The Structure and Function of Large Biological Molecules

Copyright and Overview

  • Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

  • Title: Lecture Presentations for Biology Eighth Edition by Neil Campbell and Jane Reece

  • Contributors: Chris Romero (Lectures), Erin Barley, Joan Sharp

Chapter 5: The Structure and Function of Large Biological Molecules

Overview: The Molecules of Life

  • All living organisms consist of four classes of large biological molecules:

    • Carbohydrates

    • Lipids

    • Proteins

    • Nucleic acids

  • In cells, small organic molecules bond to form larger structures.

  • Macromolecules are defined as large molecules made from thousands of covalently connected atoms.

Concept 5.1: Macromolecules as Polymers

  • A polymer is a long molecule made up of many similar building blocks known as monomers.

  • Three out of the four classes of organic molecules that compose life are polymers:

    • Carbohydrates

    • Proteins

    • Nucleic Acids

Formation and Breakdown of Polymers

  • Condensation reaction (Dehydration reaction): Occurs when two monomers bond, resulting in the loss of a water molecule.

    • This synthesis process involves:

    • Short polymer + Unlinked monomer → Longer polymer (with loss of H₂O)

  • Hydrolysis: The reverse process of condensation, where polymers are broken down into monomers by the addition of water.

    • In this reaction:

    • Polymer + H₂O → Monomer + Monomer

Diversity of Polymers

  • Each cell contains thousands of different macromolecules, showcasing the vast variety of polymers derivable from a small set of monomers.

Concept 5.2: Carbohydrates as Fuel and Building Material

  • Carbohydrates encompass sugars and their polymers.

  • Monosaccharides: The simplest form of carbohydrates and single sugars.

Sugars

  • The molecular formula of monosaccharides typically adheres to multiples of CH₂O.

  • Example: Glucose (C₆H₁₂O₆) is the most commonly occurring monosaccharide.

Classification of Monosaccharides

  • Monosaccharides are classified based on several criteria:

    • The location of the carbonyl group:

    • Aldose (aldehyde sugar)

    • Ketose (ketone sugar)

    • The number of carbon atoms in the carbon skeleton:

    • Hexoses: 6 carbon atoms (e.g., Glucose, Galactose, Fructose)

    • Pentoses: 5 carbon atoms

    • Trioses: 3 carbon atoms

Structural Forms of Monosaccharides

  • In aqueous solutions, many sugars form ring structures rather than existing in linear forms.

    • This ring formation is significant as it alters their reactivity and functionality, serving as a primary fuel for cellular metabolism and a precursor for building other molecules.

Disaccharides

  • A disaccharide forms when two monosaccharides are joined by a condensation reaction, resulting in a glycosidic linkage.

    • Example of glycosidic linkages:

    • Sucrose (Glucose + Fructose)

    • Maltose (Glucose + Glucose)

Polysaccharides

  • Polysaccharides: Long polymers of sugars designed for storage (e.g., Starch, Glycogen) or structural integrity (e.g., Cellulose).

  • The combination of sugar monomers and glycosidic linkages prominently affects the structure and functionality of polysaccharides.

Storage Polysaccharides

  • Starch: A storage polysaccharide in plants consisting entirely of glucose monomers, exists in helical structures.

  • Glycogen: An animal polysaccharide allowing energy reserve, primarily stored in liver and muscle cells.

Structural Polysaccharides

  • Cellulose: A major structural component of plant cell walls. Notably, enzymes that digest starch cannot hydrolyze cellulose's beta linkages, leading to insoluble fiber in human diets.

  • Chitin: Another structural polysaccharide found in arthropod exoskeletons and in the cell walls of fungi.

Concept 5.3: Lipids, a Diverse Group of Hydrophobic Molecules

  • Lipids are distinct among the large biological molecules as they do not form polymers.

  • They are hydrophobic due to their composition mainly of hydrocarbons, which are linked via nonpolar covalent bonds.

