There is still controversy over this today. When we look at cells through the microscope we can see that many of them are made of smaller parts. With a small microscope we can see that they are surrounded with a cell membrane and have something on the inside, called cytoplasm. Some organisms are made of only one cell. 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 .