Anyway, as its name suggests, unicellular organisms consist of only one cell. They are also generally smaller and simpler creatures due to this property.
Generally, unicellular organisms belong to the umbrella of prokaryotes or prokaryotes. They are called prokaryotes because they are not as specialized, unlike the more complex eukaryotes. Unicellular organisms and prokaryotes do not have a structure called a nucleus. Also, they are very limited in volume because they cannot handle certain surface area to volume ratios. As a result, single-celled organisms are mostly microscopic. They are so tiny that they cannot be seen with the naked eye.
Apart from having no nucleus, prokaryotes are those cell nuclei that do not have an internal organ body and are covered by an organic envelope called a membrane. They also often inhabit life-threatening habitats, such as very acidic environments and areas full of radiation. Examples of unicellular organisms are bacteria and archaea.
On the other hand, multicellular organisms are organisms that house multiple or more cell types. These organisms are generally larger, have more specialized functions, and are classified as eukaryotes. These organisms are called eukaryotes because they have a nucleus and their DNA is different from the rest of the cell. Due to these facts, they can actually grow to larger sizes; they can carry out more complex activities or functions, and their cells interact harmoniously forever.
While these creatures can grow exponentially to staggering sizes, some of them are also classified as microscopic (Myxozoa). In general, common examples of multicellular organisms are as follows: animals, plants, fungi, humans, and, as mentioned above, a special type of parasitic animal called Myxozoa.
1. Unicellular organisms have one cell whereas multicellular organisms are composed of many different types of cells.
2. Unicellular organisms are mostly prokaryotes whereas multicellular organisms are usually classified as eukaryotes.
3. Unicellular organisms are generally smaller (often always microscopic) and less complex than their more obvious and complex multicellular counterparts.