Modeling Diffusion, Facilitated Diffusion, and Active Transport

Biology in Action

Students will be able to explain the role of cell membranes as a highly

selective barrier by modeling the process of diffusion,

osmosis, facilitated diffusion, and active transport and collaborative discussion.

Learning Objective

1. Why might you compare the cell membrane to the gates of a stadium?

2. How does the cell membrane allow only some molecules to pass through?

3. What does it mean to stay that a cell is in equilibrium?

4. Describe and give an example of passive diffusion?

5. How does water cross the cell membrane?6. Why do some molecules like sodium and

potassium require energy to cross the cell membrane?

7. Discuss 2 reasons why semipermeability is important to a cell.

Pre-assessment

Modeling Passive diffusion

• Create a cell membrane from yarn (or other provided material) to create a semipermeable boundary• Set up your model by showing oxygen molecules as shown on either side of the membrane•Move molecules until they reach equilibrium (in equilibrium the number of molecules inside the cell should be the same as molecules outside the cell)

Let’s get started!

Molecules are moving not just across the membrane but also in and out of the cell

Molecules move from an area of high concentration to low concentration

No assistance is needed (no protein carrier or ATP) because the molecules are small enough and are nonpolar (non charged)

Passive Diffusion

• Set up your model cell with sodium and water molecules as shown on either side of the membrane

• Demonstrate osmosis by moving the water molecules in your model until they reach equilibrium

Modeling Osmosis

• Diffusion (movement of molecules from high concentration to low concentration) of water• Water (polar molecule) moves

through the phospholipid bilayer because of its small size without assistance

Osmosis

Osmosis is a red blood cell

• Set up your model with the addition of a channel protein in the membrane and glucose molecules as shown (channel proteins are embedded within the membrane)

• Move glucose molecules through protein channel until they reach equilibrium

Modeling Facilitated Diffusion

• A form of passive transport (no energy required) that allows molecules that are too large to pass through the membrane to pass through with the help (facilitation) of a channel protein.• Movement is still from an area of

high concentration to low concentration• Glucose is an example of a

molecule that uses facilitated diffusion

Facilitated Diffusion

• Set up your model by showing sodium, water, and ATP as shown on either side of the membrane

• Move the sodium molecules across the membrane using the channel protein (in active transport, molecules require energy in the form of ATP to cross the channel)

Note: 1 ATP can move 3 sodium molecules!

Modeling Active Transport

• The use of ATP to move molecules through a channel protein to cross a membrane• Movement of molecules is from an

area of low concentration to an area of high concentration• Ions (charged particles) like sodium

and potassium use active transport to move in or out of a cell

Active Transport

Passive Transport vs Active Transport