Biological Membrane Transport - Online Tutor, Practice Problems & Exam Prep

Biological membrane transport involves the movement of molecules across selectively permeable membranes. Small, uncharged, nonpolar molecules like O₂ and CO₂ can freely diffuse, while large, charged, or polar molecules, such as glucose and ions, require facilitated diffusion or active transport. Understanding the permeability of membranes is crucial, as it influences cellular processes. The transport mechanisms can be categorized into molecular transport for small molecules and macromolecular transport for larger entities, highlighting the complexity of cellular interactions and the importance of membrane dynamics in biological systems.

Biological Membrane Transport

Hey, guys. In this video, we're going to begin our introduction to biological membrane transport, which of course you guys have already covered in your old biology courses. And so this should be a piece of cake for you guys. Recall from those old biology courses that molecules have a natural tendency to diffuse down, or with, their concentration gradients from areas of high concentration down to areas of low concentration. If we take a look at our image down below on the left-hand side, we'll see a little reminder of this idea from our old biology courses. Notice over here we have a beaker that is filled with water molecules, and this guy here is taking this dropper filled with red food coloring dye and adding one drop of red food coloring dye to our beaker. Initially, in this relatively small area right here, there's quite a high concentration of red food coloring dye, whereas initially in other areas of the beaker, there's quite a low concentration of the red food coloring dye. But of course, as time progresses here from left to right, because molecules have this natural tendency to diffuse down their concentration gradients, we know that these red food coloring dye molecules here are going to diffuse from the area of high concentration to areas of low concentration, like what we see here. And they will continue to do that until they reach chemical equilibrium, which just means that the red food coloring dye molecules are evenly distributed throughout the beaker. And so that's a pretty straightforward idea from our old biology courses.

Now also recall from our old biology courses that biological membranes are actually selectively permeable, which allows them to act as barriers to prevent or block diffusion from occurring as it naturally occurs. And so we know that selectively permeable is the same thing as semi-permeable from, again, our old biology courses. Permeable is a word that's just referring to how penetrable it is. How easily something can cross in and out. And of course, selectively and semi are both words that are indicating that only some things can cross in and out. The term selectively permeable or semi-permeable, as it applies to biological membranes, just means that biological membranes are incredibly picky about what they allow to cross in and out of the cell membrane. And so if we take a look at the image down below on the right-hand side, we'll see a little reminder of these two ideas from our old biology courses. What you'll notice is right here in the middle, what we have is our biological membrane. And, of course, we know biological membranes are semipermeable. And so even though there is quite a high concentration of these red molecules over here and a low concentration of the red molecules over here, and they would love to diffuse right through, notice that the membrane is acting as a barrier for these guys, so, they're actually not able to cross. Notice that the membrane is saying, "Excuse me, do you red guys over here have an appointment to cross me?" And of course, some molecules will not be able to cross, but other molecules like this yellow guy down here are able to cross through the membrane extremely easily. And so, what is it that allows some molecules to cross the membrane easily and other molecules have a difficult time crossing the membrane? We'll talk about that in our next lesson video. So, I'll see you guys there.

Biological Membrane Transport

So which molecules can freely cross a membrane without any facilitation? Well, it turns out that most of the molecules that actually can freely diffuse or freely cross a biological membrane without any facilitation whatsoever are going to be really, really small molecules that are uncharged and non-polar or hydrophobic. Whereas most of the molecules that cannot freely diffuse without facilitation are going to be the complete opposite. They are going to be large, charged, either positively or negatively charged, and they're going to be polar and/or hydrophilic. And so if we take a look at our example down below, we'll be able to see the diffusion of many different types of molecules across a membrane.

Focusing over here on the left-hand image, first notice that we're showing you a biological membrane right here, and we have the outside of the cell labeled as the blue background and the inside of the cell or the cytosol labeled with the yellowish background. Notice that in our first category of molecules that we have over here on the left, we have O₂ or oxygen gas, CO₂ or carbon dioxide gas, and N₂ or nitrogen gas, and these are small uncharged and non-polar molecules that are hydrophobic, and these are all features that allow these molecules to freely diffuse across the membrane without any facilitation. And so these gases that we see here are able to freely diffuse across the membrane, and we're representing that with a big thick arrow to represent that they're able to easily cross this membrane since they have all of these features that we see here.

