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
Current time:0:00Total duration:12:59

Video transcript

oh hello there uh I'm at the gym I don't know why you're here but I'm going to do some push-ups so you can join me on the floor if you want now I'm not doing this just to show off or anything I'm actually doing this for science okay oh you see what happened there my arms move my shoulders moved my back and stomach muscles move my heart pumped blood to all those different places is pretty neat huh well it turns out that how we make and use energy is a lot like sports or other kinds exercising can be hard work and a little bit complicated but if you do it right comes with some tremendous payoffs but I'm like hitting a ball with a stick it's so marvelously complicated and awesome that we're still unraveling the mysteries of how it all works and it all starts with a marvelous molecule that is one of your best friends a TV [Music] today I'm talking about the energy process our cells and other animal cells go through to provide themselves with power cellular respiration is how we derive energy from the food that we eat pacifically from glucose since most of what we eat ends up is glucose here's the chemical formula for one molecule of glucose in order to turn this glucose into energy we're going to need to add some oxygen six molecules of it to be exact through cellular respiration we're going to turn that glucose and oxygen into six molecules of co2 six molecules of water and some energy that we can use for doing all of our push-ups so that's all well and good but here's the thing we can't just use that energy to run a marathon or something first our bodies have to turn that energy into a really specific form of stored energy called ATP or adenosine triphosphate you've heard me talk about this before people often refer to ATP as the currency of biological energy think of it as an American dollar it's what you need to do business in the u.s. you can't just walk into a Best Buy with a handful of Chinese yen or Indian rupees and expect to be able to buy anything with them even though they're technically our money same goes with energy in order to be able to use it ourselves need energy to be transferred into adenosine triphosphate to be able to grow move create electrical impulses in our nerves and brains everything while back for instance we talked about how cells use ATP to transport some kinds of materials in and out of its membranes to jog your memory about that you can watch that episode right here now before we see how ATP is actually put together let's look at how cells can cash in on the energy that's stashed in there well adenosine triphosphate is made up of a nitrogenous base called adenine with a sugar called ribose and three phosphate groups attached to it so one thing you know about these three phosphate groups is that they are super uncomfortable sitting together in a row like that like three kids on a bus who hate each other all sharing the same seats so because the phosphate groups are such terrible company for each other ATP is able to do this nifty trick where it shoots one of the phosphate groups off the end of the seat creating ad or adenosine diphosphate because now there are just two kids sitting on the bus seat in this reaction when the third jerk kid is kicked off the seat energy is released and since there are a lot of water molecules just floating around nearby a no H pairing that's called a hydroxide from one of the h2s comes over and takes the place of that third phosphate group and everybody is much happy by the way when you use water to break down a compound like this it's called hydrolysis hydro from water and lysis from the Greek word for separate so now that you know how ATP is spent let's see how it is minted nice and new by cellular respiration like I said it all starts with oxygen and glucose in fact textbooks make a point of saying that through cellular respiration one molecule of glucose can yield a bit of heat and 38 molecules of ATP now it's worth noting that this number is kind of a best-case scenario usually it's more like 29 or 30 ATP's but whatever people are still studying this stuff so let's stick with that number 38 now cellular respiration isn't something that just happens all at once glucose is transformed into ATP's over three separate stages glycolysis the Krebs cycle and the electron transport chain traditionally these stages are described as coming one after the other but really everything in the cell is kind of happening all at the same time but let's start with the first step glycolysis or the breaking down of the glucose glucose of course is a sugar you know this because it's got an O at the end of it and I call this this is just the breaking up of glucose is six carbon ring and two to three carbon molecules called pyruvic acids or pyruvate molecules now in order to explain exactly how glycolysis works I need about an hour of your time a giant cast of finger puppets each playing a different enzyme and though it would paid me to do it I would have to use words like phospho glucose on erase but a simple way of explaining it is like this if you want to make some money you got to spend some money glycolysis needs the investment of two ATP's in order to work and in the end it generates for ATP s4 in net profit if you will of two ATP's in addition to those four ATP's glycolysis results in two pyruvates and two super energy rich morsels called NADH which are sort of the love children of a B vitamin called na Plus pairing with energized electrons and a hydrogen to create storehouses of energy that will later be tapped to make ATP does keep drive while the awesome stuff we're making here let's let's keep score so far we've created two molecules of ATP and two molecules of NADH which will be used power more ATP production later now worried about oxygen like I mentioned oxygen is necessary for the overall process of cellular respiration but not every stage of it glycolysis for example can take place without oxygen which makes it an anaerobic process in the absence of oxygen the pyruvates formed through glycolysis gets rerouted into a process called fermentation if there's no oxygen in the cell it needs more of that nad plus to keep the glycolysis going so fermentation frees up some nad Plus which happens to create some interesting byproducts for instance and some organisms like yeast the product of fermentation is ethyl alcohol which is the same thing as all of this lovely stuff but luckily for our day-to-day