Up until now, we have talked a lot about the structure of different membranes found throughout, or around, the cell. We have also mentioned the fact that one very important function of any membrane is to control, or regulate, what gets into and out of a cell or organelle. In this final section on membranes, we will spend a good deal of time discussing the details of membrane transport.
What you must understand if you want to comprehend membrane transport are the concepts of concentration gradients and diffusion.
To start, let’s look at an example you are likely to be familiar with. Imagine you are sitting in the corner of a stuffy room reading a snazzy lesson about cells on Shmoop. Now imagine that someone in the opposite corner of the room is getting ready for a big date, and he is spritzing on some cologne. Even if there are no air currents in the room, eventually you will be able to smell the cologne all the way in your corner. Ickfest.
The reason you will be able to smell the spritz—try it out if you don’t believe us—has to do with the concept of a concentration gradient.
When the cologne is sprayed, the cologne molecules are highly concentrated on one side of the room. Without getting into all of the gory physics, the smelly molecules will immediately begin to disperse, or diffuse, into areas of the room where there is a lower, or zero, concentration of cologne. Eventually, the molecules will be diffused evenly throughout the room, all the way over into the opposite corner where you are sitting.
The main idea is that molecules naturally move from areas of high concentration to low concentration. In other words, molecules move down their concentration gradient in the process of diffusion.
You can see diffusion at work by placing a single drop of red food coloring in a big glass bowl of water. In a matter of minutes, all of the water in the bowl will be tinged red. The reason? The red dye molecules in the food coloring have moved from a high concentration, or the spot where the drop of food coloring hit the water, to a lower concentration, diffused throughout the bowl. We are thinking that you could come up with your own set of examples of every day diffusion, too.
Now, let’s apply these concepts of concentration gradients and diffusion to the membranes of cells. Recall that big molecules and charged ions cannot simply cross the plasma membrane, even though some small molecules and noncharged ions can cross freely. The membrane feature of allowing some things to cross while keeping others out is called semi-permeability. All membranes are semipermeable.
Molecules that freely cross cell membranes do so through the process of simple diffusion. That is, they move from a high concentration outside the cell to a lower concentration inside the cell, or vice versa. Carbon dioxide (CO2) and oxygen (O2) are both molecules that can move across cell membranes through simple diffusion.
When you breathe in oxygen, the red blood cells in your lungs have a low concentration of oxygen and a high concentration of carbon dioxide inside. When the fresh oxygen molecules in your lungs come into contact with your red blood cells, they diffuse rapidly across your red blood cell membranes into the cells, or down their concentration gradient. At the same time, carbon dioxide molecules diffuse rapidly out of the red blood cells, down their concentration gradient, and into your lungs.
When water molecules move freely across a cell membrane, the process is called osmosis, which is just a special type of simple diffusion. The diffusion is so simple that it'd be like balancing a biology book on your head and instantly learning all of the material. You can see where that would be handy, right?
Osmosis is incredibly important to sustaining life. We generally consider water as a solvent, or the medium in which other things are dissolved (there we go again with the use of sol–). So understanding which direction water will move across a cell membrane in a given situation can be a bit tricky.
To help you out, we have devised a short exercise you can work through as you consider a situation involving osmosis across a cell membrane. You can thank us later.
Why should you care about osmosis? One relevant example should convince you that the process is essential to keeping you alive. Let’s look at red blood cells again. These important cells are suspended in a fluid called blood plasma. Blood plasma is composed of water and solutes, including salts. The cytoplasm of your red blood cells is also composed of water and solutes, including salts. By the way, salts are solutes that cannot freely cross the cell membrane.
Let’s say the concentration of salts in the plasma is higher than the concentration of salts in your red blood cells. Using the exercise above, predict which direction the water molecules will move.
We'll give you a moment.
Time's up! If you predicted correctly, you said that water molecules would move out of the red blood cells and into the blood plasma by osmosis. When this occurs, red blood cells shrivel up and become unable to carry oxygen or carbon dioxide. Definitely not good.
Now let’s say that the concentration of salts is higher in the cytoplasm than in the blood plasma. Which direction will water molecules move?
We'll give you another moment (this time without the theme music).