Now moving on to the next category of molecules, what we have is a water molecule, a steroid hormone, specifically testosterone, and a glycerol molecule. And what you'll notice is that these are small molecules. They are uncharged, but they do have polar groups on them. And so what this means is that they're starting to mix and match some of the features from above. So they are small and they are uncharged, but they have some polar groups. And so because they have most of the features that allow them to freely diffuse without facilitation, they are able to freely diffuse without facilitation, and so we're representing that with a green arrow. But notice that because they have some polar groups on them as well, they're not able to diffuse across the membrane quite as easily as this first group over here, and so that's why the green arrow is not as thick. The thick one over here represents they can cross really easily, and this one here because it's thin represents that they can cross, but not quite as easily as these other ones over here. And that's again because they have polar groups on them.

Now moving on to this next category of molecules over here, what you'll notice is we have a glucose molecule, some ions, and an amino acid here, specifically glycine, and these molecules are either charged or they are highly polar molecules. Glucose, even though it does not have any charges on it, it has a lot of polar molecules in all of these hydroxyl groups, and all of those polar molecules favor the molecule not being able to freely diffuse without facilitation. Of course, the chloride ion, sodium ion, and potassium ion all have charges on them, and so again that's favoring they cannot freely diffuse without facilitation. The amino acid glycine, with ionizable groups here also has charges on it as well, and so amino acids, ions, and glucose are all examples of molecules that cannot freely diffuse across the membrane.

Now, in our last category down here, what we have are large macromolecules, and of course being large is a feature of not being able to freely diffuse without facilitation. And so these large macromolecules could include polypeptides or large proteins, polysaccharides, and large nucleic acids, and they're simply just too large to squeeze between the phospholipids so they can't squeeze between the phospholipids and therefore cannot diffuse across the membrane easily, especially without facilitation.

Over here on the right-hand side, what we're showing you guys is a scale to represent the same exact ideas that we just talked about. And so if we take a look at this scale, what we'll see is it's a membrane permeability coefficient scale with units of centimeters per second. And basically, all I want you guys to know here is that the higher the value is on this scale, the higher the permeability is, and low permeability means it cannot freely diffuse, and high permeability means that it can freely diffuse without facilitation. And so what you'll notice, looking at the molecules that are on this scale, the oxygen gas, carbon dioxide gas, and nitrogen gas that we talked about over here, have a really high membrane permeability coefficient, which means that they can diffuse really easily across the membrane, just like what we already discussed. Notice that water, testosterone, and glycerol, all still can freely diffuse across the membrane but not quite as easily as these gases. Glucose, even though it is small and uncharged, it's really polar with a lot of polar groups. So it's not able to freely diffuse across the membrane. Then you can see all of the ions, the chloride, potassium, and sodium ions all have charges. So again, they're not able to freely diffuse across the membrane, and glycine, an amino acid that has these charges as well. So again, it's not able to freely diffuse across the membrane with a really low permeability coefficient. And really it's just that idea here, not that you need to memorize any of these numbers or values or anything like that. It's just this idea of showing that if you are small, uncharged, and non-polar hydrophobic, you'll be able to diffuse pretty easily, and if you are the complete opposite large, charged, and polar hydrophilic, you won't be able to diffuse as easily, and really that's the main takeaway of this video. And so that concludes this video, and I'll see you guys in our practice video.

Which molecule most easily diffuses across a biological membrane's lipid bilayer, without facilitation?

• A. H₂O.
• B. O₂.
• C. H₂PO₄^–.
• D. Glucose.
• E. Na⁺.

Rank the unfacilitated transmembrane diffusion of the following molecules from least → most permeable.

• A. Acetamide → Urea → Butyramide.
• B. Urea → Acetamide → Butyramide.
• C. Butyramide → Acetamide → Urea.
• D. Acetamide → Butyramide → Urea.
• E. Urea → Butyramide → Acetamide.

