Chapter 5 The Structure and Function of Large Biological molecules :REVIEW

Chapter 5: The Structure and Function of Large Biological Molecules

Overview

This chapter explores the four major classes of macromolecules—carbohydrates, lipids, proteins, and nucleic acids—focusing on their structures, functions, and the chemical processes involved in their synthesis and breakdown. Understanding these molecules is fundamental to the study of biology, as they are essential to the structure and function of all living organisms.

Polymer Assembly and Disassembly

Condensation (Dehydration) and Hydrolysis Reactions

• Dehydration (Condensation) Reaction: A chemical reaction in which two monomers are covalently bonded to each other with the removal of a water molecule. This process builds polymers from monomers.

• Hydrolysis Reaction: A chemical reaction that breaks the bonds between monomers by the addition of water, disassembling polymers into monomers.

• Function: Dehydration reactions are responsible for assembling macromolecules, while hydrolysis reactions are responsible for their breakdown during digestion and cellular metabolism.

• Linkages Formed or Broken:

  • Carbohydrates: Glycosidic linkages

  • Proteins: Peptide bonds

  • Lipids: Ester linkages

  • Nucleic Acids: Phosphodiester bonds

• General Equation for Dehydration Synthesis: $\mathrm{Monomer}_1 + \mathrm{Monomer}_2 \xrightarrow{\text{dehydration}} \mathrm{Polymer} + \mathrm{H}_2\mathrm{O}$

• General Equation for Hydrolysis: $\mathrm{Polymer} + \mathrm{H}_2\mathrm{O} \xrightarrow{\text{hydrolysis}} \mathrm{Monomer}_1 + \mathrm{Monomer}_2$

Carbohydrates

Structures and Functions of Simple and Complex Carbohydrates

• Monomer: Monosaccharides (e.g., glucose, fructose)

• Polymer: Polysaccharides (e.g., starch, glycogen, cellulose)

• Linkage: Glycosidic linkage (covalent bond formed by a dehydration reaction between two monosaccharides)

• Functions:

  • Starch: Storage polysaccharide in plants; composed of glucose monomers.

  • Glycogen: Storage polysaccharide in animals; highly branched structure.

  • Cellulose: Structural polysaccharide in plant cell walls; composed of β-glucose monomers.

• Solubility: Simple sugars are generally soluble in water; complex polysaccharides vary in solubility.

• Example: Table sugar (sucrose) is a disaccharide composed of glucose and fructose.

Lipids

Structures and Functions of Three Kinds of Lipids

• Types of Lipids:

  • Fats (Triglycerides): Composed of glycerol and three fatty acids; main function is energy storage.

  • Phospholipids: Composed of glycerol, two fatty acids, and a phosphate group; major component of cell membranes; amphipathic (having both hydrophilic and hydrophobic regions).

  • Steroids: Characterized by a carbon skeleton consisting of four fused rings; examples include cholesterol and hormones.

• Linkage: Ester linkage (formed between the hydroxyl group of glycerol and the carboxyl group of fatty acids)

• Saturated vs. Unsaturated Fatty Acids:

  • Saturated: No double bonds between carbon atoms; solid at room temperature (e.g., butter).

  • Unsaturated: One or more double bonds; liquid at room temperature (e.g., olive oil).

• Impact on Membrane Fluidity: Unsaturated fatty acids increase membrane fluidity due to kinks in their hydrocarbon chains, while saturated fatty acids decrease fluidity.

• Example: Phospholipid bilayer forms the structural basis of all cell membranes.

Proteins

Structures and Functions of Amino Acids and Proteins

• Monomer: Amino acids (20 different types, each with a unique R group/side chain)

• General Structure of an Amino Acid:

  • Central carbon (α-carbon) bonded to an amino group (–NH2), a carboxyl group (–COOH), a hydrogen atom, and an R group (side chain).

