1.3 Introduction to Biological Macromolecules

Macromolecules are large biological polymers made by linking repeating monomers (e.g., monosaccharides, amino acids, fatty acids + glycerol, nucleotides). Cells build them by dehydration synthesis (removing H and OH to form a covalent bond—e.g., glycosidic linkages, peptide bonds, ester linkages, phosphodiester bonds) and break them by hydrolysis (adding water; hydrogen goes to one monomer, OH to the other). Macromolecules are essential because they carry out and store information (nucleic acids), provide structure and catalysis (proteins), store energy and make membranes (lipids), and supply quick energy/structure (carbohydrates). These anabolic and catabolic reactions are core CED concepts for Topic 1.3 and show up on the exam in multiple-choice and free-response questions (Unit 1 content, 8–11% of MC). For a focused review, use the Topic 1.3 study guide (https://library.fiveable.me/ap-biology/unit-1/intro-biological-macromolecules/study-guide/GbUEgZQ9FSaSLCMi5Emo) and practice problems at (https://library.fiveable.me/practice/ap-biology).

Dehydration synthesis (a condensation reaction) builds bigger molecules by joining monomers and removing the equivalent of a water molecule. One monomer loses a hydrogen ion (H+) and the other loses a hydroxyl group (OH–); those lost pieces combine to form H2O, and a new covalent bond links the two monomers. Repeating this polymerization makes polysaccharides (glycosidic linkages), proteins (peptide bonds), lipids (ester linkages), and nucleic acids (phosphodiester bonds). It’s an anabolic process (builds complexity) and is the reverse of hydrolysis, which adds water to break those covalent bonds into monomers. On the AP exam you should be able to describe this reaction and name examples (CED LO 1.3.A). For a clear Topic 1.3 review, see the Fiveable study guide (https://library.fiveable.me/ap-biology/unit-1/intro-biological-macromolecules/study-guide/GbUEgZQ9FSaSLCMi5Emo) and practice more problems at (https://library.fiveable.me/practice/ap-biology).

Hydrolysis and dehydration synthesis are opposite chemical reactions that build and break macromolecules (LO 1.3.A). Dehydration synthesis (a condensation reaction) joins two monomers by forming a covalent bond and removing the equivalent of a water molecule: an H is taken from one monomer and an OH from the other. Repeated dehydration reactions polymerize monomers into polymers (e.g., glycosidic linkages in polysaccharides, peptide bonds in proteins, ester bonds in fats, phosphodiester bonds in nucleic acids). Hydrolysis does the reverse: water is added across the bond, the bond is cleaved, an H attaches to one monomer and an OH to the other—breaking polymers into monomers (catabolism). On the exam, expect to describe which reaction adds/removes water and identify resulting bond changes. For a quick review, see the Topic 1.3 study guide (https://library.fiveable.me/ap-biology/unit-1/intro-biological-macromolecules/study-guide/GbUEgZQ9FSaSLCMi5Emo) and more practice at the unit page (https://library.fiveable.me/ap-biology/unit-1) or practice problems (https://library.fiveable.me/practice/ap-biology).

Think of monomers as single LEGO bricks and polymers as the finished LEGO build. A monomer is a small molecule (like a glucose, an amino acid, or a nucleotide). When monomers join by covalent bonds they form a polymer (polysaccharide, protein, or nucleic acid). Building polymers from monomers uses dehydration synthesis (condensation): a H is removed from one monomer and an OH from another, releasing a water molecule—that’s polymerization. Breaking polymers back into monomers is hydrolysis: water is added, H goes to one monomer and OH to the other, cleaving the bond. Key bonds: glycosidic (sugars), peptide (proteins), ester (lipids), phosphodiester (nucleic acids). This is exactly what AP LO 1.3.A tests—know definitions, be able to describe dehydration vs hydrolysis, and name linkages. For a quick review check the Topic 1.3 study guide (https://library.fiveable.me/ap-biology/unit-1/intro-biological-macromolecules/study-guide/GbUEgZQ9FSaSLCMi5Emo) and practice questions (https://library.fiveable.me/practice/ap-biology).

