Membrane Permeability (College Board AP® Biology): Study Guide

Plasma membranes separate the internal environment of the cell from the external environment

The hydrophobic interior of membranes means that they are selectively permeable

Selective permeability allows cell surface membranes to control which substances can cross between the external and internal environment

• Small, nonpolar molecules, including N₂, O₂ and CO₂ , freely pass across the membrane

  • These molecules can interact with the nonpolar fatty acid tails of the phospholipids and are small enough to pass easily between them

• Large, polar molecules, e.g. glucose, and ions, e.g. Na⁺, cannot pass between the phospholipids

  • The charged ions or polar substances cannot interact with the hydrophobic interior of the membrane

  • Phospholipids are tightly packed together, blocking the passage of larger molecules

• Small, polar, uncharged molecules, e.g. H₂O and NH₃ (ammonia), pass through the membrane in small amounts

  • While these molecules do not easily interact with the hydrophobic interior of the membrane, they are small enough to pass between the phospholipids in small volumes

Large, polar molecules and ions move across the membrane through embedded channels and transport proteins

• This means that the types of these substances that can cross are determined by the number and type of transport proteins that are present

Bacteria, archaea, fungi and plant cells have cell walls in addition to cell membranes

Cell walls have a structural role, providing cells with some rigidity

While cell walls are freely permeable to small molecules, they can also provide a permeability barrier for some substances

• Large molecules may be unable to cross

The rigid nature of cell walls limit the volume of water that can be taken up by a cell, preventing osmotic lysis