Raisins in 100 ml of water overnight

If you decide to try this experiment on your own, don’t let the raisins sit longer than overnight, as some stuff will start growing in the water.

For the easy part, the name of the process is osmosis. It is a specific type of diffusion where only the solvent (usually water) moves across a selectively permeable membrane, such as the cell membrane of these raisins.

When a solute, such as the sugar that is found in the raisins, is added to one side of the membrane, here the inside of the raisin, it forms hydrogen bonds with the water molecules that are also inside the raisin. That means, those water molecules, which now form a hydration shell with the sugar molecules inside the raisin, are no longer available to diffuse across the membrane. Therefore, the sugar (and other) molecules inside the raisin, in effect, reduce the concentration of free water molecules inside the raisin, so that the concentration of free water molecules in the container is now much higher than the concentration of free water molecules inside the raisin.

Because the concentration of free water molecules outside the raisin is higher than the concentration of free water molecules inside the raisin, water will tend to move into the raisin at a higher rate than the rate at which it moves out of the raisin. If there’s more water going into the raisin than coming out, it should make sense that the raisin will become larger.

Source: Peter H. Raven, et al. Biology, 7e. Boston: McGraw Hill (2005), p. 116.

Some vocabulary words concerning osmosis:

• Osmotic Pressure: This is defined as the force that a dissolved substance exerts on a semipermeable membrane, through which it cannot penetrate, when separated by it from pure solvent. In our example, the dissolved stuff inside the raisin exerts a force on the membrane around the raisin, because that stuff cannot penetrate through the membrane. It is said, in physical chemistry, that there is osmotic pressure on the membrane for water to move into the raisin. This amount of osmotic pressure is related to the difference in concentration of solute, which affects the concentration of free water molecules, between the inside and outside of the raisin.
• Hypotonic: The prefix “hypo-” means “lower,” so a solution that is hypotonic has a lower osmotic pressure than another solution with which it is compared. This word (as well as its cousins, isotonic and hypertonic) only works in a comparison of two solutions: one solution is hypotonic to the other; you cannot just call a solution hypotonic without referring to another solution.

Source: osmotic pressure. Dictionary.com. Dictionary.com Unabridged (v 1.1). Random House, Inc. (accessed: November 16, 2008).

Example in protists:

Most organisms that live in water are hypertonic (hyperosmotic) to their environment, and if they had not developed ways around this, water would flow into their bodies, just like it does with the raisins, and they would eventually bloat, and possibly explode.

An example of a solution that works can be found in the single-celled eukaryote Paramecium, which is a protist that lives in water:

Inside their cell is an organelle called a contractile vacuole, which stores water and extrudes it. (The vacuole is enclosed in a membrane, but it has a small pore that opens to the outside of the cell.)

It continually pumps water out of the cell by contracting in a rhythmic way. You might think of it as a vacuole with a beat. Once again, music saves the day for the Paramecium.

Example in mammals:

Mammals blood bathes our cells in an isosmotic solution. That is, our blood has the same concentration of dissolved substances as our cells. If this were not the case, our cells would bloat (and eventually burst) or shrivel.

Consider the experiement in which red blood cells are placed in three different solutions: one is hypotonic to our normal blood, one isotonic, and one hypertonic. What will happen to the red blood cells?

Here is a picture of a normal red blood cell:

When it is placed in isosmotic solution, there will be no net movement of water into or out of the cell. That is, water will be moving across the cell membrane, but it will move into the cell at the same rate as it moves out of the cell. The solution is isosmotic to the inside of the cell.

However, if the RBC is placed in solution that is hypotonic (hyposmotic) to the interior of the cell, water will diffuse into the RBC at a faster rate than it moves out of the cell, and the cell will swell up. This will cause it to burst, a situation that is incompatible with life.

On the other hand, if the cell is placed in a hypertonic solution, water will tend to move out of the cell at a faster rate than it moves in, causing the cell to shrivel up. This situation is also incompatible with life.

How do mammals keep their RBCs from exploding?

That’s the big question, of course. Humans, for example, have a lot of protein floating around in our blood, like albumin. This protein raises the solute concentration in the blood so that it is isosmotic to the solute concentration inside the RBCs. Too little protein could cause the blood to be hypotonic to the inside of the cells, which would not be good.

Example in amphibians:

Frogs absorb water through their skin by osmosis. The process is very similar to that described in this example for raisins. (They also absorb oxygen through their skin, which is, of course, called diffusion.)

Example in plants:

Unlike animal cells, plant cells have cell walls that apply osmotic pressure and keep the cells from bursting when placed in hyposmotic environments, which plants usually find themselves in. Think about it: soil doesn’t really dissolve in water, so it doesn’t contribute to the osmotic pressure around a plant cell, and usually, plants are in pure water, like the raisins in the container in this experiment.

Water will tend to move into plant cells at a greater rate than it moves out of plant cells, and the cells will tend to swell up from all the water that diffuses in. All the water coming into the cell causes a build-up of hydrostatic pressure, which tends to expand the cell, kind of like filling a balloon with water. It also tends to resist the osmotic pressure being applied by the concentration gradient, which tends to make water move into the plant cell.

The hydrostatic pressure where the plasma membrane presses firmly against the cell wall is called turgor pressure, and this makes the cell rigid. If a plant doesn’t get enough water, this turgor pressure is reduced, and the plant wilts.

Summary of examples:

So, in summary, this is how animals and plants stay alive despite have concentration differences between their cells and the environment in which those cells live: Animals make the blood isotonic with the environment by building proteins that float around in the blood and form hydrogen bonds with water molecules. Plants have cell walls that balance the extreme osmotic pressure enough to keep the cell from bursting.