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  • Voiceover: Let's talk about this thing called metabolism.

  • I remember being told in medical school that metabolism was

  • the sum total of all of the chemical reactions in the human body,

  • but I never actually knnew what that meant.

  • I like to think of metabolism as kind of like the balance

  • in the body between the reactions that build things up,

  • and another way of saying that is anabolism.

  • On the other side of the scale,

  • the reactions that break things down.

  • The medical way of saying that is catabolism,

  • or catabolism.

  • So, metabolism is kind of the balance between

  • building things up, or repairing or storing inside the body

  • and breaking things down, usually for energy needs.

  • How are the things that we eat

  • broken down for energy or stored within the body?

  • Let's start by looking at carbohydrates.

  • So, we eat our carbohydrates,

  • and they are in the form of starches or sugars,

  • and we saw in our last video that those could get

  • broken down into their kind of component parts.

  • The major component, or the major single unit currency

  • of carbohydrates is called glucose.

  • That glucose, and this is very big-picture,

  • can get broken down further into a compound called pyruvate.

  • The pyruvate then gets broken down into

  • a very important molecule that's kind of the center

  • of making energy in the body,

  • and that molecule is called acetyl-CoA.

  • Now, I think of acetyl-CoA kind of like one of those

  • little trampolines you see in the gym,

  • and basically acetyl-CoA can be moved around

  • to different parts.

  • We'll put the trampoline there,

  • and then we'll put it over there,

  • and then we'll put it over there.

  • Basically, different chemcals bounce on and off

  • of that little portable trampoline

  • as they're going around this cycle.

  • This cycle is called the TCA cycle.

  • That stands for tricarboxylic acid cycle,

  • or it's sometimes called the Krebs cycle.

  • Essentially, what you need to know about this cycle

  • is that as the acetyl-CoA moves around the cycle

  • to form all of these slightly different compounds,

  • things are bounced off,

  • and the things that are bounced off

  • are these high-energy carriers.

  • I'll just write high-enegy carriers.

  • What happens to the high-energy carriers next

  • is that they basically bounce down a staircase

  • called the electron transport chain.

  • As they go down that staircase,

  • they give off a chemical called ATP.

  • ATP is the currency of energy in the human body.

  • ATP is the same as energy in the human body.

  • Now, just to add some detail here,

  • water is also given off,

  • and I'll write the chemical formula for water, which is H2O,

  • and in order for this cycle, this TCA cycle to move around,

  • oxygen is required, and carbon dioxide, CO2,

  • and water, again, are given off.

  • Oxygen is also necessary for this step to happen,

  • and this is really neat because this is essentially

  • the reason why we need to breathe.

  • Now let's look at what happens to the protein that's in our diet.

  • Protein was one of the 3 macro nutrients

  • that we looked at in the last video,

  • and the proteins in our diet are broken down

  • by our digestive tract as we saw

  • into unit parts called amino acids.

  • Depending on what kind of amino acid you're looking at,

  • the amino acid can either enter the system here,

  • at this level, or it can enter here,

  • depending on which kind of amino acid it is.

  • When the amino acid makes this transition,

  • something is given off,

  • and that something is the amino group.

  • So the amino group, or the amine, is given off,

  • and that amine becomes one of the waste products

  • in our urine.

  • That's how we get rid of those amine groups.

  • Let's look now at what happens to the fats

  • that we eat in our diet.

  • Fats, basically, are made up of a little glycerol backbone,

  • so I'm gonna write here "glycerol",

  • and long chains of what we call fatty acids.

  • There are three of them attached to each glycerol backbone.

  • What happens when the body needs to use those for energy

  • is that it breaks them down into units of 2 carbons each,

  • and I'll write here fatty acids,

  • and those 2 carbon units can only enter the system over here,

  • at the level of acetyl-CoA.

  • Whereas this small glycerol backbone goes in over here.

  • Those are the breaking down reactions.

  • That's this side of the equation of metabolism,

  • but what about the building up reactions?

  • Well, the amino acids that come from the protein in our diet

  • can be used to make things like muscle

  • and other lean tissue in our bodies.

  • They can be used for repair.

  • They can be used as protein carriers in our blood,

  • and the carbohydrate that we eat,

  • and the glucose in our body,

  • can also be stored as something called glycogen,

  • which is the storage form of carbohydrates in our bodies.

  • The fats that we've eaten,

  • if there are too many of those in our diet,

  • they can also be stored as adipose tissue,

  • so they're put back together and stored in our body.

  • The balance between the breaking down reactions,

  • or the catabolic reactions,

  • and the building up reactions,

  • the balance is controlled by hormones in our body.

  • There are several hormones that lead to

  • breaking down of tissues.

  • Things like glucagon leads to the breakdown of

  • the stores of glycogen in our body.

  • Other hormones are the kind of fight or flight hormones.

  • Epinephrine is one of them and glucocorticoids.

  • These are sort of the stress hormones in our body.

  • These guys all favor the breaking down, or catabolism,

  • of the tissues.

  • The hormones that lead to the building up,

  • or the anabolism, in our bodies,

  • the major one is insulin.

  • I'm sure you've all heard of it.

  • Insulin leads to storage, or things being put away.

  • There's also a part to be played by the sex hormones,

  • or the sex steroids,

  • and then there are some hormones that are anabolic,

  • or favor the building up of proteins,

  • and those are the hormones thyroxin and growth hormone,

  • I'll just write GH.

  • Those guys favor the anabolism or the building up

  • of protein stores, so protein synthesis is favored by these guys.

  • But, they also work on this side.

  • they favor the breakdown of fats and carbohydrate stores.

  • Thyroxin and GH, growth hormone,

  • those guys favor the breaking down of our fat

  • and carbohydrate stores.

  • The last thing I want to add here

  • is that some of these reactions are reversible,

  • so glucose going to pyruvate can be reversed.

  • It can go back up in the other direction,

  • and that's how we can make glycogen

  • from, let's say, these amino acids that make pyruvate.

  • This kind of amino acid can be used to make glycogen.

  • The backbone of the fatty acids called glycerol

  • could be used to make glycogen

  • or to make glucose to feed the brain,

  • but this reaction, this particular one here,

  • is an irreversible, one-way reaction.

  • That means that fatty acids can never go up

  • to make pyruvate and be used as glucose for the brain,

  • or they can never be used to replenish glycogen stores

  • in the body.

  • Another important thing that we have to understand

  • is that this cycle will only go around

  • if ATP is needed by the body.

  • If we're using energy, we're doing exercise,

  • we're demanding ATP and energy from our system,

  • then this cycle will continue going around.

  • But if we don't,

  • let's say we're sedentary and we don't use energy,

  • then acetyl-CoA is going to build up,

  • and it's gonna favor the return in this direction

  • towards adipose tissue.

  • This is how excess carbohydrate,

  • if it's taken in in the diet,

  • can lead to excess adipose tissue, excess fat,

  • if we eat too much fat in our diet,

  • that can lead to adipose tissue,

  • and even if we eat too much protein,

  • it'll just be deaminated and converted to acetyl-CoA,

  • which can be converted to adipose tissue.

Voiceover: Let's talk about this thing called metabolism.

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B2 amino cycle acetyl glycogen body metabolism

Basics of Metabolism

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    Cheng-Hong Liu posted on 2015/01/13
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