2.3: Macromolecules

Biotech Focus

The study of how macromolecules are made can be difficult. Initial studies in this area were made possible through radiolabeling. In radiolabeling, radioactive isotopes, like carbon-14 (¹⁴C), sulfur-35 (³⁵S) and phosphorus-32 (³²P), are added to cells so that these isotopes are incorporated into the macromolecules being made. For DNA synthesis, researchers will rely upon ³²P incorporation into nucleotides. Protein synthesis uses ³⁵S, while carbohydrate synthesis can be studied using ¹⁴C. Thanks to its sensitivity, radiolabeling allows researchers to track, quantify, and analyze the behavior of macromolecules in biological systems.

Introduction

A macromolecule is a large, complex molecule that is essential to the viability and function of cells. There are four major classes of biological macromolecules, carbohydrates, lipids, proteins, and nucleic acids, and each is an important component of the cell. Combined, these molecules make up the majority of a cell’s dry mass. They perform a wide variety of functions, including energy storage, structural support, cellular communication, and genetic information transfer. Understanding the structure of a macromolecule is key to comprehending complex biological processes, from cellular metabolism, to the transfer of genetic information.

Figure \(\PageIndex{1}\): Portrait of a man made out of fruits and vegetables by Giuseppe Arcimboldo. (Arcimboldo, Public Domain)

Concept in Action

Video: Macromolecules

Learning Objectives

By the end of this section, you will be able to:

• Describe the relationship between a monomer and a polymer
• Understand the synthesis of macromolecules
• Explain dehydration and hydrolysis reactions

Synthesis of macromolecules

Biological macromolecules are large molecules, necessary for life, that are built from smaller organic molecules. Biological macromolecules are organic, meaning they contain carbon and are bound to hydrogen, and may contain oxygen, nitrogen, and additional minor elements. Most (but not all) biological macromolecules are polymers. A polymer is a molecule made up of smaller subunits, called monomers. Think of a polymer as being like a necklace, a series of beads strung together. Typically all the monomers in a polymer tend to be the same, or at least very similar to each other, linked to one another over and over again to build up the larger macromolecule. The monomer used depends on the macromolecule. Table \(\PageIndex{1}\) outlines the types of monomers used to construct a carbohydrate, protein, and nucleic acid.

Table \(\PageIndex{1}\): Types of monomer forming organic molecules

Monomer	Polymer (Macromolecule)
"Bead"	"Necklace"
Monosaccharide	Carbohydrate
Amino Acid	Protein
Nucleotide	Nucleic Acid (DNA and RNA)

Monomers combine with each to make the polymer through the formation of covalent bonds. In doing so, the monomers release water molecules as byproducts. This type of reaction is known as dehydration synthesis, which means “to put together while losing water.” When performed in a test tube, the release of water results in condensation on the side of the test tube. As such, dehydration synthesis reactions are also known as condensation reactions.

Figure \(\PageIndex{2}\): Dehydration synthesis reactions create bonds through the removal of a water molecule. In the dehydration synthesis reaction depicted above, two molecules of glucose combine together to form the disaccharide maltose. A hydroxyl (OH) is removed from carbon 1 of one glucose and combines with a hydrogen (H) removed from carbon 4 of the second glucose, to create a molecule water (dotted circle). A new covalent bond forms between the two glucose molecules. Additional monomers may join by the same process, growing the chain and forming a polymer. (Dehydration synthesis by Kareen Martin; CC BY 4.0)

In a dehydration synthesis reaction (Figure \(\PageIndex{2}\)), the hydrogen of one monomer combines with the hydroxyl group of another monomer, releasing a molecule of water. At the same time, the monomers share electrons and form covalent bonds. As additional monomers join using dehydration synthesis, the growing chain of repeating monomers forms the polymer. Since dehydration reactions involve the formation of new bonds, they require an input of energy. Different types of monomers can combine in many configurations, giving rise to a diverse group of macromolecules. Even one kind of monomer can combine in a variety of ways to form several different polymers: for example, glucose monomers are the constituents of starch, glycogen, and cellulose.

Because polymers form through the removal of water molecules, water is required to break them down. The bonds found within a polymer can be broken through a reaction called hydrolysis, which means “to split with water”. The hydrolysis reaction is a reaction in which a water molecule is added to a bond, resulting in the breaking of the bond (Figure \(\PageIndex{3}\)); one product of the reaction gains a hydrogen atom (H+) and the other gains a hydroxyl molecule (OH–). Because hydrolysis reactions break bonds, they will release energy.

Figure \(\PageIndex{3}\): Hydrolysis reactions break bonds through the addition of a water molecule. In the hydrolysis reaction shown here, the addition of water breaks down the disaccharide maltose to form two glucose monomers. One of the new molecules gains a hydrogen atom, while the other gains a hydroxyl (-OH) group. Note that this reaction is the reverse of the dehydration synthesis reaction. (Hydrolysis by Kareen Martin; modified from Synthesis of Biological Macromolecules; CC BY 4.0)

Dehydration and hydrolysis reactions are catalyzed, or “sped up,” by specific enzymes. In biological organisms, these enzymes are specific to the macromolecule. For instance, the carbohydrates amylose, sucrose, lactose, and maltose are broken down by the enzymes amylase, sucrase, lactase, and maltase, respectively. DNA polymers are created by a class of enzymes called DNA polymerases. A discussion of each of the four biological macromolecules, how they are made and how they are degraded, can be found in the remaining pages of this chapter.

Key Concepts

Biological macromolecules are important cellular components and perform a wide array of functions necessary for the survival and growth of living organisms.

Some important concepts to remember are:

• a polymer is a large macromolecule made up of subunits called monomers
• the monomers of a polymer are joined together through a dehydration synthesis reaction, which removes a molecule water to form the bond
• a monomer can be removed from a polymer through a hydrolysis reaction, which breaks a bond through the addition of a water molecule
• the four major classes of biological macromolecules are carbohydrates, lipids, proteins, and nucleic acids
• most biological macromolecules can be considered polymers

Glossary

Catalyst - a substance that speeds up chemical reactions without being consumed, often refers to enzymes in biological systems

Dehydration - to remove water

Dehydration synthesis - a chemical reaction in which two molecules combine to form a larger molecule, while removing a water molecule (H₂O); also known as a condensation reaction

Enzyme - a biological catalyst that speeds up a chemical reaction; not consumed by the chemical reaction

Hydrolysis - to break with water ("hydro" = water; "lysis" = to break)

Macromolecule - a large molecule; often used interchangeably with polymer

Monomer - a small molecule that when combined with one another forms a polymer

Polymer - a large molecule made up of repeating units called monomers