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Lecture 23 (Molecular Trafficking (★ affected by polarity (nonpolar…

- - - - bilayer represents an energetic barrier for the transport of polar molecules.
        
        energy barrier must be lowered for the molecules to be able to pass.
        → active transport
        
        transport proteins create "pores" in the membrane through which polar molecules can pass through.
        
        due to the majority of the bilayer being nonpolar.
    - - like dissolves in like ─ transported via nonmediated diffusion
      - more permeable to the membrane.
  - - - proportional to the rate of transport for molecules as it creates a stronger drive for diffusion.
- - - - energy may come from ATP hydrolysis or spontaneous cotransport of another molecule down its concentration gradient.
        
        cotransport:
        movement of two molecules
        by a transporter
        
        symport:
        both molecules move in the same direction
        
        antiport:
        molecules are transported in opposite directions.
- - - - open to both the intracellular and extracellular space.
        
        forms a physical pore within the membrane.
        
        contains polar sidechains involved in directing the movement of the molecules.
      - can open and close spontaneously or be regulated / gated.
      - rate of transport can approach the rate of diffusion: 10⁷ ~ 10⁸ mol/s
        → relatively non-selective and rapid
    - - open to either the intracellular or extracellular space
        
        dependent on the location of target molecule.
      - binding of a molecule induces a conformational change
        
        determines which side the transporter opens to.
      - rate of transport around 10² ~ 10³ mol/s
        → more selective and therefore less rapid
        
        selectivity based upon permeability coefficient ─ bigger PC = more permeable.
      - have several features in common w/ enzymes.