Molecules that can diffuse directly through the cell membrane typically share certain characteristics. These include:
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Small Nonpolar Molecules: These include gases like oxygen (O₂) and carbon dioxide (CO₂). Their small size and nonpolar nature allow them to pass freely through the lipid bilayer of the membrane.
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Fatty Acids: Long-chain fatty acids are also nonpolar and can diffuse through the membrane, though larger or charged fatty acids may require transport proteins.
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Lipid-Soluble Molecules: Molecules that are hydrophobic (water-repelling) and can dissolve in the lipid bilayer, such as steroid hormones (like cortisol and testosterone), can pass through the membrane directly.
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Very Small Polar Molecules: While most polar molecules cannot easily diffuse through the membrane, small polar molecules like water (H₂O) can do so to a limited extent, although they often rely on specific channels (aquaporins) for faster movement.
Overall, the ability of a molecule to diffuse directly through the cell membrane is largely determined by its size, polarity, and solubility in lipids.