Modeling Diffusion, Facilitated
Diffusion, and Active Transport
Learning Objective
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.
Pre-assessment
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.
Let’s get started!
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)
Passive Diffusion
 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)
Modeling Osmosis
• 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
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 is a red blood cell
Modeling Facilitated Diffusion
• 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
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
Modeling Active Transport
• 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!
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
Passive Transport vs Active Transport