Examples found in pond water are algae and protozoa . Yeast is an example of a single celled fungi. The simplest cells are prokaryotes . They have a circular piece of DNA in a nucleoid region , but not a true nucleus. Examples are bacteria and cyanobacteria or blue-green algae that make up pond scum in August.
More advanced cells are eukaryotes with a nucleus and organelles .
All of the plant and animal cells are eukaryotes . With our microscopes we can see a nucleus in a cheek cell. Sometimes we can see a small dot inside the nucleus that is the nucleolus . The cell membrane encloses cell contents. It's made of a phospholipid bilayer with protein pores and carbohydrate attachments.