Biology / Bio 2727 · Atom · ~20 seconds
Surface-to-Volume Ratio Limits Size
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As a cell or organism grows larger, its surface-to-volume ratio falls, so diffusion can no longer supply its interior fast enough.
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A single-celled amoeba relies on diffusion alone; a larger body evolves circulatory and respiratory systems instead.
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The limit is diffusion distance, not cell number — a large body works around it through specialized transport.