What are the similarities and differences between unicellular and multicellular organisms?

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mrsq1227 eNotes educator| Certified Educator

Unicellular and multicellular organisms are similar in many ways, but the main difference comes in the form of their cellular makeup. 

Unicellular organisms are made up of only one cell (the prefix "uni" means one). All life processes are carried out in one single cell! Unicellular organisms are often considered simple creatures and are far less complex than most multicellular organisms. Examples of unicellular organisms include bacteria (E. coli, Salmonella, etc.), yeast (Candida), and amoebas.

Multicellular organisms are made up of more than one cell (the prefix "multi" means many). Multicellular organisms are often composed of millions or trillions of cells (in the case of animals and humans) and are considered to be more advanced on an evolutionary scale. 

It is an advantage for an organism to be multicellular rather than unicellular. Multicellular organisms have the ability to grow larger, to be more complex, and to differentiate cells. This means multicellular organisms have different types of cells used for specialized purposes (Ex: nerve cells, muscle cells, brain cells, etc.).

Even though unicellular organisms differ from multicellular organisms in terms of cellular makeup, they do have quite a few things in common. Both unicellular and multicellular organisms share the characteristics of life: they grow, respond to stimuli, maintain homeostasis (an internal balance), reproduce, pass on genetic material to offspring, and obtain or use energy. 

afagnano | Student

Although life can be both big and small, there are still similarities that we can learn from. Unicellular and multicellular organisms show us both the vast differences seen in the natural world, but also bridge the gap with their similarities. In order to be considered alive, an organism must meet certain criteria. They have to be able to reproduce, respond to stimuli, maintain homeostasis, acquire and use energy, grow and develop, and evolve/adapt.

Unicellular organism, or single celled organism, are extremely small. These microscopic beings span across all domains of life (Bacteria, Archaea, and Eukarya). Although they are tiny and much simpler than multicellular organisms, they are still amazing creatures. Everything that they need to sustain life can be found in that one cell. These organisms can be heterotrophic (must eat to gain energy), decomposers, or autotrophic (make their own food) and can also reproduce all on their own (asexual reproduction). 

Multicellular organisms are made up of many cells (thousands to millions). Some cells organize themselves to make different tissues (smooth muscle, nervous tissue, etc.) and different organs(liver, stomach, intestines, etc.) in the bodies of the organisms they make up. Other cells exist as single cells (red and white blood cells) that still work as a unit to aide the body in everyday life. These specialized cells would not be able to exist on their own like their unicellular counterparts, because they rely on the bodies they inhabit to make survival possible. These can also be autotrophic, heterotrophic, or decomposers.

They are similar in the sense that they show organization. Single celled organisms are filled with organelles, which are just like tiny organs. These organelles need to work together in order for the cell to exist. This same type of organization can be seen in multicellular organisms, just on a larger scale. These cells come together into organs that make life possible for the organism. Both types of organisms also meet the criteria of life mentioned above. 

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