Week 2. Glycolysis Visualized

by | Lessons

Can you sprint without breathing?

Yes you can, but why?

This question is aimed to provoke you to visualize exercise and nutrition as an active event – but more specifically, to see how a cell itself uses glucose anaerobically at any moment – at rest and during any type of exercise.

I make this possible using models of muscle cells (seen at the top of this page) which show how the aerobic and anaerobic sections of a muscle cell use food as fuel.

IMPORTANT!  Tap or click the image to dismiss or recover the captions.


Thus, the primary goal of this lecture is to picture a cell itself working anaerobically level while sprinting, resting, biking, lifting weights, doing anything. In turn, this will help you understand nutrition and training for any physical endeavor more completely.

This begins by visualizing last week’s concept – glycogen depletion from working muscles – to the inside of a muscle cell ‘as it happens’.


Muscles hardly utilize glycogen during light exercise – so depletion is minimal over time. The heavier or greater the exercise intensity, the greater the quantity of glycogen depletion.


First, a quick reminder of what last week demonstrated:

  • Muscle glycogen is a rich supply of blood glucose repackaged and stored in muscle cells.
  • A graph showing cyclists exercising for 2 hours at high intensity reflected how muscle cells deplete glycogen very quickly, which utilizes great quantities of glucose, but we did not picture how a cell transforms glucose – nor did we consider what glucose is transformed into.

Now we look within the cell itself transforming glucose into a substance called lactate – using cell models to picture how it works.

Check out the gallery below to see how this goes!

IMPORTANT!  Tap or click the image to dismiss or recover the captions.


Next, let’s see how the rate of glycolysis in a cell corresponds to real world walking and running – in MPH from rest to 27.8mph.

Matching Glycolysis (and Lactate Production) to Real World Walking and Sprinting in MPH.

The lactate production in the cells and the graphs below correspond to speeds ran by humans in mph – from rest up to the world’s fastest sprint speed ever recorded – 27.8 mph by Usain Bolt.

The bold face type lists the world record speeds ran by humans in all races from the 5K to the 100m dash.

The takeaway messages:

The intensity of glycolysis in a cell at any moment ALWAYS corresponds to a ‘like intensity’ you perform at in the physical world.

Glucose converts to lactate in cells always in some amount.

The crude screencast video below shows everything just explained.

Know Your Anatomy: Know The True Powerhouse of the Cell

The candles represent mitochondria of the cell where oxygen is used. Mitochondria produce ‘aerobic power’ – or aerobic metabolism – which is insufficient to produce power to run speeds from around 15mph to 27mph.

Most writers call mitochondria the “powerhouse of a cell” – which is incorrect in terms of applied physics of exercise or the concrete reality of producing top speed and power. Mitochondria do not produce high intensity power.

Thus, aerobic metabolism cannot produce sprinting type power, instead it produces relatively slower but sustained movement, topping out around 13mph in the world record marathoner, Dennis Kimetto, as graphed below:

Dennis Kimetto, world record marathoner ran just under 13mph, around 85% VO2 max. Lactate production is near inflection point. Running world record times for the 5k or 3k requires running ≈15mph over the entire distance, which is near vVO2 max or 100% VO2 max, but unsustainable because the speed exceeds lactate threshold. Hence the narrow red zone from 5k to 3k spans this red area.

Water represents the anaerobic section of a cell called the cytosol. Here, glucose is the only fuel substrate used.

The cytosol is where it’s at for producing very high intensity ‘glycolytic’ power or glycolysis. Glucose transforms into lactate in the cytosol.

Fat and protein (amino acids) do not undergo anaerobic glycolysis.

Putting it All Together

“Rate of lactate production” means we’re really talking about the rate of glycolysis in a cell.

You do not feel lactate or acid produced at rest or during low to moderate intensity exercise – but it is happening in your cells as you read this. Lactate production results from being alive while cells use blood sugar.

Very high rates of glycolysis indicate:

  1. High rate of glycogen depletion
  2. Blood sugar is ‘sucked’ out of blood into muscle cells, which can make a person momentarily dizzy or light headed during intense exercise.
  3. The liver speeds up releasing glucose into the blood stream to keep blood sugar steady.*
  4. A Very high carb intake is necessary to replete muscle glycogen

*Sipping on sugar drinks do the same as the liver does – and may help delay running out of fuel in long distance events.

Why is Lactate Often Called Lactic Acid?

High exercise intensities actually do make cells acidic. But this is actually a result of hydrogen ions (H+) produced in a cell during high intensity exercise.

Adding H+ ions to a watery thing increases its acidity, and since cells are made mostly from water, exercise in general increase a cells acidity.

Both acidity (and lactate) skyrocket during very high intensity exercise, which shuts a muscle down and in turn, you are forced to slow down.

At any rate, many people refer to the simultaneous rise in lactate and H+ as if its the same thing. However…

Lactate is actually a ‘buffer’ against the acidic condition in a cell, meaning:

Lactate binds with H+ ions and transports H+ out of the muscle cell into the blood.

