Glossary of Scientific Terms

O

Organelle
Subcellular structure in eukaryotic cells (e.g. plants and animals) providing specialized function within cells. Organelles are separated from each other and the cytoplasm of the cell by membranes.

Organic
Compounds that contain carbon, such as vitamins, carbohydrates, proteins and fats, but not minerals. Organic is a chemical term designating compounds containing a carbon skeleton plus hydrogens, oxygen, and in smaller and variable amounts nitrogen, phosphorus, or sulfur.

Organism
The individual member of a species; can be a single cell or a multicellular organism. Organisms are the biological unit of reproduction and while cells of single cell organism are autonomous (bacteria, archaea), individual cells of multicellular organisms (fungi, plants, animals) are not.

Ortholog
The term ortholog is used to indicate an evolutionary related gene existing in two or more different organism. Orthologous genes have a high degree of similarity or sequence identity (see similarity). Orthology is a important way of assessing an organisms evolutionary history. For instance, some two thirds or all human genes have orthologs in the fruit fly Drosophila melanogaster. Humans share 99% of their genes with chimpanzee. Thus the degree of orthology correlates with the evolutionary relatedness between organisms (see also paralogs).