Hydrolysis needs water because the reaction literally uses a water molecule to split the covalent bond between monomers. In hydrolysis, the H+ from H2O attaches to one monomer and the OH– attaches to the other, restoring each monomer’s functional group and breaking the polymer’s bond (peptide, glycosidic, ester, or phosphodiester depending on the macromolecule). That’s the chemical opposite of dehydration synthesis, where a H and an OH are removed to form a bond. Enzymes (like hydrolases) speed this process and lower the activation energy, so hydrolysis happens quickly and selectively in cells. This is exactly what the CED describes for LO 1.3.A: water adds across a bond to cleave polymers into monomers. For a focused review of these reactions and examples, check the Topic 1.3 study guide (https://library.fiveable.me/ap-biology/unit-1/intro-biological-macromolecules/study-guide/GbUEgZQ9FSaSLCMi5Emo) and practice problems at Fiveable (https://library.fiveable.me/practice/ap-biology).

During dehydration synthesis (also called a condensation reaction) a hydrogen atom (H) is removed from one monomer and a hydroxyl group (OH) is removed from another. Those two pieces combine to form a water molecule (H2O) that’s lost from the reacting molecules. Losing H and OH allows the two monomers to form a new covalent bond (for example a glycosidic linkage between sugars, a peptide bond between amino acids, or an ester linkage in lipids). Repeating this process polymerizes many monomers into a macromolecule. The reverse reaction—hydrolysis—adds a water molecule back, with the H going to one monomer and the OH to the other, breaking the bond (CED LO 1.3.A.2 and 1.3.A.1). For a quick review tied to the AP curriculum, check the Topic 1.3 study guide (https://library.fiveable.me/ap-biology/unit-1/intro-biological-macromolecules/study-guide/GbUEgZQ9FSaSLCMi5Emo) and try practice questions (https://library.fiveable.me/practice/ap-biology) to see this process in exam-style items.

Hydrolysis is how cells break polymers into monomers by cleaving covalent bonds with water: the H from H2O adds to one monomer and the OH adds to the other (CED LO 1.3.A). Cells use specific hydrolase enzymes (proteases/peptidases for peptide bonds, amylase for glycosidic linkages, lipase for ester linkages) to catalyze those reactions, lowering activation energy and working at optimal pH (e.g., trypsin in the small intestine). Digestion happens extracellularly (mouth, stomach, small intestine) and intracellularly (lysosomes): enzymes hydrolyze large food polymers stepwise into absorbable monomers (amino acids, monosaccharides, fatty acids + glycerol), which cells then transport and use for energy or anabolism. This is classic catabolism vs. dehydration synthesis (polymerization) in the CED. For a quick refresher, check the Topic 1.3 study guide (https://library.fiveable.me/ap-biology/unit-1/intro-biological-macromolecules/study-guide/GbUEgZQ9FSaSLCMi5Emo) and more unit review (https://library.fiveable.me/ap-biology/unit-1). Practice problems are at (https://library.fiveable.me/practice/ap-biology).

Dehydration synthesis happens all the time in your body whenever monomers are joined to build polymers (that’s polymerization—an anabolic process). Real examples: - Building proteins: ribosomes link amino acids by peptide bonds, removing H and OH to release water each time a bond forms. - Making polysaccharides: cells form glycosidic linkages (e.g., glucose → glycogen in liver/muscle) by dehydration synthesis. - Forming fats: adipocytes link glycerol to fatty acids via ester linkages to make triglycerides, releasing water. - Assembling nucleic acids: DNA/RNA are made by forming phosphodiester bonds between nucleotides with loss of water. These are the exact reactions contrasted with hydrolysis on the AP CED (LO 1.3.A.1–1.3.A.2), so you should be able to name the bond formed (peptide, glycosidic, ester, phosphodiester) and explain that water is removed. For a quick review, see the Topic 1.3 study guide (https://library.fiveable.me/ap-biology/unit-1/intro-biological-macromolecules/study-guide/GbUEgZQ9FSaSLCMi5Emo) and more unit review (https://library.fiveable.me/ap-biology/unit-1). Practice applying these examples with problems at (https://library.fiveable.me/practice/ap-biology).

