Among unicellular eukaryotes, cell sizes differ greatly. Amoeba and Paramecium organisms are animal-like protists that are heterotrophic, have no cell wall, and are several times larger than most human cells. What might be some reasons why these unicellular organisms have larger cells than cells with similar traits (heterotrophic, lacking cell walls) that are found in multicellular organisms?

Such unicellular organisms have large cells to carry out all essential physiological functions like obtaining food, reproducing, movement, and others.

Unicellular organisms have different sizes generally have larger cell than multicellular organisms such as animal like protists. Animal like protists are large single-celled organisms, known as Protozoa that include Amoeba and Paramecium.

Thus, these cells have to perform all the physiological processes so required relatively bigger cell to function properly.

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the answer would be..

b. sucking