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.
Gerunds are distinguished grammatically from other uses of a verb's-ing form: the present participle (which is a non-finite verb form like the gerund, but is adjectival or adverbial in function), and the pure verbal noun or deverbal noun. gerund behaves as a verb within a clause (so that it may be modified by an adverb or have an object); but the resulting clause as a whole (sometimes consisting of only one word, the gerund itself) functions as a noun within the larger sentence.