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.
Diffusion
Physics 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 cells
Recall 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.
Image 6: (a) Closed sodium- and potassium- ion channels, (b) Open potassium-ion channel
Pump proteins force the movement of molecules against their free-energy gradient. This is done by coupling the fight against free-energy (such as moving Na+ from low to high concentrations) with the breakdown of an energy-storage molecule like ATP (full name: adenosine triphosphate). Perhaps the most famous example of a pump protein is the Na+-K+ ATPase, or “sodium-potassium pump.” It’s diagrammed out below, but it runs like this:
- Sodium ions bind to the protein (they have affinity for some sites);
- ATP is hydrolyzed – breaks – to ADP and a phosphate that remains attached to the protein;
- The break and addition of the phosphate forces the protein to change its shape and release sodium ions to the other side of the membrane (why do you think the sodium ions leave?);
- Potassium ions now bind to the protein, where there is affinity;
- The phosphate is cleaved off, forcing the protein to change its shape; and
- The potassium ions leave onto the other side of the membrane (why?), and we repeat.
Image 7: The sodium-potassium pump, the most famous example of active transport
Diffusion across membranes can happen for any molecule. Sometimes we need more than just a protein to transport these molecules, which is when bulk transport is utilized. Inside the cell, we can rely on diffusion…but it’s really difficult. Cells are massive compared to their internal parts, so we can speed up diffusion by attaching molecules to transporters within the cell. You can look up vesicular transport yourself – it’s pretty amazing to watch. (Have you seen the video produced by Harvard about what goes on inside of a cell? No? Watch it!)
Osmosis
Everything can diffuse. When we deal with water, we call it osmosis. Knowledge does not transfer by osmosis, although it is – in a sense – moved by diffusion. () There are a few differences between osmosis and regular, run-of-the-mill diffusion:
- Osmosis requires a semi-permeable membrane (hey…cells have that!);
- Osmosis goes from high water potential to low water potential;
- Water potential is dictated by pressure, temperature, the dissolved nature of solutes, the concentrations of solutes, location, etc.
Osmosis is such a big deal that there could be courses devoted just to it. However, we don’t have the time. But you know quite a bit about it, as you know how to dehydrate foods (add salt – draws out water – osmosis!), or force water to become purer (“reverse osmosis,” which isn’t a thing, but that’s what people call it), or why you shouldn’t go drinking salt water.
In Class
We are going to walk through four simulations today found online. They are
• Diffusion factors
• Diffusion across a semipermeable barrier
• Diffusion across semipermeable barriers
• Diffusion across a membrane
Sample Solution