The Quick and Dirty of Metabolism

Written by: Sunny Liu
Edited by: Ulya Aligulova


Here’s a puzzle. What’s something you love, need, and crave, which you can get some from people? It may involve salty or sweet fluids, and you likely moan in pleasure when it’s good. Mmmmm

I am referring to food. Don’t be dirty.

By the Law of Conservation of Energy, we know that we cannot create or destroy energy. How does the body take food and convert it into energy?

Our body is an amazing factory of food conversion into energy. Despite differences across Chinese, Thai, Italian, Spanish, and all other types of cooking, the body doesn’t care. There are three macronutrients: carbohydrate, protein, and lipid (fats).

When you eat something, digestion occurs. This is the process in which these complex macromolecules are broken down into smaller, simpler molecules to be absorbed. Carbohydrates are broken down into monosaccharides (simple mono-unit sugars), proteins are broken down into amino acids, and fats are hydrolyzed and re-absorbed as fats. During metabolism, these simple molecules are further oxidized, transferring energy to cells in the form of Adenosine Tri-Phosphate (ATP), the “energy currency” of the cell.

ATP is like a rechargeable battery. And, just like batteries, it needs to be stable in physiological conditions, and allow for controlled release. We can’t afford having ATP release energy spontaneously like Samsung Note 7 batteries – we’d die. Thankfully, ATP is relatively stable in neutral conditions in absence of catalysts, but can rapidly form Adenosine Di-Phosphate (ADP) and Adenosine Mono-Phosphate (AMP) under the right conditions to release large amounts of energy to power the cellular processes a cell needs to survive. Excessive ADP and AMP can then be recycled back to ATP to power the cell.

In fact, the recycling of ADP and AMP is so efficient that there is only a teaspoon worth of ATP in your body at any point of time, which is especially impressive considering that your body uses your body weight’s worth of ATP in a day. This regenerative process is called “metabolism”.

All human cells generate ATP primarily via three processes: Glycolysis, Electron Transport Chain, and Oxidative Phosphorylation. The latter two requires oxygen and takes place in the mitochondria (THE POWERHOUSE OF THE CELL). Thus, some cells, such as the Red Blood Cells, are only capable of the former due to the lack of mitochondria. Muscle cells, due to insufficient oxygen during intense exercise, can increase glycolysis up to 4000 times!

These three metabolic process involve the stepwise oxidation of fuels, such as glucose (and other simple molecules) to smaller molecules, such as carbon dioxide. Each step releases a small amount of energy.

Imagine walking down three floors to meet a professor to discuss a recommendation letter. It is inadvisable to jump down – you’d likely break something (or worse, fall on the professor. Not the preferred way of getting a recommendation letter). Moreover, if you need to deliberate about meeting the professor, you can start walk upwards a few steps, pause halfway, before deciding to continue downwards. On the other hand, if you jumped, you can’t just “un-jump” halfway. There’s no undo button. Similarly, having control of these processes are extremely important – they regenerate ATP that powers the cell. With too little ATP, cells are unable to go about important cellular processes to stay alive. With too much ATP, cells are unable to stop excessive cellular processes, ironically killing themselves. As Goldilocks taught us, we want to be in neither of the extreme end.

One of the first episodes of Rick and Morty discussed the Anatomy Park. The writers got something right – the human physiology is a fascinating piece of machinery. The processes of our physiology is nothing short of beautiful. So the next time you’re feeling down, just remember that it took nature millions and millions of years to develop you from a simple unicellular organism. Your body is amazing!

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