Chapter 3: The Molecules of Cells – Structure and Function of Biological Molecules

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Chapter 3: The Molecules of Cells

Big Ideas Overview

This chapter explores the molecular foundations of life, focusing on the structure and function of organic compounds, carbohydrates, lipids, proteins, and nucleic acids. Understanding these molecules is essential for grasping cellular processes and biological diversity.

Chapter 3: Big Ideas - Organic Compounds, Carbohydrates, Lipids, Proteins, Nucleic Acids

Introduction to Organic Compounds

Life’s Molecular Diversity and Carbon

Organic compounds are molecules primarily composed of carbon atoms. Carbon's unique ability to form four covalent bonds allows for the creation of large, complex, and diverse molecules essential for life.

Carbon Skeletons: Carbon chains form the backbone of most organic molecules, varying in length, branching, and ring formation.
Isomers: Molecules with the same molecular formula but different structures, leading to different properties.
Hydrocarbons: Compounds consisting only of carbon and hydrogen.

Methane molecule and tetrahedral geometry Varieties of carbon skeletons: length, double bonds, branching, rings

Key Chemical Groups in Biological Molecules

Functional Groups and Their Properties

The chemical behavior of organic molecules is determined by their functional groups. These groups confer specific properties and reactivity, often making molecules hydrophilic and biologically active.

Hydroxyl group (–OH): Found in alcohols; increases solubility in water.
Carbonyl group (–C=O): Found in sugars; can be at the end (aldehyde) or within (ketone) a carbon skeleton.
Carboxyl group (–COOH): Found in carboxylic acids; acts as an acid.
Amino group (–NH2): Found in amino acids; acts as a base.
Phosphate group (–OPO32–): Found in nucleotides and ATP; involved in energy transfer.
Methyl group (–CH3): Affects gene expression and molecular shape.

Chemical Group	Example
Hydroxyl	Alcohol (–OH)
Carbonyl	Aldehyde/Ketone (–C=O)
Carboxyl	Carboxylic acid (–COOH)
Amino	Amine (–NH2)
Phosphate	Organic phosphate (–OPO32–)
Methyl	Methylated compound (–CH3)

Table of important chemical groups

Building Large Biological Molecules

Polymers and Monomers

Cells construct macromolecules (polymers) from smaller units (monomers) through dehydration reactions, and break them down by hydrolysis. Enzymes catalyze these processes.

Dehydration Reaction: Joins monomers by removing water.
Hydrolysis: Breaks polymers into monomers by adding water.

Dehydration and hydrolysis reactions

Carbohydrates

Monosaccharides, Disaccharides, and Polysaccharides

Carbohydrates are essential for energy storage and structural support. They range from simple sugars (monosaccharides) to complex polymers (polysaccharides).

Monosaccharides: Simple sugars (e.g., glucose, fructose) with the general formula CnH2nOn.
Disaccharides: Formed by joining two monosaccharides via dehydration (e.g., maltose).
Polysaccharides: Long chains of monosaccharides; include starch (plants), glycogen (animals), cellulose (plants), and chitin (fungi, insects).

Glucose and fructose structures Ring form of glucose Formation of maltose from two glucose molecules Starch, glycogen, cellulose structures

Lipids

Fats, Phospholipids, and Steroids

Lipids are hydrophobic molecules important for energy storage, membrane structure, and signaling. They include fats, phospholipids, and steroids.

Fats (Triglycerides): Composed of glycerol and three fatty acids. Saturated fats have no double bonds; unsaturated fats have one or more double bonds.
Phospholipids: Major component of cell membranes, with hydrophilic heads and hydrophobic tails.
Steroids: Include cholesterol and hormones; characterized by four fused rings.

Dehydration reaction linking fatty acid to glycerol Unsaturated fats: olive oil and salmon Saturated fats: butter and steak Phospholipid structure Cholesterol structure

Proteins

Structure and Function

Proteins are versatile macromolecules involved in catalysis, transport, defense, signaling, structure, and movement. Their function depends on their shape, which is determined by four levels of structure.

Primary structure: Sequence of amino acids.
Secondary structure: Coiling (α-helix) or folding (β-sheet) stabilized by hydrogen bonds.
Tertiary structure: Three-dimensional shape formed by interactions among R groups.
Quaternary structure: Association of multiple polypeptide chains.
Denaturation: Loss of protein shape and function due to environmental changes.

Ribbon model of lysozyme protein General structure of an amino acid Formation of a peptide bond Levels of protein structure

Nucleic Acids

DNA and RNA Structure and Function

Nucleic acids are polymers of nucleotides, which store and transmit genetic information. DNA is a double helix, while RNA is usually single-stranded.

Nucleotide: Consists of a sugar, phosphate group, and nitrogenous base.
DNA: Double-stranded; stores hereditary information.
RNA: Single-stranded; involved in protein synthesis.

Human Evolution Connection: Lactose Tolerance

Genetic Adaptation to Diet

Lactose tolerance in adults is a result of genetic mutations that allow continued expression of the lactase enzyme. This adaptation is linked to the domestication of dairy animals and has evolved independently in several human populations.

Summary Table: Biological Molecules and Their Functions

Molecule	Monomer	Function
Carbohydrates	Monosaccharide	Energy storage, structure
Lipids	Fatty acid, glycerol	Energy storage, membranes, signaling
Proteins	Amino acid	Catalysis, structure, transport, defense
Nucleic Acids	Nucleotide	Genetic information, protein synthesis