Concept 5.2: Membrane structure results in selective permeability

Introduction

• The biological membrane is an example of a supramolecular structure

  • Supramolecular structure - many molecules ordered into a higher level of organization

• Steady traffic of small molecules and ions move across the plasma membrane in both directions

  • Sugars, amino acids, and other nutrients enter the cell, and metabolic waste products leave it

    • The cell takes in oxygen for use in cellular respiration and expels carbon dioxide

    • The cell regulates its concentration of inorganic ions, such as Na+, K+, Ca2+, and Cl- by shuttling them one way or another in the plasma membrane

• Cell membranes are selective in their permeability

  • Substances do not cross the barrier randomly

  • The cell is able to take up some small molecules and ions and exclude others

The Permeability of the Lipid Bilayer

• Nonpolar molecules are hydrophobic

  • They can dissolve in the lipid bilayer of the membrane and cross through it easily without the help of membrane proteins

• The hydrophobic interior of the membrane slows down the flow of ions and polar molecules (hydrophilic) through the membrane

  • Polar molecules (glucose and other sugars) pass only slowly through a lipid bilayer

  • Water, a polar molecule, does not cross rapidly 

    • A charged atom or molecule is even less likely to penetrate the hydrophobic interior of the membrane 

• The lipid bilayer is only one aspect of the cell’s selective permeability

  • Proteins built into the membrane play key roles in regulating transport

Transport Proteins

• Specific ions and a variety of polar molecules can’t move through cell membranes on their own

  • These hydrophilic substances can avoid contact with lipid bilayer by passing through transport proteins

    • Transport proteins - a transmembrane protein that helps a certain substance or class of closely related substances to cross the membrane

      • A transport protein is specific for the substance it moves, allowing only a certain substance to cross the membrane

  • Some transport proteins called channel proteins function by having a hydrophilic channel that certain molecules or atomic ions use as a “tunnel” through the membrane

    • Aquaporin - a channel protein in the plasma membrane that specifically facilitates osmosis, the diffusion of free water across the membrane

    • Most aquaporin proteins consist of 4 identical polypeptide subunits

      • Each polypeptide forms a channel that allows single-file passage of up to 3 billion water molecules per second

    • Carrier proteins hold onto their passengers and change shape in a way that shuttles them across the membrane