To solve the problem, we need to determine whether there is a concentration gradient of water in tube A. The solution on the left in tube A has blue dots. Wherever there are blue dots there is no water. Therefore, the concentration of water, the amount of water per unit of volume, will be greater on the right, where there are no dots, than on the left, where there are dots. We know that water, like other substances, will diffuse in the direction of its concentration gradient. This means that water will flow from the right side of the membrane to the left side. The result will be the situation in tube C. (If you answered this correctly, give yourself a big pat on the back. This is not an easy problem, so don't be discouraged if you missed it.) Since there will always be dots on the left and not on the right, there will always be a concentration gradient causing the net movement of water molecules to be from right to left. Ultimately, the pressure of the column of water on the left will push back on the membrane with a force equal to the diffusion force of water molecules and the net flow will stop. As many molecules will diffuse across the membrane as are pushed back through the membrane by the weight of the column.

Before we can go farther, I need to define three terms; solution, solvent and solute. A solution is a homogenous molecular mixture of two or more substances. Note that a solution need not be, a liquid. When we have a solution, the substance that has the greatest concentration is the solvent. It is the substance that dissolves the other substance(s) in the solution. Substances that are found in lesser concentration in solutions are solutes. Solutes are the substances dissolved by solvents. Whew! Lets look at some examples to try to clarify this. When you put a spoonful of sugar in a cup of water, the result is a solution. The water is the solvent and the sugar is the solute. A dyed blouse or shirt is a solution. The fabric is the solvent and the dye is the solute. Suppose you have a cup of coffee with sugar in it. __________ is the solvent and _________ and __________ are the solutes. Fill in the blanks and check your answers by clicking here.

Now lets return to our discussion of diffusion. What factors can influence the rate of diffusion?

  1. Temperature. The more heat molecules have, the faster they move and the faster they will diffuse.
  2. The state of the solvent; i.e. whether the solvent is a solid, liquid or gas. Diffusion occurs much faster in gases than in liquids and much faster in liquids than in solids. This happens because the molecules in a gas are farther apart than the molecules in a liquid and there is more room for the solute to diffuse between molecules. Similarly, molecules in a liquid are farther apart than molecules in a solid, so solutes dissolve faster in a liquid. (If you've ever spilled ink onto your carpet, the ink appears to diffuse through the carpet pretty quickly. Nevertheless, ink will diffuse in water much more quickly than in your carpet. However, inky water won't upset you as much as your ruined carpet will.)
  3. The size of the molecules. The larger the molecule the slower it will diffuse. (You should spill ink that has large ink particles in it if you are going to spill ink. You might be able to clean it out before it travels too far.)
  4. The steepness of the diffusion gradient. The greater the difference in concentration of a solute between one end of the gradient system and its concentration at the other end, the faster it will dissolve.
  5. Permeability. Diffusion can occur across membranes. We have already learned that membranes can be selective in their permeability, allowing some substances to pass through and barring the passage of others. Membranes are more or less permeable depending upon the degree to which they allow materials to pass through them. For example, potassium ions (K+) might pass through a membrane quite rapidly, but the passage of sodium ions (Na+) may be impeded. The same membrane might be completely impermeable to large protein molecules.
  6. Membrane structure. When we are dealing with membranes, it is important to note that the phospholipid structure of the membrane influences the rate of diffusion of materials through it. The hydrophobic, nonpolar region of the membrane allows nonpolar molecules to move across readily but blocks the passage of polar molecules. It also helps if molecules are small, e.g. gases such as oxygen, carbon dioxide and nitrogen pass through rapidly. Interestingly, water, which is polar, moves across membranes with comparative ease. The mechanism by which water does this is not completely understood. Some water passes through the lipid barrier. Some flows through membrane channels that are used primarily for ions, but there are not enough of these in cells to account for water diffusion. There are some cell types, however, that have specific water pore proteins, called aquaporins, that exist solely to increase the cell's water permeability.


We have now explored membranes, permeability and diffusion. It is time to learn about osmosis. Osmosis is a special case of diffusion. It is simply the diffusion of water through a selectively permeable membrane. This means that water moves through a selectively permeable membrane in the direction of its concentration gradient. For osmosis to occur there must be:

  1. A selectively permeable membrane between two water solutions, or between a water solution and pure water.
  2. The concentration of solute in the two solutions must be unequal; i.e. there must be more solute on one side of the membrane than on the other side.
  3. The membrane must be impermeable to the solute, but permeable to water.

