Gatorade
 Membrane Properties and Rehydration
Dr. Matt Hermes

Sports Science

Rehydration

Energy  Replacement

Quiz

Problems

Micro/Macro
and Symbolic
Representation
Chemistry uses macroscopic, large scale observationsto help describe and understand matter at the unseeable, molecular level.  And then we represent both the microscopic and macroscopic with often complex symbolic representation.

Chemists use demonstrations of microscopic molecular behavior in order to demonstrate chemical concepts. In the diffusion demonstration, we look at red and blue dots spreading across a teal-blue surface. The authors want us to see the blue as water in three dimensions and the red and blue dots as positive and negative ions. The erratic scattering of the dots represents the impact of the dots with water molecules that are unseen! So there is much to think about in a graphic demonstration of a chemical concept.

While these dynamic symbols remind us of the active movement of microscopic particles, what our macroscopic view is quite different. In the bent tube in which we have put the sugar solution and pure water, all we see is a slow increase in the volume of the sugar solution.

During exercise, we sweat.   As we sweat, our semipermeable skin allows small molecules, water and the ionic salts, to pass through. Larger molecules of the extracellular fluid (the fluid betweeen our cells), remain behind.  These molecules are held inside the body because the transport through our semipermeable skin to the outside of our bodies is difficult and must be aided by transport mechanisms that depend on cell wall structure.

Within the body, transfer of water through semipermeable membranes controls many biological processes. And in 1965, Dr. Robert Cade, Dr. Dana Shires, Dr. Jim Free, and Dr. Alex deQeusada at Florida recognized the simple chemical principle involved. The principles of osmotic behavior depend on rates of passage of molecules across membranes.

To illustrate osmotic behavior, let us think about an experiment.  Let us place a semipermeable membrane as a separator between two long, vertical tubes containing liquids. We will pour pure water in one tube and a sugar solution at 1M in the other. We quickly observe that the level of liquid in the sugar solution tube rises while the level in the water-filled tube falls.

Only water can pass across the membrane -- the sugar molecules are too large. But water can pass through in either direction. So water must be passing from the pure water side into the solution more rapidly than water on the sugar side is passing back into the pure water.

We can explain this phenomenon if we consider what is called chemical kinetics. A process will have an increased rate if the concentration of the reactive species is increased. Consider the water and sugar solution. On which side is the concentration of water higher? On which side is the concentration of water lower? Does it not make sense that the rate of flow of water from the pure water side will be higher than that from the solution side toward the water? In effect, the sugar molecules dissolved in the water get in the way of flow of water from the solution.

So water flows faster into the sugar solution than out until two things occur: First, the solution is diluted so that water flow back into pure water speeds up. Second, a pressure develops on the sugar side which forces water faster from the solution side into the pure water. Osmosis is quite dynamic with pressures and concentrations changing minute to minute. But we can calculate the presure on the solution side that would establish and equilibrium flow of water at the same rate in both directions across the membrane:

Osmotic Pressure = RT x Molarity

This relationship holds for any pair of solutions. Regardless of the nature of the dissolved particles, water will tend to flow from the more dilute solution into the more concentrated, seeking to equalize the concentrations across the semipermeable membrane.

This simple relationship drove the inventors of Gatorade in their definition of the hydrating beverages. We can understand that salt tablets would be about the worst treatment for dehydration. The body would struggle to supply water from its already depleted supply to the small bowel to dissolve the salt tablets. Orange juice or soft drinks, at 10% concentration of sugars (a concentration much higher than body fluids), would also draw water from the extracellular fluid.

Chemical Concepts
Here are some concepts of solution chemistry that the inventors of Gatorade used in developing the sports drink:

We will see how Dr. Cade and his associate researchers used these simple chemical concepts to invent and develop Gatorade and we will use these principles ourselves to evaluate and decide on issues of testing and ownership of Gatorade