Syllabus Edition

First teaching 2023

First exams 2025

|

Diffusion (CIE AS Biology)

Revision Note

Test yourself
Phil

Author

Phil

Last updated

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

Simple Diffusion Across a Cell Membrane Diagram

Simple diffusion across the cell membrane

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

Factor How the factor affects the rate of diffusion
'Steepness' of the concentration gradient
  • This is the difference in the concentrations of the substance of both sides of the surface
  • If there are more molecules on one side of a membrane than on the other, at any one moment more molecules will move randomly across the membrane from that side than from the other side
  • A greater difference in concentration means a greater difference in the number of molecules passing in the two directions and therefore a faster rate of diffusion
Temperature
  • Molecules and ions have more kinetic energy at higher temperatures
  • They move faster, resulting in a higher rate of diffusion
Surface area
  • The greater the surface area across which diffusion is taking place, the greater number of molecules or ions that can cross it at any one moment and therefore, faster diffusion occurs
  • The surface area of cell membranes can be increased by folding (eg. microvilli in an intestinal epithelium cell or cristae in a mitochondrion)
  • As a cell increases in size, the surface area to volume ratio decreases, which slows down the rate that substances can diffuse through a cell as the distance required to move becomes too great
Properties of molecules or ions
  • Large molecules diffuse more slowly than smaller ones as they require more energy to move
  • Uncharged and non-polar molecules diffuse directly across the phospholipid bilayer
  • Non-polar molecules diffuse more quickly than polar ones because they are soluble in the non-polar central layer of the phospholipid bilayer

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

Channel proteins

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

Carrier protein in facilitated diffusion

A carrier protein changing shape during facilitated diffusion

Examiner 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.

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Phil

Author: Phil

Expertise: Biology

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.