Passive Transport: Diffusion and Osmosis - Online Tutor, Practice Problems & Exam Prep

Passive transport includes diffusion and osmosis. Diffusion is the movement of molecules from high to low concentration without energy, primarily involving small, uncharged, nonpolar molecules. Facilitated diffusion, assisted by channel and carrier proteins, also moves molecules down their concentration gradient. Osmosis specifically refers to water movement across semipermeable membranes, influenced by solute concentration. Aquaporins facilitate rapid water transport through hydrogen bonding. Solutions can be hypertonic (cell shrinks), hypotonic (cell swells), or isotonic (equal flow). Understanding these processes is crucial for cellular function and homeostasis.

Diffusion

Hi. In this video, we're going to be talking about passive transport, focusing specifically on diffusion and osmosis. So first, let's talk about diffusion. Diffusion is the movement of molecules towards equilibrium, and it's unassisted, considered passive transport, passive movement, and generally, it is limited to small uncharged nonpolar molecules. This is because these types of molecules require no energy input to cross the membrane, as long as they are moving the molecules from an area of high concentration to an area of low concentration.

Now with simple diffusion, what we're going to see is this term called a partition coefficient. This measures the nonpolar solubility of something in a nonpolar solvent. Generally, if the molecule trying to pass the membrane has greater lipid solubility, meaning that it dissolves really well in nonpolar solvents or things like oil, then it is generally going to diffuse faster across the membrane. This is a measurement used to describe things that will move across the membrane through diffusion. With simple diffusion, you can see here, you have molecules moving both ways across the membrane. There's nothing assisting them, and they are just able to move.

Facilitated diffusion is different because that is the assisted movement of molecules. However, these molecules are still moving from an area of high concentration to an area of low concentration. They just can't get through the membrane without some kind of assistance. One of the interesting things is if you remember the Michaelis-Menten equation that we talked about way back in chapter 3 for enzymes, what you will notice is that Michaelis-Menten equation can actually be used to look at the kinetics of diffusion, especially facilitated diffusion. You may see these equations or things in your book, and if you do and decide that you want to know how to calculate them, feel free to go back to those equations and review them.

Facilitated movement is assisted by proteins, two classes of them called channel proteins and carrier proteins. Channel proteins move molecules by providing a channel through which they pass, which makes sense. Carrier proteins move molecules by binding to the molecule and undergoing some type of conformational change, which propels the molecule to the other side of the membrane. We have a molecule come in, undergo a conformational change of the protein, which propels the molecule inside the cell.

Transport proteins are further classified by how many molecules they can transport at once. A uniporter transports one molecule at a time, a symporter transports two, but it does so in the same direction, and an antiporter transports two, but in opposite directions. The uniporter is going to be the fastest method of facilitated diffusion because it is really only transporting one, and it is much easier than binding two separate molecules. In the uniporter, transport one molecule here, molecule A across the membrane. The symporter transports A and B and does so in the same direction. And, the antiporter does A and B, but does so in opposite directions. So those are the three classes of transporters in facilitated diffusion. Now let's move on.

GLUT1 Transporter

Hi, in this video I'm going to be presenting two different examples of transporters that you're going to read about a lot in your textbook that are examples of facilitated facilitated diffusion. So the first is the glucose transporter. Now, there are many types of glucose transporters, but the one that you are probably going to read about most is the GLUT1 uniport. So, first, do you remember what a uniport does? Right. It transfers a single molecule across the membrane. And so for the GLUT1 or the glucose transporter that’s moving glucose, duh, from an area of high concentration to an area of low concentration. So, what you see here is glucose, you have all these glucose molecules, and a GLUT transporter moves them to an area of low concentration. Now, if we were to just kind of guess, what cells in the body do you think have a lot of glucose transporters? So think about where in your body there's going to be a lot of glucose that needs to get into the cell. Right. So that's going to be in your gut cells, in your stomach cells, in your intestines. These cells are going to need a lot of glucose transporters because you've just eaten a meal with a lot of glucose in it, and those cells need that glucose, so it has to be able to transport them into cells. So glucose transporters are a really important example of this, of facilitated diffusion.

Now, another one is the sodium-calcium antiporter and unfortunately, I don't have a great image of this, but, first, do you remember what an antiporter does? Right. It transfers two molecules and does them in different directions. So here in this example, we have sodium, and that's going this way, and we have our calcium, and that's going this way. And, this antiporter is extremely important in regulating muscle contraction, and we're going to talk about, in a lot of different topics later that calcium is really important for cell biology and especially, you know, regulating different signals, but in order for it to do that, it has to be able to get into the cell. So there are actually a lot of different ways calcium gets into the cell for a lot of different functions. And so for this one, calcium is really important in regulating muscle contraction through being transported through this sodium-calcium antiport. So those are two examples of facilitated diffusion, but now let's move on.