  • The blood never becomes acidic by a buildup of ‘lactic acid’, because when H+ is buffered by lactate, its acidifying power is canceled out.

Most people just call this stuff made during high intensity exercise lactic acid.  But it is really two things put together, lactate and H+.

At any rate, the exponential increase of lactate (measured in blood) can be considered essentially the same event as the exponential production of H+ ions in muscles cells.

We can’t poke and prod into cells to measure lactate, but we can draw blood to measure the combined form: lactate/H+.

This is why I label the y-axis of the graph: ‘acidity in the cell’ and ‘lactate in the blood’.

Even at rest, your cells produce a bot of lactate. When starting to walk, you see a bump in acid levels then fall if you continue to move slowly as shown. But as intensity increases, acid levels rise slowly and steadily.


Anaerobic Glycolysis is the Muscle’s Form of Fermenting Sugar

Glucose is a simple sugar made of 6 carbon atoms.   C-C-C-C-C-C (or C6 in shorthand)

Lactate is made when glucose splits (ferments) into two 3-carbon chunk molecules – or two lactates – as shown simply in the graphic, but animated within a cell in the video below.


Fermentation means ‘splitting’ carbon food stuffs into smaller carbon chunks – particularly carbohydrates or sugars.

  • Splitting carbohydrates and sugars in grapes, grains, potatoes, honey and many more foods into smaller carbon chunks is technically called fermentation. Alcohol is only one product among several others produced by fermenting carbohydrates.
  • Glycolysis is just a muscle cell’s form of fermenting glucose (into lactate).

Common “ferments of sugars” (ferments are products of fermenting carbohydrates) include alcohol, acetic acid (in kombucha and vinegar), lactate in muscle cells, butyric acid in butter, and the acid that gives Swiss cheese its distinct smell… propionic acid.

Muscles can ferment only one sugar – glucose – which is made of 6 carbon atoms.

C-C-C-C-C-C (for simplicity)


The ‘split’ of glucose or first step of glycolysis is not actually into lactate, but rather into a 3-carbon chunk called pyruvate*.

  • *Think a ‘pyromanic’ makes a fire to associate this idea: if pyruvate is used aerobically, then it will combine with oxygen in the mitochondria during aerobic metabolism. When this happens, CO2 is made.
  • When pyruvate is used anaerobically it will ferment into lactate.

Thus, glucose is actually a hybrid fuel. When pyruvate is used anaerobically this still means glucose is fermented into lactate. I skipped the pyruvate step because that would have presented another road leading to examination of aerobic metabolism, which is not covered in depth in this lecture.

At any rate, glucose first splits into pyruvate – and if you are performing very high intensity exercise – it is correct to think glucose ferments into lactate at an exponential rate, which will lead to muscle failure when lifting weights or force you to slow down when running.

The difference between anaerobic and aerobic metabolism is a matter of splitting a set carbon atoms versus combining carbon atoms with some other atom.

First consider anaerobic metabolism in terms of splitting carbons.

At any level of anaerobic metabolism, glucose splits in half to produce energy, sort of like splitting an atom (in a fission bomb) releases energy. After all said and done fermenting glucose, 6 carbon atoms have split into two sets of 3 carbons, but 6 carbon atoms still remain.

Aerobic metabolism combines carbon with oxygen (O2).

Aerobic metabolism is a form of combustion. Carbon (Fuel) + O2 –> CO2 + H2O + Heat

For practical purposes, imagine high intensity anaerobic exercise splits glucose in half at an exponential rate akin to splitting an atom in a fission bomb. In each case, tremendous energy is released.

Unlike an atom bomb, a hydrogen bomb or a fusion bomb fuses atoms together to make something new – akin to how the carbon atoms from glucose and pyruvate will eventually fuse with O2 to make CO2.

In a hydrogen bomb two hydrogen atoms fuse together to make helium (just as our sun does), which produces way more energy and destruction than a fission bomb.

Two fission (atom) bombs ended World War II.





Blank bar coasters.

Giant Metabolism Wheel

Reading: Lactate: Not Guilty as Charged

Questions & Thoughts for Students

Which meat/muscle on a chicken – dark or white – is made especially for anaerobic power?

If you cut blood flow off to your cells, say by applying a tourniquet or interrupting blood flow to your leg, how do the cells produce energy?

Early or primitive life forms such as bacteria transform energy in a more rudimentary way (anaerobically via fermentation) compared to later developed, complex lifeforms. Thus, the cytosol of a human is the ‘bacterial’ or primitive section of the cell that ferments glucose into lactate.

Glycolysis = fermentation of sugar within the ‘bacterial’ section of the cell.

The mitochondria is considered the ‘advanced’ section of the cell because it’s found in later developed life forms from dinosaurs to mammals which use oxygen, beyond primitive lifeforms’ limited way to produce energy.