Lesson 3.4 Passive and active transport
3.4.2 Facilitated diffusion (also referred to as facilitated transport)
Recall that a cell membrane has several proteins embedded within it. Some of these proteins are involved in transporting materials. When a substance needs to use such a protein to enter or leave the cell, and no energy is required, the mode of transportation of that substance is called facilitated diffusion.
An example of facilitated diffusion is the movement of water across a cell membrane, also known as osmosis. Within the cell membrane is a type of protein referred to as an aquaporin. Its sole purpose is to help with the movement of water. No other molecule will be able to enter or leave through this protein because it is specifically designed for the water molecule. Note that the term “osmosis” is used only to describe the facilitated diffusion of water.
Activity:
- On a half-sheet of paper, list what you know about osmosis.
- Find a partner and share your thoughts.
3. View the video clip linked below explaining the process of osmosis, taking notes and listing questions as needed.
4. Report out on your work as directed by your instructor.
Under ideal circumstances, a cell will be in an environment where the concentration of water molecules outside of it is the same as that found within it. Remember, at equilibrium, the water molecules still move, but there is no net difference in the numbers of molecules of water between the outside of the cell and the inside of the cell. This allows for efficient cell functioning.
However, there are cases where this dynamic equilibrium doesn’t exist, and the cell is significantly affected.
Lab activity: Osmosis lab
Follow all the directions provided by your instructor for this activity, including applicable lab safety directions.
Egg Osmosis lab by Jodi Evans / CC BY-NC-SA 3.0
Activity:
- Observe and discuss Figure 3.4.2 below. Answer the following questions:
- What type of cell is present?
- Explain how water is moving with regard to the cell for each of the three diagrams.
- Which of the diagrams illustrates a dynamic equilibrium? Explain your reasoning.
- Based on your understanding of osmosis, explain why water molecules would need to move out of a cell.
- Based on your understanding of osmosis, explain why water molecules would need to move into a cell.
- Follow your instructor’s directions in terms of checking your answers and finding the answers to any questions you may have.
Figure 3.4.2
The terms listed above each illustration in Figure 3.4.2 refer to concentration of water molecules outside of the cell (in solution) relative to the water concentration inside the cell (the cytoplasm). Under ideal conditions, the environment around the cell is isotonic, where the water molecule concentration is relatively even on either side of the cell membrane.
Cell shrinkage occurs when cells are immersed in a hypertonic solution. This type of solution has significantly fewer water molecules than what is found in the cells. As a result, the water molecules in the cell must move out into solution. In some cases, cells will shrink to a point from which they will not recover.
Cell expansion will occur when cells are immersed in a hypotonic solution. This type of solution has more water molecules than what is found in the cells. As a result, the water molecules in the solution must move into the cell. If there is enough of a difference in concentration of water molecules, it is possible that a cell will burst.
Activity:
- Take notes and list any questions you have while viewing the video:
Osmosis: A Solute and Solvent Love Story
- Share your notes and questions as directed by your instructor.
It is important to note that water is not the only substance that requires facilitated transport. Glucose also has specific protein channels to go through in the cell membrane (Figure 3.4.2).
Figure 3.4.3
Figure 3.4.3 is a diagram illustrating the movement of glucose molecules across a plasma (cell) membrane using the process of facilitated diffusion.
[1] Tonicity by Mariana Ruiz Villarreal / Public Domain
