Dehydration synthesis or a condensation reaction | Biology | Khan Academy

Summary

This educational video script delves into the chemistry of glucose, a simple sugar, and its role as a fundamental building block for more complex carbohydrates. It explains the process of dehydration synthesis, where two glucose molecules combine to form a disaccharide, specifically maltose, through a nucleophilic attack and the release of a water molecule. The script further explores the concept of monosaccharides, disaccharides, and the progression to polysaccharides, such as starch and glycogen, highlighting their importance in both dietary intake and energy storage within living organisms.

Takeaways

• 🍬 Glucose is a simple sugar with a specific molecular structure that plays a vital role in forming more complex carbohydrates.
• 🔗 The script explains the process of linking two glucose molecules through a nucleophilic attack involving the oxygen atom and the number one carbon of another glucose molecule.
• 💧 The reaction described is a dehydration synthesis, where a water molecule is removed as two glucose molecules are joined together to form a larger molecule.
• 🔄 This type of reaction is also known as a condensation reaction, which results in the formation of a disaccharide from two monosaccharides.
• 🍺 Maltose, or malt sugar, is the specific disaccharide formed when two glucose molecules undergo this condensation reaction.
• 🔬 The process involves the partial positive charge on the number one carbon due to electronegativity of oxygen atoms and the partial negative charge on the oxygen of the other glucose molecule.
• 🔍 The script details the formation of a bond between the glucose molecules, the release of a water molecule, and the subsequent neutralization of charges.
• 🌱 Polysaccharides are large molecules made up of many monosaccharides linked together, which are important in both plant starch and animal glycogen storage.
• 📚 The concept of monomers and polymers is introduced, with glucose being an example of a monomer that can polymerize to form polysaccharides.
• 🌾 Starch is a polysaccharide found in plants, made up of many glucose units, and serves as a form of energy storage.
• 🏃 Glycogen is the animal equivalent of starch, stored in cells as an immediate energy source and is also composed of glucose units.
• 🔬 The script provides a detailed molecular explanation of how simple sugars can be combined to form more complex carbohydrates, highlighting the importance of understanding molecular interactions.

Q & A

What is the primary focus of the video script?

The video script focuses on explaining the process of how glucose molecules can be used as building blocks to form more complex sugars and carbohydrates.

What is the significance of glucose in the context of this video?

Glucose is significant as it serves as the starting point for the formation of more complex carbohydrates through a process called dehydration synthesis.

What is a nucleophilic attack in organic chemistry?

A nucleophilic attack in organic chemistry is when a nucleophile, an atom or molecule with a lone pair of electrons, donates this pair to an electrophile, typically a carbon atom with a partially positive charge, to form a new chemical bond.

Why is the number one carbon on the left-hand glucose molecule susceptible to a nucleophilic attack?

The number one carbon is susceptible to a nucleophilic attack because it is attached to two electronegative oxygens, which give it a partially positive charge, making it an electrophile.

What role does the electronegativity of oxygen play in the formation of a bond between two glucose molecules?

The electronegativity of oxygen causes it to attract electrons from the carbon it is bonded to, creating a partially positive charge on the carbon and a partially negative charge on the oxygen, facilitating the nucleophilic attack and bond formation.

What happens when the oxygen atom in one glucose molecule uses its lone pair to form a bond with the number one carbon of another glucose molecule?

When the oxygen atom uses its lone pair to form a bond with the number one carbon of another glucose molecule, it results in the breaking of the bond between the carbon and a hydrogen atom, which can then form a hydronium ion or a water molecule, depending on the reaction conditions.

What is the term for the process where two glucose molecules are linked together to form a larger molecule?

The process is called dehydration synthesis, also known as a condensation reaction, where a water molecule is removed as two glucose molecules are linked together.

What is the resulting disaccharide formed when two glucose molecules undergo dehydration synthesis?

The resulting disaccharide formed is maltose, which is also known as malt sugar.

What is the difference between a monosaccharide and a disaccharide?

A monosaccharide is a single sugar molecule, while a disaccharide is a carbohydrate composed of two monosaccharide molecules linked together.

