What is it called when two monomers are joined together?

Explanation:

Multiple monomers together form a molecule called a polymer.

Table of Contents Show

Monomers, Polymers, and Dehydration Synthesis
Dehydration Synthesis

In organic macromolecules (there may be other instances, but this is the best example), they are formed through a dehydration synthesis reaction (also called a condensation reaction). In this type of reaction, a net loss of water, makes 2 monomers attach to eachother

Monomers, Polymers, and Dehydration Synthesis

Some biological molecules are relatively small and may contain a handful of atoms bound together. Others are large and unwieldy and can contain hundreds or thousands of atoms.

If you were trying to correctly assemble a molecule that big, you would probably want to start by putting together some smaller fragments and then carefully link those fragments together at the end. Picture it like piecing together patches of a quilt. Or if quilts aren't your thing, think of it like trying to find out who among your Facebook friends knows a guy who knows a guy who knows a guy who can introduce you to Matthew Lewis. Longbottom love!

Um...not that we've ever done that.

In the molecular world, the small subunits that ultimately link together to form larger molecules are called monomers, which literally means "single unit" (mono = one). When a bunch of monomers join together into a much larger molecule, they form a polymer, meaning "many units" (poly = many).

How does this "linking together" happen? Are we talking about one monomer grabbing another monomer’s little molecular hand and never letting go? Not exactly, though that would make for an adorable webcomic. We would call it "Monamours." Clever, no? Anyway...

There is a process by which this joining usually occurs, and it's called dehydration synthesis. The process begins when two monomers line up next to each other. Just when you think they’re going to start line dancing, a hydrogen (H) from one monomer binds with a hydroxyl group (OH) from the other monomer, and voilà! A water molecule is born: H+ + OH- = H2O.

While this is happening, the two monomers are binding to each other where they were bound to their respective hydrogen (–H) or hydroxyl (–OH) groups. (We add a dash to molecular groups, to show that they are attached to something else.) Having bonded, our lonely monomers are now a single polymer. This blissful union is presided over by an enzyme, which is mainly there to help speed things along.

The name of this whole process is dehydration synthesis because monomers are literally coming together and synthesizing a polymer by dehydrating, or removing a water molecule.

Need a picture? We were thinking that, too. On the chemical bond level of things, here is what happens when your body makes a triglyceride.

First, here's what one dehydration reaction looks like:

Great. Let's do it again. And again. This is called dehydration synthesis:

Super. Why is everything so straight and awkward-looking? Fine, we will fix it. Here is the final product. Happy? (Psst. When there are zigzag lines with no atom in sight (/\/\/\), this means that carbons and hydrogens are the only connecting atoms. Yes, chemists are lazy.)

Brain Snack

Some anti-HIV drugs, called nucleotide analogs, work by interfering with the synthesis of DNA from monomers. Check out the structure of this drug and see if you can figure out why.

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Learning Objectives

Explain dehydration (or condensation) reactions

Dehydration Synthesis

Most macromolecules are made from single subunits, or building blocks, called monomers. The monomers combine with each other via covalent bonds to form larger molecules known as polymers. In doing so, monomers release water molecules as byproducts. This type of reaction is known as dehydration synthesis, which means “to put together while losing water. ” It is also considered to be a condensation reaction since two molecules are condensed into one larger molecule with the loss of a smaller molecule (the water.)

In a dehydration synthesis reaction between two un-ionized monomers, such as monosaccharide sugars, the hydrogen of one monomer combines with the hydroxyl group of another monomer, releasing a molecule of water in the process. The removal of a hydrogen from one monomer and the removal of a hydroxyl group from the other monomer allows the monomers to share electrons and form a covalent bond. Thus, the monomers that are joined together are being dehydrated to allow for synthesis of a larger molecule.

Figure \(\PageIndex{1}\): A dehydration synthesis reaction involving un-ionized moners..: In the dehydration synthesis reaction between two molecules of glucose, a hydroxyl group from the first glucose is combined with a hydrogen from the second glucose, creating a covalent bond that links the two monomeric sugars (monosaccharides) together to form the dissacharide maltose. In the process, a water molecule is formed.

When the monomers are ionized, such as is the case with amino acids in an aqueous environment like cytoplasm, two hydrogens from the positively-charged end of one monomer are combined with an oxygen from the negatively-charged end of another monomer, again forming water, which is released as a side-product, and again joining the two monomers with a covalent bond.

Figure \(\PageIndex{1}\): A dehydration synthesis reaction involving ionized monomers.: In the dehydration synthesis reaction between two amino acids, with are ionized in aqueous environments like the cell, an oxygen from the first amino acid is combined with two hydrogens from the second amino acid, creating a covalent bond that links the two monomers together to form a dipeptide. In the process a water molecule is formed.

As additional monomers join via multiple dehydration synthesis reactions, the chain of repeating monomers begins to form a polymer. Different types of monomers can combine in many configurations, giving rise to a diverse group of macromolecules. Three of the four major classes of biological macromolecules (complex carbohydrates, nucleic acids, and proteins), are composed of monomers that join together via dehydration synthesis reactions. Complex carbohydrates are formed from monosaccharides, nucleic acids are formed from mononucleotides, and proteins are formed from amino acids.

There is great diversity in the manner by which monomers can combine to form polymers. For example, glucose monomers are the constituents of starch, glycogen, and cellulose. These three are polysaccharides, classified as carbohydrates, that have formed as a result of multiple dehydration synthesis reactions between glucose monomers. However, the manner by which glucose monomers join together, specifically locations of the covalent bonds between connected monomers and the orientation (stereochemistry) of the covalent bonds, results in these three different polysaccharides with varying properties and functions. In nucleic acids and proteins, the location and stereochemistry of the covalent linkages connecting the monomers do not vary from molecule to molecule, but instead the multiple kinds of monomers (five different monomers in nucleic acids, A, G, C, T, and U mononucleotides; 21 different amino acids monomers in proteins) are combined in a huge variety of sequences. Each protein or nucleic acid with a different sequence is a different molecule with different properties.

Key Points

During dehydration synthesis, either the hydrogen of one monomer combines with the hydroxyl group of another monomer releasing a molecule of water, or two hydrogens from one monomer combine with one oxygen from the other monomer releasing a molecule of water.
The monomers that are joined via dehydration synthesis reactions share electrons and form covalent bonds with each other.
As additional monomers join via multiple dehydration synthesis reactions, this chain of repeating monomers begins to form a polymer.
Complex carbohydrates, nucleic acids, and proteins are all examples of polymers that are formed by dehydration synthesis.
Monomers like glucose can join together in different ways and produce a variety of polymers. Monomers like mononucleotides and amino acids join together in different sequences to produce a variety of polymers.

Key Terms

covalent bond: A type of chemical bond where two atoms are connected to each other by the sharing of two or more electrons.
monomer: A relatively small molecule which can be covalently bonded to other monomers to form a polymer.