  General Formula: $\mathrm{H}_2\mathrm{N} - \mathrm{C}(\mathrm{H})(\mathrm{R}) - \mathrm{COOH}$

• Polymer: Polypeptide (protein)

• Linkage: Peptide bond (formed by dehydration reaction between amino group of one amino acid and carboxyl group of another)

• Levels of Protein Structure:

  • Primary: Sequence of amino acids; held together by peptide bonds.

  • Secondary: Local folding into α-helices and β-pleated sheets; stabilized by hydrogen bonds.

  • Tertiary: Overall 3D shape; stabilized by interactions among R groups (hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bridges).

  • Quaternary: Association of multiple polypeptide chains; stabilized by the same interactions as tertiary structure.

• Directionality: New amino acids are added to the carboxyl (C) terminus of the growing polypeptide; the first amino acid is at the amino (N) terminus.

• Enzymes: Proteins that act as biological catalysts, speeding up chemical reactions without being consumed.

• Characteristics of Enzymes:

  • Highly specific for their substrates

  • Lower activation energy of reactions

  • Can be regulated by inhibitors or activators

• Example: Hemoglobin (oxygen transport), amylase (starch digestion), catalase (breakdown of hydrogen peroxide)

Nucleic Acids

Structures of DNA and RNA and Their Component Nucleotides

• Monomer: Nucleotide (composed of a five-carbon sugar, a phosphate group, and a nitrogenous base)

• Polymer: Polynucleotide (DNA or RNA)

• Linkage: Phosphodiester bond (between the phosphate group of one nucleotide and the sugar of the next)

• DNA vs. RNA:

  Feature	DNA	RNA
  Nitrogenous Bases	A, T, C, G	A, U, C, G
  Sugar	Deoxyribose	Ribose
  Strandedness	Double-stranded (helix)	Single-stranded
  Function	Genetic information storage	Protein synthesis, gene regulation
  Linkage	Phosphodiester bond	Phosphodiester bond
  Base Pairing	Hydrogen bonds (A-T, C-G)	Hydrogen bonds (A-U, C-G)

• Directionality: New nucleotides are added to the 3’ end of the growing polynucleotide chain. The first nucleotide is at the 5’ end, and the last added is at the 3’ end.

• Base Pairing: In DNA, A pairs with T and C pairs with G; in RNA, A pairs with U and C pairs with G.

• Example: DNA (deoxyribonucleic acid) stores genetic information; RNA (ribonucleic acid) functions in protein synthesis and gene regulation.

Macromolecule Comparison Chart

Summary Table of the Four Major Classes of Macromolecules

Type	Monomer	Polymer	Linkage	Functional Groups	Examples	Solubility in Water?	Purpose
Carbohydrates	Monosaccharide	Polysaccharide	Glycosidic	Hydroxyl, Carbonyl	Starch, Glycogen, Cellulose	Yes (simple); varies (complex)	Energy storage, structure
Lipids	Fatty acids, Glycerol	Fats, Oils, Phospholipids, Steroids	Ester	Carboxyl, Hydroxyl, Phosphate (phospholipids)	Triglycerides, Cholesterol	No (mostly insoluble)	Energy storage, membranes, signaling
Proteins	Amino acid	Polypeptide	Peptide	Amino, Carboxyl, R group	Enzymes, Hemoglobin	Varies	Catalysis, structure, transport
Nucleic Acids	Nucleotide	Polynucleotide	Phosphodiester	Phosphate, Sugar, Nitrogenous base	DNA, RNA	Yes	Information storage, transfer

Additional Info

• Amphipathic Molecules: Molecules that have both hydrophilic and hydrophobic regions (e.g., phospholipids).

• Predicting Solubility: Macromolecules with polar or charged groups (e.g., carbohydrates, nucleic acids) are generally water-soluble; those with large nonpolar regions (e.g., many lipids) are not.

• Identifying Linkages: Recognizing the type of bond (glycosidic, ester, peptide, phosphodiester) is key to identifying the class of macromolecule.