2.4 Membrane Permeability

Explain how the structure of biological membranes influences selective permeability.

• Plasma membranes separate the internal environment of the cell from the external environment. Selective permeability is the result of the plasma membrane having a hydrophobic interior.
• Small nonpolar molecules, including N , O2 2 , and CO2 , freely pass across the membrane. Hydrophilic substances, such as large polar molecules and ions, move across the membrane through embedded channels and transport proteins.
• The nonpolar hydrocarbon tails of phospholipids prevent the movement of ions and polar molecules across the membrane. Small polar, uncharged molecules, like HO2 or NH3 (ammonia), pass through the membrane in small amounts.

Describe the role of the cell wall in maintaining cell structure and function.

• Cell walls of Bacteria, Archaea, Fungi, and plants provide a structural boundary as well as a permeability barrier for some substances to the internal or external cellular environments and protection from osmotic lysis.