Rucete ✏ AP Biology In a Nutshell
2. Macromolecules
This chapter explains the major biological macromolecules—carbohydrates, lipids, proteins, and nucleic acids—and how their structure and polymerization relate to their function in living organisms. It also covers the four levels of protein structure and compares DNA and RNA.
Biological Macromolecules
• Essential elements: nitrogen, carbon, hydrogen, oxygen, phosphorus, sulfur (NCHOPS).
• Carbon is the versatile backbone, forming single, double, or triple bonds and various structural arrangements (linear, branched, ring).
• The structure and function of each macromolecule depend on the types of monomers and their linkages.
Making and Breaking Macromolecules
• Monomers link via dehydration synthesis (removal of water) to form polymers.
• Polymers are broken down via hydrolysis (addition of water).
Carbohydrates
• Made of sugar monomers (monosaccharides); polymers may be linear or branched.
• Function in energy storage (starch in plants, glycogen in animals) and structural support (cellulose in plants).
• The type of glycosidic linkage determines whether the carbohydrate stores energy or provides structure.
Lipids
• Nonpolar molecules used for energy storage, insulation, and membrane structure.
• Fatty acids: – Saturated: no double bonds, solid at room temperature, found in animals. – Unsaturated: one or more double bonds, liquid at room temperature, found in plants.
• Phospholipids: made of glycerol, two fatty acids, and a phosphate group. Amphipathic (hydrophilic head, hydrophobic tails). Critical for membrane formation.
• Steroids: flat, nonpolar lipids derived from cholesterol (e.g., testosterone, estrogen).
Proteins
• Polymers of amino acids, which include an amino group, carboxylic acid group, hydrogen, and unique side chain (R-group).
• R-groups determine polarity and charge; classify amino acids as nonpolar, polar, acidic, or basic.
• Functions: enzymes, structural support, cell signaling, transport, recognition.
Protein Structure
• Primary structure: linear sequence of amino acids linked by peptide bonds.
• Secondary structure: local folding due to hydrogen bonding; forms α-helices and β-pleated sheets.
• Tertiary structure: overall 3D shape formed by interactions between R-groups (hydrophobic, ionic, hydrogen bonding, disulfide bridges).
• Quaternary structure: multiple polypeptide chains assembled into a functional protein (e.g., hemoglobin).
• Shape determines function; denaturation (due to heat, pH, etc.) disrupts shape and function.
Nucleic Acids
• DNA and RNA are polymers of nucleotides.
• Nucleotides consist of a sugar, phosphate group, and nitrogenous base.
• DNA: deoxyribose sugar, bases A-T-C-G, double-stranded, stores genetic information.
• RNA: ribose sugar, bases A-U-C-G, single-stranded, various functions including protein synthesis.
• Phosphodiester bonds link nucleotides between the sugar and phosphate group.
• DNA strands are antiparallel and complementary (A pairs with T, C with G via hydrogen bonds).
Structural Comparisons
• Carbohydrates, proteins, and nucleic acids are polymers; lipids are not true polymers.
• Dehydration synthesis links monomers for all macromolecules except lipids, which form via ester linkages.
• Structure determines function at all levels, from subunit composition to 3D conformation.
In a Nutshell
Biological macromolecules—carbohydrates, lipids, proteins, and nucleic acids—are built from specific monomers and perform diverse functions in living systems. Their structure, from the chemical bonds to overall shape, is key to their function. Understanding the formation, breakdown, and organization of these macromolecules is fundamental to studying biochemistry and molecular biology.