Diffusion (CIE A Level Biology)

• Diffusion is a type of transportation that occurs across the cell membrane
• It can be defined as:

The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration.

• The molecules or ions move down a concentration gradient
• The random movement is caused by the natural kinetic energy of the molecules or ions

Simple Diffusion Across a Cell Membrane Diagram

Diffusion across the cell membrane

• As a result of diffusion, molecules or ions tend to reach an equilibrium situation (given sufficient time), where they are evenly spread within a given volume of space
• The rate at which a substance diffuses across a membrane depends on several factors:

Diffusion Factors Table

Facilitated diffusion

• Certain substances cannot diffuse through the phospholipid bilayer of cell membranes. These include:
  • Large polar molecules such as glucose and amino acids
  • Ions such as sodium ions (Na⁺) and chloride ions (Cl^-)

• Because most substances in biology are water-soluble, facilitated diffusion is required by a large majority of molecules that diffuse across membranes
• These substances can only cross the phospholipid bilayer with the help of certain proteins
• This form of diffusion is known as facilitated diffusion
• There are two types of proteins that enable facilitated diffusion:
  • Channel proteins
  • Carrier proteins

• Channel and carrier proteins are highly specific (they only allow one type of molecule or ion to pass through)

Channel proteins

• Channel proteins are water-filled pores
• They allow charged substances (eg. 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 Protein Diagram

A channel protein (open and closed)

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 concentrations on each side of the membrane
• Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)

Carrier Protein Diagram

A carrier protein changing shape during facilitated diffusion