Chapter 5: The Structure and Function of Large Biological Molecules

Chapter 5: The Structure and Function of Large Biological Molecules

Introduction to Macromolecules

Large biological molecules, or macromolecules, are essential to life and include carbohydrates, proteins, nucleic acids, and lipids. Most macromolecules are polymers, long chains of repeating units called monomers. The diversity and complexity of life arise from the unique properties and arrangements of these molecules.

Polymers and Monomers

Polymers are long molecules made of similar or identical building blocks called monomers. Carbohydrates, proteins, and nucleic acids are all polymers, while lipids are not true polymers. The synthesis and breakdown of polymers involve specific chemical reactions:

• Dehydration reaction: Joins two monomers by removing a water molecule, forming a new bond.

• Hydrolysis: Breaks a bond between monomers by adding a water molecule, disassembling the polymer.

Carbohydrates

Monosaccharides: Simple Sugars

Carbohydrates include sugars and their polymers. The simplest carbohydrates are monosaccharides, which generally have molecular formulas that are multiples of CH2O. Glucose (C6H12O6) is the most common monosaccharide. Monosaccharides are classified by the location of their carbonyl group (aldose or ketose) and the number of carbons in their skeleton.

Structure and Function of Monosaccharides

Monosaccharides can exist as linear chains or ring forms in aqueous solutions. They serve as major fuel for cells and as raw materials for building other molecules.

Disaccharides and Glycosidic Linkages

A disaccharide is formed when two monosaccharides are joined by a dehydration reaction, creating a glycosidic linkage. Common disaccharides include maltose and sucrose.

Polysaccharides: Storage and Structural Roles

Polysaccharides are polymers of sugars with storage or structural functions. Their properties depend on the types of monomers and the positions of glycosidic linkages.

• Starch: Storage polysaccharide in plants, composed of glucose monomers (amylose and amylopectin).

• Glycogen: Storage polysaccharide in animals, stored mainly in liver and muscle cells.

• Cellulose: Major component of plant cell walls; differs from starch in the type of glycosidic linkage (β instead of α).

• Chitin: Structural polysaccharide found in the exoskeleton of arthropods and cell walls of fungi.

Lipids

Overview of Lipids

Lipids are a diverse group of hydrophobic molecules that do not form true polymers. They are primarily composed of hydrocarbon regions and include fats, phospholipids, and steroids.

Fats: Structure and Function

Fats are constructed from glycerol and fatty acids. Glycerol is a three-carbon alcohol, and fatty acids are long hydrocarbon chains with a carboxyl group. Three fatty acids join to glycerol by ester linkages to form a triacylglycerol (triglyceride).

• Saturated fatty acids: No double bonds; solid at room temperature (e.g., animal fats).

• Unsaturated fatty acids: One or more double bonds; liquid at room temperature (e.g., plant and fish oils).

Fats function as energy storage, insulation, and cushioning for organs.

Phospholipids

Phospholipids consist of two fatty acids and a phosphate group attached to glycerol. The hydrophobic tails and hydrophilic head allow phospholipids to form bilayers, which are fundamental to cell membrane structure.

Steroids

Steroids are lipids with a carbon skeleton of four fused rings. Cholesterol is an important steroid in animal cell membranes and a precursor for other steroids.

Proteins

Functions of Proteins

Proteins are the most diverse macromolecules, accounting for more than 50% of the dry mass of most cells. They serve as enzymes, provide structural support, transport substances, and play roles in defense, storage, communication, and movement.

Amino Acids and Polypeptides

Proteins are polymers of amino acids, which have a central carbon, an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group). The properties of amino acids are determined by their R groups. Amino acids are linked by peptide bonds to form polypeptides.

Levels of Protein Structure

The structure of a protein determines its function and is organized into four levels:

• Primary structure: Unique sequence of amino acids.

• Secondary structure: Coils (α helix) and folds (β pleated sheet) due to hydrogen bonding.

• Tertiary structure: Overall 3D shape formed by interactions among R groups (hydrogen bonds, ionic bonds, hydrophobic interactions, van der Waals forces, and disulfide bridges).

• Quaternary structure: Association of multiple polypeptide chains (e.g., collagen, hemoglobin).

Protein Structure and Disease

A change in primary structure can affect protein function. For example, sickle-cell disease results from a single amino acid substitution in hemoglobin, causing abnormal cell shape and function.

Protein Denaturation

Physical and chemical conditions (pH, salt, temperature) can cause proteins to lose their native structure, a process called denaturation. Denatured proteins are usually biologically inactive.

Protein Folding and Determination

Protein folding is complex and often assisted by cellular machinery. Techniques such as X-ray crystallography and NMR spectroscopy are used to determine protein structures. Bioinformatics uses computational tools to predict and analyze protein structures.

Nucleic Acids

Roles of Nucleic Acids

Nucleic acids store, transmit, and help express hereditary information. The two types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). DNA directs its own replication and the synthesis of RNA, which in turn directs protein synthesis (gene expression).

Structure of Nucleic Acids

Nucleic acids are polymers called polynucleotides, made of monomers called nucleotides. Each nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups. The nitrogenous bases are divided into pyrimidines (cytosine, thymine, uracil) and purines (adenine, guanine).

DNA and RNA Structure

DNA consists of two antiparallel polynucleotide strands forming a double helix, with complementary base pairing (A with T, G with C). RNA is usually single-stranded and uses uracil instead of thymine.

Genomics and Proteomics

Genomics is the study of whole sets of genes and their interactions, while proteomics is the study of large sets of proteins. Advances in sequencing technology and bioinformatics have revolutionized biological research and applications.

Summary Table: Major Classes of Large Biological Molecules

Class	Monomer	Polymer	Bond Type	Main Functions
Carbohydrates	Monosaccharide	Polysaccharide	Glycosidic linkage	Energy storage, structure
Proteins	Amino acid	Polypeptide	Peptide bond	Catalysis, structure, transport, signaling
Nucleic acids	Nucleotide	Polynucleotide	Phosphodiester bond	Information storage and transfer
Lipids	Fatty acid, glycerol	Not true polymers	Ester linkage	Energy storage, membranes, signaling

Additional info: This summary integrates foundational concepts from Chapter 5 of Campbell Biology, focusing on the structure and function of large biological molecules, and is suitable for college-level biology students preparing for exams.