Background
Odds are you have learned about diffusion and osmosis a few times before—but that doesn’t mean we shouldn’t revisit these topics. It turns out there is an extraordinary amount of cellular energy devoted to regulating these two processes. Cells are literally filled with protein-based highways that allow for the movement of molecules packaged up in vesicles (membrane bubbles), far more filled than any picture you’ve seen lets you imagine. Given this simple fact, let’s look at these processes a bit more.
DiffusionPhysics tells us a lot of what we need to know in this case. Following laws of thermodynamics, everything moves until we reach a particular temperature, absolute zero, at which point…other things happen. This movement is at random, thanks to some physics ideas, and we know that there is definitely an energy component.
Sometimes that energy component is easy to see – let’s add electricity during electrophoresis – but sometimes it’s not as obvious. There is energy associated with concentrations, or how much stuff you pack into a space. We can measure concentrations with various units you learned in chemistry. Sometimes the energy source is from a cellular molecule, such as ATP, which needs to undergo a chemical reaction in order to be accessed. There could be pressure gradients (gradient: difference), too. That’s wind! All of these processes involve a concept called free energy, which is the only driver of reactions. We just say “ok” and move on.
Image 1: Fundamental differences between passive and active transport
Part of diffusion is we go from states of order to chaos naturally. This is where we get the concept of concentration driving diffusion (though not perfectly accurate): a bunch of things in one location is order. But what happens? They move away and spread out. That’s chaos.
Image 2: Distinguishing between net diffusion and equilibrium
Moving into and out of cellsRecall cells are membrane bubbles that maintain their internal environment (saltwater) differently than the outer saltwater. The membrane is rather important and worthy of our analysis.
Our membrane – all cell membranes, actually – are composed of a phospholipid bilayer, with the outsides of this bilayer being attracted to water, and the middle layer being pushed away by water (hydrophobic is actually a misnomer of how the interactions occur). What does this do? We have a sort of bubble formed, where an insanely thin layer of fatty acids separate the saltwater environments. That’s how your cells are formed. That’s how all cells are formed. Is your mind blown yet?! Without that layer of fatty acids, no life as we know it.
Image 3: Diagrammatic view of the plasma membrane with a fluid-mosaic model approach
Some molecules are capable of getting close enough to this fatty layer and can slip through. Those are nonpolar molecules, typically fats that were capable of being transported in water with help, but also gases. These move according to their own diffusion gradients, and thus follow what is known as passive diffusion. We don’t really get much say in controlling these except by sequestering the molecules with other molecules. (Tada, now you know how we move oxygen and carbon dioxide in the lungs and circulation!)
Image 4: Fundamental differences between ionic/polar and nonpolar diffusion through the membrane
If you don’t turn out to be one of these molecules, you’re going to need assistance. For that, proteins (hey…a tie-in from last lab!) are necessary. These protein transporters are found stuck inside of the phospholipid bilayer and would be known as an integral protein. These transporters would be divided into two camps: facilitator proteins and pump proteins. Let’s look at each one.
Facilitator proteins serve to simply allow the movement of molecules along their free-energy gradients. These can be open all the time (“channel” proteins), or they can have an open-close switch (“gate” proteins). A famous example of a channel protein is called an aquaporin, which you could probably guess what it lets move: water.
Image 5: The aquaporin
Gate proteins typically can be opened or closed through the manipulation of their environment. How does this happen? As you recall, protein’s ability to fold depends on its environment – if we change the environment, we can change its structure (such as opening or closing a gate!). We can do this by changing the charges near the gate, or by adding a chemical that can force the gate open or closed. Easy.
Sample Solution