If you predicted correctly, you said that water molecules would move from the blood plasma into the red blood cells by way of osmosis. The result? The red blood cells would swell and eventually burst, or lyse. Again, this does not sound like too much fun.
Since our red blood cells, under normal conditions, are not shriveled and do not regularly burst, what does this say about the concentration of solutes, especially salts, in the blood plasma compared to the concentration of solutes in the cytoplasm of our red blood cells?
If you are starting to grasp the concepts of diffusion, concentration gradients, and osmosis, you would say that the concentration of salts in the plasma is equal to the concentration of salts in the cytoplasm of red blood cells (again, under normal conditions). This statement is correct.
With this background in osmosis, you are ready to tackle one of the more confusing concepts regarding movement of materials across cell membranes: the concept of tonicity. Simply put, tonicity is a term that describes the concentration of solutes on both sides of a cell membrane. Because it describes two concentrations, whenever you describe the tonicity of a solution on one side of a cell membrane—like, say, the tonicity of the cytoplasm of a red blood cell—you are also describing the tonicity of the solution on the other side—the tonicity of the blood plasma.
To describe the tonicity of solutions, we add a prefix to the word "tonic" to say whether the concentration of solutes on one side of a membrane is high, low, or equal to the concentration of solutes on the other side. These prefixes are:
To solidify this concept, let’s look at the red blood cell example again. In the first situation, we said that the concentration of solutes, or salts, in the cytoplasm of the red blood cells was lower than the concentration of solutes in the blood plasma. In terms of tonicity, we could simply say the cytoplasm of the red blood cells was hypotonic to the blood plasma. Much tidier, right?
In the second scenario, we said that the concentration of solutes in the cytoplasm of the red blood cells was higher than the concentration of solutes in the blood plasma. In this situation, how would you describe the tonicity of the red blood cell cytoplasm relative to the blood plasma? That's right—hypertonic.
Under normal conditions, when red blood cells are not shriveled or bursting, what is the tonicity of the red blood cell cytoplasm relative to the blood plasma? Yep, isotonic.
With the information above, if your teacher gives you the tonicity of a cell relative to its surroundings, you should be able to predict which direction water molecules will move across the cell membrane. Being able to do so should also give you a nice, warm, fuzzy feeling. It might also make you really appreciate your red blood cells.
But what about the molecules that are so freakin' big and/or charged that they cannot simply diffuse across cell membranes? How do they get into and out of cells and organelles? In situations where simple diffusion across the membrane is not possible, membrane channel proteins play an important role.
When the concentration of big molecules is higher outside the cell than inside, and if the right proteins exist in the membrane, these big molecules can move down their concentration gradient through channel proteins into the cell. This process of diffusion through a membrane protein is called facilitated diffusion, and it can occur in either direction, into or out of a cell, depending on where the concentration of molecules is higher.
In addition to facilitated diffusion, cells are able to move big and/or charged molecules through membrane proteins against, or up, the molecules’ concentration gradients. Impressive. Because doing this goes against the natural flow of molecules down their concentration gradients, cells must put energy into the process, generally in the form of ATP. Who knew ATP was so darn useful? For this reason, the movement of molecules up their concentration gradient is called active transport.
One example of active transport you should now be familiar with occurs in the mitochondria, where protons (hydrogen ions, H+) are actively pumped against their concentration gradient from the matrix into the intermembrane space.
On the other hand, the movement of molecules down their concentration gradient across a membrane is called passive transport. To recap, types ofpassive transport include:
The movement of protons from the intermembrane space back into the matrix through ATP synthase is an example of passive transport. However, this type of passive transport is coupled, or connected, to the active transport that occurred just before this to pump protons into the intermembrane space.
In fact, the whole purpose of pumping protons into the intermembrane space in the first place is to create a concentration gradient down which protons can flow back into the matrix through ATP synthase to make ATP.
In the end, it is clear that the transport of materials across cell membranes is a critically important function of those membranes. Without such transport, cells and the organisms they comprise would quickly die. And we would not want that, seeing as we are part of that group of organisms.
Cystic fibrosis is a genetic disease that is fundamentally about a disorder in membrane transport. The protein affected is called the Cystic Fibrosis Transmembrane Conductance Regulator.