productivity our muscles don't make alcohol when they don't get enough oxygen if that were the case working out would make us drunk which actually would be pretty awesome but instead of ethyl alcohol they make lactic acid which is what makes you feel sore after that workout that kicked your butt so your muscles used to pull the oxygen they had and they had to kick into anaerobic respiration in order to get the energy that they needed and so you have all this lactic acid building up in your muscle tissues ah ah ah back to the score now we have made two molecules of ATP through glycolysis but your cells really need the oxygen in order to make the other thirty sub molecules that they need and that is because the next two stages of cellular respiration the Krebs cycle and the electron transport chain are both aerobic processes which means that they require oxygen and so we find ourselves at the next step in cellular respiration after glycolysis comes the Krebs cycle so while glycolysis occurs in the cytoplasm where the fluid medium within the cell that all the organelles hang out and the Krebs cycle happens across the inner membrane of the mitochondria which are generally considered the power centers of sell the krebs cycle takes the products of glycolysis those carbon-rich pyruvates and reworks them to create another two atps per glucose molecule plus some energy and a couple of other forms which I'll talk about in a minute here's how first one the pyruvate is oxidized which basically means that it's combined with oxygen the carbons off the three carbon chain bonds with an oxygen molecule and leaves the cell as co2 what's left is a two carbon compound called acetyl coenzyme a or acetyl co a then another nad plus comes along picks up a hydrogen and becomes NADH so our two pyruvates create another two molecules of NADH to be used late as in glycolysis and really all life enzymes are essential here they are the proteins that bring together the stuff that needs to react with each other and they bring them together in just the right way these enzymes for example bring together a phosphate with an adp to create another ATP molecule for each pyruvate enzymes also help join the acetyl co a and a four carbon molecule called oxaloacetic acid I think that's how you pronounce it together and they form a six carbon molecule called citric acid and I'm certain that that's how you pronounce that one because yeah it's the stuff that's in orange juice fun fact the krebs cycle is also known as the citric acid cycle because of this very by-product however it is usually referred to by the name of the man who figured it all out hans krebs an ear nose and throat surgeon who fled Nazi Germany to teach biochemistry at Cambridge where he discovered this incredibly complex cycle in 1937 for being such a total freakin genius he was awarded the Nobel Prize for medicine in 1953 anyway the citric acid is then oxidized over a bunch of intricate steps cutting carbons off left and right to eventually get back to oxaloacetic acid which is what makes the krebs cycle a cycle and as the carbons get cleaved off the citric acid there are leftovers in the form of co2 or carbon dioxide which are exhaled by the cell and eventually by you you and I as we continue our existence as people are exhaling the products of the krebs cycle right now good work this video by the way using a lot of ATP making it now each type of carbon comes off of the citric acid some energy is made but it's not ATP it's stored and a whole different kind of molecular package this is where we go back to NAD+ and it's sort of colleague fa d ni v + NF ad r chummy little enzymes that are related to B vitamins derivatives of niacin and riboflavin which you might have seen in the vitamin aisle if B vitamins are good at holding on to high-energy electrons and keeping that energy until it can get released later and the electron transport chain in fact they're so good at it that they show up in a lot in those high-energy vitamin powders that the kids are taking these days na DS and fa da's big awkward batteries that pick up hydrogen and energized electrons from each pyruvate which in effect charges them up the addition of hydrogen turns them into nadh and fadh2 respectively each pyruvate yields 3 nadh s and 1 fadh2 per cycle and since each glucose has been broken down into two pyruvates that means that each glucose molecule can produce six nadhs and two fadh2s the main purpose of the krebs cycle is to make these powerhouses for the next and final step the electron transport chain now go to the time when you're saying sweet pyruvate sandwiches Hank aren't we supposed to be making ATP here let's make it happen Cap'n what's the holdup well friends your patience is finally paying off because when it comes to ATP's the electron transport chain is the real moneymaker in a very efficient cell it can net a whopping 34 ATP's so remember all those NADH s and fadh2 s that we made in the Krebs cycle well their electrons are going to provide the energy that will work as a pump along a chain of channel proteins across the inner membrane of the mitochondria where the krebs cycle occurred these proteins will swap these electrons to send hydrogen protons from inside the very center of the mitochondria across its inner membrane the outer compartment of the mitochondria once they're out the protons want to get back to the other side of the intermembrane because there's a lot of other protons out there and as we've learned nature always tends to seek a nice peaceful balance on either side of a membrane so all of these anxious protons are allowed back in through a special protein called ATP synthase and the energy of this proton flow drives this crazy spinning mechanism that squeezes some ADP and some phosphates together to form ATP so the electrons from the ten nadh --is that come out of the krebs cycle have just enough energy to produce roughly three ATP's each and we can't forget our friend the fadh2 s we have two of them and they make two ATP's each and voila that is how animal cells the world over make ATP through cellular respiration now just to check let's reset our ATP counter and do the math for a single glucose molecule once again we made two ATP's for each pyruvate during glycolysis we made two during the Krebs cycle and then during the electron transport chain we made about 34 and that is just for one molecule of glucose imagine how much your body makes and uses every single day
Biology is brought to you with support from the Amgen Foundation