If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Myosin and actin

Learn how proteins, specifically myosin and actin, use ATP to produce movement in muscles. Understand the role of ATP hydrolysis in this process. This is a key part of how muscles function, converting chemical energy into mechanical energy.. Created by Sal Khan.

Want to join the conversation?

  • winston default style avatar for user Adi H
    what does atp stand for and what is it
    (21 votes)
    Default Khan Academy avatar avatar for user
  • mr pants teal style avatar for user Dean
    What happens to the movement of either the Myosin or Actin when the actin strand runs out of "rungs?" I understand how the energy is released and moves the Myosin into the "cocked" position, but when the Myosin has been re-cocked and moved all the way down the Actin strand, what happens?
    (23 votes)
    Default Khan Academy avatar avatar for user
    • starky ultimate style avatar for user Geoff Futch
      As I understand it, this is still an area of ongoing research. But it would seem that the myosin heads have nowhere else to go and are confined within their respective sarcomeres, so while they may "try" to move farther, they will simply attach and reattach in place. What we have observed with certainty is a phenomenon known as "Active Insufficiency" within the muscles, whereby a muscle is unable to produce optimal amounts of force at extreme lengths. When fully lengthened, there is not enough overlap between actin and myosin to generate a lot of contractile force. Conversely, when extremely shortened, myofibrils have nowhere else to go and once again cannot generate as much force as they could in middle ranges of length (this is the scenario you're talking about).
      (20 votes)
  • leaf green style avatar for user fthanedar
    How much energy does this one actin produce? And where in the muscle is actin and myosine?
    (4 votes)
    Default Khan Academy avatar avatar for user
    • leafers sapling style avatar for user Peter Collingridge
      The actin doesn't produce energy, it is like a long fibre. The myosin uses energy to produce force. One myosin molecule with two heads produces about 1.4 picoNewtons (0.0000000000014 Newtons) of force when it changes conformation. Actin and myosin form fibres that are across the whole length of the muscle cell.
      (27 votes)
  • starky tree style avatar for user Sonu
    1 Where do we get ATP?
    2 Why does Myosin climb the Actin "Rope"?
    3 What does Myosin do after it finishes climbing the Actin "Rope"?
    (11 votes)
    Default Khan Academy avatar avatar for user
    • piceratops tree style avatar for user Jacob
      For one thing, we generate ATP through a process called cellular respiration. Cellular respiration consists of 3 main steps which can be divided further down. Step one is called glycolysis, which produces a net of 2 ATP molecules, 2 NADH molecules, and 2 pyruvate. Step two is called the Krebs cycle/ the citric acid cycle, which produces 2 ATPs, one for each pyruvate that enters this cycle, in addition to 8 NADH molecules and 2 FADH2 molecules. The third and final step is called the ETC/ the electron transport chain. This accumulates all the NADH molecules and also the FADH2 molecules formed in the Citric acid cycle and in Glycolysis, forming up to 34 ATP molecules. These electrons come from the hydrogens on the formed NADH and FADH2 molecules, which accumulate the remaining hydrogen protons to form a gradient, which drives the ATP Synthase molecule to form ATP from ADP and inorganic phosphate.
      (3 votes)
  • blobby green style avatar for user Jake
    Does anyone have any info on the mechanics that transport proteins use to move along the cytoskeleton? I know that they walk along the cytoskeleton microtubules using ATP but I wondered if it was the same process (but with different proteins).
    (7 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Captain Claw
    Did anyone find this explanation quite vague? As if he struggled to explain it properly. I think KA can use animations to explain things like this.
    (10 votes)
    Default Khan Academy avatar avatar for user
  • piceratops ultimate style avatar for user Leighton
    It seems to me that this movement is one way. What happens when the myosin reaches the end of the actin?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Satwik Pasani
      Nice question. What you're missing is the traction on actin. The actin is attached to the ends of sarcomeres (Z bands, unless I am mistaken). These bands are then in series with whatever force is being applied on the muscle and transmit this force onto the attached actin.
      Therefore, whenever the myosin detaches, due to the traction on actin, it moves a little to its original position, allowing the cycle to continue. This is what happens in an isometric (where the muscle doesn't shorten) contraction.

