The Structure and Function of Large Biological Molecules

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The Structure and Function of Large Biological Molecules

Introduction to Biological Macromolecules

Large biological molecules, or macromolecules, are essential for life and include carbohydrates, lipids, proteins, and nucleic acids. These molecules are fundamental to the structure and function of cells, and each class has unique properties and roles in biological systems.

Carbohydrates: Serve as fuel and building material.
Lipids: Diverse group of hydrophobic molecules, important for energy storage and membrane structure.
Proteins: Perform a wide range of functions, including catalysis, transport, and structural support.
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 macromolecules (except lipids) are polymers, long chains made from repeating units called monomers. The synthesis and breakdown of polymers involve specific chemical reactions:

Dehydration Reaction: Joins monomers by removing a water molecule, forming a covalent bond.
Hydrolysis: Breaks bonds between monomers by adding a water molecule.

Dehydration and hydrolysis reactions in polymers

Carbohydrates

Monosaccharides: Simple Sugars

Monosaccharides are the simplest carbohydrates and serve as the building blocks for more complex sugars. They have molecular formulas that are usually multiples of (CH2O)n. Glucose (C6H12O6) is the most common monosaccharide.

Aldoses: Sugars with an aldehyde group (e.g., glucose, ribose).
Ketoses: Sugars with a ketone group (e.g., fructose, ribulose).

Structures of aldoses and ketoses

Ring Structures of Sugars

In aqueous solutions, many monosaccharides form ring structures, which are more stable than linear forms. These rings are the predominant forms found in cells.

Linear and ring forms of glucose

Disaccharides and Glycosidic Linkages

Disaccharides are formed when two monosaccharides are joined by a dehydration reaction, creating a covalent bond known as a glycosidic linkage. Common disaccharides 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 storage or 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.

Structures and functions of starch, glycogen, and cellulose Alpha and beta glucose linkages in starch and cellulose Chitin structure and function

Lipids

Overview and Types of Lipids

Lipids are hydrophobic molecules that do not form true polymers. The main types of lipids are fats, phospholipids, and steroids.

Fats (Triglycerides)

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

Synthesis of a fat molecule (triglyceride) 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 by assembling into bilayers in aqueous environments.

Phospholipid structure and bilayer formation Phospholipid bilayer

Steroids

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

Steroid structure (cholesterol)

Proteins

Functions and Diversity of Proteins

Proteins are the most diverse macromolecules, accounting for more than 50% of the dry mass of most cells. They serve as enzymes, structural components, transporters, and more.

Enzymatic proteins: Catalyze chemical reactions.
Defensive proteins: Protect against disease.
Storage proteins: Store amino acids.
Transport proteins: Move substances across membranes.
Hormonal proteins: Coordinate organismal activities.
Receptor proteins: Respond to chemical stimuli.
Contractile and motor proteins: Enable movement.
Structural proteins: Provide support.

Examples of protein functions More examples of protein functions

Amino Acids: The Building Blocks of Proteins

Amino acids are organic molecules with an amino group, a carboxyl group, and a variable side chain (R group). There are 20 different amino acids, classified by the properties of their side chains (nonpolar, polar, acidic, or basic).

General structure of an amino acid Structures of the 20 amino acids

Polypeptides and Peptide Bonds

Amino acids are linked by peptide bonds to form polypeptides. Each polypeptide has a unique linear sequence, with an amino (N) terminus and a carboxyl (C) terminus.

Peptide bond formation and polypeptide structure

Levels of Protein Structure

The function of a protein depends on its specific 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 side chains (hydrophobic interactions, hydrogen bonds, ionic bonds, disulfide bridges).
Quaternary structure: Association of multiple polypeptide chains.

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

Protein Denaturation

Proteins can lose their native structure (denature) due to changes in pH, salt concentration, temperature, or other environmental factors. Denaturation disrupts protein function, but some proteins can renature if conditions return to normal.

Denaturation and renaturation of a protein

Nucleic Acids

Types and Functions of Nucleic Acids

Nucleic acids store and transmit hereditary information. The two main types are:

Deoxyribonucleic acid (DNA): Stores genetic information and directs its own replication.
Ribonucleic acid (RNA): Functions in gene expression, including carrying instructions from DNA to ribosomes for protein synthesis.

Structure of Nucleic Acids

Nucleic acids are polymers called polynucleotides, made from nucleotide monomers. Each nucleotide consists of a nitrogenous base (purine or pyrimidine), a pentose sugar (deoxyribose in DNA, ribose in RNA), and a phosphate group.

Structure of nucleic acids and nucleotide components

DNA: Double helix formed by two polynucleotide strands running in opposite directions (antiparallel).
RNA: Usually single-stranded.

Summary Table: Major Classes of 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	Genetic information storage and transfer
Lipids	Fatty acid, glycerol	Not true polymers	Ester linkage	Energy storage, membranes, signaling