1. Phospholipid Bilayer:
* The membrane is primarily composed of phospholipids, which have a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. This creates a barrier where the hydrophobic tails face inwards, forming a selectively permeable membrane.
* Hydrophilic molecules: Small, uncharged polar molecules like water and urea can pass through the membrane, but larger ones have difficulty.
* Hydrophobic molecules: Small, uncharged nonpolar molecules like oxygen, carbon dioxide, and lipids can easily pass through the membrane.
2. Membrane Proteins:
* Transport proteins: These proteins facilitate the passage of specific molecules that cannot cross on their own. There are two main types:
* Channel proteins: Act like tunnels, allowing specific molecules to pass through based on size and charge.
* Carrier proteins: Bind to specific molecules and change shape to transport them across the membrane.
* Receptor proteins: Bind to specific signaling molecules, triggering a response within the cell.
3. Selective Permeability:
* Size: Smaller molecules are generally able to cross the membrane more easily than larger ones.
* Charge: Charged molecules (ions) have difficulty crossing the membrane due to the hydrophobic interior.
* Polarity: Polar molecules (those with uneven charge distribution) face more resistance than nonpolar molecules.
In summary: The cell membrane's phospholipid bilayer structure and embedded proteins work together to create a selectively permeable barrier that allows some molecules to pass through more readily than others. This selectivity is essential for maintaining the cell's internal environment and carrying out its functions.
