The Structure and Function of Large Biological Molecules

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The Molecules of Life

Overview

All living organisms are composed of four major classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids. These molecules are essential for cellular structure and function.

Macromolecules are large molecules made up of thousands of covalently bonded atoms.
Molecular structure determines function, making the two inseparable.

Macromolecules: Polymers and Monomers

Polymer Formation and Breakdown

Macromolecules are often polymers, which are long chains of repeating units called monomers.

Three major classes—carbohydrates, proteins, and nucleic acids—are polymers.
Dehydration (condensation) reactions join monomers by removing a water molecule.
Hydrolysis breaks polymers into monomers by adding water.
Enzymes catalyze both dehydration and hydrolysis reactions.

Polymer synthesis and breakdown Dehydration reaction in polymer synthesis Hydrolysis reaction in polymer breakdown

Carbohydrates: Fuel and Building Material

Monosaccharides, Disaccharides, and Polysaccharides

Carbohydrates are sugars and their polymers, serving as energy sources and structural materials.

Monosaccharides are single sugar units (e.g., glucose, C6H12O6).
Classified by carbonyl group location (aldose or ketose) and number of carbons.
Often form rings in aqueous solutions.
Disaccharides are formed by dehydration reactions between two monosaccharides, creating a glycosidic linkage.
Polysaccharides are long chains of monosaccharides with storage (starch, glycogen) or structural (cellulose, chitin) roles.

Classification of monosaccharides Aldoses and ketoses Linear and ring forms of glucose Abbreviated ring structure of glucose Dehydration reaction in disaccharide synthesis

Storage and Structural Polysaccharides

Starch: Plant storage polysaccharide, composed of glucose monomers.
Glycogen: Animal storage polysaccharide, stored in liver and muscle cells.
Cellulose: Structural polysaccharide in plant cell walls, composed of β-glucose monomers.
Chitin: Structural polysaccharide in arthropod exoskeletons and fungal cell walls.

Starch and glycogen structure Alpha and beta glucose ring structures Starch and cellulose linkages Cellulose microfibrils in plant cell wall Chitin structure and applications

Lipids: Hydrophobic Molecules

Fats, Phospholipids, and Steroids

Lipids are a diverse group of hydrophobic molecules, not true polymers.

Fats are made from glycerol and fatty acids, joined by ester linkages to form triglycerides.
Saturated fats have no double bonds and are solid at room temperature; unsaturated fats have one or more double bonds and are liquid.
Phospholipids have two fatty acids and a phosphate group attached to glycerol, forming cell membrane bilayers.
Steroids have a four-ring carbon skeleton; cholesterol is a key steroid in animal cell membranes.

Fatty acid and glycerol structure Dehydration reaction in fat synthesis Fat molecule (triglyceride) structure Saturated and unsaturated fat molecules Saturated fat structure Unsaturated fat structure Phospholipid structure Phospholipid structural formula and space-filling model Phospholipid bilayer arrangement Steroid structure (cholesterol)

Proteins: Structure and Function

Protein Functions

Proteins are the most versatile macromolecules, performing a wide range of functions.

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

Type of Protein	Function	Examples
Enzymatic proteins	Selective acceleration of chemical reactions	Digestive enzymes
Structural proteins	Support	Collagen, keratin
Storage proteins	Storage of amino acids	Ovalbumin, casein
Transport proteins	Transport of substances	Hemoglobin
Hormonal proteins	Coordination of activities	Insulin
Receptor proteins	Response to stimuli	Receptors in nerve cells
Contractile and motor proteins	Movement	Actin, myosin
Defensive proteins	Protection against disease	Antibodies

Overview of protein functions

Amino Acids and Polypeptides

Amino acids are organic molecules with amino and carboxyl groups, differing by their side chains (R groups).
Amino acids are linked by peptide bonds to form polypeptides.
Proteins are composed of one or more polypeptides.

Amino acid structure

Protein Structure

Proteins have four levels of structure:

Primary structure: Unique sequence of amino acids.
Secondary structure: Coils (α helix) and folds (β pleated sheet) due to hydrogen bonding.
Tertiary structure: Three-dimensional shape formed by interactions among R groups.
Quaternary structure: Association of multiple polypeptide chains.

Example: Hemoglobin is a globular protein with quaternary structure, consisting of four polypeptide chains.

Nucleic Acids: Hereditary Information

DNA and RNA

Nucleic acids store and transmit genetic information.

DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are polymers of nucleotides.
Nucleotides consist of a nitrogenous base, a pentose sugar, and a phosphate group.
DNA directs its own replication and the synthesis of RNA, which in turn directs protein synthesis.

Example: The sequence of bases in DNA encodes genetic information, which is transcribed to mRNA and translated into proteins.

Emergent Properties and Organization

Review

Higher levels of biological organization result in emergent properties, emphasizing the importance of molecular structure and organization in the chemistry of life. Additional info: Molecular biology techniques, such as X-ray crystallography and bioinformatics, are used to study protein and nucleic acid structures. Bioinformatics and protein structure analysis