Think of dehydration synthesis as removing the parts of water (H and OH) from two monomers so the monomers can share electrons and form a covalent bond. One monomer loses an H (a proton) and the other loses an OH; those two pieces combine to make H2O and leave a new bond between the monomers (e.g., a peptide bond between amino acids, a glycosidic linkage between sugars, or an ester linkage in lipids). Enzymes (and energy input like ATP) usually drive this anabolic reaction because forming covalent bonds is endergonic. The reverse reaction is hydrolysis: adding water breaks the bond (H goes to one fragment, OH to the other). On the AP exam you should use CED terms—dehydration synthesis/condensation, hydrolysis, polymerization, monomer/polymer, and covalent bond—when describing how macromolecules are built and broken down. For a short review, check the Topic 1.3 study guide (https://library.fiveable.me/ap-biology/unit-1/intro-biological-macromolecules/study-guide/GbUEgZQ9FSaSLCMi5Emo) and try practice questions (https://library.fiveable.me/practice/ap-biology).

Polymerization is the process of joining many small molecules (monomers) into a large chain or network (a polymer) by forming covalent bonds. In biology this happens mainly by dehydration synthesis (a condensation reaction): a H is removed from one monomer and an OH from another, releasing a water molecule and creating a linkage (e.g., glycosidic bonds in carbohydrates, peptide bonds in proteins, ester linkages in lipids, phosphodiester bonds in nucleic acids). The reverse is hydrolysis, which adds water to break those covalent bonds and split polymers into monomers. On the AP exam you should use the CED terms (dehydration synthesis, hydrolysis, monomer, polymer, anabolism, catabolism) and be able to identify specific linkages. Review examples and practice questions on the Topic 1.3 study guide (https://library.fiveable.me/ap-biology/unit-1/intro-biological-macromolecules/study-guide/GbUEgZQ9FSaSLCMi5Emo), the Unit 1 overview (https://library.fiveable.me/ap-biology/unit-1), and lots of practice problems (https://library.fiveable.me/practice/ap-biology).

Good question—the name just describes what's happening to water, not whether something’s made or broken. In dehydration synthesis (aka condensation), two monomers form a new covalent bond and, in the process, a hydrogen ion (H+) is removed from one monomer and a hydroxyl (OH) is removed from the other. Those two pieces combine to make one H2O molecule that’s lost from the reactants—so you “dehydrate” the pair while synthesizing (building) a larger polymer. Examples: glycosidic linkages between sugars, peptide bonds between amino acids, ester linkages in lipids (CED 1.3.A.2). Hydrolysis is the reverse: water is added to break the bond (CED 1.3.A.1). For AP you should be able to describe both reactions and name the groups involved—see the Topic 1.3 study guide (https://library.fiveable.me/ap-biology/unit-1/intro-biological-macromolecules/study-guide/GbUEgZQ9FSaSLCMi5Emo) and practice questions (https://library.fiveable.me/practice/ap-biology).