      In isotonic (where the muscle shortens) contraction, there is indeed shortening of the sarcomere, which can happen only until a limit. Beyond that, there is no binding site for myosin, and only isometric contraction is possible there.

      It is a bit complicated by the large number of series and parallel elastic elements, but this is the gist of what is happening.
      (6 votes)
  • blobby green style avatar for user nidhi.verma907
    what is the difference between myosin and actin
    (3 votes)
    Default Khan Academy avatar avatar for user
  • leaf green style avatar for user umaima
    when the muscle relaxes does myosin head return to previous rung of actin (where ATP came in)? Hope you understand my question
    (3 votes)
    Default Khan Academy avatar avatar for user
    • leafers ultimate style avatar for user megan.walusiak
      In a resting state (in absence of stimulation at the neuromuscular) a spiral protein called tropomyosin wraps around the actin filament blocking the places that the myosin head binds on actin. Tropomyosin will only reveal myosin binding sites when Troponin, a protein bound to Tropomyosin, binds to calcium ions.

      However, when a person dies, ATP in the muscle cells becomes used up. This causes several things to occur. The cell can no longer tightly control the concentration of calcium ions. The free calcium ions will interfere with tropomyosin/troponin regulation of myosin/actin binding. This allows myosin to bind to actin. In the absence of ATP, myosin will stay bound to actin causing the muscle cells to stiffen. This is known as Rigor Mortis.
      (4 votes)
  • blobby green style avatar for user Janvier Habiyaremye
    Where do calicium ions come from?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • hopper cool style avatar for user Irmak
      They're emitted from the sarkoplasmic reticulum (SR for short), which is a type of endoplasmic reticulum.

      Muscle fibers are long cells. The SR is actually their endoplasmic reticulum.
      (The endoplasmic reticulum is a part of cells.)

      In the video "Role of the sarcoplasmic reticulum in muscle cells", you might find more about the subject.

      Hope that helped!
      (4 votes)

