Why can't single cells grow very large? Or Big organisms like human beings are multi- cellular?

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Why can't single cells grow very large? Or Big organisms like human beings are multi- cellular? Why can't such big organisms be a single large cell?

Answer: Cell is the fundamental unit of life. All metabolic activities (necessary for life) takes place inside the cell. All raw materials required for these metabolic activities enter through the cell surface
via cell membrane.

Greater the surface are, large amount of raw material can enter (this is the case of uni-cellular micro-organisms). As the size of cell grows larger, the Surface Area to Volume (SA/V) ratio decreases, it means raw material required for the cell to survive will not be sufficient. 

Thus, SA/V ratio limits the size of the cell. Therefore big organisms like human beings are mulch-cellular.

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