)
An organism that requires oxygen for cellular respiration and cannot live without it.

obligate anaerobe

(an-ur-obe)
An organism that cannot use oxygen and is poisoned by it.

oceanic zone

The region of water lying over deep areas beyond the continental shelf.

oligotrophic lake

A nutrient-poor, clear, deep lake with minimum phytoplankton.

omnivore

[L. omnis, all + vorare, to devour]
A heterotrophic animal that consumes both meat and plant material.

oncogene

(on-koh-jeen) [Gk. onkos, tumor + genos, birth, race]
A gene found in viruses or as part of the normal genome that is involved in triggering cancerous characteristics.

ontogeny

(on-toj-en-ee)
The embryonic development of an organism.

oocyte

(o-uh-sight) [Gk. oion, egg + kytos, vessel]
A cell that gives rise by meiosis to an ovum.

oogamy

(oh-og-um-ee)
A condition in which male and female gametes differ, such that a small, flagellated sperm fertilizes a large, nonmotile egg.

oogenesis

(oo-oh-jen-eh-sis)
The process in the ovary that results in the production of female gametes.

open circulatory system

An arrangement of internal transport in which blood bathes the organs directly and there is no distinction between blood and interstitial fluid.

operant conditioning

A type of associative learning that directly affects behavior in a natural context; also called trial-and-error learning.

operator

A segment of DNA that interacts with a repressor protein to regulate the transcription of the structural genes of an operon.

operon

(op-ur-on) [L. opus, operis, work]
A unit of genetic function common in bacteria and phages, consisting of coordinately regulated clusters of genes with related functions.

opportunistic species

Species characterized by high reproduction rates, rapid development, early reproduction, small body size, and uncertain adult survival.

opsonization

An immune response in which the binding of antibodies to the surface of a microbe facilitates phagocytosis of the microbe by a macrophage.

orbital

[L. orbis, circle, disk]
In the current model of atomic structure, the volume of space surrounding the atomic nucleus in which an electron will be found 90 percent of the time.

order

A taxonomic grouping of related, similar families; the category below class and above family.

organ

[Gk. organon, tool]
A specialized center of body function composed of several different types of tissues.

organ-identity gene

A plant gene in which a mutation causes a floral organ to develop in the wrong location.

organ of Corti

The actual hearing organ of the vertebrate ear, located in the floor of the cochlear canal in the inner ear; contains the receptor cells (hair cells) of the ear.

organelle

(or-guh-nel) [Gk. organon, instrument, tool]
One of several formed bodies with a specialized function, suspended in the cytoplasm and found in eukaryotic cells.

organic

[Gk. organon, instrument, tool]
Pertaining to (1) organisms or living things generally, or (2) compounds formed by living organisms, or (3) the chemistry of compounds containing carbon.

organic chemistry

The study of carbon compounds (organic compounds).

organic compound

A chemical comound containing the element carbon and usually synthesized by cells.

organism

An individual living thing, such as a bacterium, fungus, protist, plant or animal.

organogenesis

(or-gan-oh-jen-eh-sis) [Gk. organon, instrument, tool + genos, origin, descent]
An early period of rapid embryonic development in which the organs take form from the primary germ layers.

orgasm

Rhythmic, involuntary contractions of certain reproductive structures in both sexes during the human sexual response cycle.

origin of replication

A specific sequence of bases in a nucleic acid molecule to which the enzymes responsible for replicating the nucleic acid bind to initiate the copying process.

osmoconformer

An animal that does not actively adjust its internal osmolarity because it is isotonic with its environment.

osmolarity

(oz-moh-lar-eh-tee)
Solute concentration expressed as molarity.

osmoregulation

Adaptations to control the water balance in organisms living in hypertonic, hypotonic, or terrestrial environments.

osmoregulator

An animal whose body fluids have a different osmolarity than the environment, and that must either discharge excess water if it lives in a hypotonic environment or take in water if it inhabits a hypertonic environment.

osmosis

(oz-moh-sis) [Gk. osmos, impulse, thrust]
The diffusion of water across a selectively permeable membrane.

osmotic potential

(oz-mot-ik) [Gk. osmos, impulse, thrust]
The tendency of water to move across a selectively permeable membrane into a solution; it is determined by measuring the pressure required to stop the osmotic movement of water into the solution.

osmotic pressure

A measure of the tendency of a solution to take up water when separated from pure water by a selectively permeable membrane.

Osteichthyes

The vertebrate class of bony fishes, characterized by a skeleton reinforced by calcium phosphate; the most abundant and diverse vertebrates.

ostracoderm

(os-trak-uh-durm)
An extinct agnathan; a fishlike creature encased in an armor of bony plates.

outgroup

A species or group of species that is closely related to the group of species being studied, but clearly not as closely related as any study-group members are to each other.

ovarian cycle

(oh-vair-ee-un)
The cyclic recurrence of the follicular phase, ovulation, and the luteal phase in the mammalian ovary, regulated by hormones.

ovarian follicle

[L. ovum, egg + folliculus, small ball]
A developing oocyte and the specialized cells surrounding it; located near the surface of the ovary; following ovulation, forms the corpus luteum.

ovary

(oh-vur-ee) [L. ovum, egg]
1) In flowers, the portion of a carpel in which the egg-containing ovules develop. (2) In animals, the structure that produces female gametes and reproductive hormones.

oviduct

[L. ovum, egg + ductus, duct]
A tube passing from the ovary to the vagina in invertebrates or to the uterus in vertebrates.

oviparous

(oh-vip-ur-us)
Referring to a type of development in which young hatch from eggs laid outside the mother's body.

ovoviviparous

(oh-voh-vy-vip-ur-us)
Referring to a type of development in which young hatch from eggs that are retained in the mother's uterus.

ovulation

The release of an egg from ovaries. In humans, an ovarian follicle releases an egg during each menstrual cycle.

ovule

(ov-yool) [L. dim. of ovum, egg]
A structure that develops in the plant ovary and contains the female gametophyte.

ovum pl. ova

(oh-vum) [L. egg]
The female gamete; the haploid, unfertilized egg, which is usually a relatively large, nonmotile cell.

oxidation

The loss of electrons from a substance involved in a redox reaction.

oxidative phosphorylation

(fos-for-eh-lay-shun)
The production of ATP using energy derived from the redox reactions of an electron transport chain.

oxidizing agent

The electron acceptor in a redox reaction.

oxygen debt

In muscle, the cumulative deficit of oxygen that develops during strenuous exercise when the supply of oxygen is inadequate for the demand; ATP is produced anaerobically by glycolysis, and the resulting pyruvic acid is converted to lactic acid, which is subsequently metabolized when adequate oxygen is available.