Let's now reexamine the earlier diagram and interpret the results in terms of osmosis. On each side of the tubes we have a water solution separated by a selectively permeable membrane. The blue dots represent solute (They could be sugar molecules). The concentration of solute is greater on the left than on the right. The membrane is impermeable to the solute. Water will move from where it is more concentrated to where the concentration of water is less (remember that is the definition of diffusion). Now you have to do a careful switch of thinking. Water will move towards the side of the membrane where there is more solute. This happens because where there is solute, there is less water. Referring to the diagram, we see that water flows from the side with 0% solute to the side with 20% solute. This decreases the concentration to 15% solute in tube B. The water from the right side of the tube will continue to dilute the solution on the left side until the pressure of the solution on the left equals the osmotic pressure; i.e. as many water molecules are pushed from left to right by pressure of the column as are diffused from right to left by osmosis.

Organisms depend upon osmosis to move water from one space into another. There is no mechanism for the active transport of water as there is for many other substances such as calcium, sodium and glucose. It is necessary to pump salt or other solutes from cells into tissue spaces and create an osmotic gradient in order to move water from cells. Conversely, an osmotic gradient is necessary to move water into cells. To understand this better we need to examine what happens to cells when they are placed in solutions with different solute concentrations. Unfortunately, this discussion involves more terms, proving once again that biologists speak a foreign language.

There are three possible relationships that cells can encounter when placed into a water solution.

  1. The concentration of solute in the solution can be equal to the concentration of solute in the cells. The cell is in an isotonic solution. (iso = same as normal)

  2. The concentration of solute in the solution can be greater than the concentration of solute in the cells. The cell is in an hypertonic solution. (hyper = more than normal)

  3. The concentration of solute in the solution can be less than the concentration of solute in the cells. The cell is in an hypotonic solution. (hypo = less than normal)








Determine the tonicity of each solution (hypertonic, isotonic or hypotonic). Click on the beaker buttons to test your answer.

Look carefully at what happened to the cells in each type of solution. The cells remained the same in the isotonic solution, burst (lysed) in the hypotonic solution and shriveled up (crenated) in the hypertonic solution. The cell remains the same size in the isotonic solution because the concentration of solute (and therefore of water) is the same in the cell and in the isotonic solution, so as many water molecules move into the cell as move out of it. The concentration of water is greater in the hypotonic solution than in the cell, causing the cell to swell and finally burst. The cell acts like a balloon filling with water. It expands until it can hold no more water and then shatters. Note that the water is moving into the greater concentration of solute. In a hypertonic solution the concentration of water is greater in the cell than in the solution, so water leaves the cell and it shrinks. Water is again moving towards the greater solute concentration.

Now let's see what happens to a plant cell in each type of solution. Remember that plant cells have a cellulose cell wall around them. In an isotonic solution, the cell will remain the same size for the reasons given above for animal cells. In an hypertonic solution, the plasma membrane of the cell will pull away from the cell wall as the cell shrinks. This situation is called plasmolysis.







In an hypotonic solution, the cell will swell, but will not burst because of the rigid cell wall. Again, think of a balloon. This time the balloon is inside of a box. You can fill it with water until the balloon pushes against the sides of the box with enough force to bend the sides of the box outwards, but you will not be able to break the balloon. The pressure on the sides of the box is not enough to break the box and the balloon contained inside remains intact. The situation is the same with a plant cell. The pressure on the cell wall is not enough to break the wall. Pressure builds up and the cell wall bends outward, but it does not break. The pressure is called turgor pressure. Turgor pressure on the walls of plant cells is what keeps plants from drooping. Plants wilt without enough water to develop turgor pressure.

Facilitated diffusion.

Remember that the middle portion of a plasma membrane is a fluid of fatty acids. Since fatty acids are non-polar, materials that are polar or charged can experience difficulty passing through this hydrophobic region. The passage of such material is assisted, i.e. facilitated, by membrane proteins which allow passage through channels inside them. This process is still diffusion. That means that no external cellular energy is required to facilitate the movement of molecules in the direction of their concentration gradient. This is still a passive transport mechanism. The aquaporins mentioned previously are among the channels for assisting diffusion, in this case the facilitated diffusion involves water which, as you recall, is polar..

Go to active transport           Go back to outline of cell transport        Go back to outline of course

copyright June B. Steinberg, 2005