Biophysics PHY201-B - Lecture 2: Membrane Transport Mechanisms

Membrane Transport Mechanisms

Facilitated Diffusion or Carrier Mediated Diffusion

• Few molecules can cross membranes by passive (simple) diffusion.
• A pure phospholipid bilayer is permeable to:
  • Gases: CO2, N2, O2, NO
  • Small uncharged polar molecules: ethanol, urea, water
• Membranes are essentially impermeable to ions and large polar molecules.
• Transport of most molecules into and out of cells requires specialized membrane proteins called transporters or carriers. This is called facilitated diffusion or carrier-mediated diffusion.

Membrane Permeability

• Small nonpolar molecules, such as gases (CO2, O2), exhibit fast diffusion.
• Water and alcohol show slower diffusion.
• Small polar molecules diffuse slowly.
• Large nonpolar molecules (e.g., benzene) diffuse slowly.
• Large polar molecules (e.g., glucose) cannot pass.
• Charged molecules (ions like Na+, Cl-) and amino acids cannot pass.

Functions of Proteins in the Cell Membrane

1. Receptors: Proteins act as receptors, informing the cell about the external environment.
2. Transport: Proteins facilitate the transport of molecules into and out of the cell.

Transport Proteins

• All transport proteins are transmembrane proteins.

1. ATP-powered pumps:

   • Transport substances and ions "uphill" against a concentration gradient, electric potential or both.
   • Requires energy, usually from ATP hydrolysis. This type of transport is called active transport.

2. Channel proteins:

   • Transport specific ions and water down their electrochemical gradient.

3. Transporters (Uniporters, Symporters, Antiporters):

   • Move a variety of molecules and ions across the membrane.
   • Uniporters: transport a single type of molecule.
   • Symporters: cotransport proteins
   • Antiporters: catalyze the movement of one molecule against its concentration gradient, driven by movement of one or more ions down an electrochemical gradient.

Secondary Active Transporter Proteins

• Move two molecules at the same time: one against a gradient and the other with its gradient.

  • Symporter: Move a molecule against its gradient while displacing one or more different ions along their gradient. The molecules move in the same direction.
  • Antiporter: (also called exchanger or counter-transporter) move a molecule against its gradient and at the same time displaces one or more ions along their gradient. The molecules move in opposite directions.

Membrane Transport: Diffusion

• The random movement of solute molecules from higher concentration areas to lower concentration areas.
• The rate of diffusion is described by Fick's law:
  • $J = -D \frac{dc}{dx}$
    • Where:
      • $J$ is the rate of movement or flux,
      • $D$ is the diffusion coefficient, and
      • $\frac{dc}{dx}$ is the concentration gradient.

Diffusion Through Membranes

• The net flux $J$ of molecules flowing through the membrane is given by:
  • $J = P (C1 - C2)$
    • Where:
      • $P$ includes the diffusion coefficient and the thickness of the membrane ($Dx$):
        • $P = \frac{D}{Dx}$

Fick’s First Law

• Diffusive flux goes from regions of high concentration to low concentration (C1 to C2).
• The expression is written as:
  • $J = -D \frac{(C2 - C1)}{Dx}$
  • Or
  • $J = -P (C2 - C1)$
• Fick’s first law is usually used to model transport processes.

Facilitated Diffusion

• The transport of solutes across membranes involves processes other than simple diffusion such as facilitated diffusion.
• The major difference is that Fick’s law no longer holds.
• The flux does not continue to increase with increasing solute concentration; instead, the system shows saturation kinetics.