Summary of Membrane Transport - Online Tutor, Practice Problems & Exam Prep

Membrane transport encompasses passive and active mechanisms. Passive transport includes simple diffusion, where molecules move through lipid bilayers without energy, and facilitated diffusion, which requires protein mediators like carriers and channels. Active transport, such as the sodium-potassium pump, uses ATP to move ions against their gradients. Secondary active transport, exemplified by the sodium-glucose symporter, relies on ion gradients. Additionally, endocytosis and exocytosis facilitate the transport of large molecules. Understanding these processes is crucial for grasping cellular function and homeostasis.

Summary of Membrane Transport

So at this point in our course, we've covered all of the forms of membrane transport that we're going to talk about in our course. And so here in this video, we're going to do a summary of membrane transport. And really, there's no new information in this video. And so if you'd like, feel free to skip this video if you already have a good understanding of all of the different types of membrane transport. And so here we're revisiting our map of the lesson on membrane transport and, of course, we know that we explored this map by exploring the leftmost branches first. And so we talked about the molecular transport of small molecules distinguishing passive transport from active transport. And of course, recall that passive transport means that there's absolutely no energy required, and that's because it's moving molecules downhill down their concentration gradients from areas of high concentration down to areas of low concentration. Whereas active transport on the other hand is going to require energy, and that's because it moves molecules uphill against their concentration gradients from areas of low concentration to areas of high concentration.

In terms of passive transport, we talked about 2 main types, simple diffusion and facilitated diffusion. Recall simple diffusion requires no protein mediator in order for the molecules to cross the membrane. And so in simple diffusion, the molecules just squeeze right between the phospholipids to get from one side of the membrane to the other side of the membrane down their concentration gradients. Whereas with facilitated diffusion, a protein mediator is required in order to allow molecules to transport across the membrane down their concentration gradients. And in terms of the protein mediator, we talked about 2 main types, the carriers and transporters, which undergo a conformational change to allow molecules to transport across the membrane down their concentration gradients. And then we also talked about pore ends and channels, which do not undergo a conformational change. Instead, they create a membrane-spanning tunnel to allow molecules to diffuse down their concentration gradients across the membrane.

Now in terms of the carriers and transporters, we talked about 2 specific biological examples, the erythrocyte glucose uniporter or GLUT1, which allows erythrocytes to uptake glucose from the blood, and then we also talked about the erythrocyte chloride bicarbonate antiporter, which allows for the chloride shift that allows our bodies to transport more carbon dioxide from the tissues to the lungs. And then in terms of the pores and channels, we talked about 5 different types of ion channels, including the leakage ion channel, which always remains open, and then we talked about these 4 gated ion channels, which do not always remain open. They actually open and close in response to various stimuli. The ligand gated ion channel, recall, opens and closes in response to extracellular ligands. The signal gated ion channel opens and closes in response to intracellular signaling molecules. The voltage gated ion channel opens and closes in response to changes in voltage, transmembrane voltage, or transmembrane potential. And then the mechanically gated ion channels open and close in response to mechanical stimuli such as touch, pressure, or sound.

Then in terms of active transport, we distinguish between primary active transport, which is driven directly by ATP, and so you can see ATP hydrolysis is directly shown here. And then we also distinguish secondary active transport, which is not directly driven by ATP. It's indirectly driven by ATP hydrolysis, and actually directly driven by a gradient. And so you can see a molecule diffuses down its gradient as another molecule diffuses against its gradient.

And so in terms of primary active transport, we talked about 5 types of ATPases, the P-type, V-type, F-type, and A-type ATPases, and the ABC transporters that you can see down below. Now in terms of the P-type ATPases, we talked about 2 specific biological examples, the sodium-potassium pump and the SERCA pump, or the calcium ion pump. And of course, in terms of secondary active transport, we talked about a very specific example in the sodium glucose importer of our intestinal epithelial cell.

And then, of course, after we talked about active transport, we shifted over to the final part of our map over here, which is macromolecular transport of really, really large molecules and small molecules as well. And so we distinguish between endocytosis and exocytosis, and recall endocytosis allows molecules to enter the cell, whereas exocytosis allows molecules to exit the cell. And in terms of endocytosis, we talked about 3 different types, phagocytosis or cellular eating of large solid molecules, pinocytosis or cellular drinking of small liquid molecules, and then receptor-mediated endocytosis, which is really just a form that uses receptor proteins. And then, of course, in terms of exocytosis, we specifically talked about neurotransmitter release and those SNARE fusion proteins.

