Macromolecules: Structure and Function

• Within cells, small organic molecules combine to create larger molecules.
• Large macromolecules consist of thousands of covalently bonded atoms.
• Macromolecules weigh over 100,000 daltons.
• The four major classes of macromolecules include carbohydrates, lipids, proteins, and nucleic acids.
• Most macromolecules are polymers constructed from smaller building blocks called monomers.
• Three classes of macromolecules—carbohydrates, proteins, and nucleic acids—form polymers.
• A polymer is a long molecule of similar or identical monomers linked by covalent bonds.
• Monomers are the repeating units that build polymers.
• Monomers connect through covalent bonds via a dehydration synthesis reaction, resulting in the loss of a water molecule.
• During the creation of a bond between monomers, each monomer contributes part of the water molecule; one provides a hydroxyl group (-OH), and the other provides a hydrogen (-H).
• Hydrolysis disassembles polymers by breaking bonds through the addition of water molecules, effectively reversing dehydration synthesis.
• During hydrolysis, a hydrogen atom attaches to one monomer, and a hydroxyl group attaches to an adjacent monomer.
• Enzymes in the digestive tract facilitate the hydrolysis of specific polymers into monomers.
• Body cells then use dehydration reactions to assemble monomers into new, specific polymers.

Carbohydrates

• Includes sugars and their polymers.
• Sugars serve as fuel and a source of carbon.
• The simplest carbohydrates are monosaccharides, also known as simple sugars.
• Disaccharides, or double sugars, consist of two monosaccharides joined by a condensation reaction.
• Polysaccharides are polymers composed of many monosaccharides.

Monosaccharides

• Molecular formulas are multiples of CH2O (e.g., glucose is C6H12O6).
• They contain a carbonyl group (-C=O) and multiple hydroxyl groups (-OH).
• Most names for sugars end in -ose.
• Glucose (an aldose) and fructose (a ketose) are structural isomers
• Monosaccharides, especially glucose, serve as a major fuel for cellular work.
• They also function as raw materials for synthesizing other monomers, like amino acids and fatty acids.
• Monosaccharides typically form rings in aqueous solutions.
• Two monosaccharides can join with a glycosidic linkage to form a disaccharide via dehydration synthesis.
• Maltose (malt sugar) forms by joining two glucose molecules.
• Sucrose (table sugar) forms by joining glucose and fructose and is the major transport form of sugars in plants.
• Lactose (milk sugar) forms by joining glucose and galactose.

Polysaccharides

• Polymers are composed of hundreds to thousands of monosaccharides linked by glycosidic bonds.
• Serve in storage and structural roles.
• Some are for sugar storage and are hydrolyzed when sugars are needed.
• Others provide building materials for cells or organisms.
• Starch is a storage polysaccharide composed entirely of glucose monomers.
• Most glucose monomers in starch are joined by 1–4 linkages.
• 1,4 glycosidic bonds lead to elongation of the polymer
• 1,6 glycosidic bonds lead to branching of polymers
• Amylose, the simplest form of starch, is unbranched and forms a helix.
• Amylopectin is a branched form of starch.
• Animals store glucose as glycogen, similar to amylopectin with extensive branching.
• Humans and other vertebrates store a day’s supply of glycogen in the liver and muscles.
• Cellulose is the most abundant organic compound and a major component of plant cell walls.
• Like starch, it is a glucose polymer.
• Parallel cellulose molecules form microfibrils, providing strength to plant cell walls.
• Cellulose passes through the human digestive tract as “insoluble fiber.”
• Some microbes digest cellulose into glucose monomers using cellulase enzymes.
• Herbivores have symbiotic relationships with cellulolytic microbes, allowing access to energy.
• Some fungi can also digest cellulose.
• Chitin, found in arthropod exoskeletons, provides structural support.
• It is similar to cellulose but contains a nitrogen-containing appendage on each glucose monomer.
• Chitin also supports the cell walls of many fungi.

Explore the formation, structure, and function of macromolecules. Learn how small organic molecules combine to form larger macromolecules, including carbohydrates, lipids, proteins, and nucleic acids. Understand the roles of monomers, polymers, dehydration synthesis, and hydrolysis in building and breaking down these essential biological molecules.