Think of water as the “tool” that either glues pieces together or pulls them apart. - Dehydration synthesis (aka condensation) builds polymers: you remove a H from one monomer and an OH from another—that “loss of water” forms a covalent bond (polymerization). Example: amino acids → peptide bonds. Keyword: anabolism (building). - Hydrolysis breaks polymers: you add H and OH from a water molecule across the bond, cleaving the covalent link into smaller monomers. Example: digestion of proteins into amino acids. Keyword: catabolism (breaking). Mnemonic: “Dehydrate to create; Hydrolyze to halve.” Remember CED LO 1.3.A: dehydration removes water to join monomers; hydrolysis adds water to cleave them. Review the Topic 1.3 study guide for quick examples and diagrams (https://library.fiveable.me/ap-biology/unit-1/intro-biological-macromolecules/study-guide/GbUEgZQ9FSaSLCMi5Emo) and drill extra practice questions at (https://library.fiveable.me/practice/ap-biology).

Water is central to both building and breaking macromolecules. In hydrolysis (catabolism), a water molecule is a reactant: its H is added to one monomer and its OH to the other, breaking a covalent bond (e.g., peptide, glycosidic, ester, or phosphodiester bonds) and splitting a polymer into smaller units. In dehydration synthesis/condensation (anabolism/polymerization), the opposite happens: a hydrogen is removed from one monomer and a hydroxyl from the other, those pieces combine to form one H2O molecule, and a new covalent bond links the monomers. So water is consumed to break polymers and produced when monomers join. This exact mechanism is part of AP LO 1.3.A (see the CED). For a quick topic review, check the Topic 1.3 study guide (https://library.fiveable.me/ap-biology/unit-1/intro-biological-macromolecules/study-guide/GbUEgZQ9FSaSLCMi5Emo) and practice problems (https://library.fiveable.me/practice/ap-biology).

1) Polysaccharide → disaccharide → monosaccharides (glycosidic bond) - Step 1: Water approaches a glycosidic linkage between two glucose units. - Step 2: The H from H2O attaches to the O on one sugar; the OH attaches to the C on the other. - Step 3: The C–O glycosidic covalent bond is cleaved and you get two smaller sugars. (Example: starch → maltose → glucose) 2) Protein → peptides → amino acids (peptide bond) - Step 1: Water donates H to the amino (–NH) side and OH to the carboxyl (–C=O) side of the peptide bond. - Step 2: The C–N peptide bond breaks; result is two free amino acids (catabolic hydrolysis). 3) Nucleic acid → nucleotides (phosphodiester bond) - Same idea: water adds across the phosphodiester bond; OH goes to one sugar, H to the other, breaking the backbone. 4) Lipid (triglyceride) → glycerol + fatty acids (ester linkages) - Water attacks ester bonds; glycerol and fatty acids are released. These are hydrolysis reactions (CED 1.3.A.1)—think H goes to one monomer, OH to the other. For more review and practice, check the Topic 1.3 study guide (https://library.fiveable.me/ap-biology/unit-1/intro-biological-macromolecules/study-guide/GbUEgZQ9FSaSLCMi5Emo) and thousands of practice questions (https://library.fiveable.me/practice/ap-biology).

You were supposed to see the chemical logic of building vs. breaking macromolecules—not magic changes. In the lab you should’ve observed that dehydration synthesis (condensation/polymerization) joins monomers by forming covalent bonds (glycosidic, peptide, ester, phosphodiester) while removing the equivalent of a water molecule. The reverse, hydrolysis, adds water across a bond and cleaves polymers into monomers. Practically that means: after a “build” treatment you’d expect evidence of larger molecules (new covalent linkages, changes in solubility/viscosity or positive biochemical tests), and after a “break” treatment you’d see smaller units restored and loss of polymer-specific signals. On the AP exam you might be asked to describe these reactions, name them (hydrolysis vs. dehydration synthesis), and link them to anabolism vs. catabolism (CED LO 1.3.A). Review the Topic 1.3 study guide (https://library.fiveable.me/ap-biology/unit-1/intro-biological-macromolecules/study-guide/GbUEgZQ9FSaSLCMi5Emo) and the full unit page (https://library.fiveable.me/ap-biology/unit-1). For extra practice, try problems at (https://library.fiveable.me/practice/ap-biology).