occipital lobe The lobe of the cerebral cortex located at the rear of the head; is responsible for receiving and processing visual information. PICTURE
oils Triglycerides that are liquid at room temperature.
oncogenes Genes that can activate cell division in cells that normally do not divide or do so only slowly. A gene that when over-expressed leads to cancer, but which normally functions in cell division.
"one gene, one enzyme hypothesis Holds that a single gene controls the production, specificity, and activity of each enzyme in a metabolic pathway. Thus, mutation of such a gene changes the ability of the cell to carry out a particular reaction and disrupts the entire pathway.
"one gene one polypeptide hypothesis" A revision of the one gene, one enzyme hypothesis. Some proteins are composed of different polypeptide chains encoded by separate genes, so the hypothesis now holds that mutation in a gene encoding a specifc polypeptide can alter the ability of the encoded protein to function and thus produce an altered phenotype.
oocyte A cell that will/is undergo/ing development into a female gamete. PICTURE
oogenesis The production of ova. The development of a diploid cell into a haploid ovum or egg cell. PICTURE
open community A community in which the populations have different density peaks and range boundaries and are distributed more or less randomly.
opposable The capability of being placed against the remaining digits of a hand or foot; e.g., the ability of the thumb to touch the tips of the fingers on that hand.
opsins Molecules in cone cells that bind to pigments, creating a complex that is sensitive to light of a given wavelength.
orders Taxonomic subcategories of classes. PICTURE
Ordovician extinction Paleozoic-aged mass extinction possibly related to glaciation in the southern-hemisphere supercontinent Gondwana.
Ordovician Period Geologic period of the Paleozoic Era after the Cambrian Period between 500 and 435 million years ago. Major advances during this period include the bony fish and possibly land plants (during the late Ordovician). PICTURE
organelles Cell components that carry out individual functions; e.g., the cell nucleus and the endoplasmic reticulum. Subcellular structures (usually membrane-bound and unique to eukaryotes) that perform some function, e.g. chloroplast, mitochondrion, nucleus. PICTURE 1 | PICTURE 2
organism An individual, composed of organ systems (if multicellular). Multiple organisms make up a population.
organs Differentiated structures consisting of tissues and performing some specific function in an organism. Structures made of two or more tissues which function as an integrated unit. e.g. the heart, kidneys, liver, stomach.
organ systems Groups of organs that perform related functions.
orgasm Rhythmic muscular contractions of the genitals (sex organs) combined with waves of intense pleasurable sensations; in males, results in the ejaculation of semen.
osmoconformers Marine organisms that have no system of osmoregulation and must change the composition of their body þuids as the composition of the water changes; include invertebrates such as jellyÞsh, scallops, and crabs.
osmoregulation The regulation of the movement of water by osmosis into and out of cells; the maintenance of water balance within the body.
osmoregulators Marine vertebrates whose body þuids have about one-third the solute concentration of seawater; must therefore undergo osmoregulation.
osmosis Diffusion of water molecules across a membrane in response to differences in solute concentration. Water moves from areas of high-water/low-solute concentration to areas of low-water/high-solute concentration. Diffusion of water across a semi-permeable barrier such as a cell membrane, from high water potential (concentration) to lower water potential (concentration).
osmotic pressure Pressure generated by water moving by osmosis into or out of a cell.
ossification The process by which embryonic cartilage is replaced with bone. PICTURE
osteoarthritis A degenerative condition associated with the wearing away of the protective cap of cartilage at the ends of bones. Bone growths or spurs develop, restricting movement and causing pain.
osteoblasts Bone-forming cells. PICTURE
osteoclasts Cells that remove material to form the central cavity in a long bone. PICTURE
osteocytes Bone cells that lay down new bone; found in the concentric layers of compact bone. Bone cell, a type of connective tissue. PICTURE
osteoporosis A disorder in which the mineral portion of bone is lost, making the bone weak and brittle; occurs most commonly in postmenopausal women.
out of Africa hypothesis Holds that modern human populations (Homo sapiens) are all derived from a single speciation event that took place in a restricted region in Africa.
ovaries 1) In animals, the female gonads, which produce eggs (ova) and female sex hormones. PICTURE 2) In þowers, part of the female reproductive structure in the carpel; contain the ovules, where egg development occurs. The lower part of the carpel that contains the ovules within which the female gametophyte develops. PICTURE
overkill The shooting, trapping, or poisoning of certain populations, usually for sport or economic reasons.
oviducts Tubes that connect the ovaries and the uterus; transport sperm to the ova, transport the fertilized ova to the uterus, and serve as the site of fertilization; also called the fallopian tubes or uterine tubes. PICTURE
ovulation The release of the oocyte onto the surface of the ovary; occurs at the midpoint of the ovarian cycle. The release of the ovum (egg) from the ovary after the peaking of luteinizing hormone concentration in the blood during the menstrual cycle. PICTURE 1 | PICTURE 2
ovule In seed plants, a protective structure in which the female gametophyte develops, fertilization occurs, and seeds develop; contained within the ovary. Structures inside the ovary of the flower within which the female gametophyte develops after megasporogenesis has produced a megaspore inside each ovule. PICTURE
ovum The female gamete, egg.
oxidation The loss of electrons from the outer shell of an atom; often accompanied by the transfer of a proton and thus involves the loss of a hydrogen ion. The loss of electrons or hydrogens in a chemical reaction. PICTURE
oxytocin A peptide hormone secreted by the posterior pituitary that stimulates the contraction of the uterus during childbirth.
ozone A triatomic form of oxygen that is formed in the stratosphere when sunlight strikes oxygen atoms. This atmospheric ozone helps Þlter radiation from the sun.
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