Classification of living organisms based on the number of cells - SS1 Biology Lesson Note

Living organisms can be classified into two broad categories based on the number of cells they possess: unicellular and multicellular organisms.

1.     Unicellular Organisms: These organisms consist of a single cell as their entire body. They can carry out all the necessary life functions within that single cell. Examples of unicellular organisms include bacteria, archaea, protozoa, and some types of algae. Despite being composed of only one cell, these organisms exhibit remarkable diversity and can perform complex processes such as metabolism, reproduction, and response to stimuli.

2.     Multicellular Organisms: These organisms are composed of multiple cells organized into specialized structures and systems. They exhibit a higher level of complexity and division of labor among different cell types. Multicellular organisms range from relatively simple forms like sponges and hydra to highly complex organisms like plants, animals, and fungi.