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In my last post we learned how a molecule of oxygen is delivered to the tissues. But how the tissues use that oxygen and the exact mechanism is a story on it's own!! To understand that completely, we need to understand a few basics of metabolism first. So this is gonna be my first series of posts and I hope you'll enjoy this journey which will end in the final post with explanation of exactly how energy is generated by the cells!!
That light in your room, that air conditioner, your laptop.....all uses energy to run. Electrical energy. The car you drive takes energy from the fuel you put in. Our body is in a sense a machine too (the most wonderful machine to be honest!!) and it needs energy to run as well!! What is the form of energy that is used in our body?
Adenosine Triphosphate, ATP
This is the energy currency that our body accepts!! You might have tons of steem, SBDs or smartcash!! Your body doesn't care!! ATP is all it wants!!
ATP is a molecule that holds energy and this energy can be mobilized (or in simpler terms made available for use!!) by hydrolysis (breaking down in presence of water) of the molecule.
There is another currency called GTP, but it's considered energy equivalent to ATP and produced in small amount anyways. So for the sake of simplicity we'll stick to ATP for this series, although i will tell you guys the point where GTP is being produced.
But where does this ATP come from? This is the answer we'll try to find as we move along this series!!
Fuel
As we need fuel to drive our car, our body needs fuel too to produce energy and we supply this fuel in the form of this wonderfully complex thing we call......ready? FOOD!!
The food we eat can be broadly be classified to 3 categories we're all familiar with :
- Carbohydrates
- Proteins
- Fat
Our body has mechanisms in place to metabolize these nutrients to generate energy or use them as building block for synthesis of necessary things...nothing too major...you know....things like your muscles, enzymes, transporter proteins.....okay I was wrong!! Every major thing your body synthesizes, most of the building block is derived from these nutrient, from what you eat, what nutrients are available. Kind of giving validity to the statement You are what you eat!!
Our focus for today will be on the metabolism of carbohydrates because discussing all the nutrients in one post.....I might as well write a book!!! Let's take it one step at a time :)
Digestion and Absorption of Carbohydrates
Believe it or not, digestion of carbohydrates begin as early as in your mouth!! Before the food has even reached your stomach or small intestines, your mouth has already started digesting the carbs by an enzyme called salivary amylase produced by you salivary glands (duh!!). So the saliva in your mouth is actually filled with enzymes, ready to attack them carbs you're chewing!!
Carb digestion takes a break as soon as you have swallowed the food and it has reached your stomach because the acidic environment in our stomach inhibits the activity of the amylase. Once the food passes into the small intestine the digestion begins again. The intestine in fact has a variety of different to digest different carbs!!! You're drinking milk? We have lactase to take care of those lactose you're taking in. We have sucrase for your sucrose....we have maltase for your maltose.....!!
After all these heavy complex carbs have been digested (broken down) into molecules that we can absorb from our gut : glucose, galactose and fructose, absorption begins. These are the forms of carbohydrates our body can metabolize to produce energy. Even all stored carbs in the body needs to be broken down first into glucose before it can be metabolized for energy!! One molecule of glucose has the energy to provide us with 32 molecules of ATP!!
Because glucose is just so much more abundant in amount in the body, we'll focus on how glucose is metabolized!!
Glycolysis
Glycolysis is the fancy term for a metabolic pathway in the cytoplasm that breaks down glucose molecules in an attempt to tap into the energy and use it to make ATP (remember how our body accepts only ATP, so the energy in these nutrients need to be converted to ATP first before it's of any use to the body). Now we're gonna look at an extremely complicated diagram that'll make you lose you minds (that's what i'm trying to do muhahahaha!!).
Image from Kaplan USMLE Step 1 Biochemistry Lecture Notes
Head spinning yet??!! Don't worry!! I'm not gonna make you go through all these crazy stuff!! You don't really need to unless you have a biochemistry exam tomorrow, in that case, what the hell are you doing here on steemit??!!
Moving on, first thing we gotta know is glucose can move in and out of the cells via the cell membrane. So there is no guarantee that it will be faithful to the cell and stay inside the cell when the cell needs to break it down to generate energy. So the first step the cell does is phosphorylates (adds a phosphate group to glucose) it by an enzyme hexokinase. (Whenever you see the term kinase, know that it is an enzyme that adds phosphate to something). Now this phosphorylated glucose is trapped inside the cell because while free glucose can freely move out of the cell, phosphorylated glucose can't.
Now series of reactions happen with complex names and all which, unless you have been a biochem student at some point in you life, will simply fly over your head!!
What I want you guys to know for today are the ones marked in the circles :
Image from Kaplan USMLE Step 1 Biochemistry Lecture Notes
Look quickly at the green circle, NADH. It's importance will not be revealed in this post. That's for the finale of this series!! **Suspense
But what I will tell you guys and will be extremely important for the finale is, NADH is the reduced form of a molecule called Nicotinamide adenine dinucleotide, NAD. I'm sure at this point you guys are like...Hey damn you 
Okay, the names....not my fault guys!! But the rest i'll help you guys out with a mnemonic!!
"OIL RIG."
Oxidation is loss of electrons, Reduction is Gain of electrons!!
Now this is gold. You must absolutely remember, at least the part that reduction is gain of electrons!! So when I say NADH is the reduced form, it means NAD has been loaded up with electrons to form NADH (there is an added hydrogen in this compound too but I will not confuse you guys further, just know that NADH is now a container of electron!!).
Now here's one mathematic twist we can't avoid. In the diagram you'll see two compounds Glyceraldehyde-3P (G3P) and DHAP. These are in fact isomers of each other and can exist in either form. But when glycolysis is proceeding at full pace, both are in fact in the glyceraldehyde-3P form. Which means, there are 2 G3Ps that's taking part in glycolysis. In the diagram we see only one G3P. So if one G3P is producing 1 NADH, glycolysis is in fact producing 2 NADH by the time we have reached the end reaction.
Let's look at the red circles. Two ATP molecules are produced along the way while we break down a molecule of glucose. Now take into consideration that the diagram was only considering one of the G3P was in the reaction. Now if we have 2, how many molecules of ATP did we get? 4 ATPs!! Great!!??
We are not in fact getting a profit of 4 ATP. If you look at the first part of the diagram, you will notice that before we even began glycolysis, we had to invest 2 molecules of ATP!! (And you thought you needed to invest only in the crypto space to be in profit....poor you!!)**
Image from Kaplan USMLE Step 1 Biochemistry Lecture Notes
So after you've invested 2 molecules of ATP, an yield of 4 ATP molecules leaves you with a profit of only 2 ATP.
Two molecules are not what we're looking for. Two just isn't enough. Let's look at the violet colored compound, Phosphoenol pyruvate, PEP for a moment. It is the most high energy compound in the cell. It in fact has much more energy than ATP itself, yet when it is broken down into **Pyruvate by the enzyme pyruvate kinase, only 1 molecule of ATP is released. Where did the rest of the energy go? Didn't I mention one molecule of glucose provides us with 32 molecules of ATP? Where did we lose the remaining 30 molecules ATP? Did the hackers attack our blockchain?
Continue this journey along with us to find out the answers....
Sources :
Lippincott's illustrated Reviews Biochemistry, 5th edition
Kaplan USMLE Step 1 Biochemistry Lecture Notes
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Peace!!