Chapter 2: Chemical Composition of the Body and Its Relation to Physiology

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Welcome back to The Deep Dive.

Today, we're really going to dig into something truly foundational.

The very chemistry that makes our bodies work.

You know, in our last deep dive about homeostasis, how our bodies maintain balance, we touched on that.

Yeah, we did.

But to really grasp those incredible mechanisms, we have to understand the basic chemistry powering it all.

Absolutely.

So our mission today is to break down the key features of atoms,

molecules, solutions, and the major organic molecules in the body, connecting them directly to their physiological roles.

Yeah.

Kind of peeling back the curtain, you know.

Exactly.

And it's so fascinating how these fundamental interactions dictate pretty much everything.

Like what specifically?

Well, like maintaining a healthy blood pH, how individual molecules bind to each other, even how your cells manage energy.

It's the essential bedrock, really.

Playing out every single second inside you.

That makes perfect sense.

Okay, so let's start right at the beginning.

The most fundamental level,

atoms.

The building blocks.

Right.

We know atoms are basic, but what's truly remarkable is that, okay, there are over a hundred known elements, but only about 24 are actually essential for human body function.

That's right.

And even fewer do most of the heavy lifting.

So what's inside an atom?

Okay, if we zoom in,

you've got three primary components.

In the tiny, dense center, the nucleus, that's where you find positively charged protons and

neutral neutrons.

Then orbiting this nucleus are the negatively charged electrons.

Orbiting how?

Just randomly?

No, not randomly.

They're arranged in specific regions called orbitals, which form distinct electron shells.

The innermost shell holds just two electrons.

The second holds up to eight and so on.

And here's a key insight.

An atom is most stable,

like chemically happiest when its outermost shell is completely full.

This drive for stability, it dictates basically all chemical interactions.

Ah, so they want to fill up that outer shell.

Precisely.

And critically, a whole atom is electrically neutral because it has an equal number of positive protons and negative electrons.

The charges balance out.

Got it.

That balance is essential.

Now what gives each element its unique identity?

You mentioned protons.

That's the atomic number.

It's the specific number of protons an element has, like a unique ID badge, as you said.

Okay.

And how do we measure their mass?

They're so tiny.

We use a special unit called Daltons.

It's basically a mass scale relative to a standard carbon -12 atom, just a way to compare these incredibly small particles.

Right, a specialized weight class.

Exactly.

And sometimes atoms of the same element can have a different number of neutrons.

Oh.

Yeah, those are called isotopes.

Many isotopes are unstable.

They emit energy or particles radiation.

We call those radioisotopes.

Unstable sounds potentially bad, but you said?

Well, here's where it gets incredibly interesting for medicine.

Despite their instability,

radioisotopes have vital

applications.

Like what?

For example, we can use focused high -energy radiation from some radioisotopes to target and kill cancer cells.

Wow.

And maybe you've heard of PT scans?

Positron emission tomography?

Vaguely, yeah.

For a P scan, we inject a substance like glucose that's been tagged with a radioactive isotope.

Cells, especially really active ones like cancer cells, take up this glucose.

Because they need energy.

Right.

And the radioactive tag emits signals that a scanner can detect.

This allows doctors to visualize organ function or pinpoint areas of high metabolic activity like tumors.

It's really quite ingenious.

So harnessing instability for diagnosis and treatment, that is ingenious.

OK, speaking of changes,

what if an atom doesn't just have a different number of neutrons but actually gains or loses electrons?

That sounds like a bigger deal.

It absolutely is.

When an atom gains or loses one or more electrons, it electrical charge is no longer neutral.

It becomes an ion.

An ion.

OK, example.

Sure.

Think of a sodium atom, Na.

It can easily lose one electron.

When it does, it has one more proton than electron, so it becomes a positively charged sodium ion written as Na plus i.

OK, loss of electron means positive charge.

Right.

Conversely, a chlorine atom, Cl, readily gains an electron.

Now it has one more electron than protons, so it becomes a negatively charged chloride ion, Cl.

