Transporters - Online Tutor, Practice Problems & Exam Prep

Transporters are proteins that facilitate the movement of specific molecules across cell membranes, utilizing mechanisms like conformational changes. Key types include ATP-driven pumps, which require ATP for active transport, and coupled pumps, which leverage concentration gradients for indirect transport. Notable examples are the sodium-glucose symporter, which aids glucose uptake, and bacteriorhodopsin, a light-driven pump that moves hydrogen ions. ATP-driven pumps are categorized into P pumps, V pumps, F pumps, and ABC transporters, each serving distinct roles in cellular function and maintaining concentration gradients.

Transporters

Hi. In this video, we're going to be talking about transporters and ATP-driven pumps. So the first class of pumps that I want to talk about are transporters. What are transporters? Well, transporters are proteins that are responsible for transporting specific molecules across the membrane, and they usually do this through some type of conformational change in the protein that allows for the molecule to pass. Transporters can be involved in passive or active transport, and there are 3 types of transporters that exist. The first is ATP-driven pumps, which we're going to talk about a lot. Essentially, these use ATP to drive transport. And then there's also coupled pumps, which use energy from concentration gradients of one molecule to transport another. So, if there's a high concentration of one molecule, it's going to easily flow through the membrane. Well, that means that something else that might not have that concentration force can also be passed. It couples it with this really high concentration gradient. And so, you may see coupled pumps or coupled transport being referred to as indirect active transport, because it does require some type of energy, right, because it needs that concentration gradient, but it's not actually requiring ATP energy. So it's indirect. Two that we've talked about before, but just want to mention again here are symports, which move two molecules in the same direction, and antiports, which move two molecules in the opposite direction. And then, finally, there is this last class called light-driven pumps, and they use energy from light to transport molecules across the membrane. So, if we are looking at the example here, we have our ATP-powered pump, which you can see uses ATP to transport things across the membrane. You have your coupled pumps, which come in two forms, the symporter, which transports two in the same direction. And you have your antiporter, which transports things in opposite directions. And then, finally, you have your light-driven pump, so that when light comes in, that provides energy to transport, in this case, hydrogen ions across the membrane.

Now I want to talk to you specifically about a couple of really important transporters that you're going to read about and you need to know about. And the first one is the sodium-glucose symporter. Remember, a symporter is going to transport two molecules in the same direction across the membrane. The sodium-glucose symporter allows for glucose uptake, or entrance into the cell cytosol, even when concentrations are high. How does it do this? Well, it uses the energy from sodium, and that uses the concentration gradient of sodium to trigger glucose uptake into the cytosol. How this works is the binding of sodium enhances the binding of glucose, and so the transporter doesn't work unless both of them are bound. That allows for the uptake of sodium and also glucose into the cell. If we think about where in the body these are going to be most useful, it's going to be areas of the body that need to take up a lot of glucose. So those are going to be cells, like gut epithelial cells, and it's found on the apical surface, which is going to be the surface that's facing inwards towards the gut. There are different transporters that exist on the other side because it doesn't need to necessarily absorb glucose there, but it may need to let glucose out of the cell and into the bloodstream for something.

If we're looking at the sodium-glucose transporter, what you see is that sodium is here, it comes in and binds, and then that triggers glucose binding into this pocket. Now that both of them are bound, there's a conformational change, which is really common among transporters. After this conformational change takes place, then they can both be released into the cytosol. That allows for glucose uptake into the cytosol even with those high concentrations. Now another one that you're going to read about is bacteriorhodopsin, and that is a protein that uses light energy to pump hydrogen ions. This is a kind of rare protein or transporter. It's found in archaea, commonly lives in Great Salt Lake in Utah, if you've ever been there. It contains this special molecule that can sense light, and once it has sensed light, it then provides that energy to move a proton to the cell exterior. This is what this looks like: the light energy comes in, it is recognized by retinol, and retinol allows for hydrogen to be pumped across the membrane and to the extracellular area. So, that's bacteriorhodopsin and it is a really important transporter. So now let's turn the page.

ATP Driven Pumps

Hello, everyone. In this lesson, we are going to be talking about the different types of ATP-driven pumps. Okay. So, what are ATP-driven pumps? Well, these are going to be certain types of proteins that actively pump substances, molecules, or atoms across cell membranes. And they're ATP driven, which means they require that energy source ATP. So there are 4 different classes of ATP-driven pumps that transport molecules against a concentration gradient, and they do this using ATP. So these are going to be molecules that are very important for building concentration gradients. So what they're going to be doing is they're going to be pumping a substance against its concentration gradient or against its electrical gradient or against a gradient of any kind. And in living organisms, when you try to push something against a gradient, you're going to have to use energy to do that. So that's where the ATP comes into play. This is going to be a form of primary active transport because it requires ATP energy. And there are going to be 4 different classes of these ATP-driven pumps, the P pumps, V pumps, F pumps, and ABC transporters, and we're going to go over those.

