Active Transport – Transport Across Cell Membranes

by Georgina Cornwall, PhD

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    00:01 So we've covered the three mechanisms of passive transport where we have basic diffusion, we have facilitated diffusion if the molecules cannot pass through the membrane on their own.

    00:16 They need a channel or a carrier. And we've also covered osmosis which is specific to the movement of water. Now, let's explore what active transport really is. Active transport is the opposite of passive in that it requires energy. We are moving things against their concentration gradient.

    00:39 So, what happens in this situation? Well there is three ways that it could really happen.

    00:45 You could have one molecule moving itself up its concentration grandient. So from low concentration to high concentration. We could have symporters which are things going in the same direction. We'll look at an example of that shortly. Glucose is transported with sodium in a symport protein and we'll see that shortly. Antiport is when we have molecules being transported in opposite directions of each other. So all of these are possible secondary I mean, all of this are possible active transport mechanisms. So moving on we will take a look at a primary active transport mechanism. One that comes up in Biological sciences and Physiology all the time. You may have heard of it before. The sodium-potassium pump.

    01:45 The sodium-potassium pump is moving sodium against its concentration. At the same time is moving potassium against its concentration in the opposite direction. So what do we call that based on our transporter types? One this way, one that way, antiport right. So there are going in opposite directions. So the sodium-potassium pump is an antiporter. Sodium, the natural state of the sodium-potassium ATPase which is what this transmembrane protein is called, is open to the inside.

    02:24 And it has three binding sites for sodium. Sodium will bind to those three binding sites, and then ATP will come along. This is where the energy investment comes. We're pushing sodium against its concentration gradient. ATP comes along. Provides a little bit of energy.

    02:43 Drops off a phosphate and leaves as a lower energy molecule. But this investment of energy causes the conformational change. It causes the ATPase to open up on the other side and dump out sodiums. Now, the next stage in the game is going to be having the potassiums bind in to their binding sites. As soon as potassiums bind into their binding sites, what we'll see is this antiporter snaps closed and drops them off on the other side of the membrane.

    03:17 So again we have the investment of ATP in order to make all of this happen. So to summarize this whole thing, first of all we've got the picture up the top where we have our sodiums and they want to go against their concentration gradient. They cannot diffuse, they cannot do facilitated diffusion because we're going up the concentration gradient from low to high.

    03:41 The sodiums jump in to their spots inside the transporter. The ATP comes along, phosphorylates the protein. It has a conformational change. And we continue through the cycle. The conformational change happens. The sodium pump opens up, grabs its potassiums, snaps close, dumps them on the inside.

    04:05 So each cycle we have 3 sodiums out, 2 potassiums in. 3 sodiums out, 2 potassiums in.

    04:13 And it will continue like that. And this is one of the primary mechanisms for creating electrochemical gradients across cell membranes or correcting for them for that matter.

    04:24 We have sodiums. Each time we have one more sodium going out than we have potassium coming in.

    04:30 So we have two positive charges coming in. Three positive charges going out. That's the electrical part. And then sodiums are going against their gradient so we're creating essentially some energy difference there across the membrane because of the active transport. Active meaning we have to use ATP. So secondary active transport is actually completely dependent on primary active transport. So we've already established that the sodium-potassium pump is pumping sodium out of the cell and potassium in. And we have established that this is creating a gradient especially for sodium where we have sodium getting higher concentration on the outside of the cell. Lower concentration on the inside of the cell. So it's in the perfect position to actually move down a concentration gradient. The only problem is it's slightly too large and polar to fit through the cell membrane itself. So we need one of those channel proteins or transport proteins. And that's exactly what we have. We're going to look at now the glucose and sodium cotransport protein. In this situation, again, we have sodium and it wants to move down its concentration gradient. Glucose is actually completely dependent on the transport of sodium.

    05:58 So glucose can move against its concentration gradient because essentially sodium is going to grab its hand and jump into the cotransport protein. So the protein that we can see here has a spot for sodium and it has a spot for glucose. And because sodium binds, glucose can bind, and that can end up pushing the gluose against its concentration gradient inside the cell.

    06:28 Now, we call this secondary active transport because the transport against the concentration gradient is completely dependent on what happens in primary active transport. The sodium gradient is created by the primary sodium-potassium active transport pump. Now we have sodium wanting to move passively down its gradient but it grabs the hand of glucose to run through its transporter together. Glucose though is going against its concentration gradient using the power that was invested by ATP in the primary active transport of the sodium-potassium pump.

    07:16 So again, we have two different things going on here. We've got the primary active transport driving the cotransport of sodium and glucose. An interesting thing to remember about this sodium and glucose cotransporter is that it is, glucose will always require sodium in order to get into cells. And this is one of the reasons that many of the sports drinks we consume come with sodium and glucose. Because we must have sodium in order to get the benefit of the glucose if we want to have some recovery during our exercise.

    About the Lecture

    The lecture Active Transport – Transport Across Cell Membranes by Georgina Cornwall, PhD is from the course Cellular Structure.

    Included Quiz Questions

    1. …the molecules against their concentration gradients by using energy
    2. …the water molecules against a concentration gradient by using energy
    3. …the water molecules down the concentration gradient by using energy
    4. …the molecules down the concentration gradient by using energy
    5. …the molecules against their concentration gradient without using any energy
    1. Na+/Glucose Cotransporter ---- Primary active transport
    2. Symport membrane transport ---- Na+/ Glucose Cotransporter
    3. Antiport ---- Na+/K+ Pump
    4. Uniporter ---- Moves a single substance in a single direction
    5. Active transport across membrane ---- Utilize energy
    1. To facilitate the active transport of glucose across the cell membrane via Na+/Glucose cotransporter
    2. To flavor the energy drinks
    3. To facilitate the quick breakdown of the glucose
    4. To promote the rapid release of energy via enzymatic breakdown of glucose
    5. To slow down the leakage of K⁺ ions from the exterior to the interior of the cell
    1. Na+/Glucose cotransporter is an example of the primary active transport system for nutrient absorption in the intestine
    2. Na⁺/Glucose cotransporter is a symport membrane transport system
    3. Na⁺/Glucose symporter utilizes the energy driven from a downhill sodium ion gradient
    4. Na+/Glucose pump is a secondary active transport system as it is dependent on the functioning of the Na+/K+ pump
    5. Na+/Glucose cotransporter pumps in the glucose and Na+ ions from external fluid to the cytoplasm of the cell
    1. It is a secondary active transport mechanism that transports, 3 sodium atoms out of the cell for every 2 potassium atoms in pumps in
    2. It is a primary active transport mechanism that transports, 3 sodium atoms out of the cell for every 2 potassium atoms in pumps in
    3. Creates both an electrical and chemical gradient across the cell membrane
    4. Can be used to drive secondary active transport, in which the gradient generated is used to transport another molecule against its concentration.

    Author of lecture Active Transport – Transport Across Cell Membranes

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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