nucleus
No doubt you've had a sore throat before, and you've probably eaten cheese or yogurt. If so, then you've already encountered the amazing world of prokaryotes. Prokaryotes are single-celled organisms that lack a nucleus. They also lack other membrane-bound organelles. Prokaryotes are tiny. They can only be viewed with a microscope see Figure below . But they are the most numerous organisms on Earth. Without them, the world would be a very different place.