Investigation 4 – Concept Day
Cellular Organization – Investigation 4
Note: In this Investigation, we wish to explain and demonstrate the important process of osmosis. You will perform experiments in the lab in which you place plant cells (either Elodea or onion) in solutions of varying salt concentrations. You will then view each preparation in the light microscope and make observations.
In this presentation, we wish to introduce you to the concepts of semipermeable membranes, isotonic, hypertonic, and hypotonic solutions. We will model what would be expected to occur under different salt concentrations so that you can interpret your observations in Lab in terms of the movement of water into or out of cells.
- This slide simply establishes the idea that salt is a solute when dissolved in water. The graph compares the amount of salt added to 100 ml of water to its final percent concentration.
Note: While you will not actually prepare the salt solutions, you certainly could if you were given enough time. However, you should nonetheless understand how these solutions are prepared and that there are increasing amounts of solute as the salt concentration increases.
- This slide introduces the extremely important concept of a semipermeable membrane. As shown in the center illustration, a semipermeable membrane allows water molecules to freely pass through it. Solute molecules, on the other hand, are not able to pass through the semipermeable membrane.
- The cell membrane of both plant and animal cells are semipermeable membranes. They separate the cell from its surrounding environment. The cell has a certain amount of dissolved solute in its cytoplasm. Depending on the concentration of solutes in the environment, water will either enter the cell from the outside or leave the cell. This is shown in more detail on the following slide.
- This slide defines three different types of solutions based on the amount of solutes they contain. An isotonic solution has the same amount of dissolved solutes as found inside the cell. A hypertonic solution has a greater amount of dissolved solutes than inside the cell. Finally, a hypotonic solution contains a lower concentration of dissolved solutes than inside the cell.
- Osmosis causes water to move into or out of the cell depending on which of the three types of solutions the cell is surrounded by. Since there is the same amount of dissolved solute outside and inside the cell in an isotonic solution, there is no net movement of water into or out of the cell under isotonic conditions.
- In a hypertonic solution, there is more dissolved solute outside the cell than inside, so water will leave the cell. It is as if the cell is trying to dilute the external solute solution.
- In a hypotonic solution, there is less dissolved solute outside the cell than on the inside. Therefore, water will begin to enter the cell. It is as if the external environment is trying to dilute the more concentrated solute solution inside the cell.
- This slide shows what can actually happen to a plant cell in isotonic, hypertonic, and hypotonic solutions. As shown, no net water movement occurs under isotonic conditions. The arrows show that water may move across the semipermeable cell membrane, but as much water enters the cells as exits.
- In the case of the hypertonic solution, water leaves the cell. As it does, look for the membrane pulling away from the cell wall and “shriveling” up. This is because the cellular volume decreases as the water leaves.
- In the case of the hypotonic solution, water begins to enter the plant cell. However, since the plant cell has a ridge and strong cell wall, the amount of water that can enter is counteracted by the force of the cell wall pushing against the further expansion of the semipermeable cell membrane. This pressure is referred to as turgor pressure and stops the plant cell from exploding in hypotonic solutions.
- This slide shows what can actually happen to an animal cell in isotonic, hypertonic and hypotonic solutions. As shown, no net water movement occurs under isotonic conditions. The arrows show that water may move across the semipermeable cell membrane, but as much water that enters the cell, the same amount exits.
- In the case of the hypertonic solution, water leaves the animal cell through its semipermeable membrane. As cell volume decrease with water loss, the entire cell may shrivel. Shifting the cell to an isotonic environment may reverse this process.
- Finally, in the case of the hypotonic solution, water from the environment rushes into the cell. The cell membrane will expand and stretch to a limit and then, if the solute concentration outside the cell is low enough, will eventually burst. This is sometimes referred to as cell lysis. Once the cell lyses, returning to an isotonic solution cannot reverse the damage.
- This slide simply repeats the information from the previous slide but is specific for human blood cells. The degree of hypotonicity to which human blood cells can withstand before lysing is a function of the strength of their cell membrane and its supporting structures.
- Some human blood diseases can be diagnosed by placing them in varying concentrations of hypotonic solutions and comparing the point at which they lyse compared to normal human blood cells. For example, Sickle Cell Anemia blood cells are measurably more sensitive to lysis in hypotonic solutions than normal human blood cells.
- This final slide points out that the typical salt concentration of the human body is 0.9% and that this is why saline IV solutions have a 0.9% NaCl concentration.
- The slide asks the question, “What do you think would happen to the cells in a patient’s body who was mistakenly given an IV of pure water (zero salt concentration)?”