And so hopefully you guys can use this map of the lesson on membrane transport as a refresher and a way to summarize the membrane transport that we talked about in our previous lesson videos. And so this here concludes this video and I'll see you guys in our next one.

Summary of Membrane Transport

Alright. So here we have another really interesting image that helps to summarize some, not all, of the forms of membrane transport that we talked about in our previous lesson videos. And so notice that the top half of this image here is dedicated to passive transport. And, of course, the bottom half of our image right here is dedicated to active transport.

In the passive transport section of our image, notice that we have simple diffusion shown here as number 1. And of course, simple diffusion requires no protein mediation, and so these molecules are capable of squeezing between the phospholipids down their concentration gradient to get from one side of the membrane to the other side of the membrane. Then notice over here for number 2, we're showing you a type of facilitated passive transport. More specifically, we're showing you a carrier/transporter, a glucose transporter such as GLUT 1, which is going to allow molecules to diffuse down their concentration gradients, using a conformational change.

Then for number 3 over here what we have is a type of pore or channel, specifically a Potassium Leakage Ion Channel. And so notice that this allows molecules, ions, to diffuse down their concentration gradients through a channel that is creating a membrane-spanning tunnel instead of a conformational change, and recall leakage ion channels always remain open.

Now down below in the bottom half, the active transport, notice over here for number 4, we're showing you a specific type of P-type ATPase, the sodium-potassium pump, which will take three sodium ions from the inside of the cell and pump them to the outside of the cell, and two potassium ions from the outside of the cell and pump them into the cell, and it does this as it hydrolyzes ATP and becomes phosphorylated itself. Now moving on to number 5 over here, what we have is another type of ATPase, the ABC transporter, also known as an MDR transporter for multi-drug resistance transporter. And notice that it has these two transmembrane domains, these two cytosolic nucleotide-binding domains, and it's capable of hydrolyzing ATP as it transports drugs and toxins outside of the cell.

Then for number 6 over here, what we have is a type of secondary active transport, which is the sodium-glucose symporter. And so notice that it's capable of transporting sodium down its concentration gradient as it transports glucose against its concentration gradient into the cell.

And so again, this image here is just meant to summarize and refresh your memories of some of the concepts that we talked about in our previous lesson videos. But really there's no new information here, and so that concludes this video and I'll see you guys in our next video.

Summary of Membrane Transport Example 1

Alright. So here we have an example problem that wants us to match each term over here on the left-hand side with the correct descriptions that we have over here on the right-hand side. And so, of course, there are many different ways to approach solving this problem. And so if you have your own strategy that works for you, then that's fantastic. We're just going to show you one potential way. And so starting at the top here with a, it says integral membrane protein. And you might recall from our previous lesson videos that integral membrane proteins are integrated into the membrane. And so they form lots and lots of hydrophobic interactions with the hydrophobic part of the membrane, which is the interior part of the membrane. And because they form so many hydrophobic interactions with the interior part of the membrane, these are actually going to interact very tightly with the membrane interior. And so number 3 here is what matches with option a. So we can go ahead and put 3 in here, and of course cross 3 off our list. Now moving on, what we have is peripheral membrane proteins, and recall, peripheral membrane proteins, as their name implies, remain on the periphery or on the perimeter or the border of the membrane. And so they are going to interact with the border of the membrane. And so, we can go ahead and put 5 here for Part B, and, of course, cross off 5 from our list.

So next, what we have in c is a channel, and recall that channel proteins are an example of facilitated passive transport or facilitated diffusion. And so facilitated diffusion is going to match with option c here, the channel. So we can put 1 here for the facilitated diffusion. Now next what we have is passive transport, and of course, passive transport we know requires absolutely no energy. And it's going to move molecules down their concentration gradients, from areas of high concentration to areas of low concentration. And so the only answer option that suggests that there is movement from high to low concentration is option 6, which says that it allows rapid movement of molecules down their concentration gradient. So for option d, we can put 6, and of course cross 6 off our list. Now next what we have is active transport, which you might recall is going to require energy. And so active transport is actually going to do the opposite of passive transport and move molecules against their concentration gradients from areas of low concentration to areas of high concentration. And so, of course, option 7 here, movement against a concentration gradient, is going to match with option e, active transport. And so we can put 7 right here and cross off 7 from our list.

