Here's why:
* Lipid bilayers are hydrophobic: This means they are repelled by water and prefer to interact with other nonpolar molecules.
* Polar molecules are hydrophilic: They are attracted to water and will have difficulty crossing the hydrophobic lipid bilayer.
* Small size: Smaller molecules can more easily slip through the gaps between the phospholipid tails in the bilayer.
Examples of molecules that pass easily:
* Oxygen (O2): Essential for cellular respiration.
* Carbon dioxide (CO2): A waste product of cellular respiration.
* Nitrogen (N2): An abundant gas in the atmosphere.
* Steroid hormones: These are lipids themselves and can easily diffuse across the membrane.
Examples of molecules that have difficulty passing:
* Water (H2O): While small, water is polar and requires special channels (aquaporins) to cross the membrane efficiently.
* Ions (Na+, K+, Cl-): These charged particles are strongly repelled by the hydrophobic interior of the membrane.
* Large polar molecules (glucose, proteins): These are too large and/or too polar to cross the membrane without assistance from transport proteins.
