Comparing Biological Macromolecules

Discuss biological macromolecules and the differences between the four classes

As we’ve learned, there are four major classes of biological macromolecules:

  • Proteins (polymers of amino acids)
  • Carbohydrates (polymers of sugars)
  • Lipids (polymers of lipid monomers)
  • Nucleic acids (DNA and RNA; polymers of nucleotides)

Let’s take a closer look at the differences between the difference classes.

Learning Objectives

  • Define the term “macromolecule”
  • Distinguish between the 4 classes of macromolecules

Now that we’ve discussed the four major classes of biological macromolecules (carbohydrates, lipids, proteins, and nucleic acids), let’s talk about macromolecules as a whole. Each is an important cell component and performs a wide array of functions. Combined, these molecules make up the majority of a cell’s dry mass (recall that water makes up the majority of its complete mass). Biological macromolecules are organic, meaning they contain carbon. In addition, they may contain hydrogen, oxygen, nitrogen, and additional minor elements.

You are What You Eat

Comparing the Biological Macromolecules

Macromolecule Basic Formula, key features Monomer Examples Uses
Proteins  CHON

−NH2 + −COOH +R group

 

 Amino acids Enzymes, some hormones Storage; Signals; Structural; Contractile; Defensive; Enzyme; Transport; Receptors
Lipids  C:H:O

Greater than 2:1 H:O (carboxyl group)

 Fatty acid and glycerol Butter, oil, cholesterol, beeswax Energy storage; Protection; Chemical messengers; Repel water
Carbohydrates C:H:O

1:2:1

 Monosaccharides Glucose, Fructose, Starch, Glycogen, Cellulose Energy storage; Structure
Nucleic Acids CHONP

pentose, nitrogenous base, phosphate

 Nucleotides DNA, RNA Genetic information

Dehydration Synthesis

Most macromolecules are made from single subunits, or building blocks, called monomers. The monomers combine with each other using 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.”

Shown is the reaction of two glucose monomers to form maltose. When maltose is formed, a water molecules is released.

Figure 1. In the dehydration synthesis reaction depicted above, two molecules of glucose are linked together to form the disaccharide maltose. In the process, a water molecule is formed.

In a dehydration synthesis reaction (Figure 1), 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, this chain of repeating monomers forms a polymer. 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.

Hydrolysis

Polymers are broken down into monomers in a process known as hydrolysis, which means “to split water,” a reaction in which a water molecule is used during the breakdown (Figure 2). During these reactions, the polymer is broken into two components: one part gains a hydrogen atom (H+) and the other gains a hydroxyl molecule (OH–) from a split water molecule.

Shown is the breakdown of maltose to form two glucose monomers. Water is a reactant.

Figure 2. In the hydrolysis reaction shown here, the disaccharide maltose is broken down to form two glucose monomers with the addition of a water molecule. Note that this reaction is the reverse of the synthesis reaction shown in Figure 1.

Dehydration and hydrolysis reactions are catalyzed, or “sped up,” by specific enzymes; dehydration reactions involve the formation of new bonds, requiring energy, while hydrolysis reactions break bonds and release energy. These reactions are similar for most macromolecules, but each monomer and polymer reaction is specific for its class. For example, in our bodies, food is hydrolyzed, or broken down, into smaller molecules by catalytic enzymes in the digestive system. This allows for easy absorption of nutrients by cells in the intestine. Each macromolecule is broken down by a specific enzyme. For instance, carbohydrates are broken down by amylase, sucrase, lactase, or maltase. Proteins are broken down by the enzymes pepsin and peptidase, and by hydrochloric acid. Lipids are broken down by lipases. Breakdown of these macromolecules provides energy for cellular activities.

Visit this site to see visual representations of dehydration synthesis and hydrolysis.

In Summary: Comparing Biological Macromolecules

Proteins, carbohydrates, nucleic acids, and lipids are the four major classes of biological macromolecules—large molecules necessary for life that are built from smaller organic molecules. Macromolecules are made up of single units known as monomers that are joined by covalent bonds to form larger polymers. The polymer is more than the sum of its parts: it acquires new characteristics, and leads to an osmotic pressure that is much lower than that formed by its ingredients; this is an important advantage in the maintenance of cellular osmotic conditions. A monomer joins with another monomer with the release of a water molecule, leading to the formation of a covalent bond. These types of reactions are known as dehydration or condensation reactions. When polymers are broken down into smaller units (monomers), a molecule of water is used for each bond broken by these reactions; such reactions are known as hydrolysis reactions. Dehydration and hydrolysis reactions are similar for all macromolecules, but each monomer and polymer reaction is specific to its class. Dehydration reactions typically require an investment of energy for new bond formation, while hydrolysis reactions typically release energy by breaking bonds.

Check Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.

Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.