Final answer:
Cell membranes control substance movement in and out of cells, including nutrients and waste. Cell size is limited by the efficiency of substance exchange across the surface.
Explanation:
Cell Membranes: Cell membranes play a crucial role in regulating the movement of substances in and out of cells. They are selectively permeable, allowing only certain substances to pass through.
Transport Functions: Nutrients, gases like oxygen, and waste products move across cell membranes. Blood, however, does not directly move in and out of cells as it is contained within blood vessels.
Importance of Cell Size: The need to efficiently exchange substances across the cell's surface places limits on cell size to ensure effective transport processes.
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