Gain of electron negative charge.

Makes sense.

And we have names for these.

Positively charged ions are called cations.

Negatively charged ions are anions.

Cations positive, anions negative.

Got it.

And these ions dissolved in water are electrolytes.

Exactly.

That's what electrolytes are.

And this is a huge deal in physiology.

Why is that?

Because these electrolytes, these charged ions dissolved in water, can conduct electricity.

They are absolutely critical for carrying electrical signals across cell membranes.

Ah, so like nerve impulses, muscle contractions.

Precisely.

That's how your muscle and nerve cells communicate and function.

It all relies on the movement of these ions.

It's pretty wild when you consider that just four elements, hydrogen, oxygen, carbon, and nitrogen make up over 99 % of all the atoms in your body.

99 % from just four.

Yep.

The rest are mineral elements like calcium and phosphorus, mainly in bone, and trace elements like iron, needed for oxygen transport, or iodine for thyroid hormones.

Each vital but in much smaller amounts.

It's a stark reminder, like you said, of the elegant simplicity yet profound complexity of our chemical makeup.

It really is.

Okay, so we have our atoms, our ions, the building blocks.

But the real magic happens when they start forming connections, right?

How do these individual atoms actually link up to create larger structures like molecules?

They do this through chemical bonds.

The strongest type is the covalent chemical bond.

Covalent?

This happens when atoms that have partially unfilled outer electron cells share one or more pairs of electrons.

Remember that drive for stability.

Filling the outer shell?

Exactly.

By sharing electrons, both atoms can effectively achieve a stable, full outer shell.

Take methane, CH.

Carbon needs four electrons to fill its outer shell.

Hydrogen needs one.

So carbon shares one of its outer electrons with each of four hydrogen atoms, and each hydrogen shares its single electron back with the carbon.

Everyone's happy, stable outer shells all around.

And that sharing creates the bond.

A very strong bond, yes.

It makes molecules like methane very stable.

Different atoms tend to form a characteristic number of covalent bonds.

Hydrogen usually forms one, oxygen two, nitrogen three, and carbon, the superstar, forms four.

Carbon forms four.

That seems important.

Hugely important.

It allows carbon to form the backbone of all the large organic molecules we'll talk about.

And sometimes, atoms share two pairs of electrons, forming a double bond, like in carbon dioxide, chiorose.

Okay, so sharing electrons makes covalent bonds.

Is it always an equal share though, like a perfectly fair partnership?

Ah, great question.

No, it's not always equal.

Some atoms are, let's say, greedier for electrons than others.

Greedier.

Well, technically we call it electronegativity.

It's a measure of an atom's ability to attract shared electrons in a covalent bond.

Okay, electronegativity.

So if two atoms in a bond have different electronegativity - Then the sharing isn't equal.

It's more like a tug of war.

The electrons spend more time orbiting the more electronegative atom.

Which must create some sort of charge difference across the bond, right?

Even if it's not a full charge, like an ion.

Precisely.

This unequal sharing creates partial negative.

We use the Greek letter delta and partial positive, plus H or a few charges across the bond.

We call these polar covalent bonds, or just polar bonds.

Polar.

Like poles on a magnet.

Kind of like that, yeah.

Tiny electrical poles on the molecule.

A classic example is the hydroxyl group, ROH, found in alcohols and sugars.

Oxygen is more electronegative than hydrogen.

So oxygen pulls the shared electrons closer.

Right, making the oxygen slightly negative and the hydrogen slightly positive, plus.

Molecules with lots of these polar bonds tend to be highly soluble in water.

And water itself, H, is the classic example of a polar molecule.

Oxygen pulls electrons from both hydrogens.

Ah, okay, so that explains why water dissolves so many things.

And the opposite of polar would be - Non -polar covalent bonds.

This happens when electrons are shared equally or very nearly equally, usually between atoms with similar electronegativities.

Think carbon -hydrogen bonds, CH, or carbon -carbon bonds, CC.

No significant charge difference there.

Exactly.

Little or no charge distribution.

