A plasmid that is attached to the cell membrane or integrated into the bacterial chromosome is called an episome (q.v.). Plasmids are extremely valuable tools in the fields of molecular biology and genetics, specifically in the area of genetic engineering (q.v.).
A gerund is a form of a verb used as a noun, whereas a participle is a form of verb used as an adjective or as a verb in conjunction with an auxiliary verb. In English, the present participle has the same form as the gerund, and the difference is in how they are used. When used with an auxiliary verb (is walking), it serves as a verb and is the present participle. When used as an adjective (a walking contradiction) it is also a participle.
Both gerunds and infinitives can be nouns, which means they can do just about anything that a noun can do. Although they name things, like other nouns, they normally name activities rather than people or objects.