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Krulwich Wonders: Lord, Save Me From The Krebs Cycle

Wednesday, September 14, 2011 - 10:57 AM


Little kids love dinosaurs, bugs and exploring the woods. Science doesn't scare them; they find it fun — until 9th grade. That's when most of us take our first biology class and everything changes. That's when we learn, not because we choose to, but because we know it might be on The Test, and too often, curiosity gets replaced by fear.

For me, of all the fear-inducing horrors of 9th grade biology, the monster in the pack, the highest mountain of meaningless memorization is — do you remember this? — the Krebs cycle.

Oooh, the Krebs cycle. If there were a single moment in high school when kids all over the country lose interest in the life sciences, it's right here: when their teacher shows them a diagram covered with mysterious acronyms, little dots and squiggly arrows pointing in different directions, and says, "This will be on the test."


Diagram of the Krebs Cycle

Diagram of the Krebs Cycle, Penn State University


What is the Krebs cycle? It was created by a German chemist, Hans Adolf Krebs (of course, he would have that middle name ... though, truth to tell, he was Jewish and got kicked out of Germany) and it describes, in excruciating detail, how, say, a hamburger (rich with protein and fat) gets broken down chemically in a series of repeating steps, till it releases the energy that fuels the teeny batteries (or mitochondria) in your cells.

How food becomes energy, is, obviously, a very important thing to know, but when it's dumped on you in Learn-All-This-Or-Else form, a lot of 9th graders just get scared. For some people, fear will push them through, but for many more, this is the beginning of the end. This is when you decide you don't want to learn any more science and that the science you're learning is being shoved down your throat like an uninvited hamburger.

But now comes the news.


We are living, you and I (and every 14-15-year-old taking biology this year), in a new world, a world that feels our pain. These days, if you want to, you can zip over to Google, or your favorite browser, write in Krebs, and up will pop any number of alternate guides to the Terrible Cycle.

And many of them will be deliberately, almost giddily friendly. Take this hip hop version, "Oxidate It Or Love It," performed by Derrick Davis, a student at Stanford, and Tom McFadden, a biology instructor there. It's a riff on "Hate It or Love It" by 50 Cent, and "On to the Next One" by Jay Z. It's totally fun to watch, and very catchy ...

But did you understand what they were singing? In its friendly, unscary way, it's just as dense as the diagram at the top of this post. Instead of being frightened and mystified, this song leaves me giddy and mystified.

But we also live in an era of extraordinary popular science writing. So here are two sample paragraphs, just as brilliant in their way as the hip hop song, from Pulitzer Prize science writer Jonathan Weiner. Here's his description of the Cycle:

To power all of its molecular machinery ... each cell contains anywhere from a few hundred to a few thousand mitochondria. And every one of those mitochondria contains a large collection of rotary motors. With every breath you take, you set off a long series of actions and chemical reactions that make those rotary motors spin around and around in every living cell of your body like zillions of turbines, windmill vanes, or airplane propellers. These rotary motors turn out a concentrated energy food, an energy-rich molecule called adenosine triphosphate, or ATP.

And this ATP, more than any other molecule in the cellular inventory, makes all the rest of the machines go. This is the fuel of all our mortal engines. Without ATP it would be useless for us to breathe in air, to drink and to eat. Without ATP, even the smallest piece of action in our bodies would slow down and stop.

You can see why Jonathan gets the big prizes. But as easy as this one is to read, it doesn't give you the details; it isn't meant to, it's a beautiful summary.

So where do you go if you want to explore the Krebs cycle in a dense-but-friendly environment? Is there any realm where Krebs can be digested by a not very sciency 9th grader?

Yes, yes and yes, I say. And here I quote the 19th century educator John Dewey: "Understanding derives from activity." Kids learn when they want to know the answer, when they care. The place to learn the Krebs cycle is any classroom where the teacher knows how to make you want to know. Teachers who teach to the test are failing their students. The videos can help. Popular science can whet the appetite, but when all is said and done, it's the teacher who shows you a hamburger and then shows you a human cell with an impossibly small battery part inside spinning a ridiculously small motor, who says "How can we break this burger down into bite size chemicals and power this motor?" If the 15 year old looks up, and says, "I don't know. How?" That's how you begin: with wonder.

A young teacher, Dan Meyer, who taught high school math for five years, has a similar critique of classroom learning (which is now a TED Talk) that he calls "Math Class Needs A Makeover." If you like, check him out. The extended quotation from Jonathan Weiner comes from his eloquent and moving book Long For This World, The Strange Science of Immortality (Harper Collins, 2010). If you want to see and hear more from Stanford's "Science Rappers," in 2009 they had a web hit with Regulatin' Genes; and for football fans, the Ohio State marching band does a Krebs cycle demo on its 50-yard line that is, in its high steppin' way, incomprehensible.