How does the concept of dehydration synthesis relate to the formation of polysaccharides?

Dehydration synthesis is the fundamental process behind the formation of polysaccharides, where many monosaccharide units are linked together, with the simultaneous removal of water molecules, to form long chains of carbohydrates.

What are some examples of polysaccharides mentioned in the script?

Examples of polysaccharides mentioned in the script include starch, which is found in plants, and glycogen, which is stored in animal cells for immediate energy.

Outlines

00:00

🔬 Exploring Glucose and Its Reactions

The video revisits the importance of glucose, a simple sugar, and its molecular structure. The focus is on understanding how glucose can act as a building block for more complex sugars and carbohydrates. The narrator explains a chemical reaction where two glucose molecules interact. Specifically, the oxygen in one glucose molecule uses its lone pair to attack the carbon in another glucose molecule, leading to bond formation. This process is driven by the partial charges on the involved atoms due to the electronegativity of oxygen. The reaction results in the formation of a bond between the two glucose molecules, highlighting the mechanism through which glucose can form more complex structures.

05:02

🧪 Dehydration Synthesis and Formation of Disaccharides

The video continues by detailing the dehydration synthesis reaction that links two glucose molecules, forming a disaccharide called maltose. This process involves the loss of a water molecule and is also known as a condensation reaction. The narrator explains the significance of monosaccharides (single sugars) and disaccharides (two sugars) and introduces the concept of polysaccharides, which are long chains of monosaccharides. These complex carbohydrates are crucial in biology, found in plants as starch and in animals as glycogen. The video emphasizes the importance of these macromolecules in energy storage and their common presence in everyday food and human cells.

Keywords

💡Glucose

Glucose is a simple sugar and an essential source of energy in living organisms. It has a specific molecular structure, which can be used as a building block for more complex sugars and carbohydrates. In the video, glucose molecules are shown in their cyclic form and are involved in a reaction to form disaccharides.

💡Molecular Structure

The molecular structure refers to the arrangement of atoms within a molecule. In the video, the molecular structure of glucose is highlighted, with the carbons numbered to explain how glucose can form bonds with other molecules. This structure is crucial for understanding how glucose acts as a building block for complex carbohydrates.

💡Nucleophilic Attack

A nucleophilic attack is a type of chemical reaction where a nucleophile (an electron-rich species) forms a bond with an electrophile (an electron-poor species). The video explains how an oxygen atom in a glucose molecule uses its lone pairs to perform a nucleophilic attack on another glucose molecule's carbon, facilitating the formation of a disaccharide.

💡Electronegativity

Electronegativity is the tendency of an atom to attract electrons in a covalent bond. In the video, the oxygen atoms in glucose are described as very electronegative, causing the carbon they are attached to have a partially positive charge. This concept is important for understanding the nucleophilic attack described.

💡Dehydration Synthesis

Dehydration synthesis is a chemical reaction that involves the loss of a water molecule when forming a new bond between molecules. The video demonstrates this process when two glucose molecules combine to form a disaccharide, releasing a water molecule in the process.

💡Disaccharide

A disaccharide is a carbohydrate composed of two monosaccharide molecules bonded together. The video shows how two glucose molecules undergo a dehydration synthesis reaction to form a disaccharide, specifically maltose.

💡Monosaccharide

A monosaccharide is the simplest form of carbohydrate, consisting of a single sugar molecule. The video starts with glucose as a monosaccharide and explains how it can be combined with another monosaccharide to form a disaccharide. Examples include glucose and fructose.

💡Polysaccharide

A polysaccharide is a carbohydrate composed of many monosaccharide units linked together. The video explains how repeated dehydration synthesis reactions can form long chains of glucose molecules, resulting in polysaccharides like starch in plants and glycogen in animals.

💡Condensation Reaction

A condensation reaction is a chemical reaction where two molecules combine to form a larger molecule, with the loss of a small molecule like water. In the video, the formation of a disaccharide from two glucose molecules through dehydration synthesis is an example of a condensation reaction.

💡Hydronium Ion

A hydronium ion (H3O+) is a water molecule with an extra proton attached, making it positively charged. In the video, the formation and involvement of a hydronium ion are discussed during the dehydration synthesis process, where a proton is transferred, and a water molecule is formed.

