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Moment of truth
The Sun, with a diameter of 1,392,000 km, is made of 94% hydrogen atoms.

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poikilotherm Biol.
A cold-blooded animal such as frogs, fishes etc. It usually having a body temperature that varies with the temperature of its surroundings.

polymerase chain reaction Biol.
Commonly known as the PCR, it is an inexpansive and useful technique of making an unlimited number of copies of any piece of DNA. The technique is fast and is important in genetic research. Since only a munite amount of DNA material is required it is also proved to be a useful technique in forensic analysis and DNA fingerprinting. For instance, PCR can be used to determine the source of the finger cells and hair cells left in the crime scene.

polynomial function Math.
A mathematical function of the type

f(x) = a0xn + a1xn-1 + a2xn-2 + ... + an-1x + an

where a0, a1, ..., an are real constants and n is a positive integer. The function is said to be a polynomial function in x of degree n. For example, f(x) = 4x4 + 2x3 - 7x2 - 11x + 8 is a polynomial function of degree 4 with a0 = 4, a1 = 2, a2 = -7, a3 = -11 and a4 = 8.

potash Chem.
A common name group of a number of inorganic potassium compounds, especially the impure form of potassium carbonate. The name derives from English words 'pot' and 'ash' refering to its discovery of white water-soluble residue of wood ash, which consists mainly of potassium carbonate. The name subsequently becomes more widely used that now includes potassium oxide (potash fertilizer) and potassium chloride (muriate of potash) which are used as fertilizers. On the other hand, potassium hydroxide, a strong alkali, is known as caustic potash or potash lye.

potential energy surface Chem.
A hypersurface landscape defined by the potential energy of a collection of atoms over all possible atomic arrangements. Abbreviated as PES, it has 3N-6 coordinate dimensions where N is the number of atoms which must be greater than 2. For 2-atom system, the PES has 1 coordinate dimension because the only degree of freedom is the bond length(or distance between the atoms) which dictates the potential energy of the atoms.

ppm Chem.
Abbreviation for parts per million (1 part in 1 000 000), a unit of concentration used to describe a minute amount of substance. It is often used in toxicology, environment and atmospheric sciences. For example, the Earth's atmosphere contain 52 ppm of helium gas.

pressure Chem., Phys.
The force acting normally on unit area of a surface. Mathematically this is defined as the ratio of force to area. The standard SI unit for pressure is pascal (Pa) or Nm-2.

pressure law Chem.
An ideal gas law which states that for a fixed amount of gas at constant volume (V), the change of pressure is directly proportional to its absolute temperature (T). This can be expressed as

pressure law

Kinetically, the increase in temperature increases the average kinetic energy of the molecules and this leads to an increase in the rate of change of momentum at the walls of the container of fixed volume. This results in an increase in pressure.

prime number Math.
A number that has no factor except itself and one. In other words, it is a number that is divisible by itself or 1. For instance 21 is divisible by 1, 3, 7 and 21 (factors), whereas 23 is a prime number which is only divisible by 1 and 23.

prion Biol., Med.
Short for proteinaceous infectious particle. It is an infecting agent that is made of only flaw proteins and does not involve virus or pathogenic organisms. Prion protein, PrP (encoded from a gene on Chromosome 20), are naturally generated in the human body and normally reside on the membranes of cells. Both primary structures of the cellular form of normal prions (PrPC) and infectious prions (PrPSc) are similar. However, they differ from each other by their tartiary structure.

It is now believed that prion diseases are the cause of certain neurological diseases such as Creutzfeldt-Jakob disease (CJD). This disease and other similar diseases are called transmissible spongiform encephalopathies (TSEs), due to the brain of an infected organism has an appearance of a sponge. The disease are though to propagate due to the ability of PrPSc cluster to convert normal prion and integrated into the cluster.

prokaryote Biol.
Also called procaryote, a primitive form of living organism with no defined nucleus or other membrane-bound organelles. It does not have complex chromosomes and the genetic material is usually consisted of a circular duplex of DNA. Examples are bacteria, blue-green algae, Actinomycetes etc. Opposite to eukaryote.

promoter Chem.
A substance that is added to increase the activity of a catalyst. However, the substance itself is not a catalyst. For instance addition of cesium as promoter that allows lower temperature operations in the Contact process for manufacturing of sulfuric acid.

pronucleus Biol.
The nucleus of an egg or a sperm cell before fertilization. However, pronucleus can also refer to the distinct nuclei (one from the egg and one from the sperm) in a fertilized egg just before they are fused together to make a single nucleus and subsequently the whole cell is divided. The pronucleus stage is useful for the scientist to study animal models and developments. For instance, in the studies of mouse models of human disease, the DNA is injected into the male pronucleus of the fertilized egg of the mouse.

proteasome Biol.
A protein that can digest a variety of proteins into short polypeptides (7-9 amino acids long peptide) and amino acids. The structures of proteasome are generally hollow and has opening at both ends to allow entry of the protein to be digested. Proteasome is usually found in the cytoplasm of living cells, digesting any redundant or damaged proteins synthesized within cells (endogenous proteins).

protein chaperone Biochem.
See chaperone

proteome Biol.
A complete set of proteins that function, and interact within a cell, tissue or whole organism. It is roughly the protein equivalent of genome. Proteomics is the study of proteome that involves identification, characterization and quantification of all proteins including understanding of protein structures at molecular levels, their functions and interactions with other proteins, so as to provide complete and comprehensive data of that living system.

proton Phys., Chem.
A baryon elementary particle which is also the atomic nucleus of the normal hydrogen (1H) atom. It has a positive charge equal in magnitude to that of an electron but is about 1836 times heavier than the electron. Proton is thought to decay with a half-life of 1032 years (much longer than the current estimate age of universe), although this has never been observed, yet.

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