Dr. Paul's Virtually Biology Show.

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O

Obligate. Required. For example obligate aerobes are organisms that require oxygen to grow. Obligate anaerobes require the absence of oxygen to grow. Contrast facultative which means can use but not required.

Oncogene. A gene that when defective is known to cause cancer.

Ommatidium(a). The basic unit of the arthropod compound eye. An arthropod eye can consist of thousands of ommatidia.

Omnivorous. Literally eating all; eating a wide variety of foodstuff. Humans are the ultimate omnivores.
Ontogeny.The unfolding of development in an individual organism.
Open circulatory system. A circulatory system where the "blood" is carried part of the time in large vessels but spends some of the time bathing the organs directly rather than flowing through capillaries as would be the case in a closed circulatory system.
Operon. A region of DNA which includes a promotor, operator and structural genes. The promotor is a site for attachment of RNA polymerase, the operator a site for repressor protein attachment, and the structural genes can be transcribed by the RNA polymerase if it can travel from the promotor to those genes for transcription.
Optimum. The value of some variable or set of variables at which some process works best at. Enzymes in our bodies for example have their maximum catalytic ability at a temperature of 37°C. So we also say that the the optimum temperature for these enzymes to function is at 37°C.
Orbital. An orbital is the shape of the probability clouds that electrons are found in at a certain energy level or shell. The way the electrons are added to a shell and its orbitals is determined by rules based on quantum mechanics. For example, each orbital can hold at most two electrons.
Organ. A body structure composed of two or more tissues.
Organelle. A distinct and large scale structure within a cell for carrying out a particular set of functions. Organelles are often surrounded by one or more plasma membranes and sometimes the definition is restricted to just such structures. We term such organelles "membrane bound organelles."
Organic compound. A compound consisting of carbon, hydrogen and often oxygen and other elements. The term originally arose because scientists thought only living things could make organic compounds. We know that organic compounds can be made from non-living chemicals. It should be noted that artificially made organic compounds are not better or worse than those from living things.
Osmoregulation. The ability of organisms to control the concentration of water and solutes in their bodily fluids. Many marine organisms for instance cannot do this since they spend their whole lives at one osmotic concentration. Such organisms are called osmoconformers. Other organisms that spend their lives on land or in fresh and salt water can regulate their internal osmotic concentration. These are called osmoregulators.
Osmosis. The diffusion of water(or some other solvent) across a semipermeable(selectively permeable) membrane.
Osmotic pressure. The pressure that would have to be inserted on the hypertonic side of a selectively permeable membrane to prevent a net flow of water across the membrane from the hypotonic side. This is a measure of a solution's ability to take on water. A really important concept in understanding fluid flow in plants...bcause after all, neither plants nor scarecrows have hearts to circulate fluids.
Ovule. In plants, the structure found in the carpels at the base of the flower that develops into the seed.

Oxaloacetate(Oxaloacetic acid). An organic acid with four carbons that is the starting material for the Kreb's citric acid cycle. Its function is to pick up two energy rich carbons from other parts of metabolism feeding into the Kreb's cycle.

Oxidation. The process by which electrons are removed from a chemical. The chemical is then said to be oxidized while whatever chemical receives the electrons is said to be reduced.
Oxygen. An element with eight protons and eight electrons and with an atomic mass averaging a bit over 16. Oxygen is important because of its strong tendency to grab electrons. Under current conditions, oxygen is a waste product of photosynthesis. In aerobic cellular respiration, oxygen is the final electron Acceptor from the electron transport systems.