The "Rule" - The Lipid Bilayer:
* The cell membrane's core is made of a lipid bilayer. This is essentially a double layer of phospholipids, with their hydrophobic (water-fearing) tails facing inwards and their hydrophilic (water-loving) heads facing outwards.
* Hydrophobic molecules can easily pass through the membrane's core. This is because they are repelled by the water on either side of the membrane and attracted to the hydrophobic tails of the phospholipids.
Molecules that Can Pass Through Unassisted:
* Small, nonpolar molecules:
* Oxygen (O2): Essential for cellular respiration.
* Carbon dioxide (CO2): A waste product of cellular respiration.
* Nitrogen (N2): Not used by most cells, but can diffuse across.
* Steroid hormones: These are lipid-based, so they can readily cross the membrane.
* Small, uncharged polar molecules:
* Water (H2O): While polar, water is small enough to slip through the gaps in the membrane. This is a very important process, called osmosis.
* Ethanol (C2H5OH): This alcohol molecule is small and slightly polar.
* Urea (NH2)2CO: A small, uncharged molecule involved in waste excretion.
Molecules that Cannot Pass Through Unassisted:
* Large molecules: Proteins, carbohydrates, and nucleic acids are too big to fit through the membrane's gaps.
* Charged molecules: Ions (like sodium, potassium, calcium) and most polar molecules (like sugars and amino acids) are repelled by the hydrophobic interior of the membrane.
Facilitated Diffusion:
* To move large or charged molecules across the membrane, cells use transport proteins. These proteins act as channels or carriers to help these molecules pass through the membrane. This process is called facilitated diffusion.
Key Point: The ability of a molecule to pass through the cell membrane unassisted depends on its size, polarity, and charge. Smaller, nonpolar molecules can pass through easily, while larger, polar, and charged molecules need help from transport proteins.
