Diffusion
Applied Science: BTEC Level 3 National Extended Diploma, topic 3.4.1
Simple Diffusion
Diffusion is the net movement of particles from an area of high concentration to an area of lower concentration.
Diffusion
- Diffusion describes the passive movement of particles in fluids (liquids and gases).
- Molecules move randomly but tend to move into any space available until it is evenly distributed.
- So, the net movement of particles will be towards the area of lower concentration or down a concentration gradient.
- Substances such as CO2, O2 and urea can move in and out of cells across cell membranes via diffusion.
E.g. Perfume
- When you spray perfume, the smell diffuses across the room from the area of high concentration (where you just sprayed it) to the area of low concentration (the other side of the room).
Factors Affecting Diffusion
The rate of diffusion can speed up or slow down based on the following factors:
Concentration gradient
- The concentration gradient is linked to the difference in concentration between two areas.
- The bigger the difference in concentration between two areas, the greater the concentration gradient and the faster the rate of diffusion.
Temperature
- The higher the temperature, the faster the rate of diffusion.
Membrane surface area
- The larger the surface area of the membrane that a substance is diffusing through (e.g. the membrane around a cell), the faster the rate of diffusion.
Thickness of membrane
- The thicker the exchange surface, the slower the rate of diffusion.
Channel or carrier proteins
- The higher the concentration gradient, the faster the rate of facilitated diffusion until an equilibrium is reached and the rate will level off.
- The number of channel or carrier proteins affects the rate of facilitated diffusion. The more proteins, the faster the rate.
Size and shape of molecules
- Small molecules diffuse faster than larger molecules at a given temperature.
- This is because more energy is required to move a larger molecule than a smaller molecule.
- The shape of molecules can also impacts the speed of diffusion.
- For example, rounded shapes move more quickly through liquid.
Test yourself on Diffusion
What do we call the net movement of particles from an area of high concentration to an area of lower concentration?
In which state of substance does diffusion NOT happen?
Which of the following substances can diffuse across cell membranes?
Which of the following does NOT affect the rate of diffusion across membranes?
What would happen to the rate of diffusion if the concentration got steeper and the temperature rose?