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7. Transport in Cells

Osmosis

Osmosis is a specific type of diffusion; it is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. Water moves in or out of a cell until its concentration is the same on both sides of the plasma membrane.

Semi-permeable membranes are very thin layers of material that allow some things to pass through them but prevent other things from passing through. Cell membranes are an example of semi-permeable membranes. Cell membranes allow small molecules such as oxygen, water carbon dioxide, and oxygen to pass through but do not allow larger molecules like glucose, sucrose, proteins, and starch to enter the cell directly.

The classic example used to demonstrate osmosis and osmotic pressure is to immerse cells into sugar solutions of various concentrations. There are three possible relationships that cells can encounter when placed into a sugar solution. Figure 5.7.45.7.4 shows what happens in osmosis through the semi-permeable membrane of the cells.

  1. The concentration of solute in the solution can be greater than the concentration of solute in the cells. This cell is described as being in a hypertonic solution (hyper = greater than normal). The net flow or water will be out of the cell.
  2. The concentration of solute in the solution can be equal to the concentration of solute in cells. In this situation, the cell is in an isotonic solution (iso = equal or the same as normal). The amount of water entering the cell is the same as the amount leaving the cell.
  3. The concentration of solute in the solution can be less than the concentration of solute in the cells. This cell is in a hypotonic solution (hypo = less than normal). The net flow of water will be into the cell.
water moves out of the cell. inside has 20% solutes (80% water). outside has 40% solutes (60% water).
Figure 5.7.4.A5.7.4.A: Hypertonic solution. A solution that has a higher solute concentration than another solution. Water particles will move out of the cell, causing crenation.
water moves equally into and out of the cell due to the same concentration of solutes inside and out (in this case, 20% solutes and 80% water)
Figure 5.7.4.B5.7.4.B: Isotonic solution. A solution that has the same solute concentration as another solution. There is no net movement of water particles, and the overall concentration on both sides of the cell membrane remains constant.
water moves into the cell. inside has 20% solutes (80% water). outside has 10% solutes (90% water).
Figure 5.7.4.C5.7.4.C: Hypotonic solution. A solution that has a lower solute concentration than another solution. Water particles will move into the cell, causing the cell to expand and eventually lyse.

Figure 5.7.55.7.5 demonstrates the specific outcomes of osmosis in red blood cells.

  1. Hypertonic solution. The red blood cell will appear to shrink as the water flows out of the cell and into the surrounding environment.
  2. Isotonic solution. The red blood cell will retain its normal shape in this environment as the amount of water entering the cell is the same as the amount leaving the cell.
  3. Hypotonic solution. The red blood cell in this environment will become visibly swollen and potentially rupture as water rushes into the cell.
Osmotic pressure on blood cells
Figure 5.7.55.7.5: Osmosis demonstration with Red Blood cells places in a hypertonic, isotonic, and hypotonic solution.

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