Diffusion and Osmosis

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Simple Diffusion 

One of the basic ways substances can get in and out of cells is by simple diffusion. Diffusion is the movement of molecules from a region of high concentration to a region of low concentration by means of random molecular motion. 


The animation illustrates diffusion of a large molecules(red balls) into water(blue balls). The random molecular motion in difuusion is because all molecules have energy of motion(kinetic energy) and are constantly colliding with each other. Here the water molecules transfer some of their kinetic energy to the larger molecules.  

The transfer of Kinetic energy involved in the collision causes a shift in the velocities of both molecules. The net result is each molecule seems to be moving in a random fashion. The whole group of large molecules that start out concentrated in a small area moves from the region of high concentration to a region of low concentration. 

Diffusion in a solution can be sped up by heating the solution. This makes sense when you consider that heat is the total amount of kinetic energy in a solution. Likewise to slow diffusion down the solution can be cooled. Be careful not to confuse diffusion with convection currents caused by unequal heating of solutions or with the kind of mixing that can come about by stirring a solution. 

Diffusion requires kinetic energy from the environment but does not require cellular energy. Hence diffusion is a form of passive transport


Omosis is diffusion of water across a semipermeable membrane. Again Osmosis like diffusion in general does not require any cellular energy but just the kinetic energy related to the heat on either side of the membrane. 

In this animation, the large molecules, the solute dissolved in the water, are in greater concentration in the solution to the left of the membrane. Here the water is the solvent. But unlike the first example, solute molecules are restricted from crossing to the right hand side by the membrane. 

The water molecules are free to move back and forth through the small openings in the membrane. Since the large molecules are in greater concentration on the left side, that means the water is in lower concentration on the left side relative to the right side of the membrane. Note that water molecules are constantly moving back and forth across the membrane but the net movement of water will be from its region of highest concentration to its region of lower concentration. 

Here are some things to consider with regard to osmosis in cells: 

1. The inside of cells is full of salts, amino acids, sugars and other solutes that cannot get through a cell membrane. What will happen to an animal cell or a single celled organism surround by relatively pure water if the cell has no other type of transport other than osmosis? The water in this case is more concentrated outside the cell. Would the same thing happen to a plant cell? 

2. What would happen if you took the same cells in put them in ocean water? Assume the water concentration is now greater inside the cells. Explain. 

3. When you forget to water your house plants they wilt. Explain this wilting in terms of osmosis. Hint: remember that soil contains water and dissolved minerals. 


pgd. 7/3/99