So first off, let's start with the P pump. P pumps are going to be called P pumps because they have a phosphorylated intermediate. P pumps are phosphorylated in the process of pumping ions across the plasma membrane. So these are going to be utilized to pump many different types of ions across the plasma membrane. And whenever the ATP binds with these pumps, these pumps become phosphorylated and they get that phosphate group, and that's why they are going to be called P pumps. They have a phosphorylated intermediate. These other types of pumps do not have a phosphorylated intermediate. So these P pumps are going to use that energy to pump different types of molecules or different types of ions across cell membranes. A great example of a P pump is actually going to be the sodium-potassium pump, which if you don't already know what this is, this is a very, very important pump in living organisms, probably the most famous pump in living organisms, and we will definitely touch more on this topic later if we haven't already learned about it. Okay. So sodium-potassium pumps are types of P pumps.

Now, let's move on to V pumps. V pumps are more specified than the P pumps, and that is because V pumps don't just pump many different types of ions across the cell membrane. They only transport hydrogen ions, and they're going to move these hydrogen ions across different types of membranes that are called vacuolar membranes. Now, this looks like, hey, these are going to be used in the vacuole membrane in plants or other organisms, which they can be utilized in the vacuole membrane, but they can also be utilized in the vesicle membrane, and they can also be utilized in other organelle membranes like the lysosome. And these V pumps are utilized to pump hydrogen ions against their gradient to build a hydrogen ion concentration, which will create high acidity in certain environments like the lysosome. So these V pumps are very important. These V pumps are also very important for the electron transport chain and they are going to be utilized to build that concentration of hydrogen ions, which will later be utilized to build ATP.

Now, these are going to be utilized to build that concentration gradient, and the pumps that take advantage of that hydrogen ion concentration gradients are going to be called the F pumps, and they are going to do the reverse of what the V pumps do. And what they're going to do is they are going to utilize those hydrogen ion concentration gradients to drive ATP synthesis. And these pumps also have another name that's very important. They're also called ATP synthase proteins. And that is because they actually generate these ATP molecules of energy from the proton motive force, which was created by the V pumps creating a hydrogen ion gradient. Okay, so those are very important in mitochondria, and they're also very important in chloroplasts. They build those ATP. They're also found in bacterial cells, and that's how they make their ATP.

The last group of transporters, ATP-driven transporters, are the ABC Transporters, and these don't move ions. What they're going to move are small molecules, and these are going to move small molecules across cell membranes or organelle membranes. And this is the largest class of ATP transporters, and they are actually also called the ABC superfamily. So, they're a superfamily because it is such a huge group of proteins. And this giant group of proteins is going to be found in things as small as bacteria all the way up to human beings and other mammals like ourselves. They're found in many different types of living things. Now, what does ABC actually stand for? Because it seems kind of arbitrary, but it's not. It actually stands for ATP Binding Cassette Transporters. So basically, this name just says that this is a group of transporters that binds ATP. So ABC transporters is their name. Now an important group of ABC transporters that I want you all to know is going to be the multidrug resistant protein group or the MDR group. And these are going to be forms of ABC Transporters that move molecules across the cell membrane, but they specifically move drug molecules across a cell membrane and they are going to allow certain types of cells to have drug resistance. Now these are going to be found in human beings. An example of the multidrug resistant protein class working in human beings is some cells have these types of proteins that actually remove chemotherapy drugs from cancer cells, which is not good for the treatment of cancer, but it's a very mechanism. So they, you can see that they actively move drugs out of the cell to allow that cell to have drug resistance. Bacterial cells will also do this and they will also move antibiotic drugs out of the bacteria, which is also another thing that we don't particularly very important group, Another example of an ABC transporter is going to be ABC 4. You don't have to know this one specifically unless your professor wants you to know. This is just an example. The ABC 4 acts as a phospholipid flipase, which if you remember a phospholipid flip is able to transition from one side of the phospholipid bilayer to the other side of the phospholipid bilayer. And what this particular type of protein is going to do is it's going to move small molecules from one side of the cell membrane to the other side of the cell membrane. So they a form of small molecule transporter.

Now before I finish off with this description, I want to let you know that these pumps can have different names, but they will all mean the same thing. There's many different ways to identify these types of pumps. So P pumps can also be called P-type ATPases. So an ATPase is just going to be a protein that utilizes ATP. And these are P-type ATPases, and they are transport proteins. So the same thing goes for the V pump. You can also call a So that's another name. The F pumps are generally going to be called ATP synthase. I don't see them called F pumps very often. ATP synthase is the most common name that I have seen in textbooks and in lessons. So I want you all to know that name as well and ABC transporters are generally just called ABC transporter proteins. They don't have another name other than that. Okay? Alright. So now I wanted to show you all what these proteins, these transporters might actually look like. So you don't have to know these structures unless your professor wants you to, but most of the time you're not going to need to know these. And I just wanted to show you what they're going to look like. We have the P pump here, the V pump, the F pump, and the ABC transporter. And as you can see, they all have unique shapes and they all have unique functions and different molecules that they do move across the cell membrane and transport utilizing the help of the energy from ATP. Okay everyone, let's go on to our next topic.

