The structural and functional units of all the living beings are cells. Cells are the building blocks of life. All living organisms are made up of cells, The cells are discovered by Robert Hooke in 1665. The size of the cells is about 10 micrometer. The cells contain cellular organelles that control the activity of the cell. On the number of cells, organisms can be classified as unicellular and multicellular organisms.
Unicellular Organisms:-
Unicellular organisms are known as singled-celled organisms. They are made up of a single cell. Organisms like the Amoeba, Paramecium are single-celled organisms. They are the oldest forms of life. They existed about 3.8 million years ago. Bacteria, protozoa, unicellular algae, and unicellular fungi are the main groups of unicellular organisms. The single cell regulates all the activity of the organism. Most of the Unicellular organisms are invisible to the naked eye.
Multicellular Organisms:-
Organisms that consist of more than one cell are known as multicellular organisms. Multicellular organisms are made up of more than one cell. These cells identify and attach to each other to form a multicellular organism. Most of the multicellular are visible to naked eye. Organisms like plants, animals, and some algae arise from a single cell and they grow up into a multi-celled organism.