Now, next, what we have is the sodium-potassium ATPase. And of course, the Sodium-Potassium ATPase we know is a type of P-type ATPase that gets phosphorylated during the process of active transport. And we also know that it is an antiport that will pump 3 sodium ions out of the cell and 2 potassium ions into the cell. And so essentially what it does is it establishes an ion concentration gradient of sodium and potassium. And so, of course, it's going to help create an electrical gradient across the membrane in order to make the inside of the cell more negative with respect to the outside of the cell. A lot of that has to do with the function of the sodium-potassium ATPase. And so 10 is going to match with that option. And of course, we can cross off 10.

Now next, what we have is a secondary transporter, and of course, we know that secondary active transport is going to essentially have one molecule travel down its concentration gradient as another molecule is traveling against its concentration gradient. And so essentially what it's doing is it's using the energy of one gradient to create another gradient. And so option 2 here is what matches best with the secondary active transporter here, and so we can go ahead and put 2 right here. Then what we have is an antiporter, and recall that antiporters are going to transport 2 molecules in opposite directions across the membrane. And so, of course, molecules moving in opposite directions is going to match with the antiporter here. So we can put 4 here and cross off 4 from our list.

Now, of course, next what we have are symporters and symporters, Because it starts with s, that can remind us that it moves molecules in the same direction across the membrane. S in symporter for the s in same. And so, of course, this is going to match with option 8, moving molecules in the same direction across the membrane. So we can go ahead and put 8 here and cross this off our list. And then, of course, last but not least, what we have is the ion channel, which is going to match with our last description here. And the ion channel can either be voltage-gated, or ligand-gated, also signal-gated or mechanical gated as well. And it could also be a leakage ion channel as well. But, number 9 here is what's going to match best with the ion channel here. And so we can cross that off our list. And now you can see that these here are the answers to our practice problem. And that concludes this practice, so I'll see you guys in our next video.

Classify each of the following transport systems according to the terms in the list on the right by putting the appropriate letter or letters in the blank next to each transport system. More than one term may apply to each transporter.

__________ GLUT1 transporter of erythrocytes.                      a) Primary active transport.

__________ Cl^–/HCO₃^– transporter of erythrocytes.                b) Secondary active transport.

__________ Na⁺/K⁺ ATPase.                                                   c) Symport.

__________ Ca²⁺ ATPase of sarcoplasmic reticulum.             d) Antiport.

__________ Glucose uptake driven by a Na⁺ gradient.            e) Uniport.

                                                                                                   f) Facilitated diffusion.