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Comments [9]

Abby from wisconsin

im a 15 year old and honestly i dont understand at all...teachers shove all this info down our throat and then expect us to learn it, when really we just memorize and spit it out on a test then fail the exam...i just want help understanding what is going on

Nov. 07 2011 08:50 PM
Michael Osier from Kalamazoo MI

Dear Jenn from NY..

I think your missunderstanding the point that this article is trying to raise. The krebbs cycle is on such a micro scale that it is very difficult for humans let alone high schoolers to to understand. Most high schoolers don't even give a thought to what energy is. If the concept was approached from a much more macro concept as from the beggings radiating to earth from the sun... then I think it becomes a much more wonderouse process of the different organisms and cyles these joules of energy journey until our cells can make use of them. Biology is taught completely backwards, cells and mitochondria and things which we cannot see with our eyes are inconceivable, but the sun is something that everyone loves. If biology was taught sun to cell rather than really big undistinguishable bold definitions in textbooks to cycles diagrams figures and youtube videos - it may invoke much more wonder into the audience.

Oct. 09 2011 10:37 PM
Jenn from New York

It seems to me there may be a misunderstanding...

The tiny motor to which the paragraph refers is the enzyme ATP synthase that by actually spinning in place (which has been visualized as a video using radiolabels and x-ray), fueled by diffusion through the ATP synthase molecule, puts the third P onto ADP creating ATP. This process is called oxidative phosphorylation through chemiosomosis and takes place AFTER the reactions in the Krebs cycle are complete (or a subset of them).

The Krebs cycle itself is a series of sequential reactions, many of which make the fuel for ATP synthase, NADH and FADH2, or hydrides.

While it can be cyclical, i.e. returning to the first ingredient at the end, Krebs' cycle does not have to cycle fully nor is it a motor. The Krebs cycle makes hydrides, not ATP/energy, from each step and can be initiated at many points without glucose to cycle through the reactions fully.

It is through the electron transport chain (ETC), where a proton (H+) gradient is formed, by which the spinning ATP synthase motor (called the tiniest motor of the cell) is fueled. It is that motor, ATP synthase, to which the paragraph above seems to refer.

If the input into Krebs is other than from glucose - i.e. from raw material of amino acids or fatty acids, only one or a subset of the reactions in Krebs, not the whole cycle, are required to make the hydrides that bring electrons to the ETC, where the proton gradient is formed and the ATP synthase motor spins.

The Krebs cycle is not a motor nor does it have to be a cycle.

ATP synthase (find videos online by googling) is the miraculous tiny motor post-Krebs to which I believe Jonathan Weiner refers...

Sep. 20 2011 08:18 PM
Kaics-sfm from Stanford, CA

I loved getting the Krebs cycle down. That rote knowledge allowed me to understand primary metabolism and its regulation. Similar, I suppose to the way memorizing chords let's you to play a tune. Very empowering, and I did end up majoring in biochemistry eventually.

Sep. 20 2011 07:46 AM
KC from Monterey, CA

"How can we break this burger down into bite size chemicals and power this motor?" If the 15 year old looks up, and says, "I don't know. How?"

I think that the wonder quickly fades when the Kreb's cycle is revealed. Back to square one, great teacher

Sep. 17 2011 01:01 PM

I no longer feel as I have to loathe the Kreb's Cycle. It will carry on without me, my acknowledgement of the process, and any number of standardized test that require the 'filling-in-of-bubbles', which will still use up ATP even if the incorrect bubbles are filled in. Thank you for that.

Sep. 16 2011 01:11 PM
S Belkin from NY

The video is a great, fun approach to learning something so complex...but does it work?

Sep. 16 2011 10:06 AM
Meredith Roehrs from Cheyenne WY

This is something I struggle with every semester- the Dreaded Cellular Respiration Lecture. It is a subject I love, because I finally do understand how all of this stuff works. I taught it just last night in Anatomy and Physiology and I hope that I conveyed some of the excitement I feel every semester when this topic comes up. It is awful, it is dreaded, but maybe with the right delivery I didn't scare all of my students away.

Sep. 15 2011 09:49 PM
Steve from Orange County, CA

Thank you Robert! Learning the Krebs cycle and the Sodium-Potassium pump were moments I realised I did not want to be a biologist. I still loved science, but I could not get past the complexities of these processes. Fortunately, my fear waned, and am now a biologist. But it took teachers who could explain these mechanisms to me before I became comfortable with them.

Sep. 15 2011 04:47 PM

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