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What's the function of cell membrane?
How does the cell membrane regulate what enters and leaves the cell?
The cell membrane has a selective permeability, meaning only certain substances can pass through. It allows small, uncharged molecules like oxygen and carbon dioxide to diffuse across, while larger molecules like proteins and sugars require transporters or channels to help them cross. The membrane also pumps certain molecules against their concentration gradient, which requires energy in the form of ATP.