Multicellular Definition

When a tissue, organ, or organism consists of many cells, it is said to be multicellular. Animals, plants, and fungi are multicellular organisms, and their cells often specialize for different purposes. 

Single-celled organisms, on the other hand, are much smaller and less complex because they contain just one cell that senses its environment, gathers nutrients, and reproduces asexually.

Examples of Multicellular Organisms

Organs and Tissues

Multicellular organisms delegate biological functions, such as barrier function, circulation, digestion, respiration, and sexual reproduction, to specific organ systems, such as the skin, heart, stomach, lungs, and sex organs. Organs are composed of many different types of cells that work together to perform specific functions. 

As an example, cardiac muscle cells have more mitochondria and produce adenosine triphosphate to power the movement of blood. Although skin cells have fewer mitochondria and do have contractile function, they contain tight barrier junction proteins and produce keratin to protect the soft inner tissues.


Organisms with more than one cell are classified as multicellular organisms. Multicellular organisms, however, haven’t always existed. In the billion years following the formation of the Earth, unicellular organisms appeared on the planet. In fact, unicellular organisms existed alone on Earth for approximately two billion years before multicellular organisms appeared approximately 600 million years ago. 

The majority of unicellular organisms reproduce asexually, while the majority of multicellular organisms reproduce sexually. In the case of human beings, for example, the egg and the sperm, which are specialized for sexual reproduction, merge to form a multicellular organism. 

Zygotes, or fertilized egg cells, are formed when a single egg gamete fuses with a single sperm gamete. Zygotes contain both sperm and egg genetic material. A zygote’s mitotic division leads to the development of all its cells. 

Each cell follows a distinct pathway towards differentiation during development, which involves proliferation and division. Despite being genetically identical, cells can perform very different functions due to differentiation.

A multicellular organism, including its organs, tissues, nerves, skin cells, respiratory epithelium, and cardiac cells, all originated from the zygote formed by merging two single-celled gametes.

Related Biology Terms

  • Tissue – Groups of similar cells with a common origin that are clustered together to perform a specialized function.
  • Gamete – Haploid cells specialized for reproduction that merge with another from the opposite sex at conception to form a diploid zygote.
  • Unicellular – An organism comprised of a single cell.
  • Zygote – A diploid cell formed by the fusion of two haploid gametes from opposite sexes.

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