Chloroplasts have circular DNA, but would it be right to say that they have plasmids? Are plasmids and circular DNA even the same thing? Thank you in advance.
A plasmid is a length of circular or linear double stranded DNA that exists independent of chromosomal DNA with a cell (i.e. extra-chromosomal DNA) and often confers a selective advantage to an organism such as antibiotic resistance. Plasmids have been identified in archaea, prokaryotes, and eukaryotes and allow horizontal gene transfer within a population, in processes such as conjugation, which proliferates the selective advantage contained within, if any exists.
Bacteria contain genomic DNA in chromosomes while a plasmid in the same bacteria contains non-genomic DNA such as genes that provide AR, virulence, pathogenicity, etc.. Bacteria may survive without plasmids depending on environment but without the chromosomal DNA survival is not possible.
Circular plasmids are often utilized in genetic engineering to transfect a target species with a desired gene for various reasons such as to enhance the performance of the organism, to understand the function of the gene, etc..
Not all plasmids are circular but all plasmids contain extra-chromosomal DNA. Chloroplast DNA is contained within a chromosome and is not a plasmid. Symbiotic theory of eukaryotic origin asserts that chloroplasts were once independent bacteria and this will perhaps allow you to develop an intuition as to why chloroplasts DNA is chromosomal rather than a plasmid. The chloroplast could not survive without the DNA contained within the chloroplast chromosome so it is not categorized as a plasmid.