Classification
Membrane Transport Systems
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Lipid-soluble substances (eg, O2, CO2, steroid hormones) cross cell membranes because they can dissolve in the hydrophobic lipid bilayer.
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Water-soluble substances (eg, Na+, Cl−, glucose, H2O) cannot dissolve in the lipid of the membrane but may cross through water-filled channels, or pores, or may be transported by carriers.