Learning Objectives
In this section, you will explore the following questions:
- How are complex macromolecule polymers synthesized from monomers?
- What is the difference between dehydration (or condensation) and hydrolysis reactions?
Connection for AP® Courses
Living organisms need food to survive as it contains critical nutrients in the form of biological macromolecules. These large molecules are composed mainly of six elements—sulfur, phosphorus, oxygen, nitrogen, carbon, and hydrogen (SPONCH)—in different quantities and arrangements. Complex polymers are built from combinations of smaller monomers by dehydration synthesis, a chemical reaction in which a molecule of water is removed between two linking monomers. (Think of a train: each boxcar, including the caboose, represents a monomer, and the entire train is a polymer.) During digestion, polymers can be broken down by hydrolysis, or the addition of water. Both dehydration and hydrolysis reactions in cells are catalyzed by specific enzymes. Dehydration reactions typically require an investment of energy for new bond formation, whereas hydrolysis reactions typically release energy that can be used to power cellular processes. The four categories of macromolecules are carbohydrates, lipids, proteins, and nucleic acids. Evidence supports scientists’ claim that the organic precursors of these biological molecules were present on primitive Earth.
Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 1 of the AP® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® Exam questions. A learning objective merges required content with one or more of the seven Science Practices.
| Big Idea 1 | The process of evolution drives the diversity and unity of life. |
| Enduring Understanding 1.D | The origin of living systems is explained by natural processes. |
| Essential Knowledge | 1.D.1 There are several hypotheses about the natural origin of life on Earth, each with supporting scientific evidence. |
| Science Practice | 1.2 The student can make claims and predictions about natural phenomena based on scientific theories and models. |
| Learning Objective | 1.27 The student is able to describe a scientific hypothesis about the origin of life on Earth. |
| Essential Knowledge | 1.D.1 There are several hypotheses about the natural origin of life on Earth, each with supporting scientific evidence. |
| Science Practice | 3.3 The student can evaluate scientific questions. |
| Learning Objective | 1.28 The student is able to evaluate scientific questions based on hypotheses about the origin of life on Earth. |
Teacher Support
Stress to the class that macromolecules are produced through dehydration synthesis and taken apart through hydrolysis. As the names imply, water is involved in both cases.
Dehydration Synthesis
As you’ve learned, biological macromolecules are large molecules, necessary for life, that are built from smaller organic molecules. There are four major classes of biological macromolecules (carbohydrates, lipids, proteins, and nucleic acids); 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 and are bound to hydrogen, and may contain oxygen, nitrogen, and additional minor elements.
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.”
Teacher Support
Explain that when something is dehydrated, water is removed from it. Identify the specific hydrogen and specific hydroxyl group in the models or illustrations that are removed from two monomers to make the water. The remaining oxygen atom is used to link the two monomers together.
Hydrolysis is the splitting or lysis of a bond between monomers within a polymer, using water. Explain that the two parts of water, the hydrogen atom and hydroxyl group, are added to the monomers after the separation from the polymer, with the result that each has a hydroxyl group where the oxygen molecule linking them was found.
Ask the students what came first, biological chemicals or intact cells? Then discuss Miller and Urey’s experiments. They can attempt to explain how these complex macromolecules could be created in the absence of life. The resulting molecules floated around in the atmosphere and eventually fell into the early oceans, then probably became incorporated into primitive cells.
In a dehydration synthesis reaction (Figure 3.2), 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, 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 with water.” Hydrolysis is a reaction in which a water molecule is used during the breakdown of another compound (Figure 3.3). 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.
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.
Link to Learning
Visit this site to see visual representations of dehydration synthesis and hydrolysis.
Everyday Connection for AP® Courses
Recreating Primordial Earth
Many people wonder how life formed on Earth. In 1953, Stanley Miller and Harold Urey developed an apparatus like the one shown in Figure 3.4 to model early conditions on earth. They wanted to test if organic molecules could form from simpler molecular precursors believed to exist very early in Earth’s history. They used boiling water to mimic early Earth’s oceans. Steam from the “ocean” combined with methane, ammonia, and hydrogen gases from the early Earth’s atmosphere and was exposed to electrical sparks to act as lightning. As the gas mixture cooled and condensed, it was found to contain organic compounds, such as amino acids and nucleotides. According to the abiogenesis theory, these organic molecules came together to form the earliest form of life about 3.5 billion years ago. (credit: Yassine Mrabet)
Science Practice Connection for AP® Courses
Think About It
How does Stanley Miller’s and Harold Urey’s model support the claim that organic precursors present on early Earth could have assembled into large, complex molecules necessary for life? What chemical “ingredients” were present on early Earth?
Teacher Support
This question is an application of Learning Objectives 1.27 and Science Practice 1.2 because students are asked to describe how the organic soup model supports the formation of complex polymers from simple organic precursors.