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Are you a morning person?
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One of us is and one of us is definitely not.
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Mainly because, when I wake up in the morning,
it just takes a while for me to get my energy
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back.
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It takes a lot of time- and coffee -for that
to happen for me.
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Cells really don’t have that luxury.
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They are busy performing cell processes all the time: active transport of many substances
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needed for their survival, for example.
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And the energy currency they need, specifically, is ATP.
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ATP stands for adenosine triphosphate.
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It’s a type of nucleic acid actually, and
it is action packed with three phosphates.
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We have a video all about ATP and how it works as an energy currency.
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So where am I going with this?
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Well, cells have to make this ATP.
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It doesn’t really matter what kind of cell
you are - prokaryote or eukaryote - you have
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to make ATP.
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The process for making that ATP can be different, however, depending on that type of cell.
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One way is called aerobic cellular respiration.
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Lots of organisms can do aerobic cellular
respiration, but this video is specifically
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going to go into aerobic cellular respiration
in eukaryotic cells.
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That means, this video is talking about the
process within cells that have membrane-bound
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organelles such as a nucleus and mitochondria.
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Eukaryotic cells include the cells you’d
find in protists, fungi, animals, and plants.
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The mitochondria- which can be found in most eukaryotic cells - are going to be kind of
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a big deal in this aerobic cellular respiration, because some of the process occurs in the
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mitochondria.
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So let’s get started.
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Remember the major goal for any organism performing
this: it’s to make ATP.
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Ok, here’s the overall look at the equation
for aerobic cellular respiration.
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Remember that reactants (inputs) are on the left side of the arrow.
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And products (outputs) are on the right side of the arrow.
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This equation, by the way, looks remarkably similar to photosynthesis.
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Look at how the reactants and products are on different sides.
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While that doesn’t mean they’re simply
opposites, it does show that they have substances
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in common.
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In photosynthesis, organisms make glucose.
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Notice how glucose is a product.
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But in cellular respiration, organisms break the glucose down to make ATP.
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Fun fact: You know how when a bean seed first germinates in the ground, it isn’t able
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to do photosynthesis yet?
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Yeah, the germinating bean is relying on glucose that it has stored and it’s doing cellular
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respiration to break it down to make ATP so it can grow.
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Of course, once it starts to mature and develop leaves, it can do photosynthesis.
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It will be able to do both photosynthesis
and cellular respiration.
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Plants make glucose in photosynthesis and they break it down in cellular
respiration:
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they can do both.
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But if you aren’t photosynthetic, such as
a human or an amoeba, you have to find a food
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source to get your glucose.
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You need glucose to get this process started, and we’re going to assume that we’re starting
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with one glucose molecule.
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Step #1 Glycolysis.
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This step takes place in the cytoplasm, and this step does not require oxygen.
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It’s considered anaerobic.
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In glycolysis, glucose, the sugar from the
equation, is converted into a more usable
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form called pyruvate.
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Glycolysis usually takes a little ATP itself
to start up.
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The net yield from this step is approximately 2 pyruvate and 2 ATP molecules.
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And 2 NADH.
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What is NADH?
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NADH is a coenzyme, and it has the ability
to transfer electrons, which will be very
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useful in making even more ATP later on.
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We’ll get to that in a minute.
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Now, an intermediate step occurs.
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The 2 pyruvate are transported by active transport into the mitochondria, specifically the mitochondrial
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matrix.
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In the mitochondria, pyruvate is oxidized.
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The 2 pyruvate are converted to 2 acetyl CoA, which will be used by the next step.
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Carbon dioxide is released, and 2 NADH are produced.
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Step #2 The Krebs Cycle - also called the
Citric Acid Cycle.
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Still in the mitochondrial matrix.
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The Citric Acid Cycle is considered an aerobic process.
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While the cycle doesn’t directly consume
oxygen, some of the events in the cycle need
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oxygen to continue.
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To learn more about this intricate cycle where the 2 acetyl CoA will enter, check out the
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further reading suggestions in the video details.
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Carbon dioxide is released.
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We also produce 2 ATP, 6 NADH, and 2 FADH2.
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FADH2 is also a coenzyme, like NADH, and it will also assist in transferring electrons
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to make even more ATP.
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Step #3 The electron transport chain and chemiosmosis.
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This is, just, a beautiful thing, really.
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In eukaryotic cells, this is still in the
mitochondria, and to be more specific, it’s
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involving the inner mitochondrial membrane.
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We do require oxygen for this aerobic step.
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This is a very complex process, and we are greatly simplifying it by saying that electrons
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are transferred from the NADH and FADH2 to protein complexes and electron carriers.
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The electrons are used to generate a proton gradient as protons are pumped across to the
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intermembrane space.
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All these protons being pumped out into this intermembrane space generates an electrical
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and chemical gradient.
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The thing is, if you remember from our cell transport video, ions (like the H+) don’t
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easily travel across membranes directly without something to travel through.
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The protons can travel through an amazing enzyme called ATP synthase.
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If I could be any enzyme, I’d be ATP synthase, because it has the ability to make ATP by
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adding a phosphate to ADP.
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ADP is a precursor to ATP.
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ADP has two phosphates, but if it obtains
a third phosphate, it becomes ATP.
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So, in chemiosmosis, the protons travel down their electrochemical gradient through a portion
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of the ATP synthase, powering it to make ATP.
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The ultimate goal.
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Oxygen is the final acceptor of the electrons.
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When oxygen combines with two hydrogens, you get H20 - water.
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If you remember from our equation, water is a listed product.
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Now, the electron transport chain and chemiosmosis step makes a lot more ATP compared to the
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other two previous steps.
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How much?
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So, I’ve noticed in my years of teaching
and the assortment of textbooks I’ve collected
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over the years, there are some different numbers in cellular respiration charts.
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In fact, you can even see them change a bit in different editions of the same book.
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And it’s made me want to emphasize that
it’s important to not just memorize a number,
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because it really is more of a range.
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I want to focus less on that number of ATP
made per glucose molecule because it depends
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on a lot of variables.
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One variable is the gradient we were talking about: how many protons were pumped across
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that mitochondrial membrane.
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You can see more variables discussed in the factual references, and those references have
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estimates ranging from 26-34 molecules of ATP per glucose molecule in the electron transport
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chain and chemiosmosis step alone.
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Then if you add the other two steps: Krebs
– otherwise known as the Citric Acid Cycle
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- and glycolysis, you could estimate a range of anywhere between 30-38 net ATP molecules
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total per molecule of glucose.
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Again, to emphasize, a range.
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Now, this was just one way of creating ATP.
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But like we had said at the beginning, all
cells have to make ATP, but the way that they
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do it can differ.
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If there's no oxygen available, some cells
have the ability to perform a process known
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as fermentation.
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It’s not as efficient, but it can still
make ATP when there isn’t oxygen.
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We have a video about that!
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We can’t emphasize enough how important the process of making ATP is for cells.
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For example, cyanide - which is found in some rat poisons - can block a step in the electron
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transport chain, which would block ATP production.
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A poison that prevents ATP from being produced can be deadly.
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With the important role that mitochondria
have in ATP production, there is also a demand
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for increased research on mitochondrial diseases.
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We are confident that the understanding of
how to treat these diseases will continue
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to improve as more people, like you, ask questions.
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Well, that’s it for the Amoeba Sisters,
and we remind you to stay curious.