Alright. So this practice problem wants us to classify each of the following transport systems that we have over here on the left-hand side, according to the terms in the list on the right that we have over here, and we're going to do that by putting the appropriate letter or letters that are associated with each of the terms, and we're going to put these letters in the blank next to each transport system. And so you can see that we have these blanks over here that we can fill in the letter or letters that are associated. And so it tells us that more than one term over here may apply to each transporter on the left. And so notice that the first transporter is the GLUT 1 transporter of erythrocytes. And so you might recall from our previous lesson videos that the GLUT 1 transporter of erythrocytes was a classic example of facilitated passive transport or facilitated diffusion. And so for sure, we know that letter f here is going to apply to the GLUT 1 transporter of erythrocytes. Now also recall that the GLUT 1 transporter of erythrocytes would transport just one molecule across the membrane in one direction, and that was a glucose molecule. But, again, because it transports 1 molecule in one direction across the membrane, that makes it a uniport. And so e is also going to apply to this answer, and so f and e are the two correct answers for that transporter. Now moving on, what we have is the Chloride Bicarbonate Transporter of Erythrocytes, and you might recall that this transporter is responsible for the chloride shift. That shifts chloride and bicarbonate in opposite directions. And again, this was another classic example of facilitated diffusion. And so, again, we can put f next to this transporter as well. But because the chloride and bicarbonate were actually shifted in opposite directions across the membrane, that actually makes it an antiporter or an antiport. And so, letter d here is going to apply to this transporter. And so these are the two answers for this transporter. Now moving on, next what we have is the Sodium Potassium ATPase, and you might recall that this was a classic example of a P-type ATPase, and ATPases utilize ATP or energy, and so that makes it a type of active transport. And because it utilizes ATP directly, that makes it a type of primary active transport. And so letter a here is going to apply to the Sodium Potassium ATPase. Now also recall that the sodium ion and the potassium ion are pumped in opposite directions across the membrane. The sodium is pumped outside of the cell whereas the potassium is pumped into the cell, and so because they're pumped across the membrane in opposite antiport, and so letter d is also going to apply here. And so these are the two answers for this part right here. And moving on, what we have is the calcium ATPase of sarcoplasmic reticulum. And so really this one is referring to the SERCA pump from our previous lesson videos. And so you might recall that the SERCA pump was another classic example of a P-type ATPase. And so because it is an ATPase and uses ATP directly, that makes it a form of primary active transport just like the Sodium Potassium ATPase, and so letter a is also going to apply here. But this time, recall that the calcium ATPase or SERCA is really going to pump just one molecule across the endoplasmic membrane in one direction, and that one molecule is, of course, a calcium ion. And because it pumps this one molecule in one direction, that makes it a uniport. And so letter e is going to apply here for this answer. And so these are the answers here for this transporter. Now last but not least, what we have is glucose uptake driven by the sodium gradient. And really this one here is referring to our sodium, glucose, symporter. And so you might recall that this was a classic example of secondary active transport because it did not utilize ATP directly. Instead, it utilizes the sodium gradient that's established by the sodium potassium ATPase. And so, because it's associated with secondary active transport, we can put letter b here. And then, of course, recall that both the sodium ion and the glucose molecule were moved in the same direction across the membrane to the inside of the intestinal epithelial cell. And so that means, because they're both moved in the same direction, that makes it a symporter. Again, you can think of the s in symport for the s in the same direction. And so we can put letter c over here. And so these are the two answers for this transporter. And so these here are the answers to this practice problem, and that concludes this practice. So I'll see you guys in our next video.

Here’s what students ask on this topic:

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

Passive transport involves the movement of molecules across cell membranes without the use of energy, moving from areas of high concentration to low concentration. This includes simple diffusion, where molecules pass directly through the lipid bilayer, and facilitated diffusion, which requires protein mediators like carriers and channels. Active transport, on the other hand, requires energy, usually in the form of ATP, to move molecules against their concentration gradients, from low to high concentration. Examples include the sodium-potassium pump and secondary active transport mechanisms like the sodium-glucose symporter.

How does the sodium-potassium pump work in active transport?

The sodium-potassium pump is a type of primary active transport that uses ATP to move sodium (Na⁺) and potassium (K⁺) ions against their concentration gradients. It pumps 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell. This process involves the hydrolysis of ATP, which provides the energy needed for the pump to undergo conformational changes and transport the ions. This pump is crucial for maintaining the cell's electrochemical gradient and overall cellular function.

What are the different types of facilitated diffusion?

Facilitated diffusion involves the use of protein mediators to transport molecules across the cell membrane down their concentration gradients. There are two main types: carriers/transporters and channels. Carriers/transporters, such as the erythrocyte glucose uniport (GLUT1), undergo conformational changes to move molecules. Channels, including ion channels like the potassium leakage ion channel, create a membrane-spanning tunnel that allows molecules to pass through without conformational changes. Channels can be further classified into leakage channels, which are always open, and gated channels, which open in response to specific stimuli.

What is secondary active transport and how does it differ from primary active transport?

Secondary active transport does not directly use ATP. Instead, it relies on the energy stored in the form of ion gradients created by primary active transport. For example, the sodium-glucose symporter uses the gradient of sodium ions (Na⁺) established by the sodium-potassium pump to transport glucose into the cell against its concentration gradient. In contrast, primary active transport directly uses ATP to move molecules against their gradients, as seen in the sodium-potassium pump.

What are the different types of endocytosis?

Endocytosis is the process by which cells internalize molecules. There are three main types: phagocytosis, pinocytosis, and receptor-mediated endocytosis. Phagocytosis, or cellular eating, involves the engulfing of large solid particles. Pinocytosis, or cellular drinking, involves the uptake of small liquid molecules. Receptor-mediated endocytosis uses receptor proteins to specifically target and internalize certain molecules. This process is crucial for nutrient uptake, immune responses, and cellular signaling.