We know living things are made of cells, and that nearly all living things are about 70% water. But how does a living thing absorb water? Each cell of a living organism is completely enclosed by the cell membrane...how can water get in and out?
The answer is osmosis! Osmosis is the passing of a solvent (usually water) through a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration, generally until the solute concentrations on both sides of the membrane are approximately equal. To understand this fully, we must understand both what solvents and solutes are, and what semi-permeability means.
A solvent is a substance, usually a liquid, in which solutes may be dissolved. In the case of cells, this is water. Other common solvents you may be familiar with include alcohol, acetone (commonly found in nail polish remover), and even air—remember gasses can be solutes also!
A solute is any substance dissolved in a solvent. In a cell there are many solutes, such as salts, sugars, and other small molecules. Simply put, a solute is the substance that is dissolved, and the solvent is the substance that does the dissolving.
A solvent is a substance, usually a liquid, in which solutes may be dissolved. In the case of cells, this is water. Other common solvents you may be familiar with include alcohol, acetone (commonly found in nail polish remover), and even air—remember gasses can be solutes also!
A solute is any substance dissolved in a solvent. In a cell there are many solutes, such as salts, sugars, and other small molecules. Simply put, a solute is the substance that is dissolved, and the solvent is the substance that does the dissolving.
A semi-permeable membrane is a thin, selective barrier that only allows certain things to pass through it, while keeping other things on one side or the other. We see semi-permeable membranes very frequently in biology—nearly every living thing has them not only as the outer membrane of their cells, but also as part of the organelles within their cells. These biological membranes allow water to pass through, without allowing most solutes to pass.
A very good example of osmosis in action is what happens when we put red blood cells into solutions with different amounts of solute. These different solutions may have more solute (hypertonic), less solute (hypotonic), or equal amounts of solute (isotonic) as compared to the red blood cells.
Put the red blood cells in a hypertonic solution, and water will flow through the membrane out of the cells, since there is a higher solute concentration on the outside of the cells. As a result, the cells will become wrinkled and shriveled up!
Put the red blood cells in a hypertonic solution, and water will flow through the membrane out of the cells, since there is a higher solute concentration on the outside of the cells. As a result, the cells will become wrinkled and shriveled up!
Put the cells in a hypotonic solution, and water will flow through the membrane into the cells, since there is now a higher solute concentration on the inside of the cells. As a result, the cells will become very full and bulge, and may even burst!
If we put the cells in an isotonic solution, the solute concentration is now balanced between the inside and the outside of the cells, allowing them to maintain their normal shape.
TRY THIS!
A good model of a cell is a typical chicken egg—it has a shell on the outside, but remove that hard shell and it also has a semi-permeable membrane on the outside!
A good model of a cell is a typical chicken egg—it has a shell on the outside, but remove that hard shell and it also has a semi-permeable membrane on the outside!
After you remove the shell using vinegar, you can do some pretty interesting things with your egg!
Here’s what you’ll need:
1. Two eggs (extras are appropriate, just in case!)
2. Two cups of vinegar
3. One 3-4 cup clear plastic cup or bowl
4. One large slotted spoon
5. Two small, wide mouthed drinking glasses
6. One cup of pure distilled water
7. One cup of corn syrup
NOTE: This experiment requires some planning ahead; preparing the eggs will take 24 to 48 hours.
Here’s what to do:
1. Place your eggs in the clear plastic cup.
2. Pour the vinegar over the top of the eggs.
3. Wait 24-48 hours for the vinegar to dissolve the egg shells! Look closely at the shells as they are dissolving in the vinegar—you should see small bubbles forming on the shell.
4. When the egg shells are fully dissolved, carefully remove them from the vinegar with the slotted spoon. Place one egg in each of the two drinking glasses.
5. Pour one cup of corn syrup over one of the eggs.
6. Pour one cup of distilled water over the other egg.
7. Allow the eggs to sit at room temperature for 3 hours to overnight.
8. Remove the eggs from their glasses with the slotted spoon, and place them carefully on the table.
What do the eggs look like? Has their appearance changed? If so, how? Based on this observation, which of these solutions is hypertonic? Hypotonic? Sketch the way the eggs look, and be sure to write down all your observations!
Image Licenses:
Creative Commons Attribution-Share Alike 3.0 Unported license.
Here’s what you’ll need:
1. Two eggs (extras are appropriate, just in case!)
2. Two cups of vinegar
3. One 3-4 cup clear plastic cup or bowl
4. One large slotted spoon
5. Two small, wide mouthed drinking glasses
6. One cup of pure distilled water
7. One cup of corn syrup
NOTE: This experiment requires some planning ahead; preparing the eggs will take 24 to 48 hours.
Here’s what to do:
1. Place your eggs in the clear plastic cup.
2. Pour the vinegar over the top of the eggs.
3. Wait 24-48 hours for the vinegar to dissolve the egg shells! Look closely at the shells as they are dissolving in the vinegar—you should see small bubbles forming on the shell.
4. When the egg shells are fully dissolved, carefully remove them from the vinegar with the slotted spoon. Place one egg in each of the two drinking glasses.
5. Pour one cup of corn syrup over one of the eggs.
6. Pour one cup of distilled water over the other egg.
7. Allow the eggs to sit at room temperature for 3 hours to overnight.
8. Remove the eggs from their glasses with the slotted spoon, and place them carefully on the table.
What do the eggs look like? Has their appearance changed? If so, how? Based on this observation, which of these solutions is hypertonic? Hypotonic? Sketch the way the eggs look, and be sure to write down all your observations!
Image Licenses:
Creative Commons Attribution-Share Alike 3.0 Unported license.