Video transcript

What I want to do in this video is try to understand how two proteins can interact with each other in conjunction with ATP to actually produce mechanical motion. And the reason why I want to do this-- one, it occurs outside of muscle cells as well, but this is really going to be the first video on really how muscles work. And then we'll talk about how nerves actually stimulate muscles to work. So it'll all build up from this video. So what I've done here is I've copy and pasted two images of proteins from Wikipedia. This is myosin. It's actually myosin II because you actually have two strands of the myosin protein. They're interwound around each other so you can see it's this very complex looking protein or enzyme, however you want to talk about it. I'll tell you why it's called an enzyme-- because it actually helps react ATP into ADP and phosphate groups. So that's why it's called an ATPase. It's a subclass of the ATPase enzymes. This right here is actin. What we're going to see in this video is how myosin essentially uses the ATP to essentially crawl along. You can almost view it as an actin rope and that's what creates mechanical energy. So let me draw it. I'll actually draw it on this actin right here. So let's say we have one of these myosin heads. So when I say a myosin head, this is one of the myosin heads right here and then it's connected, it's interwound, it's woven around. This is the other one and it winds around that way. Now let's just say we're just dealing with one of the myosin heads. Let's say it's in this position. Let me see how well I can draw it. Let's say it starts off in a position that looks like that and then this is kind of the tail part that connects to some other structural and we'll talk about that in more detail, but this is my myosin head right there in its starting position, not doing anything. Now, ATP can come along and bond to this myosin head, this enzyme, this protein, this ATPase enzyme. So let me draw some ATP. So ATP comes along and bonds to this guy right here. Let's say that's the-- and it's not going to be this big relative to the protein, but this is just to give you the idea. So soon as the ATP binds to its appropriate site on this enzyme or protein, the enzyme, it detaches from the actin. So let me write this down. So one, ATP binds to myosin head and as soon as that happens, that causes the myosin to release actin. So that's step one. So I start it off with this guy just touching the actin, the ATP comes, and it gets released. So in the next step-- so after that step, it's going to look something like this-- and I want to draw it in the same place. After the next step, it's going to look something like this. It will have released. So now it looks something like that and you have the ATP attached to it still. I know it might be a little bit convoluted when I keep writing over the same thing, but you have the ATP attached to it. Now the next step-- the ATP hydrolizes, the phosphate gets pulled off of it. This is an ATPase enzyme. That's what it does. Let me write that down. And what that does, that releases the energy to cock this myosin protein into kind of a high energy state. So let me do step two. This thing-- it gets hydrolized. It releases energy. We know that ATP is the energy currency of biological systems. So it releases energy. I'm drawing it as a little spark or explosion, but you can really imagine it's changing the conformation of-- it kind of spring-loads this protein right here to go into a state so it's ready to crawl along the myosin. So in step two-- plus energy, energy and then this-- you can say it cocks the myosin protein or enzyme to high energy. You can imagine it winds the spring, or loads the spring. And conformation for proteins just mean shape. So step two-- what happens is the phosphate group gets-- they're still attached, but it gets detached from the rest of the ATP. So that becomes ADP and that energy changes the conformation so that this protein now goes into a position that looks like this. So this is where we end up at the end of step two. Let me make sure I do it right. So at the end of step two, it might look something like this. So the end of step two, the protein looks something like this. This is in its cocked position. It has a lot of energy right now. It's wound up in this position. You still have your ADP. You still have your-- that's your adenosine and let's say you have your two phosphate groups on the ADP and you still have one phosphate group right there. Now, when that phosphate group releases-- so let me write this as step three. Remember, when we started, we were just sitting here. The ATP binds on step one-- actually, it does definitely bind, at the end of step one, that causes the myosin protein to get released. Then after step one, we naturally have step two. The ATP hydrolyzes into ADP phosphate. That releases energy and that allows the myosin protein to get cocked into this high energy position and kind of attach, you can think of it, to the next rung of our actin filament. Now we're in a high energy state. In step three, the phosphate releases. The phosphate is released from myosin in step three. That's step three right there. That's a phosphate group being released. And what this does is, this releases that energy of that cocked position and it causes this myosin protein to push on the actin. This is the power stroke, if you imagine in an engine. This is what's causing the mechanical movement. So when the phosphate group is actually released-- remember, the original release is when you take ATP to ADP in a phosphate. That put it in this spring-loaded position. When the phosphate releases it, this releases the spring. And what that does is it pushes on the actin filament. So you could view this as the power stroke. We're actually creating mechanical energy. So depending on which one you want to view as fixed-- if you view the actin as fixed, whatever myosin is attached to it would move to the left. If you imagine the myosin being fixed, the actin and whatever it's attached to would move to the right, either way. But this is where we fundamentally get the muscle action. And then step four-- you have the ADP released. And then we're exactly where we were before we did step one, except we're just one rung further to the left on the actin molecule. So to me, this is pretty amazing. We actually are seeing how ATP energy can be used to-- we're going from chemical energy or bond energy in ATP to mechanical energy. For me, that's amazing because when I first learned about ATP-- people say, you use ATP to do everything in your cells and contract muscles. Well, gee, how do you go from bond energy to actually contracting things, to actually doing what we see in our everyday world as mechanical energy? And this is really where it all occurs. This is really the core issue that's going on here. And you have to say, well, gee, how this thing change shape and all that? And you have to remember, these proteins, based on what's bonded to it and what's not bonded to it, they change shape. And some of those shapes take more energy to attain, and then if you do the right things, that energy can be released and then it can push another protein. But I find this just fascinating. And now we can build up from this actin and myosin interactions to understand how muscles actually work.