Highlights

Importance of glucose as a simple sugar.

Molecular structure of glucose.

Using glucose as a building block for more complex sugars and carbohydrates.

Nucleophilic attack in organic chemistry explained using glucose molecules.

Electronegativity of oxygen atoms creating partial charges on carbons.

Formation of a bond between oxygen and carbon through nucleophilic attack.

Formation of hydroxide anion and its interaction with hydrogen ions.

Linking of two glucose molecules to form a more complex structure.

Dehydration synthesis: losing a water molecule to form a larger molecule.

Difference between neutral oxygen and positively charged oxygen.

Hydronium ion formation from water molecules.

Formation of disaccharides from monosaccharides through dehydration synthesis.

Definition and importance of saccharides and carbohydrates.

Polysaccharides: long chains of glucose molecules and their biological significance.

Starch in plants and glycogen in animal cells as examples of polysaccharides.

Transcripts

- [Voiceover] In the previous video, we talked about

the importance of glucose as a simple sugar.

We talked about its molecular structure.

What I wanna do in this video is study how glucose

can be, how we can use it as a building block

for more complex sugars and more complex carbohydrates.

So this right over here, I've copy and pasted

two glucose molecules, we can number their carbons.

This is one, two, three, four, five, six.

One, two, three, four, five, six.

We have them in their cyclic form.

And what we're going to do is explore

what would happen if this oxygen right over here,

I'll highlight it in this magenta color,

were to use one of its lone pairs,

one of its lone pairs, to do,

what's in organic chemistry, referred to

as a nucleophilic attack on the number one carbon

on the left-hand glucose molecule.

And the reason why that could happen

is this number one carbon right over here,

it's attached to two oxygens.

Oxygens are very electronegative, they like to

hog electrons when they're in a covalent bond.

So that's gonna give this carbon

a partially positive charge.

And this oxygen is very electronegative.

It's gonna hog the electrons from this hydrogen

and the number four carbon

on the right-hand glucose molecule,

so it's gonna have a partially negative charge.

And so it is going to be nucleophilic.

It's going to be attracted to, I guess you could say,

the carbon nucleus, to the partially positive charge

right over here, and so as it does,

it's gonna use a lone pair to form a bond.

It's gonna share it with the carbon,

and then the carbon can let go of another bond.

So it could let go of, it could let go of

both of these electrons in that bond.

Now you could say maybe that just goes

back to the oxygen and it forms a hydroxide anion.

Or we could imagine, well, maybe it'll be used,

maybe it forms a hydroxide anion first,

or maybe that bond immediately goes

and picks up a hydrogen ion out of the solution

from another, from a hydronium ion sitting someplace.

So this could be used to form

a bond with this hydrogen ion, which is really,

this is just a proton here.

You take an electron away from hydrogen,

it's just going to be a proton.

Well, what's that going to do?

Well, that's going to link these two glucose molecules.

And it's gonna link it just like this,

and it's important to keep track of our molecules here.

So this oxygen is now going to be this oxygen.

It's now going to be that oxygen.

This bond between the number four carbon

on the right-hand side of that oxygen

is this bond right over here.

This, where we took this electron pair

to form this bond with the number one carbon,

that is, let me do it in that magenta color.

That is this bond, this bond right over here.

The oxygen, this oxygen, is now this oxygen right over here.

And this electron pair is now formed a bond

with this hydrogen, so we could say,

oh, that could be, let me do that blue,

that could be, that could be this bond right over here.

Now the one difference is, based on how I've drawn it,

this oxygen, or sorry, this oxygen, the way I've drawn it,

it's attached to the number one carbon here,

the number four carbon here.

We have that over, we've already done that over here.

Number one carbon on the left molecule,

number four carbon on the right molecule.

But we also have it bonded,

we also have it bonded to a hydrogen.

So just the way I've done it right now,

it's still bonded to a hydrogen.

It's going to have a net positive charge.

Over here, it was neutral.

It was neutral right over here, but then

it's now sharing its electrons.