Molecules rich in these non -polar bonds, like fats and oils, are less soluble in water.

We call them hydrophobic water -fearing.

Hydrophobic, so they don't mix well with water.

Right.

They're essential for things like cell membranes, but because our body fluids are mostly water, these non -polar molecules often need special carrier molecules to transport them around.

Okay.

Covalent bonds.

Sharing electrons, either polar or non -polar.

What about ionic bonds?

You mentioned ions earlier.

Ionic bonds are different.

They're not about sharing.

They're strong electrical attractions between oppositely charged ions, like magnets attracting.

Okay.

Think of table salt, sodium chloride, and ACL.

In solid salt, the positive sodium ions, Na +, and negative chloride ions, ACL, are held together tightly by this strong electrical attraction.

But salt dissolves in water.

Ah, yes, because water is so polar.

The partially negative oxygen ends of water molecules are attracted to the positive sodium ions, and the partially positive hydrogen ends are attracted to the negative chloride ions.

Water molecules essentially surround the ions, pulling them apart and away from each other.

That's dissolving.

So the polar water overcomes the ionic attraction.

Clever.

Okay.

Strong covalent bonds, strong ionic bonds.

What about hydrogen bonds?

You mentioned them briefly.

Right.

Hydrogen bonds are much weaker individually.

Think of them, as you said, like very gentle magnetic attractions between neighboring polar molecules.

How do they form?

It's an electrical attraction between a partially positive hydrogen atom in one polar molecule and a partially negative atom, usually oxygen or nitrogen, in another nearby polar molecule.

So plus hydrogen attracted to a oxygen or nitrogen.

Exactly.

We usually draw them with dashed lines to show they're weaker.

Individually, a hydrogen bond is only about 4 % as strong as a polar covalent bond.

Only 4%.

That doesn't sound like much.

Individually, no.

But collectively, that's the key.

Oh, strength in numbers.

Absolutely.

If we connect this to the bigger picture,

hydrogen bonds, when present in large numbers, become incredibly important.

They significantly influence how molecules interact, and crucially, they determine the precise three -dimensional shape of large biological molecules like proteins and DNA.

And that shape is critical for function, right?

The whole lock and key idea.

Precisely.

The specific shape dictated by all these bonds, including many hydrogen bonds, allows molecules to interact very specifically with other molecules.

It's essential for almost everything they do.

And speaking of shape,

molecules aren't just flat drawings on a page.

Not at all.

They're three -dimensional.

And they're often flexible.

Atoms can rotate around single covalent bonds, almost like wheels on an axle.

So they can bend and twist.

Yes.

This flexibility allows molecules, especially long chains, to assume various shapes.

A long carbon chain might fold up into a ring, for example.

This raises an important question, then.

Why is this flexibility in shape so vital?

It really underlines a fundamental principle of biology and physiology.

Structure dictates function.

This is true right down at the molecular level.

A protein -specific 3D shape, for instance, determines exactly what job it can do in the cell.

Change the shape, you change the function.

Structure dictates function.

Okay.

Before we leave bonds, you mentioned ionic molecules, too.

Right.

Sometimes it's not the whole molecule that's an ion, but specific groups of atoms within a larger molecule can gain or lose a proton, becoming charged.

A carboxyl group, RcOOH, can release its H plus a becoming RcOO.

An amino group, RNHers, can accept an H plus a becoming RNHers plus a.

And these reactions can go both ways.

Yes.

They're typically reversible.

And these ionizable groups are super important for how molecules interact and, again, for maintaining the delicate chemical balance, especially pH, in our body fluids.

Which brings us nicely to part three, the body's fluid environment.

Solutions and pH.

The watery world within.

Exactly.

So in any solution, you have the solutes, the things dissolved, and the solvent, the liquid they're dissolved in.

Yeah.

And in our bodies.

Water is the undisputed champion solvent.

It makes up about 60 % of your total body weight.

Nearly all the chemical reactions sustaining life happen with molecules dissolved in water.

Why is water so good at this?