Osmosis

In this video, we're going to be focusing on osmosis. So, osmosis is the diffusion of water across semipermeable membranes, and water movement depends on the solute concentration. So, if you remember back to intro chemistry, what a solute is, it is the thing that, you know, dissolves in water. And so, water moves from a low solute concentration, which would mean that it has a high water concentration, to a higher solute concentration or low water concentration. We use solute to describe water movement just because it makes more sense than using water. And so when in a hypotonic solution, a cell swells, and in a hypertonic solution, they shrink. Now, there is another word here, isotonic, and that is solutions that have similar concentrations in cells, and you're probably familiar with these terms from your intro classes. Now, osmotic pressure is another term that you're going to read about, and that pressure is or that term describes the pressure needed to stop water flow across the membrane. So, whether that is concentration or actual physical pressure, it's just, you know, the conditions that allow for the water flow to stop moving across the membrane. Now, in osmosis, water has to get across the membrane. So, how does it do that? Well, it does it through these proteins called aquaporins, and they are channel proteins that allow for water to cross the membrane. So, how does water actually flow through aquaporins? Well, the surface of the inside of the channel through which water flows actually has the ability to form hydrogen bonds with water. And so, as a water molecule enters aquaporin, it forms hydrogen bonds, and therefore displaces the water molecule that was sitting there before, which then goes on to create another hydrogen bond. So, these water molecules just create all these different hydrogen bonds as they move up the aquaporin and across the membrane. And so, the important aspect of this is that conformational changes are not needed, and so there is really fast transport because the water molecules are just breaking and reforming these hydrogen bonds as they move through the aquaporin. Now, you can imagine that different cell types and different organisms need different amounts of aquaporins because they react differently with water. So, for instance, a frog has to have a completely entirely different number of aquaporins than humans do because we live in very different water environments. So, aquaporins are really important. So, when we are looking at hypertonic, hypotonic, and isotonic solutions, here are examples of what they look like. So, in our hypertonic, the water is flowing out, the cells are shrinking. In the isotonic, there is an equal amount of water flow into and out of the cell, and in a hypotonic solution, we have water flowing into the cell, which is really causing the swelling of the cell. So that is osmosis, so now let's move on.

Which of the following describes the diffusion of water across a membrane?

• A. Simple Diffusion
• B. Facilitated Diffusion
• C. Osmosis
• D. Uniporters

The plasma membrane by itself is impermeable to all charged molecules.

• A. True
• B. False

Which of the following transport proteins transports two molecules across the membrane in the same direction?

• A. Uniport
• B. Symport
• C. Antiport
• D. Biport

Carrier proteins that transport molecules via facilitated diffusion require energy from ATP.

• A. True
• B. False

Isotonic is a term that describes what?

• A. The cytoplasmic side of a membrane has a higher solute concentration
• B. The extracellular side of a membrane has a higher solute concentration
• C. Both sides of a membrane have equal solute concentrations

Here’s what students ask on this topic:

What is the difference between simple diffusion and facilitated diffusion?

Simple diffusion is the unassisted movement of small, uncharged, nonpolar molecules from an area of high concentration to an area of low concentration across the cell membrane. It does not require energy or assistance from proteins. Facilitated diffusion, on the other hand, involves the movement of molecules down their concentration gradient with the help of specific transport proteins, such as channel proteins and carrier proteins. These proteins assist molecules that cannot easily cross the membrane on their own, such as larger or polar molecules. Both processes are forms of passive transport and do not require energy input.

How do aquaporins facilitate the process of osmosis?

Aquaporins are specialized channel proteins that facilitate the rapid transport of water molecules across cell membranes. They work by forming hydrogen bonds with water molecules as they enter the channel. This bonding displaces the water molecules already in the channel, allowing them to move through quickly. The process does not require conformational changes in the protein, making it highly efficient. Aquaporins are crucial for maintaining water balance in cells, especially in environments where rapid water movement is necessary, such as in kidney cells or plant roots.

What are the roles of channel proteins and carrier proteins in facilitated diffusion?

In facilitated diffusion, channel proteins and carrier proteins assist in the transport of molecules across the cell membrane. Channel proteins provide a hydrophilic pathway through which specific molecules or ions can pass. They are like tunnels that allow substances to move down their concentration gradient. Carrier proteins, on the other hand, bind to the molecules they transport and undergo a conformational change to move the molecules across the membrane. This process is still passive and does not require energy, but it is specific to the molecules being transported.

What is the significance of the partition coefficient in simple diffusion?

The partition coefficient is a measure of a molecule's solubility in nonpolar solvents, such as lipids. It indicates how well a molecule can dissolve in the lipid bilayer of the cell membrane. Molecules with a high partition coefficient dissolve easily in nonpolar solvents and, therefore, diffuse more rapidly across the cell membrane. This is significant because it helps predict the rate at which different molecules can pass through the membrane via simple diffusion. Molecules with low partition coefficients, which are less soluble in lipids, will diffuse more slowly.

How does osmotic pressure influence water movement in cells?

Osmotic pressure is the pressure required to stop the flow of water across a semipermeable membrane due to osmosis. It is influenced by the solute concentration on either side of the membrane. When the solute concentration is higher on one side, water moves towards that side to balance the concentration, creating osmotic pressure. In cells, this pressure can cause water to move in or out, leading to cell swelling (in hypotonic solutions) or shrinking (in hypertonic solutions). Osmotic pressure is crucial for maintaining cell volume and overall homeostasis.