Lab #5: Osmosis, Tonicity, and Concentration. Background. The internal environment of the human body consists largely of water-based solutions. A large number of different solutes may be dissolved in these solutions. Since movement of materials across cell membranes is heavily influenced by both differences in the concentration of these various materials across the cell membrane and by the permeability of the lipid bilayer to these materials, it is critical that we understand how the concentration of a particular solute is quantified, as well as how differences in concentration influence passive membrane transport. Diffusion, Osmosis, and Tonicity Simple diffusion. Particles in solution are generally free to move randomly throughout the volume of the solution. As these particles move about, they randomly collide with one another, changing the direction each particle is traveling. If there is a difference in the concentration of a particular solute between one region of a solution and another, then there is a tendency for the substance to diffuse from where it is more concentrated to where it is less concentrated. This is because of the random collisions among particles which eventually will evenly distribute the solute throughout the volume of the solution. Thus net diffusion occurs “down” a concentration gradient, from an area of high concentration to an area of low concentration, until a state of equilibrium is reached throughout the volume of the solution. At this equilibrium (typically when the concentration is uniform), there will be no more net diffusion of solute from one area to another, although random movement of particles will continue. If such diffusion is unimpeded by the presence of some barrier (e.g., a membrane that is impermeable to the solute), it is referred to as simple diffusion. In the case of cells, solutes that can readily pass through the lipid bilayer of cell membranes (i.e., small uncharged molecules or moderate-sized nonpolar molecules) are transported across cell membranes via simple diffusion. For example, the exchange of gases such as O2 and CO2 across the plasma membrane occurs through simple diffusion. Osmosis Like some other small, uncharged molecules, water (H2O) can pass quite readily through a cell membrane, and thus will diffuse across the membrane along its own concentration gradient independent of other particles that may be present in the solution. However, often solutes that are dissolved in water (e.g., glucose, Na+, Cl- , etc.) cannot pass through the lipid bilayer of the cell membrane, and thus cannot move across the cell membrane even if there is a difference in concentration. The “concentration” of water in a solution is inversely related to solute concentration – the greater the concentration of total solutes in the solution (regardless of what those specific solutes are), the lower the number of water molecules per unit volume of the solution. Thus water tends to diffuse from more dilute areas (i.e., with lower solute concentration) to more concentrated areas (i.e., with higher solute concentrations). Cells are surrounded by a semi-permeable membrane which will allow water to pass Fig 5.1. An example of simple diffusion. Molecules of red dye gradually diffuse from areas of higher concentration to areas of lower concentration until the concentration of dye is uniform throughout the volume of the solution.through but prevent many hydrophilic solutes from passing through. Thus, the diffusion of water into or out of the cell is driven by differences in the total concentration of solutes that cannot pass through the plasma membrane. The diffusion of water across a semi-permeable membrane such as a cell membrane is termed osmosis. Osmosis is thus a special case of diffusion involving only the movement of water (the solvent of the solution) across a barrier permeable to water but impermeable to certain solutes. It is the difference in the solute concentration across a semi-permeable membrane that provides the driving force for osmosis (Fig 5.2). Therefore, osmosis can only occur under specific conditions where a) two aqueous solutions are separated by a membrane that is permeable to water but impermeable to at least one of the solutes in the solution1 and b) there is a difference in the total concentration of impermeable solutes between the two solutions. If water flows from one solution into another, then the volume of the second solution will tend to increase while that of the first solution decreases (Fig 5.2). This change in volume can only occur if the semi-permeable membrane (or one of the other walls of the container holding each solution) is compliant enough to accommodate this change in volume. If water is redistributed across the membrane but the membrane will not stretch or reposition itself to accommodate an increase in volume, then pressure will build up inside of the solution gaining water, and likewise decrease in the solution that is losing water. As more water is drawn into the more concentrated solution, pressure will build up, pushing outward on the walls of the container. Eventually, this pressure exerting outward will become high enough to equal the force that is driving water from the less concentrated solution to the more concentrated solution. At this point, no further osmosis will occur, since these two equal and opposite forces acting on the movement of water are canceling each other out. The amount of force that would need to be exerted on a solution to prevent osmotic uptake of water by that solution is called the osmotic pressure. In effect, it is a measurement of how strongly a solution will draw water into itself from an adjacent solution across a semi-permeable membrane. Osmotic pressure is directly related to the total solute concentration of a solution. As solute concentration increases, more and more pressure would need to be exerted on a solution to prevent osmotic uptake of water from an adjacent solution. To illustrate this, consider a situation where we have a solution containing a particular concentration of solutes that is separated from a pure water by a semi-permeable membrane (Fig 5.3). The membrane and walls of the container are rigid, but the top of the container is open, so the solutions can change in volume by changing vertical height. Notice that as water moves from the pure water into the solution, the height of the pure water decreases and