Membrane Permeability

The plasma membrane is selectively permeable.This means it selectively prevents or allows the passage of different molecules across the membrane.Lipid soluble molecules such as steroid hormones can dissolve in the bilayer and pass through the membrane.Small polar molecules and ions such as sodium, potassium or chloride ions must pass through hydrophilic channels in the membrane by diffusion or through carrier proteins by facilitated diffusion or active transport.Small non-polar molecules such as oxygen and carbon dioxide can pass freely through the phospholipid bilayer by diffusion.Water molecules can pass through the membrane by diffusion. Although water is polar it is a very small molecule.Large molecules such as proteins cannot pass through the membrane. They may be imported into the cell by endocytosis or exported out of the cell by exocytosis.