    • Major lipids include:

    • Fats

    • Phospholipids

    • Steroids

Formation of Fats

  • Fats consist of glycerol (a three-carbon alcohol) bound to fatty acids via ester linkages, forming triacylglycerols (triglycerides).

Types of Fatty Acids

  • Saturated fats: No double bonds between carbon atoms; examples include stearic acid.

  • Unsaturated fats: At least one double bond in the fatty acid chain; tends to be liquid at room temperature (e.g., oleic acid).

  • Saturated fats (animal fats) are solid at room temperature, while most unsaturated fats (plant fats) are liquid.

Functions of Fats

  • The primary role of fats is to serve as energy storage, and they also function in cushioning organs and insulating the body.

Phospholipids

  • Phospholipids: Comprised of two fatty acids and a phosphate group bound to glycerol.

    • When placed in water, phospholipids form bilayers due to hydrophobic interactions, essential for forming cell membranes.

Steroids

  • Steroids: Lipids with a carbon skeleton exhibiting four fused rings; cholesterol serves as an important structural component of cell membranes, although high blood cholesterol levels may lead to health risks.

Concept 5.4: Proteins and Their Diversity

  • Proteins are recognized for their diversity of structures, leading to varied functions.

    • Classes of proteins include:*

    • Enzymatic proteins

    • Defensive proteins

    • Storage proteins

    • Transport proteins

    • Hormonal proteins

    • Receptor proteins

    • Motor proteins

General Function of Proteins

  • Enzymatic: Catalyze biochemical reactions (e.g., digestive enzymes).

  • Protective: Inactivate pathogens (e.g., antibodies).

  • Structural: Provide support (e.g., collagen in connective tissues).

Amino Acids as Monomers

  • Amino acids are organic molecules with:

    • A carboxyl group (-COOH)

    • An amino group (-NH₂)

    • Differing side chains known as R groups which determine the characteristics of each amino acid.

Protein Structure

  • Proteins show several structural levels:

    • Primary Structure: Sequence of amino acids in a polypeptide.

    • Secondary Structure: Coils and folds through hydrogen bonding in the backbone (α-helix and β-pleated sheet).

    • Tertiary Structure: 3D shape determined by interactions among R groups.

    • Quaternary Structure: Multiple polypeptide chains forming a complex macromolecule.

Protein Folding and Denaturation

  • Denaturation: Alterations that disrupt the structure of proteins due to variables such as pH, temperature, or salt concentration.

  • Chaperonins: Specialized proteins assist in the proper folding of other proteins within cells.

Concept 5.5: Nucleic Acids as Genetic Material

  • Nucleic acids (DNA and RNA) store, transmit, and regulate genetic information.

    • DNA provides instructions for its own replication and for protein synthesis via mRNA.

    • There are two families of nucleic acid:

    • Deoxyribonucleic Acid (DNA)

    • Ribonucleic Acid (RNA)

Structure of Nucleic Acids

  • Nucleotides: Basic unit of nucleic acids, made up of a nitrogenous base, a five-carbon sugar, and a phosphate group.

    • Nitrogenous Bases:

    • Purines: Adenine (A) and Guanine (G)

    • Pyrimidines: Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA)

    • Nucleic acids exhibit a polynucleotide structure with phosphodiester linkages connecting nucleotides.

DNA vs. RNA Characteristics

  • DNA:

    • Double-stranded helix with antiparallel sugar-phosphate backbones.

    • Nitrogenous bases pair via hydrogen bonds (A with T and G with C).

  • RNA: Typically single-stranded, playing roles in protein synthesis and gene expression.

Themes in Molecular Biology

  • The themes of emergent properties emphasize organization's importance in understanding the chemistry of life.

    • Higher levels of molecular organization produce new properties.

Review and Distinctions

  • After studying, you should be able to:

    • List and describe the major classes of biological macromolecules.

    • Recognize the formation of glycosidic linkages and differentiate between sugars.

    • Differentiate between types of fats and their properties.

    • Outline the levels of protein structure and the factors influencing protein shape and functionality.

    • Distinguish between the structures and functions of nucleic acids (DNA and RNA).