CIE AS Biology (9700) exams from 2022

Revision Notes

4.2.1 Diffusion

Diffusion & Facilitated Diffusion

  • 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
  • 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)
  • 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
  • They are highly specific (they only allow one type of molecule or ion to pass though)

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

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
  • In the diagram below, there is a net diffusion of molecules or ions into the cell down a concentration gradient

Exam Tip

Remember – the movement of molecules from high concentration to low concentration is diffusion. If this movement requires the aid of a protein (for example because the molecule is charged and cannot pass directly through the phospholipid bilayer) this is facilitated diffusion.


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