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When I was a kid, swimming was one of my favorite
things to do.
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I was on the swim team starting when I was
four years old and then on and off throughout
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grade school, although I wasn’t especially
fast.
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I just loved the water.
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I used to dream that there was a way I could
be like a fish.
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When I was little, I had a misconception that
fish didn’t need oxygen.
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Later on, I learned that, no, most fish have
gills that allow them to extract the oxygen,
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which they need, from the water.
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So then I just thought it’d be really cool
if I just had gills.
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But, alas, no gills for Pinky.
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Oxygen is a really big deal---so many organisms---from
fish to plants to humans---need oxygen.
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And yes, even though plants make oxygen in
photosynthesis…they still perform cellular
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respiration and therefore plants still need
oxygen themselves.
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There’s often a misconception that plants
don’t need oxygen; that’s just not true.
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So why do these organisms need oxygen?
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It’s similar to why you need oxygen.
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If you’ve ever wondered why you need to
breathe, which is done by the respiratory
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system in your body, zoom into the cell level.
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Cells in your body use the oxygen you inhale
to perform cellular respiration.
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The formula here requires inputs, otherwise
known as reactants, to make ATP.
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And oxygen is one of those reactants in the
overall equation that is needed to break glucose
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down in forming ATP.
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Why ATP?
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ATP stands for adenosine triphosphate.
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It is action packed with three phosphates.
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It has the ability to power many cellular
processes.
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Typically it’s coupled to other things that
it may be powering.
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Now after losing the phosphate, the molecule
is ADP, adenosine diphosphate, because it
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has 2 phosphates.
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In cellular respiration though, there are
enzymes that can add another phosphate to
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it and convert it back into ATP again.
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This particular formula of cellular respiration
is aerobic, meaning overall, it requires oxygen.
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It is pretty complex, and we have a video
breaking down steps.
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But that’s not what this video is about.
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This video is about what happens when there
is no oxygen.
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Because cells still need to make their ATP.
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So what kind of cells can handle the no oxygen
thing?
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Well many types of bacteria can.
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Many types of archaea.
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Yeast, which is a fungus that could be helpful
like making your bread rise.
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Your muscle cells can, for a while anyway.
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These are all just some examples.
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Now these organisms handle the lack of oxygen
in different ways.
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Some organisms such as some types of bacteria
or archaea can do anaerobic respiration---
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they can continue to perform glycolysis, krebs,
and the electron transport chain just like
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aerobic cellular respiration.
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But since there is no oxygen to be that final
electron acceptor at the end of the electron
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transport chain, they use something else.
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Sulfate for example.
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These organisms are specifically adapted to
be able to use a different electron acceptor
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in this anaerobic respiration.
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Another option is the organism may just stick
with doing just glycolysis, which doesn’t
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require oxygen, and then the addition of some
way to get their NAD+ back----we’ll talk
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about what that means in a minute.
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This process is called fermentation and that’s
what we’re going to focus on.
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Fermentation is a way to be able to handle
the little to no oxygen issue: it allows for
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glycolysis to happen and for glycolysis to
keep going.
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That means making ATP when there is no oxygen.
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And while you won’t make as much ATP in
this process as you would aerobic cellular
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respiration, you can’t be too picky when
oxygen isn’t around.
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Recall what glycolysis is from our cellular
respiration video: in glycolysis, you take
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glucose---a sugar---and it gets converted
into pyruvate.
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This takes a little ATP cost to actually start
it up, but overall, you make 2 net ATP per
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glucose molecule and you also produce 2 NADH.
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What’s that?
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Recall that NADH is a coenzyme and an electron
carrier.
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We also need to mention that NADH didn’t
just *poof* appear as a product.
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No, because NAD+ actually was reduced to NADH
when it gained electrons.
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And if the words reduced and oxidized are
confusing…you can remember the famous LEO
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GER mnemonic: Lose electrons= oxidized.
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Gained electrons=reduced.
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So NADHNAD+ is oxidation because it loses
electrons and NAD+NADH is reduction because
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it gains electrons.
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Now NADH, the electron carrier, would normally
be delivering the electrons gained to the
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electron transport chain if this was aerobic
cellular respiration.
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Once losing their electrons, NADH would be
oxidized into NAD+ and be ready to be involved
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all over again in glycolysis.
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But there’s no electron transport chain
step in this fermentation process.
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So we’ve got to regenerate the NAD+ somehow—NAD+
is needed here after all for glycolysis to
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continue.
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Fermentation therefore adds another little
step to the end of glycolysis---a step to
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help regenerate NAD+.
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This happens because fermentation allows NADH
to give its electrons to an electron acceptor
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which, in the two fermentation examples we
are going to give, will either be a derivative
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of pyruvate or pyruvate itself.
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So here we go with two types of fermentation
which both result in different products from
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pyruvate.
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Alcoholic fermentation: as done by some types
of yeast.
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So first glycolysis which yields 2 net ATP,
2 pyruvate, and 2 NADH.
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Now we need the step to regenerate the NAD+
so we can keep doing glycolysis.
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The 2 pyruvate is used which will ultimately
produce carbon dioxide and 2 ethanol (alcohol),
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but the derivative of pyruvate shown here,
acetaldehyde, can act as an electron acceptor
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in this process so that the 2 NADH can be
oxidized to 2 NAD+ so that glycolysis can
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start all over.
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Since ethanol (alcohol) is a waste product
in this process.Yeasts also can do alcoholic
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fermentation in making bread, and the carbon
dioxide product we mentioned is involved with
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helping the bread rise!
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The tiny amount of alcohol produced in the
short fermenting time of bread will evaporate
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in the baking process.
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Lactic acid fermentation: as can be done by
cells such as your muscle cells for example!
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While your muscle cells can do aerobic cellular
respiration, they can shift to lactic acid
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fermentation if they experience an oxygen
debt.
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This could happen if you are working out very
intensely where your blood is unable to deliver
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a sufficient amount of oxygen to them for
their demand.
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Just like with alcoholic fermentation, we
start with glycolysis that yields 2 net ATP,
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2 pyruvate, and 2 NADH.
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But now we need the step to regenerate the
NAD+, and this step is a bit different from
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alcoholic fermentation.
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The 2 pyruvate on the reactant side will ultimately
yield 2 lactate.
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The pyruvate can act as an electron acceptor
allowing NADH to be oxidized to NAD+ so that
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glycolysis can start over.
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By the way, this lactate product or specifically
its other form lactic acid, has often been
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blamed for the muscle soreness that occurs
the day after intense exercise- in many of
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my teaching years this was the hypothesis
with this- but actually there’s some recent
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research that may dispute this product as
the cause of muscle soreness.
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Check out our further reading suggestions
in our video details to learn more!
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Lactic acid fermentation is also done by bacteria
that are involved in making yogurt and lactic
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acid can contribute to its sour taste.
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So overall, fermentation is a pretty remarkable
process.
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Although, it does make us appreciate oxygen
because despite how absolutely awesome fermentation
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may be….…it just can’t make as much
ATP as aerobic cellular respiration.
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Well, that’s it for the Amoeba Sisters and
we remind you to stay curious!