It goes back to its properties we discussed, its polarity and its ability to form hydrogen bonds.

This lets it surround and separate ions and other polar molecules.

Plus, water itself participates in many chemical reactions.

Like?

Like hydrolysis, where a water molecule is used to break down large molecules.

The opposite is dehydration, sometimes called condensation, where water is removed to link smaller molecules together to build larger ones.

Building up and breaking down with water involved.

And water also influences things based purely on the concentration of solutes, regardless of what they are.

These are called colligative properties, like osmosis water moving across membranes based on solute concentration differences.

Okay.

So how well something dissolves in water, its solubility.

You mentioned hydrophilic and hydrophobic.

Right.

Hydrophilic means water -loving.

These are molecules with plenty of polar bonds or ionized groups.

They dissolve easily in water, like sugars or salt.

And hydrophobic.

Water -fearing.

These are molecules rich in non -polar bonds, mainly carbon and hydrogen, like oils and fats.

They don't dissolve well in water, they tend to clump together, separating from the water.

Like oil and vinegar dressing.

Exactly.

But then there's a really interesting third category,

amphipathic molecules.

Amphipathic.

Sounds like they can't make up their mind.

Ah, in a way.

They have both a polar hydrophilic region and a non -polar hydrophobic region on the same molecule.

Best of both worlds.

It's incredibly useful.

When you mix amphipathic molecules with water, they do something clever.

They spontaneously arrange themselves into structures, like clusters called micelles or sheets.

They position their polar hydrophilic ends facing outwards, interacting with the water, while their non -polar hydrophobic ends cluster together on the inside, hidden away from the water.

That is clever.

Why is that important?

It's absolutely crucial.

This property is the basis for our cell membranes, which are made of phospholipid bilayers, amphipathic molecules forming a barrier.

It's also key for transporting non -polar substances, like fats, through our watery blood.

Wow, okay.

Amphipathic molecules are key players.

Now, when we talk about solutions, we need to talk about concentration.

How much solute is actually in the solvent?

Right.

We can express it simply as mass per volume, like grams per liter, gl.

You see that sometimes.

But that doesn't tell us the number of molecules, does it?

A gram of small molecules is way more molecules than a gram of huge ones.

Exactly.

For physiology, the number of molecules often matters more than the total mass.

So first, we need the molecular weight of a substance that's just the sum of the atomic masses of all the atoms in one molecule.

Okay.

Calculate it from the periodic table.

Then we use the concept of a mole.

One mole of any compound is the amount in grams that's numerically equal to its molecular weight.

The magic is, one mole always contains the same huge number of molecules, Avogadro's number.

So a mole is like a standard counting unit for molecules.

Precisely.

And this lets us define concentration in terms of molarity, m, which is moles of solute per liter of solution.

This gives us a direct comparison of the number of solute molecules between different solutions.

That seems much more useful for chemical reactions.

It is.

In the body,

concentrations are often quite low.

So we frequently use smaller units like millimoles, mm, thousandths of a mole, micromoles, m, millions, or even nanomoles, nm, billionths per liter.

Okay.

Molarity gives us the number count.

Now, a really critical aspect of concentration in the body is acidity.

Yes.

The concentration of hydrogen ions, H plus E is.

Remember, H plus is basically just a free proton.

Right.

And acids release H plus is.

Correct.

An acid is any molecule that releases hydrogen ions when dissolved in solution.

Think hydrochloric acid, HCl, in your stomach, or a lactic acid produced during intense exercise.

A base, conversely, is a substance that accepts H plus ions, removing them from solution.

Bicarbonate is a key base in our blood.

Are all acids the same strength?

No.

Strong acids, like HCl, ionize completely in water, releasing all their H plus Rzs.

Weak acids, like lactic acid or carbonic acid, only ionize partially, releasing just some of their H plus Rzs.

So if we connect this to the bigger picture,

the hydrogen ion concentration, the acidity is critical for life.

Absolutely vital.

It's usually expressed using the pH scale.

Ah, pH.

