Tonicity

Terminology

These three terms here are very important terms that I use and that your book uses. These words, much like many other terms we learn, are familiar. Yet, they take on completely new definitions in a class like this. So let’s start with the components here. In this class more often than not we are concerned with a solute. And for us a solute will usually be a gas or a solid. If you take a chemistry class that’s not true liquids will also be included. For our purposes in this class the solvent is usually waterThe universal solvent essential for life. or more specifically blood. We are constantly concerned with gases and solids, as they are distributed throughout our bodies in watery blood. And here comes the word that is deceptively familiar to us: solutionA homogeneous mixture of two or more substances.. A solution is a combination of solute and solvent. In factA statement based on direct observation that is repeatedly confirmed., in this class, we can use percentages. For example, coffee is 90% water. It is also 10% essence of coffee. Sea water is a solution of sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. chloride and water. In fact, seawater contains about 37 percent salt. The math will tell you the percent of the solvent. This is crucial. The percent of a solvent plus the percent of a solute must always equal 100. So if seawater is actually 37% salt then it would be 63% water.

Tonicity

Solute is usually more important to us in this class because solutes help us determine this term tonicityThe ability of a solution to affect the water balance in a cell.. This is the measure of solutes not the measure of solvents. However using some simple math if I were given the percent of solute I could calculate the percent of solvent. We need to apply these terms of Tonicity: hypotonic isotonicA solution with the same solute concentration as the inside of a cell, maintaining equilibrium. and hypertonicA solution with a higher solute concentration than the inside of a cell, causing water to leave th. These terms are relative terms. They are not absolute and do not tell you anything about the mathematical percentage of solutes in solutions. Let me show you with an example. This beaker over here on the left contains a 10% solution. The one in the middle contains a 15% solution. The one on the right contains 20%. The first beaker is hypotonic to the second beaker. And that is because it contains a lower percentage of solutes. The second speaker is hypertonic to the first beaker and that’s because it contains a greater percentage of solutes. This would be similar to lining up three people of different ages. You would use the terms younger and older to compare them. As a bonus in terms of Tonicity, we have this term isotonic which means having the same tonicity. This is what your cellsThe basic structural and functional units of life. strive for isotonicity.

Colloid Osmotic Pressure

We will talk about the tonicity of your bodily fluids: urineThe liquid waste excreted by the kidneys., blood plasmaThe liquid component of blood., and cerebrospinal fluid(CSF) – A fluid that cushions and nourishes the brain and spinal cord.. We will use the term osmotic pressureThe force exerted by water moving across a membrane due to osmosis..  We might use the term colloid osmotic pressureThe pressure exerted by proteins (mainly albumin) in the blood that pulls water into the capillaries. You may receive test results about fluids that list the osmolarityA measure of solute concentration in fluid; affects fluid movement between compartments. of the fluids. Now for us in this class these things all mean pretty much the same thing. They mean the attractiveness of the solutes in a solution. When there are solutes in a solution the solutes attract water moleculesGroups of atoms bonded together.. So this is like the attractiveness of solutes. And the more solutes they are the more attractive they are to water. So now looking at this term colloidA mixture where small particles are dispersed but not dissolved in a liquid., it means substance. Osmotic indicates that there’s some type of water movementA fundamental property of life involving motion of the body or its parts.. The term pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. indicates something about a gradient. 

Osmosis

Finally, we get to the concept of osmosis. It has a very robust definition. We can kind of leave it at the movement of water. I appreciate it when students share amusing stories. They tell me they fell asleep on their book and thought they could study by way of osmosis. But you can’t. Osmosis is the movement of water, not the movement of knowledge. More specifically, osmosis is the movement of water across a semi permeable membrane. This means that some cells somewhere has to be involved. Many of you may have heard this saying before that water follows salt. Is correct because as we found with colloid osmotic pressure salts or solutes are attractive to water. Water molecules like to stick to solute molecules. For water is always drawn to an area of hypertonicity. Another perspective is that water is drawn to hypertonic areas. This helps make the inside and outside of the cell an isotonic mixture. Isotonicity would be the target homeostasisThe maintenance of a stable internal environment in the body. that a cell is looking for. When a cell’s insides are isotonic to the outside, it exchanges freely with the environment around it. Nope notice the cell in the center of Panel of this picture. Indicating water movement are going both in and out of the cell. Students often mistakenly think that when the inside and outside tonicity is the same, there is no exchange in the cell. However, that is incorrect. At a state of homeostasis, the cell freely exchanges with the environment around it.

So the arrows in these pictures kind of indicate to us where the areas of hypertonicity may be. With the cell here on the left, water is being drawn out of the cell. This means that the outside of this cell is hypertonic to the inside. With the panel on the right, the arrows indicate that water is moving into the cell. Therefore, the inside of the cell is the area of hypertonicity. Again I know this from the saying water follows salt which means that water will flow to areas of hypertonicity.  Sometimes there are students who feel I am saying the exact wrong thing here. I want to acknowledge that it is very easy to misunderstand this. Over years of teaching, I have learned something important. I can’t say much to convince you that water flows to an area of hypertonicity. Right now just believe me that it’s true. I assure you that in about three days you’ll wake up. It will be 3:00 AM in the morning and you’ll say I get it.