1. The Cell Membrane and Diffusion

2. Imagine… Imagine if you had to live inside a plastic bag.
How long would you survive?
Not long without holes so oxygen could enter. Soon you would need a way to get water and food through the plastic.
You would also need a way of removing wastes, such as urine and carbon dioxide.
In some ways the cell membrane is like the plastic bag but much more complex.

3. The Cell Membrane
The cell membrane controls what enters or exits the cell.
The cell membrane is called a selectively permeable membrane because only certain particles can pass through it.
Permeable means “let through.” Impermeable means “not let through.” The cell membrane is permeable to some materials and impermeable to others.

4. The Cell Membrane
The cell membrane is made of two layers of fat particles.
Proteins are fixed into these layers. Some of these proteins form pathways, called channels, that open and close to let different particles pass through.
Particles that easily pass through the membrane include water, oxygen and carbon dioxide.
Particles that do not easily pass through the membrane (because they are too large) include sugars and fats.

5. The Cell Membrane

6. Fluid Movement in Cells: Diffusion
When you place a blob of ink in water the ink gradually spreads out and colours the whole beaker of water.
At first, the concentration of particles is highest where the ink blob first dropped.
Then, the particles spread out through the container.
When the ink particles are released into the water, they collide with the water particles. The ink and water particles move and bounce off each other.
The particles spread out as they move.
They move until there is an equal concentration throughout.

7. Diffusion
Particles move from where they are highly concentrated (many in one place) to where they are less concentrated (few in one place).
This process is called diffusion.
When two areas have different concentrations of a certain particle, we say that there is a concentration gradient.
Diffusion can happen only when there is a concentration gradient.

8. Diffusion Diffusion is important to cells.
Many materials a cell needs enter the cell by diffusion. Some wastes leave the cell by diffusion.
In your body, blood carries oxygen to cells and carries carbon dioxide waste away from cells through diffusion.
Oxygen moves from red blood cells where it is highly concentrated, to other body cells where it is less concentrated.
Carbon dioxide moves out of cells where it is highly concentrated, into the blood where it is less concentrated.