Heard of it, but what is it exactly?

pH is technically the negative logarithm, base 10, of the free hydrogen ion concentration in moles per liter.

Okay, math.

But what does that mean practically?

Practically, it's a scale from 0 to 14.

A pH of 7 .0 is neutral, like pure water.

Anything below 7 .0 is acidic, meaning there's a higher concentration of H plus seri.

Anything above 7 .0 is alkaline or basic, meaning a lower concentration of H plus seri.

And it's a logarithmic scale, right?

So small changes mean a lot.

Exactly.

Each whole number change on the pH scale represents a tenfold change in H plus concentration.

So pH 6 is 10 times more acidic than pH 7.

pH 5 is 100 times more acidic than pH 7.

Wow, okay.

So tiny changes in pH mean big changes in acidity.

Huge changes, and our bodies maintain a very, very narrow homeostatic pH range.

The extracellular fluid, like blood plasma, is normally around pH 7 .4.

Slightly alkaline.

Inside cells, it's usually closer to 7 .0, 7 .2.

That sounds incredibly tightly controlled.

It has to be.

Even small deviations from this narrow range can drastically alter the function of enzymes and other proteins remember how important their shape is, and can have severe, even life -threatening consequences.

Survival depends on keeping pH in check.

It's amazing how crucial that balance is.

Right, so all these chemical principles, atoms, bonds, water, pH, they all come together in the four major classes of organic molecules essential for life.

The molecules of life built around carbon.

Carbon.

With its ability to form four strong, covalent bonds, it's the star player, isn't it?

Making this incredible diversity possible.

It really is.

And many of these organic molecules are huge macromolecules, often polymers.

Polymers meaning chains.

Exactly.

Long chains made up of smaller, repeating subunits called monomers.

These monomers are linked together, usually through those dehydration reactions we mentioned, where water is removed.

Okay, so what are the four big classes?

Carbohydrates, lipids, proteins, and nucleic acids.

The four pillars.

Let's start with carbohydrates.

Okay.

Carbs make up only about 1 % of body weight, but they're vital.

Think energy.

Where bodies go to for quick energy.

Pretty much.

They're composed of carbon, hydrogen, and oxygen, often in a 1 .2 .1 ratio, like CaOro for glucose.

They have lots of those polar hydroxyl OH groups.

Which makes them?

Water -soluble hydrophilic.

Monosaccharides are the simplest sugars.

The monomers like glucose, which is our primary blood sugar, the main direct energy source for cells.

Fructose and galactose are others.

Simple sugars.

What about bigger ones?

Link two monosaccharides via dehydration and you get a disaccharide.

Sucrose, table sugar, lactose, milk sugar, maltose, malt sugar are examples.

Link many monosaccharides together, hundreds or thousands, and you get polysaccharides.

Long chains.

Very long chains.

Plants store glucose as starch.

Animals, including us, store glucose as glycogen, mainly in the liver and muscles.

Glycogen acts as a crucial energy reservoir.

When your blood sugar dips, like during fasting, your liver breaks down glycogen to release glucose back into the blood.

Energy storage.

Got it.

Okay, next pillar.

Lipids.

Are water -fearing friends,

but no less vital.

Right.

Mostly carbon and hydrogen, very nonpolar, low water solubility.

They make up about 15 % of body weight.

What do they do?

Several key things.

Long -term energy storage, forming the core structure of cell membranes, and acting as signaling molecules.

There are four main subclasses.

Okay, first.

Fatty acids.

These are long chains of carbon atoms, typically 16 or 18 carbons long, bonded to hydrogens with an acidic carboxyl group, Darc -COH, at one end.

Chains of C and H, are they all the same?

No.

They can be saturated, meaning all the carbon bonds in the chain are single bonds.

The chain is saturated with hydrogen atoms.

Or they can be unsaturated, meaning there's at least one double bond in the chain.

Saturated versus unsaturated fats.

We hear about those.

Right.

Unsaturated can be monounsaturated, one double bond, or polyunsaturated, multiple double bonds.

