Active Transport: Sodium-potassium Atpase – Biological Membranes

by Kevin Ahern, PhD

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    00:02 Active transport in contrast to facilitated diffusion is a process that requires the input of energy. Now, the input of energy is essential because, what active transport systems are doing is moving molecules from an area of lower concentration, into an area of higher concentration. Now I showed in one of the previous lectures, the sodium-potassium ATPase that moves ions against concentration gradients on both sides of the cell. So I’ll move to this fairly quickly, but suffice it to say that the sodium-potassium ATPase is embedded in the lipid bilayer, just like you saw in the earlier slides, and here it's using energy, specifically energy of ATP, to move ions against the direction they would naturally move.

    00:44 So we see for example the loading of the sodium, we see the ATP cleavage that allows the chamber to open and release the sodium outside of the cell. We see the binding of the potassium coming into the chamber. We see the release of the phosphate at the bottom of the thing, and the release the phosphate is accompanied by the release of potassium ions. In both cases the ions are moving against a concentration gradient.

    01:13 Now the importance of the sodium/potassium ATPase is several fold, okay. First it generates a resting potential. What is a resting potential? We can think about it as a voltage. The voltage exists because if we think about this, what happened in the previous slide, three sodiums went out, two potassiums came in. That meant that every time that pump functioned, we added one more positive charge to the outside, three out two in. That increased the voltage across the membrane, and the voltage change across the membrane can have significance because potential energy can ultimately be harvested to make other forms of energy.

    01:54 Another thing that is very important about the sodium-potassium ATPase is its used to actually fuel nerve transmission. Now nerves function in a remarkable way. When a nerve is stimulated, what happens is that the nerve opens up a chamber that allows sodium to move across its membrane. Now, if there exists a gradient, which there does because of the sodium-potassium ATPase, then the sodium will come rushing into the nerve cell. When sodium comes rushing into the nerve cell, the voltage across that nerve cell has changed, and since voltage is a very serious consideration for the transmission of information along a nerve cell, we’ve initiated the process. The process that I've just described to you is carried forward many times along the nerve cell, until the signal is ultimately received by the brain.

    02:53 Because there's already a pre-existing gradient that exists as a result of the sodium/potassium ATPase, a nerve cell is ready to fire instantaneously.

    03:02 Now the sodium gradient is also used as an energy source to pump other ions. I said that a gradient or a charge gradient such as a voltage gradient or resting potential is potential energy, that potential energy is used in this example to move other ions, and I’ll show an example in just a minute.

    03:24 Now the last thing about the sodium/potassium ATPase, it's very important to consider, is the need of the sodium-potassium ATPase to help the cell to maintain osmotic balance.

    03:33 What does that mean? Well osmotic balance occurs when there is an equal distribution of things across the membrane. Most students have worked in biology labs for example with membranes that allow diffusion of molecules across them. If you take a membrane that allows that and you put some material in it that cannot, for example, cross the barrier, then what happens when you put that membrane into a solution. Well the material that can't move across the membrane is at a higher concentration inside of the chamber than it is on the outside.

    04:08 So what happens is water starts moving in to equalize the concentration of the material that is inside the chamber, however the material inside the chamber can't get out.

    04:19 So water keeps coming in, the membrane keeps getting bigger and bigger and bigger, and if the pressure is great enough, the membrane will burst. That is what osmotic pressure is all about. Now cells have the same problem, because you remember that a lipid bilayer is permeable to water and there are things inside a cell that can't make it out, but water can make it in. That means that cells are under a continual osmotic pressure.

    04:53 Well how come cells don't burst? The reason that they don't burst is cells use a few tricks including the sodium-potassium ATPase to help balance that osmotic pressure. So when we talk about the sodium-potassium ATPase being the cost or the price of being alive, what the price is, is to avoid the bursting that would happen if water were to freely move into the cell.

    About the Lecture

    The lecture Active Transport: Sodium-potassium Atpase – Biological Membranes by Kevin Ahern, PhD is from the course Biochemistry: Basics.

    Included Quiz Questions

    1. The movement of molecules across the membrane against their concentration gradients using energy.
    2. The movement of sodium ions across the membrane along their concentration gradients using energy.
    3. The movement of sodium ions across the membrane against their concentration gradients without using energy.
    4. The movement of water molecules across the membrane against its concentration gradient using the energy in the form of NADH2.
    5. The movement of water molecules across the membrane along its concentration gradient using the energy in the form of NADH2.
    1. The Na+/K+ pump uses energy in the form of ATP to transport the Na+ and K+ ions along their concentration gradients.
    2. The Na+/K+-ATPase pump is embedded in the lipid bilayer.
    3. The Na+/K+ pump puts three Na+ ions out of the cell for every two K+ ions taken into the cell.
    4. The Na+ /K+ -ATPase pump generates a resting potential.
    5. The Na+/K+-ATPase is necessary to maintain osmotic balance within the cell.
    1. The cells will swell and lyse due to higher osmolarity inside the cell.
    2. The cell will begin to lose water and shrink.
    3. The cell will keep on working as it does under normal conditions.
    4. The cell will start to grow and will divide into two cells.
    5. The cell will start to synthesize heat shock proteins to cope with the high salt concentrations around the cell.

    Author of lecture Active Transport: Sodium-potassium Atpase – Biological Membranes

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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