2.1.6 Diffusion & Facilitated Diffusion

• Diffusion is a type of transport that can occur across a cell membrane
  • Note that diffusion can also happen within a cellular compartment; the presence of a membrane is not essential
• It can be defined as the net movement of a substance from a region of its higher concentration to a region of its lower concentration
  • Net movement refers to the overall direction of movement; in reality molecules in a liquid or gas move in all directions, but most molecules move from a high to a low concentration
    • The molecules of a liquid or a gas move randomly due to the kinetic energy of the molecules or ions
• The molecules or ions move down a concentration gradient
• Diffusion is a passive process meaning that it does not require energy in order to occur
  • Diffusion can be described as passive transport
• Diffusion will continue until the concentration of a substance on each side of a membrane has become equal; this is known as equilibrium
• Some molecules are able to diffuse directly between the phospholipids of a membrane; this can be described as simple diffusion
  • Molecules that can move by simple diffusion tend to be small and non-polar
    • Small molecules can fit between the phospholipids
    • Non-polar molecules are able to interact with the non-polar tails of the phospholipids

Diffusion can occur across a cell surface membrane. The image above shows the progression of glucose diffusion; on the left of the diagram there is a steep concentration gradient, and by the end of the process on the right of the diagram the glucose concentration has reached equilibrium.

• The rate at which a substance diffuses across a membrane depends on several factors
  • Concentration gradient
  • Temperature
  • Surface area
  • Properties of substance involved

Factors Affecting Diffusion Table

Facilitated diffusion

• Some substances cannot diffuse directly through the phospholipid bilayer of cell membranes, e.g.
  • Large, polar molecules such as glucose and amino acids
  • Ions such as sodium ions (Na⁺) and chloride ions (Cl^-)
• These substances can only cross the phospholipid bilayer with the help of transport proteins; this form of diffusion is known as facilitated diffusion
• There are two types of transport protein that enable facilitated diffusion
  • Channel proteins
  • Carrier proteins
• They are highly specific, meaning that they only allow one type of molecule or ion to pass through

Channel proteins

• Channel proteins are pores that extend through the membrane from one side to the other
• They allow charged substances, e.g. ions, to diffuse through the cell membrane
• The diffusion of these ions does not occur freely; most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
  • This allows the channel protein to control the exchange of ions

Channel proteins are pores in the cell membrane; they can open and close.

Carrier proteins

• Unlike channel proteins which have a fixed shape, carrier proteins can switch between two shapes
• This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape
• The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane
  • Net diffusion of molecules or ions into or out of a cell via carrier proteins will occur down a concentration gradient

Carrier proteins change shape to transport molecules from one side of a membrane to the other