Fatty acids themselves can be used as an energy source, and they're also used to synthesize important signaling molecules called eicosanoids.

We should also briefly mention trans fatty acids, an unhealthy type, often produced artificially.

Okay, fatty acids.

Second lipid subclass.

Triglycerides.

These are formed by linking three fatty acids to a three carbon alcohol molecule called glycerol.

This is how most fat is stored in your body.

Triglycerides are the main storage form of fat.

Yes.

Primarily in adipose tissue or fat cells.

They represent a huge energy reserve.

The type of fatty acids involved, saturated or unsaturated, affects whether the triglyceride is a solid fat or a liquid oil at room temperature.

Saturated fats tend to be solid, unsaturated ones liquid.

Makes sense.

Third subclass.

These sound important for membranes.

Phospholipids.

You're right, they're crucial for membranes.

They're structurally similar to triglycerides, but instead of a third fatty acid, they have a phosphate group attached to the glycerol and usually a charged nitrogen containing molecule attached to the phosphate.

So one part is different.

How does that change things?

It makes them amphipathic.

The two fatty acid tails are non -polar hydrophobic, but the phosphate and charged group form a polar hydrophilic head.

Amphipathic again, like we discussed.

Exactly.

And the structure is perfect for forming cell membranes.

They arrange themselves into a lipid bilayer with the hydrophobic tails facing inwards and the hydrophilic heads facing the watery environment inside and outside the cell.

The fundamental structure of membranes.

Okay, fourth lipid subclass.

Steroids.

These have a distinctly different structure.

Four interconnected rings of carbon atoms.

Cholesterol is the most well -known steroid.

Cholesterol gets a bad rap sometimes.

It does, but it's actually essential.

It's a component of cell membranes, and it's the precursor molecule from which all steroid hormones are synthesized, like cortisol, testosterone, estrogen.

While they might have a polar OH group, overall, steroids are not very water -soluble.

Okay, carbs for energy, lipids for storage, membrane signals.

Now,

proteins.

The workhorses, as you called them, often considered of first -rank importance.

They do seem to do almost everything, about 17 % of body weight.

Around that, yeah.

And their functions are incredibly diverse.

They regulate genes, transport substances across membranes, act as enzymes to speed up chemical reactions, function as receptors for cell signaling, enable movement through muscle contraction, provide structural support like collagen, and form antibodies for defense.

The list goes on.

Truly the workhorses.

What are they made of?

They are polymers made from monomer subunits called amino acids.

There are 20 common types of amino acids used to build proteins in humans.

20 different building blocks.

What's their basic structure?

Each amino acid has a central carbon atom bonded to an amino group, NHRs, a carboxyl group, COOH, a hydrogen atom, and a unique fourth group called the side chain, or R group.

And that R group is different for each of the 20.

Yes, the side chain varies, giving each amino acid its unique properties.

Some are non -polar, some polar, some charged.

We have to get some of these, the essential amino acids, from our diet because our bodies can't make them.

Okay, so how do these amino acids link up?

They join together via dehydration synthesis, forming a covalent bond called a peptide bond between the carboxyl group of one amino acid and the amino group of the next.

Forming a chain.

Exactly.

A chain of amino acids linked by peptide bonds is called a polypeptide.

A functional protein consists of one or more polypeptide chains that are folded into a specific, precise, three -dimensional shape.

Some proteins also have carbohydrates attached.

Those are called glycoproteins.

You said before that 3D shape is everything for proteins.

Structure dictates function.

Absolutely fundamental.

We actually describe protein structure on four levels.

Four levels.

Okay, let's break that down.

Level one.

Primary structure.

This is simply the sequence, the specific order, and number of amino acids in the polypeptide chain.

It's determined by the genetic code.

Just the list of amino acids in order.

Got it.

Level two.

Secondary structure.

This refers to local regular folding patterns within the polypeptide chain.

The two main types are the alpha helix, which is like a coil, and the beta -pleated sheet, which is like a folded ribbon.

Coils and sheets.

What holds them together?

Hydrogen bonds forming between atoms in the polypeptide backbone, not the side chains.

