Topic 2.6 - Selective Permeability

Selective Permeability

Plasma membranes are selective, which allows for some substances to pass through (in and out) easier than others.

Those who have easy passage are:

• Small, nonpolar hydrophobic molecules

  • Hydrocarbons

Examples: Carbon dioxide, oxygen, nitrogen

Those who have difficult passage are: (also may require protein assisted passage)

• Hydrophilic molecules

• Polar molecules

• Large molecules

• Ions (b/c they are charged)

  • Have to get through the nonpolar region of the bilayer

Examples: Sugars, water (assisted passage).

There are two main types of transport: passive and active

• Differentiated by whether they need energy or not

Passive transport: Occurs when a solute is moving with its concentration or electrochemical gradient (think of a toll-free bridge)

• EG: Going from higher to lower levels of concentration

  • In passive transport you only go down the gradient

• Involved in diffusion (the import/export of materials)

  • Spontaneous process resulting from the constant motion of particles

    • Moving down the concentration gradient is moving high → low (like moving down a slide)

  • Different rates of diffusion (molecule by molecule basis)

  • Only applies if the molecule can actually go in.

• Osmosis (diffusion of water through a selectively permeable membrane)

  • Water requires help from aquaporins because it’s polar.

  • Maintaining balance is crucial for survival (your cells will explode if there’s too much).

• Facilitated diffusion

  • Diffusion via transport proteins

    • Still passive b/c no ATP gets used, just happens on its own b/c they’re just moving down their concentration gradient

  • Increases rate of diffusion for small ions, water, and carbohydrates

  • Channel and carrier proteins

    • Each protein is specific for substances it can assist.

    • Channel proteins provide a pathway for molecules and ions to pass

      • Hydrophilic

      • Many are gated (closing and opening depending on whether there’s a stimulus)

      • Aquaporins: Specialized channel proteins for water

    • Carrier proteins undergo conformational changes for substances to pass

      • EX: Changing shape to fit the shape of a molecule

      • Still, no ATP is being used here.

      • Helps sugars, nucleosides, and amino acids.