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Table of Contents
Table of Contents
Hypertonic vs. Hypotonic Solutions
Hypertonic vs. Hypotonic Solutions
Defining the Two Sides of Our Barrier
Defining the Two Sides of Our Barrier
Now that we've spent some time looking at this barrier and its selective permeability, let's take a look at either side of said barrier. Generally, in the context of AP Biology, you will see that on one side of a plasma membrane is an extracellular fluid ("outside cell") and on the other is an intracellular fluid ("within cell"). Sometimes the intracellular side may be labeled as the cytoplasm, which is aqueous solution found throughout the cell. This makes sense - the plasma membrane is the barrier that surrounds the cell. So anything on one side is outside the cell and anything on the other side is within the cell.
Both sides of the cell membrane are generally aqueous, so it would be useful to review some basic terminology from chemistry here. Remember that a solution is made up of two things: the solvent and the solute(s). Often a solution in a cell with be made up of many different solutes within the same solvent (which is generally water). However, we often simplify diagrams to have only the solutes on which we are focused.
Concentration/Osmolarity
Concentration/Osmolarity
You have definitely heard the term concentration (aka osmolarity) many times throughout your day-to-day life, but I want to hone in on the scientific applications of the term here. Concentration basically represents how much solute there is per unit solution. Sometimes it is best to think about this in the context of a very common and obvious solution you are familiar with: salt water.
If you've ever gone swimming in the ocean, you know what salty water taste likes... but how salty is it? Well that depends and, in fact, you are able to taste the differences. If you've ever gotten a sore throat, a common home remedy calls for gargling salt water. To gather this, most people take some water and just pour some table salt into it. Most of the time, people who do this find that this water tastes a little salty, but not nearly as much as ocean water. This does depend on how much salt you added to your tap water, of course, but I am speaking about the average case here.
So, just using the term concentration as you colloquially do, which of the waters - the tap water with some salt added or the ocean water - tastes more concentrated? That is the ocean water. The saltier one is the more concentrated one. That's it - you've used the term concentration correctly for AP Biology and it was that easy!
Assigning Numbers to Concentration
Assigning Numbers to Concentration
That's great and all, but we're scientists. We like numbers when we can have them. And you can't always taste something to tell which is more concentrated. So there is a mathematical way to represent concentration:
Concentration = amount of solute / volume of solution
The unit for concentration, or osmolarity, is M (stands for molar). 1 M = 1 mol/L for reference (just look at the equation, amount of solute is measured in moles, and volume in L, so the resulting units are mol/L, which we have dubbed 'M').
Comparing Two Concentrations
Comparing Two Concentrations
Concentration's significant in AP Biology cannot be understated, and the reason for that will be clear as we cover the next few units. But for now, let's consider some situations in which we might compare two different solutions.
In this image, you can see that there are two solutions, and the solute in question is the same for both solutions. However, the flask on the left (labeled the concentrated one) has a deeper color, and the reason for that would be clear if we could zoom in on a microscopic view of the solution's particles. That is represented by the image in the box.
When you compare the concentrated solution to the dilute solution, you can see that for the same given area (or volume in reality), there are more solutes (the blue particles) in the concentrated solution. So, the concentration (amount of solute/volume of solution) is higher in the left flask.
Hypertonic vs. Hypotonic
Hypertonic vs. Hypotonic
When we are comparing two solutions (such as the intracellular and the extracellular sides of a plasma membrane), the one that is more concentrated is referred to as being hypertonic (think about the word 'hyper' and picturing all those particles. Those particles are really cramped in there and just want to get out, so they're going to be bumping into each other a lot. The dilute solution is referred to as being hypotonic ('hypo' means under or below) because it has a lower concentration. If two solutions have the same concentration, they are said to be isotonic ('iso' means same).
This image shows the same cell placed into 3 different solutions - one hypotonic, one isotonic, and one hypertonic. The quickest way to assess whether a solution is hyper or hypotonic is to look at how much space surrounds each particle. In the hypotonic solution, there is more space for the solutes to move around. In hypertonic solutions, there is less room for each particle on average. In reality, placing a cell into different solutions is very dangerous, and the reasoning for that will be clear when we discuss diffusion.
Concentration Gradients - Just Another Battery
Concentration Gradients - Just Another Battery
The Tendency of Particles to Spread Out
The Tendency of Particles to Spread Out
Now that we can define concentration, let's look at why understanding this is so crucial to AP Biology. In order to do that, I want you to think about what happens if you put a single drop of food coloring into a glass of water. As you can see in these sequential pictures, the drop will slowly start to spread out. It starts off very dark when it is first dropped in (look at how dark the drop is before it hits the water) - that is where it is most concentrated. As it hits the water, the color starts to lighten as it spreads throughout the whole container of water. Why did it do this?
It is often easiest to think or phrase this as the solute (dye particles) 'wanting' to spread out. In reality, of course, particles have no wants, but they tend to spread out because they knock into each other frequently when they are concentrated, sending the particles off in different directions
This 'tendency' of the particles to move in a particular way is similar to how you 'tend' to fall toward the Earth if you are in the air. There is a potential energy (gravitational potential energy) within you. The higher you go, the higher the potential energy.
It is important to note that particles will still be crossing this line back and forth, but the net movement will be zero. So, for every one particle that goes from left to right, another one goes from right to left.
The tendency of the particles to move in such a way, much like gravitational potential energy, represents a form of potential energy. Potential energy is energy that we can turn into something useful, like kinetic energy. But more on that in a bit...
When there is a line separating a hypertonic solution from a hypotonic one, the particles will 'want' to move from the hypertonic solution to the hypotonic solution - remember, particles like some space from one another. So, if enough time passes, the two sides of the membrane will be isotonic in the end. That potential energy that existed was transferred into kinetic energy, moving the particles.
What If the Solutes Cannot Cross the Line?
What If the Solutes Cannot Cross the Line?
Well now we have an interesting situation: there are some particles that want to spread out, but cannot. They are pent up in the hypertonic solution and want to get out. That is a lot of potential energy in that situation. Think about it like a dam, as that is precisely the same situation. You have a LOT of particles on one side that desperately want to get to the other side. That dam is the barrier holding it back. Think about how much potential energy there is simply by imagining what occurs if a dam breaks. Floods from broken dams do an insane amount of damage because of the potential energy held within them. In fact, that is precisely why we build dams - we slowly (emphasis on slowly!) let some of the water through and use that kinetic energy to spin turbines, harnessing the energy for our own purposes.
The Concentration Gradient
The Concentration Gradient
So let's zoom back in to cells despite how interesting dams are. If there are more solutes per unit volume on one side (that's the hypertonic side shown on the left here), they 'want' to move to the other side in order to equalize the concentrations on either side. But what happens if that barrier won't let them do so? There is potential energy there, just like the dam. In fact, this is precisely how batteries operate. Chemical reactions within one side of the battery create a LOT of solutes on that side. They want to move, and the battery lets them do so through a particular path. That is an electrical current - charged particles moving, and their kinetic energy of that motion is used for some work, as physicists refer to it, or the use of energy to do something.
This is a concentration gradient. a difference in concentration (osmolarity) across a membrane. I want you to think of this simply as a battery. This is a charged battery - when one side is hypertonic and the other is hypotonic. When that barrier is lifted and the solutions become isotonic, that is effectively a dead battery. If we want to reuse that battery, we're going to have to charge it back up. We will be exploring this quite a bit shortly.
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