These structures are linked by less structured random coil regions.

Interestingly, hydrophobic side chains within these secondary structures can help anchor proteins within lipid membranes.

Okay.

Primary is the sequence, secondary is local folding.

Level three.

Tertiary structure.

This is the overall unique three -dimensional shape of a single entire polypeptide chain.

It's the final folded conformation that makes the protein functional.

How does it achieve that specific shape?

It results from interactions between the various amino acid side chains, the R groups.

These interactions include hydrogen bonds, ionic bonds between charged side chains, hydrophobic interactions,

non -polar side chains clustering together away from water, and sometimes strong covalent bonds called the sulfide bonds between specific sulfur -containing amino acids.

Weaker van der Waals forces also played a role.

So all those bond types we talked about come into play to shape the protein.

Exactly.

They all contribute to the final stable tertiary structure.

And the fourth level.

Is there more?

Yes.

For some proteins.

Quaternary structure applies only to proteins that are composed of multiple polypeptide subunits.

It describes how these individual subunits associate and bind together to form the final functional multimeric protein.

Hemoglobin, the oxygen carrier in red blood cells, is a classic example.

It has four polypeptide subunits.

Four subunits working together.

Primary, secondary, tertiary, quaternary.

That's complex.

This raises an important question.

What happens if this intricate structure is altered?

You mentioned mutations earlier.

Right.

A mutation is a change in the genetic code that leads to a change in the primary structure.

Maybe just one amino acid is substituted for another.

Just one.

Even just one.

This change in the primary sequence can have a ripple effect, potentially altering the secondary, tertiary, and even quaternary structures because it changes the interactions between side chains.

And if you change the structure?

You almost always change or even abolish the protein's function.

It's a profound illustration of that structure function principle.

Wow.

Okay.

Final pillar.

Nucleic acids.

The information molecules.

The blueprints and instruction manuals for life.

They only account for about 2 % of body weight, but they handle storage, expression, and transmission of all our genetic information.

Correct.

There are two main classes, deoxyribonucleic acid or DNA and ribonucleic acid or RNA.

Both are polymers like proteins and polysaccharides.

Polymers of what?

What are their monomers?

Their monomers are called nucleotides.

Each nucleotide has three parts.

A phosphate group, a five -carbon sugar, and a nitrogen -containing base.

Phosphate -sugar base.

Okay.

How do DNA and RNA differ?

Key differences.

In DNA, the sugar is deoxyribose.

It has four possible bases.

Adenine A, guanine G, these two are called purines, and cytosine, cytamine T, these two are pyrimidines.

AGCT in DNA.

And structurally, DNA typically exists as a double helix, two long nucleotide chains coiled around each other like a spiral staircase.

The famous double helix.

What holds the two strands together?

Hydrogen bonds.

Specifically, hydrogen bonds between the bases on opposite strands, and there's a strict rule for pairing.

Really?

Yes.

Base pairing rules.

Adenine A always pairs with thymine T, forming two hydrogen bonds.

Guanine G always pairs with cytosine C, forming three hydrogen bonds, A with T, G with C.

Always.

That sounds important.

Critically important.

This specificity is the key to how genetic information is accurately stored, replicated, and passed on.

These hydrogen bonds can be broken by enzymes or heat, allowing the strands to separate for processes like DNA replication.

That's called denaturation.

Okay, so that's DNA.

How is RNA different?

First, the sugar in RNA is ribose, not deoxyribose.

Second, RNA is usually a single nucleotide chain, not a double helix.

Third, one of the bases is different.

RNA uses uracil U instead of thymine T.

U instead of T.

So how does base pairing work in RNA?

Uracil U pairs with adenine A.

So the pairing possibilities involving RNA are Au and Gc.

And what's RNA's main job?

RNA molecules are involved in decoding the genetic information stored in DNA and using it as instructions to link amino acids together in the correct sequence to synthesize proteins.

It's the messenger and translator for the DNA blueprint.

DNA holds the master plan.

RNA carries out the instructions to build proteins.

