Diffusion

What is diffusion?
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.

Is diffusion a spontaneous process (no energy required)? Well, the answer is yes.
What are the factors affecting diffusion rate? One of the factors is temperature.

Diffusion is a spontaneous process (no input of energy required). This explains the below observations:
- Substances tend to spread from an area where they are more concentrated to an area where they are less concentrated
- Two or more substances can become evenly distributed (reach equilibrium) even without external interventions.

Concentration Gradient

First of all, what is concentration?
- Concentration = amount of substance (mass g/kg) / volume of fluid (cm3/ml)
- It is a measure of the amount of a substance in a specific volume.

The change in concentration between 2 regions is known as the concentration gradient.

In other words, the the concentration gradient between points A and B is difference in concentration between points A and B.

Relationship between concentration gradient and diffusion
- Particles diffuse down the concentration gradient.
- The larger the concentration gradient, the faster the rate of diffusion.

Applications of diffusion in Biology

This is a picture of an amoeba, a unicellular organism.

To stay alive, this organism needs to obtain nutrients and remove waste efficiently by diffusion.

"Chemical substances must be able to move from one place to another in order to keep the living organisms alive and growing."
For example, food substance that were absorbed need to :
- Move from one cell to another
- Move in & out of the cell
- Move from one part of the cell to another
One way which these movements would occur is by diffusion.
eg. Substance have to diffuse across membranes.

There are 2 types of membranes:
- Permeable membrane which allows ALL substances to pass through
- Partially permeable membrane which allows SOME substances to pass through; small particles can pass through, large particles cannot pass through

Diffusion across a permeable membrane :

Diffusion across a partially permeable membrane :

The visking tubing encloses a solution of starch while the beaker contains iodine solution.

Starch reacts with iodine to form a dark blue complex.



What type of membrane is the visking tubing? Explain. 
Partially permeable membrane. Iodine solution can enter the visking tubing while starch cannot pass through the visking tubing into the beaker of iodine solution.


Other example of diffusion in Biology include :
- Movement of carbon dioxide during photosynthesis
- Movement of oxygen and carbon dioxide in animals

Conclusion
- Diffusion is an important process where substances are moved without use of energy.
- It is the NET MOVEMENT of particles 9or molecules; or ions) from a region of HIGHER concentration to a region of LOWER concentration.
- Thus the movement is down a concentration gradient.
- The movement is random.
- The GREATER the concentration gradient, the FASTER the rate of diffusion.

Here's a quiz for you to try out!

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html