Movement of substance

Diffusion

What is diffusion?

Imagine that you are sitting in the living room reading a book. Your sister accidently spills a bottle of perfume in the bedroom. Gradually you become aware of a pleasant scent spreading through the living room.

QUESTION

• How has the scent of the perfume spread from the bedroom to the living room and into your nose?

All matter, including

perfume, is made up of

particle such as atoms,

molecules and ions.

Individual particles of

perfume evaporate from the surface of

the spills, that is, they become a

gas.

The particles of the gas are constantly

moving. They bump into one

another and bounce about.

This causes the gas to spread outwards

throughout the house, until it

reaches your nose and you smell it. The gas spreads

outwards through a process called

diffusion.

What is concentration gradient and how is it related to diffusion?

• The particles of fluids (liquids and gases) possess movement (kinetic energy). They are continually moving about. As their movement is random, the particles will move (diffuse) down the concentration gradient and become evenly spaced out after some time. Thus, the smell of perfume diffuses throughout the house.

• Diffusion can thus be described as the random movement of particles down a concentration gradient.

• The steeper the concentration gradient for a substance, the faster the rate of diffusion will move.

• This is an important rule: The steeper the concentration gradient for a

substance, the faster the rate of diffusion is for the substance.

Definition of diffusion

• Diffusion is the net movement of particles (atoms, ions or molecules) from a region where they are of higher concentration to a region where they are of lower concentration, that is, down a concentration gradient.

BRIEF RECAP

• The difference in concentration between two regions is known as the concentration gradient.

• Diffusion is the net movement of atoms, molecules or ions down a concentration gradient.

• The steeper the concentration gradient for a substance, the faster the rate of diffusion.

How can we show diffusion of a dissolved substance?

1.Drop a Potassium permanganate (KmNO4) into a gas jar full of water. Cover the jar and allow it to stand for a few days.

2.Observe the changes in the color of the water. Using the idea of a concentration gradient, explain why the purple colour gradually spreads throughout the water in the jar. When does diffusion stops?

Practical demonstration

KMnO4

Practical Demonstration

Practical Demonstration

• As you saw in the clip, the KMnO4 move from a region of high concentration to regions of low concentration in the surrounding water.

• In other words, the solute particles diffuse evenly throughout the liquid, forming a solution.

• Temperature affects the rate of diffusion.

Diffusion across membrane

• Figure 3.4 illustrates the process of diffusion through a permeable membrane. A permeable membrane allows both the solvent (water) and the solutes (the dissolved substance) to pass through it.

• The dissolved particles of copper sulphate and potassium iodide will diffuse across the membrane. Eventually, there will be equal concentration of all particles on both sides of the membrane.

Diffusion across membrane

Partially permeable membrane

• The cell surface membrane is not like the membrane in fig 3.4. It allows some substances through but not others.

• For example, oxygen and carbon dioxide can pass through the cell surface membrane but proteins cannot.

• Diffusion is therefore an important way by which oxygen and carbon dioxide move into and out of cells. Proteins enter in some other way.

• Amoeba gets oxygen by diffusion. Dissolved oxygen from water diffuses through the cell membrane.

• Amoeba also removes the carbon dioxide it produces by diffusion.CO2 diffuses out of the cell surface membrane into the water.

• The cells of our lungs exchange oxygen and carbon dioxide through diffusion.

• Plant cells such as root hair cells also take in oxygen and remove carbon dioxide through diffusion.

Summary

• Solutes diffuse independently of each other in solution.

• Some substances can diffuse across the cell surface membrane

• The following cells exchange oxygen and carbon dioxide through diffusion- amoebae, plant cells and the cells of the human lungs.

Sneak peek on the next lesson