Biological Membrane Transport

In this video, we're going to introduce our map of the entire lesson on biological membrane transport. It's important to note that molecular transport across biological membranes can occur in a wide variety of ways, as you can see by our membrane transport map below. We categorize membrane transport into two different branches. The left branch, which involves molecular transport, or the transport of very small molecules. Then we have the right branch, which involves macromolecular transport, or the transport of very large molecules, and small molecules as well.

Just like the lipid map that we used in our previous lesson videos to cover lipids, this map here on biological membrane transport is a reflection of our entire lesson on biological membrane transport. You can use it to make predictions about what topics we're going to cover and the order of the topics that we're going to cover. You can make a prediction as to what topic we're going to cover next.

The way that this map works is that we're always going to explore the leftmost branches first, and then after we've explored the leftmost branches, we'll zoom out and start to explore other branches. We're first going to explore the left branch with molecular transport of small molecules. Then we're going to explore more passive transport, starting with simple, then moving to facilitated transport, exploring this path, zooming out, and exploring all these paths. Once we've explored all of those, we'll zoom out and explore active transport, again, starting with the left and making our way to the right. Finally, once we cover both passive and active transport, we'll zoom all the way back out and explore the right branch with macromolecular transport, again exploring the leftmost branches first and then making our way to the right.

This here concludes our introduction to our map of the entire lesson on biological membrane transport, and you should continuously refer to this map as we move along in our lesson. I'll see you guys in our next video where we'll start to explore our left branch over here. So I'll see you guys there.

Here’s what students ask on this topic:

What is the difference between passive and active transport in biological membranes?

Passive transport involves the movement of molecules across a biological membrane without the use of cellular energy (ATP). It relies on the natural tendency of molecules to move from areas of high concentration to areas of low concentration, a process known as diffusion. Examples include simple diffusion and facilitated diffusion, where molecules move through protein channels or carriers. Active transport, on the other hand, requires energy to move molecules against their concentration gradient, from areas of low concentration to high concentration. This process involves specific transport proteins, such as pumps, that utilize ATP to function. Examples include the sodium-potassium pump and proton pumps.

Which molecules can freely diffuse across the cell membrane?

Small, uncharged, and nonpolar molecules can freely diffuse across the cell membrane without any facilitation. Examples include oxygen (O₂), carbon dioxide (CO₂), and nitrogen (N₂) gases. These molecules are hydrophobic and can easily pass through the lipid bilayer of the membrane. Additionally, small uncharged molecules with some polar groups, such as water (H₂O), steroid hormones like testosterone, and glycerol, can also diffuse across the membrane, although not as easily as the nonpolar gases.

Why can't large or charged molecules freely cross the cell membrane?

Large or charged molecules cannot freely cross the cell membrane due to their size and polarity. The lipid bilayer of the membrane is hydrophobic, making it difficult for hydrophilic (water-attracting) molecules to pass through. Large molecules, such as proteins and polysaccharides, are too big to fit between the phospholipids. Charged molecules, including ions like Na⁺, K⁺, and Cl^-, are repelled by the hydrophobic core of the membrane. These molecules require specific transport proteins, such as channels or carriers, to facilitate their movement across the membrane.

What is facilitated diffusion and how does it differ from simple diffusion?

Facilitated diffusion is a type of passive transport that involves the movement of molecules across a biological membrane through specific transport proteins, such as channels or carriers. Unlike simple diffusion, which occurs directly through the lipid bilayer, facilitated diffusion requires these proteins to help molecules that are either too large, polar, or charged to pass through the membrane on their own. Despite this assistance, facilitated diffusion does not require energy and still moves molecules down their concentration gradient, from areas of high concentration to low concentration.

How do transport proteins assist in the movement of molecules across the cell membrane?

Transport proteins assist in the movement of molecules across the cell membrane by providing specific pathways or mechanisms for molecules that cannot freely diffuse through the lipid bilayer. There are two main types of transport proteins: channel proteins and carrier proteins. Channel proteins form pores in the membrane that allow specific ions or molecules to pass through by diffusion. Carrier proteins, on the other hand, bind to the molecules they transport, undergo a conformational change, and then release the molecules on the other side of the membrane. Both types of proteins facilitate the movement of molecules down their concentration gradient in facilitated diffusion, while some carrier proteins also participate in active transport, moving molecules against their concentration gradient using energy from ATP.