Structure and Function of Large Biological Molecules: Carbohydrates, Lipids, Proteins, and Nucleic Acids

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

The Structure and Function of Large Biological Molecules

Overview of Biological Macromolecules

All living organisms are composed of four major classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids. These molecules are essential for life, serving as structural components, energy sources, and carriers of genetic information.

Carbohydrates: Serve as fuel and building material.
Lipids: Diverse group of hydrophobic molecules, including fats, phospholipids, and steroids.
Proteins: Perform a wide range of functions due to their structural diversity.
Nucleic acids: Store, transmit, and help express hereditary information.

Overview of the four classes of biological molecules

Macromolecules: Polymers and Monomers

Polymer Formation and Breakdown

Most biological macromolecules (except lipids) are polymers, long chains made from repeating units called monomers. The synthesis and breakdown of polymers involve two key reactions:

Dehydration reaction: Joins monomers by removing a water molecule, forming a covalent bond.
Hydrolysis: Breaks polymers into monomers by adding a water molecule.

Dehydration and hydrolysis reactions in polymer synthesis and breakdown

Carbohydrates

Monosaccharides: Simple Sugars

Monosaccharides are the simplest carbohydrates, typically having molecular formulas that are multiples of (CH2O)n. They can be classified by the location of their carbonyl group (aldose or ketose) and the number of carbons in the skeleton (triose, pentose, hexose).

Glucose (C6H12O6) is the most common monosaccharide.
Monosaccharides often form ring structures in aqueous solutions.

Classification of monosaccharides: aldoses and ketoses Linear and ring forms of glucose

Disaccharides and Glycosidic Linkages

Disaccharides are formed when two monosaccharides are joined by a glycosidic linkage through a dehydration reaction. Common examples include maltose (glucose + glucose) and sucrose (glucose + fructose).

Synthesis of maltose and sucrose via dehydration reactions

Polysaccharides: Storage and Structural Roles

Polysaccharides are large polymers of sugars with important storage and structural functions:

Starch: Storage polysaccharide in plants, composed of α-glucose monomers.
Glycogen: Storage polysaccharide in animals, highly branched.
Cellulose: Structural polysaccharide in plant cell walls, composed of β-glucose monomers.
Chitin: Structural polysaccharide in arthropod exoskeletons and fungal cell walls.

Comparison of starch, glycogen, and cellulose structures Alpha and beta glucose ring structures and their linkages in starch and cellulose Structure of chitin and its biological role

Lipids

Fats: Structure and Function

Fats are constructed from glycerol and fatty acids. Their main function is energy storage. Fats can be classified as saturated (no double bonds, solid at room temperature) or unsaturated (one or more double bonds, liquid at room temperature).

Saturated fats: Found in animal fats.
Unsaturated fats: Found in plant and fish oils.

Synthesis of a fat molecule from glycerol and fatty acids Comparison of saturated and unsaturated fats

Phospholipids

Phospholipids consist of two fatty acids and a phosphate group attached to glycerol. They are amphipathic, with hydrophobic tails and a hydrophilic head, and form the basis of cell membranes.

Structure of a phospholipid Phospholipid bilayer formation

Steroids

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

Structure of cholesterol, a steroid

Proteins

Amino Acids and Polypeptides

Proteins are polymers of amino acids, which have a central carbon (α carbon) bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group). There are 20 different amino acids, classified by the properties of their side chains.

Nonpolar side chains: Hydrophobic
Polar side chains: Hydrophilic
Electrically charged side chains: Acidic (negative) or basic (positive)

General structure of an amino acid Classification of amino acids by side chain properties

Protein Structure: Four Levels

The function of a protein is determined by its structure, which is organized into four levels:

Primary structure: Unique sequence of amino acids.
Secondary structure: Local folding into α-helices and β-pleated sheets, stabilized by hydrogen bonds.
Tertiary structure: Overall 3D shape, determined by interactions among R groups (hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bridges).
Quaternary structure: Association of multiple polypeptide chains.

Primary structure of a protein Secondary structure: alpha helix and beta pleated sheet Tertiary structure of a protein Quaternary structure: transthyretin protein

Protein Denaturation

Proteins can lose their native structure due to changes in pH, temperature, or other environmental factors, a process known as denaturation. Denatured proteins lose their biological function.

Denaturation and renaturation of a protein

Nucleic Acids

DNA and RNA: Structure and Function

Nucleic acids are polymers called polynucleotides, made of monomers called nucleotides. Each nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphate group. There are two types of nucleic acids:

DNA (Deoxyribonucleic acid): Double-stranded, stores genetic information.
RNA (Ribonucleic acid): Single-stranded, involved in protein synthesis.

Components of nucleic acids: nucleotide structure

Structure of DNA and RNA Molecules

DNA consists of two polynucleotide strands forming a double helix, held together by hydrogen bonds between complementary bases (adenine-thymine, guanine-cytosine). RNA is typically single-stranded and can fold into complex shapes.

DNA double helix and transfer RNA structure Key features of DNA structure and chemical details

Summary Table: Comparison of Biological Macromolecules

Class	Monomer	Polymer	Main Functions
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structural support
Lipids	Fatty acid, glycerol	Triacylglycerol, phospholipid, steroid	Energy storage, membrane structure, signaling
Proteins	Amino acid	Polypeptide	Catalysis, transport, structure, signaling
Nucleic acids	Nucleotide	Polynucleotide (DNA, RNA)	Genetic information storage and transfer