Makes sense.

Exactly.

Okay, this is a fantastic overview of the chemistry.

Now, let's bring these principles into the real world with a Chapter 2 clinical case study.

It was about a young man with severe abdominal pain while mountain climbing.

This isn't just abstract theory, right?

These chemical principles have direct, sometimes dramatic, impacts on our health.

Absolutely.

This case is a perfect illustration if we connect this back to what we've discussed.

The patient had sickle cell trait, or SCT.

Sickle cell.

What's the underlying chemical issue?

It's a genetic mutation affecting hemoglobin that protein with quaternary structure in red blood cells that carries oxygen.

Specifically, it's a mutation in the gene for the beta subunits of hemoglobin.

What kind of mutation?

A single amino acid substitution in the primary structure.

A glutamic acid residue, which is normally present, is replaced by a valine residue at a specific position in the beta chain.

Glutamic acid replaced by valine.

Why is that specific change so bad?

Because glutamic acid has a charged polar side chain, while valine has a non -polar hydrophobic side chain.

This single change completely alters the chemical properties at that spot on the protein's surface.

So changing one amino acid affects the structure?

Profoundly.

It creates a sticky hydrophobic patch on the outside of the hemoglobin molecule.

This change in primary structure ripples through, affecting the tertiary and quaternary structures and critically changing the intermolecular bonding forces.

How does that cause problems, especially at high altitude?

At high altitudes, oxygen levels are lower.

When hemoglobin molecules carrying less oxygen, deoxyhemoglobin, encounter this low oxygen environment in someone with SCT, that sticky hydrophobic patch causes the altered hemoglobin molecules to clump together or polymerize.

They form long, rigid rods inside the red blood cells.

Rods inside the cells.

Yes, and these rigid rods distort the normally flexible disc -shaped red blood cells into a crescent or sickle shape.

Ah, hence the name sickle cell.

And what are the consequences of that shape change?

Several serious problems.

First, the spleen recognizes these abnormally shaped cells and removes them from circulation faster than normal, leading to anemia and often painful enlargement of the spleen.

Second, and acutely dangerous, these rigid, sickled cells can't squeeze through tiny blood vessels, capillaries, like normal cells can.

They get stuck.

Blocking blood flow.

Exactly.

They block blood flow, depriving tissues and organs of oxygen.

This causes episodes of severe pain, like the abdominal pain the climber experienced, and can lead to serious organ damage over time.

Wow.

All from one single amino acid change.

It's a truly profound example.

A microscopic change at the most fundamental chemical level, one atom difference in a side chain, leading to one amino acid swap cascades up through the levels of protein structure, alters bonding forces, changes cell shape, and causes devastating body -wide physiological effects.

It perfectly illustrates how critical primary structure and the resulting conformation are for protein function.

An incredible, if sobering, illustration of everything we've discussed.

Indeed.

And that brings us toward the end of this deep dive into the chemical composition of the body.

We've journeyed from the smallest atoms and their intricate bonds.

Covalent, ionic, hydrogen.

Explore the absolutely crucial role of water as the body's solvent, its polar nature.

Hydrophilic, hydrophobic, amphipathic.

And then delved into the four major classes of organic molecules, carbohydrates, lipids, proteins, and nucleic acids, the building blocks and the functional powerhouses.

Chemistry that makes physiology happen.

It really is also interconnected.

It is.

And perhaps this raises an important final thought for you, our listeners, to ponder.

Considering the immense complexity and the absolute precision required for all these chemical interactions we've talked about, the bonding, the shapes, the pH balance.

Just think about the constant,

tireless, intricate work your body is performing every single second, simply maintaining this delicate chemical equilibrium.

Without us even thinking about it.

It's truly a marvel of biological engineering playing out right inside you right now.

Something to appreciate.

Absolutely.

Puts a whole new perspective on just living.

Thank you so much for joining us on this deep dive today.

Last minute lecture team.

And a warm thank you from the last minute lecture team for joining us on this deep dive into the chemical foundations of the body.