Lesson 3.4 Passive and active transport
3.4.1 Introduction to cell transportation
As you have already seen, the cell membrane separates the internal environment
of the cell from the external environment of the cell. Cells must maintain homeostasis in order to function and to do this, cells need to be able to regulate what come into and what goes out of it. A cell membrane, being semi-permeable, fulfills this need.
Activity:
- Before moving on to the next section, please view the video on the cell membrane and homeostasis:
Homeostasis (and the Cell Membrane King)
- Discuss what you viewed with a partner.
- Follow your instructor’s directions to report out on your discussion.
Diffusion
To better understand how materials cross the cell membrane, we will first look at a process called diffusion.
If you take a drop of food coloring, add it to a cup of clear water, and allow it to remain undisturbed for a few minutes, you would notice that the food coloring begins to spread into the water. Left for a long enough time period, all of the water will have color (Figure 3.4.1). This is a simple example of diffusion.
Figure 3.4.1
Figure 3.4.1 shows the process of diffusion of a drop of purple food coloring into water over a period of time.
Diffusion occurs because molecules that make up a particular substance (solid, liquid, gas) will move from areas of higher concentration to areas of lower concentration. This is due to the kinetic energy of the molecules. As the molecules move, they collide with one another, and move randomly until equilibrium is established. At equilibrium, the molecules will still have motion. However, there is no net change in the concentration of molecules. It is a process that does not require any energy to occur and is considered a form of passive transport.
For cells, passive transport allows some materials to enter and leave without the cell having to use ATP. Examples of these materials would include gases (such as oxygen and carbon dioxide) ,small uncharged molecules, water, anything able to dissolve in lipid (fat), sex hormones, and many others.
Lab activity: Diffusion
Follow all the directions provided by your instructor for this activity, including applicable lab safety directions.
Gummy bear diffusion lab by Cottrell / CC BY-SA 3.0