ATP Pumps

So in this video, I'm going to be going over a couple of examples of ATP-driven pumps. The first one that I want to talk about is the calcium pump, which is really responsible for driving muscle relaxation. The calcium pump is going to be a P-class pump. It's found in what's known as the sarcoplasmic reticulum, a specialized form of endoplasmic reticulum. It is really responsible for causing muscle relaxation, and it does this by pumping calcium from the inside of the cell in the cytosol into the sarcoplasmic reticulum lumen. As the calcium is pumped from the cytosol into the lumen, it triggers various signaling pathways that cause muscle relaxation. We'll go over this last point as we walk through the example. But, calcium and ATP bind to the pump, and this results in a conformational change that opens and releases calcium into the SR lumen. While I do not have a great image of this, imagine the cytosol here and our lumen on this side; calcium will move into the lumen by binding with ATP. This isn't necessarily where it binds, but just to get an idea: both have to bind, and this provides the energy to transport the calcium through to the other side into the lumen by causing some type of protein conformational change.

Now another one that you're going to hear a lot about is the sodium-potassium pump. This pump is responsible for removing sodium and potassium, but it does so against concentration gradients. In the cytosol, there's going to be a high amount of potassium and a low amount of sodium, whereas in the extracellular space, there's going to be a low amount of potassium and a high amount of sodium. This pump is responsible for pumping sodium ions out and potassium ions into the cell, and every time it uses a molecule of ATP, it actually transports 3 sodium ions out and 2 potassium ions into the cell. This is an important pump because it creates a steep concentration gradient of sodium across the plasma membrane, which is then used by the cell for various processes. You can see ATP is bound. So, for every molecule of ATP, there are going to be 3 sodium ions, as shown here (1, 2, 3), and also 2 potassium ions (1, 2). The ATP becomes hydrolyzed; it breaks apart, becoming ADP, and that transports the sodium out of the cell and the potassium into the cell. Eventually, that's exactly what happens, and the process then can repeat itself.

So, that is the sodium-potassium pump. So now, let's turn the page.

Which of the following is not considered a transporter?

• A. V ATP pump
• B. Light driven pumps
• C. Coupled pumps
• D. Ion channels

Transporters always require energy to move solutes across a membrane?

• A. True
• B. False

Which of the following classes of ATP-drive pumps can synthesize ATP when reversed?

• A. P pumps
• B. V pumps
• C. F pumps
• D. ABC Transporters

The sodium-potassium pump works by doing what?

• A. Pumping one sodium ion into the cell, while pumping one potassium ion out of the cell
• B. Pumping one sodium ion out the cell, while pumping one potassium ion into the cell
• C. Pumping three sodium ion into the cell, while pumping two potassium ion out of the cell
• D. Pumping three sodium ion out the cell, while pumping two potassium ion into the cell

Here’s what students ask on this topic:

What are the different types of ATP-driven pumps and their functions?

ATP-driven pumps are proteins that actively transport molecules across cell membranes using ATP. There are four main types:

P pumps: These pumps, like the sodium-potassium pump, are phosphorylated during the transport process and move ions across the plasma membrane.

V pumps: These pumps transport hydrogen ions across vacuolar membranes, creating acidic environments in organelles like lysosomes.

F pumps: Also known as ATP synthase, these pumps use the proton gradient created by V pumps to synthesize ATP, crucial in mitochondria and chloroplasts.

ABC transporters: These pumps move small molecules across cell membranes and are involved in processes like drug resistance and phospholipid flipping.

How does the sodium-glucose symporter work?

The sodium-glucose symporter is a type of coupled transporter that moves sodium (Na⁺) and glucose into the cell simultaneously. It uses the energy from the sodium concentration gradient to drive glucose uptake, even when glucose levels are high in the cytosol. Sodium binding enhances glucose binding, causing a conformational change in the transporter, allowing both molecules to enter the cell. This symporter is crucial in cells that require high glucose uptake, such as gut epithelial cells.

What is the role of bacteriorhodopsin in cellular transport?

Bacteriorhodopsin is a light-driven pump found in certain archaea, such as those in the Great Salt Lake. It uses light energy to pump hydrogen ions (H⁺) across the cell membrane. The light is absorbed by a molecule called retinal, which triggers the transport of H⁺ to the cell exterior. This process helps generate a proton gradient, which can be used for ATP synthesis or other cellular processes.

What are coupled pumps and how do they function?

Coupled pumps, also known as cotransporters, use the energy from the concentration gradient of one molecule to transport another molecule across the membrane. They can be classified into symporters, which move two molecules in the same direction, and antiporters, which move two molecules in opposite directions. An example is the sodium-glucose symporter, which uses the sodium gradient to drive glucose uptake into cells.

What is the significance of the sodium-potassium pump in cells?

The sodium-potassium pump (Na⁺/K⁺ ATPase) is a crucial P pump that maintains the electrochemical gradient across the plasma membrane. It actively transports three sodium ions out of the cell and two potassium ions into the cell, using ATP. This pump is essential for various cellular functions, including maintaining cell volume, generating action potentials in neurons, and regulating osmotic balance.