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Medicine LibreTexts

4.3A: Passive Uptake/Transport

There are three forms of passive uptake/transport:

  1. Simple Diffusion
  2. Osmosis
  3. Facilitated Diffusion

Below is more information of each type of uptake/transport.

Simple Diffusion

Simple diffusion is the movement of solutes from an area of higher concentration (with the concentration gradient) to an area of lower concentration without the help of a protein, as shown below.

Figure 4.311 Simple diffusion

Osmosis

Osmosis is similar to simple diffusion, but water moves instead of solutes. In osmosis water molecules move from an area of lower concentration to an area of higher concentration of solute as shown below. The effect of this movement is to dilute the area of higher concentration.

Figure 4.312 Osmosis

Another example illustrating osmosis is the red blood cells in different solutions shown below.

Figure 4.313 Effect of salt solution concentration on red blood cells1

We will consider the simple example of salt as the solute. If the solution is hypertonic, that means that there is a greater concentration of salt outside (extracellular) the red blood cells than within them (intracellular). Water will then move out of the red blood cells to the area of higher salt concentration, resulting in the shriveled red blood cells depicted. Isotonic means that there is no difference between concentrations. There is an equal exchange of water between intracellular and extracellular fluids. Thus, the cells are normal, functioning red blood cells. A hypotonic solution contains a lower extracellular concentration of salt than the red blood cell intracellular fluid. As a result, water enters the red blood cells, possibly causing them to burst.

Facilitated Diffusion

The last form of passive absorption is similar to diffusion in that it follows the concentration gradient (higher concentration to lower concentration). However, it requires a carrier protein to transport the solute across the membrane. The following figure and video do a nice job of illustrating facilitated diffusion.

Figure 4.314 Facilitated diffusion examples2