It's now sharing both of those electrons

in a covalent bond, and so you can think of it as

it's giving away an electron to this carbon,

so it's going to have a net positive charge.

But then to get back to neutral, you could imagine,

well, maybe some type of a water molecule

could grab that ion, so maybe this one right over here.

This one right over here could grab that hydrogen,

and then these electrons, both of them,

and it would just grab the hydrogen nucleus of the proton,

and so these two electrons could go back

to this oxygen and then this oxygen would become neutral.

And so what we would be left with,

actually, let me just erase this,

is that this hydrogen would now be attached

to this oxygen, and we would have a hydronium ion.

And this is reasonable.

We essentially had some hydronium.

We had a hydrogen proton out here before and we still do.

Now it's attached to a water,

so we've take a proton and we've given back a proton,

so we have a net-net kind of added charge

or taken charge away from the system.

But the important thing that we just saw is

as these two things essentially attached,

we lost a water molecule, or I guess net-net,

this system lost a water molecule.

It took up a charge to do it, to build that water molecule,

but the thing that really kind of escaped

from both of these two molecules is this,

is this right over here.

This H is this H, this oxygen is this oxygen.

And this hydrogen is this hydrogen right over here.

And so this type of a reaction in which

we're synthesizing a more complex molecule,

a longer chain of glucose molecules,

this is called a dehydration synthesis.

So what we just did, this right over here

is called a dehydration synthesis.

Why are we calling it a dehydration synthesis?

Well, we've just taken a water out.

If you imagine losing water, we talk about

you're getting dehydrated.

And why synthesis?

Well, we put two things together.

We synthesized a larger molecule.

Sometimes this would be called a condensation reaction.

Condensation reaction.

And by doing this, these two glucose molecules

are able to form a disaccharide now.

So each individually, they were monosaccharides,

so this one on the right, that's a monosaccharide.

What does monosaccharide mean?

Well, it means, mono means single or one

and saccharide comes from the Greek word for sugar.

The Greek word for sugar is,

I'm gonna mispronounce it, is sakcharon.

When people talk about something being saccharine,

they're saying something is very, very sweet.

The Greek word for sugar is sakcharon.

So saccharide means it's a sugar, it's a single sugar.

So that meaning there, sugar.

And the general term saccharide refers to

not just the simple sugars, monosaccharides,

but it could mean two of these things put together.

And there's other simple sugars, fructose and others.

Or it could mean a huge number of these put together.

You could have polysaccharides.

And that whole class, saccharides,

we also associate with carbohydrates.

Now we went from two monosaccharides to right over here.

This is a disaccharide.

This is a disaccharide, we have two.

Two monosaccharides were involved.

This is a disaccharide, and this particular disaccharide

is maltose, or malt sugar. Maltose.

So the whole point of this video is to see

how you can start with these simple sugars,

these monosaccharides, and form disaccharides.

In fact, you could keep going.

You could keep having dehydration synthesis,

condensation reactions to keep adding

more and more monosaccharides to build

longer and longer chains.

So if you were to keep doing that,

it you were to keep building chains of these things,

now you're getting into the world of polysaccharides.

Polysaccharides, or many simple sugars,

many monosaccharides, many monosaccharides put together.

And this is the case for sugar, but this is something

that you'll see often in chemistry,

where you have a single unit, here's a single sugar,

but if we talk in more general terms,

we would call it a monomer.

And then if we have a bunch of these monomers put together,

we would call it a polymer.

Now polysaccharides are super important,

and you have probably eaten some polysaccharides today,

and you probably have some, in fact,

I'm sure you have some polysaccharides

stored in your cells right now.

If you put a bunch of glucose molecules,

if we were to keep this process going

and we were to have a bunch of glucose molecules together,

when you find it in plants, it'll often be

in the form of a starch.

So a polysaccharide that you'll find in a plant

is a starch, a bunch of glucoses put together

in your own cells to have a immediate energy store,

a bunch of glucoses put together is glycogen.

So these macromolecules, these polysaccharides

that are made up of a bunch of simple sugars,

a bunch of monosaccharides put together,

these are very common in biology.

You have eaten them and you are storing them

in your body right now.