Chapter 5 Biology

The Structure and Function of Large Biological Molecules

Introduction

Large biological molecules, also known as macromolecules, are essential for life. They include carbohydrates, lipids, proteins, and nucleic acids. Each class of macromolecule has unique structures and functions that are fundamental to cellular processes and the maintenance of life.

The Molecules of Life

• All living organisms are composed of four major classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids.

• Most of these macromolecules are polymers, built from repeating subunits called monomers.

Concept 5.1: Macromolecules are Polymers, Built from Monomers

• Polymer: A long molecule consisting of many similar or identical building blocks linked by covalent bonds.

• Monomer: The repeating units that serve as the building blocks of a polymer.

• Carbohydrates, proteins, and nucleic acids are polymers; lipids are not true polymers.

The Synthesis and Breakdown of Polymers

• Dehydration Reaction: Synthesizes a polymer by removing a water molecule, forming a new bond.

• Hydrolysis: Breaks down a polymer by adding a water molecule, breaking a bond.

Equation for Dehydration: $\text{Monomer}_1 + \text{Monomer}_2 \xrightarrow{\text{Dehydration}} \text{Polymer} + H_2O$

Equation for Hydrolysis: $\text{Polymer} + H_2O \xrightarrow{\text{Hydrolysis}} \text{Monomer}_1 + \text{Monomer}_2$

Concept 5.2: Carbohydrates Serve as Fuel and Building Material

• Carbohydrates: Include sugars and polymers of sugars.

• The simplest carbohydrates are monosaccharides (simple sugars).

• Carbohydrate macromolecules are polysaccharides, polymers composed of many sugar building blocks.

Monosaccharides

• Have molecular formulas that are usually multiples of $(CH_2O)_n$.

• Glucose ($C_6H_{12}O_6$) is the most common monosaccharide.

• Monosaccharides can exist in linear or ring forms; in aqueous solutions, ring forms are predominant.

• Serve as major fuel for cells and as raw material for building molecules.

Disaccharides

• Formed when a dehydration reaction joins two monosaccharides.

• The covalent bond formed is called a glycosidic linkage.

• Examples: Maltose (glucose + glucose), Sucrose (glucose + fructose).

Polysaccharides

• Polymers of sugars with storage and structural roles.

• Storage polysaccharides: Starch (plants), Glycogen (animals).

• Structural polysaccharides: Cellulose (plant cell walls), Chitin (exoskeleton of arthropods, cell walls of fungi).

Polysaccharide	Monomer	Function
Starch	α-glucose	Energy storage in plants
Glycogen	α-glucose	Energy storage in animals
Cellulose	β-glucose	Structural support in plant cell walls
Chitin	Modified glucose	Structural support in arthropods and fungi

Concept 5.3: Lipids are a Diverse Group of Hydrophobic Molecules

• Lipids are not true polymers but are grouped together because they are hydrophobic.

• Major types: fats, phospholipids, and steroids.

Fats

• Constructed from glycerol (a three-carbon alcohol) and fatty acids (carboxyl group attached to a long hydrocarbon chain).

• Major function is energy storage.

• Saturated fats: No double bonds; solid at room temperature; mostly animal fats.

• Unsaturated fats: One or more double bonds; liquid at room temperature; mostly plant and fish fats.

Phospholipids

• Composed of two fatty acids and a phosphate group attached to glycerol.

• Fatty acid tails are hydrophobic; phosphate group is hydrophilic.

• Form bilayers in water, which are the basis of cell membranes.

Steroids

• Lipids with a carbon skeleton consisting of four fused rings.

• Cholesterol is an important steroid, component of animal cell membranes, and precursor for other steroids.

Concept 5.4: Proteins Include a Diversity of Structures, Resulting in a Wide Range of Functions

• Proteins account for more than 50% of the dry mass of most cells.

• Functions include catalysis (enzymes), defense, storage, transport, cellular communication, movement, and structural support.

• All proteins are constructed from 20 different amino acids.

Amino Acid Monomers

• Amino acids are organic molecules with amino and carboxyl groups, and a variable side chain (R group).

• The properties of amino acids are determined by their R groups.

Polypeptides (Amino Acid Polymers)

• Amino acids are linked by peptide bonds to form polypeptides.

• Each polypeptide has a unique linear sequence of amino acids, with an amino end (N-terminus) and a carboxyl end (C-terminus).

Protein Structure and Function

• The specific activities of proteins result from their intricate three-dimensional architecture.

• A functional protein consists of one or more polypeptides precisely twisted, folded, and coiled into a unique shape.

Four Levels of Protein Structure

• Primary structure: Unique sequence of amino acids.

• Secondary structure: Coils and folds (α-helix, β-pleated sheet) resulting from hydrogen bonds between backbone atoms.

• Tertiary structure: Overall shape determined by interactions among R groups (hydrophobic interactions, ionic bonds, hydrogen bonds, disulfide bridges).

• Quaternary structure: Association of multiple polypeptide chains.

What Determines Protein Structure?

• Protein structure can be affected by alterations in pH, salt concentration, temperature, or other environmental factors.

• Loss of native structure is called denaturation, which usually results in loss of function.

Concept 5.5: Nucleic Acids Store, Transmit, and Help Express Hereditary Information

• Two types: Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA).

• DNA stores genetic information and directs its own replication.

• DNA directs synthesis of messenger RNA (mRNA), which controls protein synthesis.

The Components of Nucleic Acids

• Nucleic acids are polymers called polynucleotides.

• Each polynucleotide is made of monomers called nucleotides.

• Each nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphate group.

Type	Nitrogenous Bases	Sugar	Strands
DNA	A, T, C, G	Deoxyribose	Double helix
RNA	A, U, C, G	Ribose	Single-stranded

The Structures of DNA and RNA Molecules

• DNA molecules have two polynucleotides forming a double helix, with complementary base pairing (A-T, C-G).

• RNA is usually single-stranded and can fold into various shapes.

Example: The sequence of bases in DNA encodes genetic information, which is transcribed into RNA and then translated into proteins, determining the structure and function of cells.