3.1 Synthesis of Biological Macromolecules

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 dehydration synthesis, which means “to put together while losing water.”

In a dehydration synthesis reaction, a hydrogen from one monomer combines with a hydroxyl group of another monomer, releasing a water molecule. 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 monomer types 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 different polymers. For example, glucose monomers are the constituents of starch, glycogen, and cellulose.

Hydrolysis

Polymers break down into monomers during hydrolysis. A chemical reaction occurs when inserting a water molecule across the bond. Breaking a covalent bond with this water molecule in the compound achieves this. During these reactions, the polymer breaks into two components: one part gains a hydrogen atom (H) and the other gains a hydroxyl group (OH) from a split water molecule.

Dehydration and hydrolysis reactions are catalyzed, or “sped up,” by specific enzymes; dehydration reactions increase the length of polymer chains, requiring energy, while hydrolysis reactions decrease their length and release energy. These reactions are similar for most macromolecules, but each monomer and polymer reaction is specific for its class. For example, catalytic enzymes in the digestive system hydrolyze or break down the food we ingest into smaller molecules. This allows cells in our body to easily absorb nutrients in the intestine. A specific enzyme helps break down each macromolecule. For instance, amylase, sucrase, lactase, and maltase contribute to the breakdown of specific carbohydrates. Enzymes called proteases, such as pepsin and peptidase, and hydrochloric acid break down proteins. Lipases break down lipids. As these macromolecules are broken down, energy is released to enable cellular activities.

FIGURE DESCRIPTIONS

Figure 1. The image depicts a chemical reaction diagram. It illustrates the formation of a disaccharide from two monosaccharide molecules through a dehydration reaction. On the left, two identical hexagonal ring structures represent monosaccharides, each with chemical groups labeled as CH2OH, OH, H, and O surrounding the rings. An arrow points to the right side of the image, indicating the reaction process. The resulting disaccharide, shown on the right, consists of two hexagonal rings connected by an oxygen bridge. The equation “+H2O” in red indicates the removal of a water molecule during the reaction.

Figure 2. The image illustrates a chemical reaction involving two hexagonal glucose molecules. On the left, two glucose rings are connected by an oxygen bridge, with specific hydroxyl (OH) groups highlighted. The connected glucose molecules are depicted in beige with varying bold black lines indicating the structure. An arrow points to the right side, representing the hydrolysis reaction where the bond between the glucose molecules is broken. The product includes two separate glucose rings, with an added red hydroxyl group to one and a red hydrogen atom to the other, indicating where water (H2O) has added. The chemical structures are annotated with hydrogen atoms, oxygen atoms, and hydroxyl groups.