Glossary of Scientific Terms

P

Paralog
Paralog refers to genes of high similarity within the same organism. Paralogous genes thus are members of a gene or protein family with similar sequence, structure and function (see also ortholog).

Partition coefficient K
The partition coefficient most often refers to the oil-water or air-water partition coefficient expressing the concentration ratio of a solute in a two-phasic system (e.g. oil and water). After thoroughly mixing an oil-water solution with a particular solute (an amino acid, lipid, hormone, gas etc.) the concentration of the solute are measured in each phase after the system comes to rest and the oil (gas) phase is well separated from the water phase. The ratio thus is an expression of the relative solubility of a molecule in oil vs water quantifying its hydrophobicity, or its ability to be soluble in oil. The oil-water partition coefficient of molecules is a good indicator if a molecule can easily diffuse across cell membranes or not. The more soluble it is in oil, the better its permeation across membranes. However, very high oil solubility is also an indicator of small molecules to function as general anesthetics, because they tend to stick in cell membranes rather than diffuse across it, thus altering the composition and physical properties of membranes and some of its proteins. If membrane solubility affects neuronal membranes, a loss of sensation or consciousness can be the result.

Pharmacogenomics
Pharmacogenomics extends the study of pharmacology to modern genetics. Knowing the full genetic complement of the human genome, the development and testing of drugs can be assessed at a global molecular level and can also take into account genetic differences between individuals, e.g., can assess the drug efficacy one patient at a time. Basing pharmacology on genomics thus will eventually allow to match drugs with patients that actually respond well to them, and avoid giving medication to patients that do not respond at all or have side effects.

Pharmacology
Pharmacology is the study of drugs and their interactions with the human body (or test animal). A branch of medicine.

Phenotype
The characteristic of a species or individual of a species that is inherited from generation to generation. Each phenotype is the result of a genotype, i.e., the genetic information stored in DNA. Most phenotypes are morphological, i.e., they describe a particular structure, size, texture, or color of an organism or part of an organism. Examples are the shape and arrangement of leaves of plants, or the legs, wings, and body segmentation of insects. These morphological phenotypes are the bases of most taxonomic classification of an organism, i.e., the organization of the evolutionary relationship among all life on Earth thought to originate from a single ancestral cell type. Phenotypes can also be functional characteristics and can best be thought of as hereditary diseases or metabolic processes (photosynthesis, skin color pigmentation).

Phospholipid
Main lipid component of cell membranes. Phospholipids are a heterogeneous type of molecule composed of glycerol, phosphate, two fatty acid residues, and 'headgroups' with different chemical properties. The organization of phospholipids in cell membranes is known as phospholipid bilayer where the fatty acid residues face the center of the membrane (hydrophobic or water insoluble) and the headgroups forming the surface of the membrane. As such, bilayers separate water filled compartments and provide an electrically insulating barrier between these two compartments. This barrier is overcome by the placement of membrane proteins penetrating the fatty acid core of the membrane and forming channels and transport pathways for metabolites. The latter are usually water soluble and/or charged and would not diffuse across cell membranes without the help of these transport proteins.

Phytochemicals
Plant molecules with beneficial effects for our physiology such as antioxidant properties. They are also known as nutraceuticals or and are found in designer foods.

Polymer
A string of units covalently linked together either in linear form or with branching points. A macromolecule made of two or more units. The units can be identical forming a homo-polymer. Glycogen and starch, the glucose storage particles in animals and plants are examples of homopolymers. Polymers can be made of strings of different units. Proteins are made of 20 different amino acids, DNA and RNA are made of 4 different types of nucleotides. These polymers are called hetero-polymers Many polysaccharides are heteropolymers and found on cell surfaces where they serve as protective layer and receptors.

Polysaccharide
Sugars or carbohydrates made up of more than one sugar unit (monosaccharide). See 'carbohydrates' for more information.

Population
The totality of a closely related number of individual organisms that belong to the same species and live in the same geographical area and interact with each other through sexual (or asexual for bacteria) reproduction.

Prebiotic
Prebiotic refers to the state of matter before life existed, but was conducive to the formation of life (the origin of life). The prebiotic 'soup' is the mixture of organic molecules in bodies of water that are thought to have spontaneous, self-assembly property for the first formation of an organic complex with self-replicating qualities.

Prokaryote
A single-celled form of life without internal membrane bound organelles known as a nucleus, a distinctive sub-cellular structure found in the larger eukaryotic cells. Prokayrotes are thought to be the oldest (primitive) forms of life on earth predating the eukaryotes. Earliest accounts of prokaryotic cell like structure date ancestral forms as anywhere from 2.2 to 3.4 billion years old. Prokaryotes have been found to form two major groups (domains) of life, the archaea and bacteria (eubacteria) that differ in genetic and biochemical structures such as the composition of the cell wall and membrane lipids, as well as chromosome organziation.

Protein
Proteins are macromolecules made from twenty different types of amino acids. Proteins constitute the active component of cells . Proteins function as enzyme in metabolism, transporters and receptors in cell membranes, hormones, antibodies, and help read, translate, and replicate the genetic information. 

Proteome
The proteome is the full set of proteins found at any given time in a cell or organisms. Proteins are the machinery of life and thus knowing all the proteins used by an organism tells about the metabolism and physiology of this organism. The proteome can change during the life cycle of an organisms and often proteins active during development are different from those used in adulthood (see also transcriptome).