glossary G -17osmoreceptor Sensory receptor that detects changes in
water volume (solute concentration) in the fluid bathing it.
osmosis (oss-moe-sis) [Gk. osmos, act of pushing] The ten-
dency of water to move across a cell membrane in response to
a concentration gradient.
osteoblast A cell that forms bone.
osteoclast A bone cell that breaks down the matrix of bone
tissue.
osteocyte A living bone cell.
osteon A set of thin, concentric layers of compact bone
tissue surrounding a narrow canal carrying blood vessels and
nerves; arrays of osteons make up compact bone.
ovarian cycle Cycle during which a primary oocyte matures
and is ovulated.
ovary (oh-vuh-ree) The primary female reproductive organ,
where eggs form.
oviduct (oh-vih-dukt) Duct through which eggs travel from
the ovary to the uterus; also called fallopian tube.
ovulation (ahv-you-lay-shun) During each turn of the men-
strual cycle, the release of a secondary oocyte (immature egg)
from an ovary.
ovum (oh-vum) (plural: ova) A mature female gamete (egg).
oxidation-reduction reaction An electron transfer from one
atom or molecule to another. Often hydrogen is transferred
along with the electron or electrons.
oxygen debt Lowered O 2 level in blood when muscle cells
have used up more ATP than they have formed by aerobic
respiration.
oxyhemoglobin A hemoglobin molecule that has oxygen
bound to it.
ozone thinning Pronounced seasonal thinning of Earth’s
ozone layer, as in the lower stratosphere above Antarctica.
palate Structure that separates the nasal cavity from the
oral cavity. The bone-reinforced hard palate serves as a hard
surface against which the tongue can press food as it mixes it
with saliva.
pancreas (pan-cree-us) Gland that secretes enzymes and
bicarbonate into the small intestine during digestion, and that
also secretes the hormones insulin and glucagon.
pancreatic islets Any of the two million clusters of endo-
crine cells in the pancreas, including alpha cells, beta cells,
and delta cells.
pandemic A situation in which epidemics of a disease break
out in several countries around the world within a given
time span.
parasympathetic nerve Of the autonomic nervous system,
any of the nerves carrying signals that tend to slow the body
down overall and divert energy to basic tasks; parasympa-
thetic nerves also work continually in opposition with sym-
pathetic nerves to bring about minor adjustments in internal
organs.nuclear envelope A double membrane (two lipid bilayers
and associated proteins) that is the outermost portion of a cell
nucleus.
nucleic acid (noo-clay-ik) A long, single- or double-stranded
chain of four different nucleotides joined at their phosphate
groups. Nucleic acids differ in which nucleotide base follows
the next in the sequence. DNA and RNA are examples.
nucleolus (noo-klee-oh-lus) [L. nucleolus, a little ker-
nel] Within the nucleus of a nondividing cell, a site where the
protein and RNA subunits of ribosomes are assembled.
nucleotide (noo-klee-oh-tide) A small organic compound
having a five-carbon sugar (deoxyribose), nitrogen-containing
base, and phosphate group. Nucleotides are the structural
units of adenosine phosphates, nucleotide coenzymes, and
nucleic acids.
nucleotide sequence The order of nucleotides in a gene; it
codes for a specific polypeptide chain.
nucleus (noo-klee-us) Of atoms, the central core consist-
ing of one or more positively charged protons and (in
all but hydrogen) electrically neutral neutrons. In cells, a
membranous organelle that physically isolates and organizes
the DNA, out of the way of cytoplasmic machinery.
obesity An excess of fat in the body’s adipose tissues, often
caused by imbalances between caloric intake and energy
output.
olfactory receptors Receptors in the nasal epithelium that
detect water-soluble or vaporized substances.
oligosaccharide A carbohydrate consisting of a short chain
of two or more covalently bonded sugar units. One subclass,
disaccharides, has two sugar units. Compare monosaccharide;
polysaccharide.
oncogene (on-coe-jeen) A gene that has the potential to
induce cancerous transformations in a cell.
oocyte An immature egg.
oogenesis (oo-oh-jen-uh-sis) Formation of a female gamete,
from a germ cell to a mature haploid ovum (egg).
opinion A subjective judgment.
organ A body structure of definite form and function that is
composed of more than one kind of tissue.
organelle In cells, an internal, membrane-bounded sac or
compartment that has a specific, metabolic function.
organic compound A compound having a carbon backbone,
often with carbon atoms arranged as a chain or ring structure,
and at least one hydrogen atom.
organ of Corti Region of the inner ear that contains the sen-
sory hair cells involved in hearing.
organ system Two or more organs that interact chemically,
physically, or both in performing a common task.
orgasm The culmination of the sex act that involves muscle
contractions and sensations of warmth, release, and relaxation.
origin The end of a muscle that is attached to the bone that
remains relatively stationary when the muscle contracts.
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