Membrane Permeability Notes

Cell Membrane Structure

• Composed of amphipathic phospholipids, forming a bilayer.
• Described by the fluid mosaic model, including proteins, steroids, and carbohydrates.

Selective Permeability

• A direct consequence of membrane structure.
• Small, nonpolar molecules ($N<em>2$, $O</em>2$, $CO_2$) pass freely.
• Hydrophilic substances (large polar molecules, ions) require transport proteins.
• Small polar molecules (e.g., water) pass in very minimal amounts.

Transport Proteins

• Channel Proteins: Hydrophilic tunnels for specific target molecules.
• Carrier Proteins: Change shape to move target molecules across the membrane.

Cell Walls

• Act as a structural boundary and permeable barrier in some cells.
• Prevent cellular rupture and maintain shape (e.g., plant cells).
• Plant Cell Walls: Composed of cellulose; allow passage of substances through plasmodesmata.
• Fungi Cell Walls: Composed of chitin.
• Prokaryote Cell Walls: Composed of peptidoglycan.
• Animal cells do not have cell walls.