Unit 1 – Life Starts Small

Module 2: Molecules of Life

This module explores the chemical foundations of life, focusing on the structure, synthesis, and function of biological macromolecules. Understanding these molecules is essential for grasping how life originated and how cells function.

Origin of Life and the Role of Macromolecules

Stages in the Origin of Life on Earth

• Abiotic Synthesis: Formation of small organic molecules (e.g., amino acids, nitrogenous bases) from nonliving matter.

• Polymerization: Linking of small molecules into polymers such as proteins and nucleic acids.

• Protocell Formation: Aggregation of polymers into membrane-bound structures (protocells) with internal environments distinct from their surroundings.

• Self-Replication: Emergence of molecules capable of self-replication, enabling inheritance and evolution.

Protocells are considered early self-replicating chemical systems that could interact with their environment.

Major Classes of Biological Macromolecules

Overview

• All living organisms contain the same four major classes of macromolecules:

  • Carbohydrates

  • Proteins

  • Lipids

  • Nucleic acids

• These macromolecules are polymers built from smaller subunits called monomers (except most lipids).

• The diversity of life arises from the variety of ways these monomers are assembled.

Functional Groups and Chemical Properties

Organic molecules have carbon skeletons with attached functional groups that determine their chemical properties and reactivity.

Polymer Synthesis and Breakdown

Building and Breaking Polymers

• Dehydration Synthesis: Monomers are joined to form polymers by removing a molecule of water. This process is catalyzed by enzymes.

• Hydrolysis: Polymers are broken down into monomers by adding water, splitting the covalent bond.

Equation for Dehydration Synthesis:

Equation for Hydrolysis:

Proteins

Structure and Function

Proteins are essential macromolecules that perform a vast array of functions in cells, including catalysis, structure, transport, and signaling.

• Proteins are polymers of amino acids linked by peptide bonds.

• There are 20 different amino acids, each with a unique side chain (R group) that determines its properties.

• The sequence and chemical nature of amino acids determine protein structure and function.

Levels of Protein Structure

1. Primary Structure: Linear sequence of amino acids in a polypeptide chain.

2. Secondary Structure: Local folding into structures such as α-helix and β-pleated sheet stabilized by hydrogen bonds.

3. Tertiary Structure: Three-dimensional folding due to interactions among R groups (side chains).

4. Quaternary Structure: Association of two or more polypeptide chains to form a functional protein.

Example: Hemoglobin is a quaternary protein composed of four polypeptide subunits.

Lipids

Types and Biological Importance

Lipids are hydrophobic molecules important for energy storage, membrane structure, and signaling.

• Fats (Triglycerides): Composed of glycerol and three fatty acids. Used for long-term energy storage.

• Phospholipids: Major component of cell membranes. Consist of a glycerol backbone, two fatty acid tails (hydrophobic), and a phosphate group (hydrophilic).

• Steroids: Lipids with four fused carbon rings. Cholesterol is a key steroid in animal cell membranes and a precursor for steroid hormones.

Saturated vs. Unsaturated Fats

• Saturated Fats: No double bonds in fatty acid chains; solid at room temperature.

• Unsaturated Fats: One or more double bonds; liquid at room temperature. Trans fats are artificially hydrogenated unsaturated fats with health risks.

Amphipathic Nature of Phospholipids

• Phospholipids have both hydrophilic (phosphate head) and hydrophobic (fatty acid tails) regions.

• This property allows them to form bilayers, the fundamental structure of biological membranes.

Carbohydrates

Structure and Function

Carbohydrates serve as energy sources and structural materials in cells.

• Monosaccharides: Simple sugars (e.g., glucose, fructose) with the general formula .

• Disaccharides: Two monosaccharides joined by a glycosidic bond (e.g., sucrose, lactose).

• Polysaccharides: Long chains of monosaccharides. Examples include starch (energy storage in plants), glycogen (energy storage in animals), and cellulose (structural component in plant cell walls).

Example: Starch and cellulose are both polymers of glucose but differ in the type of glycosidic linkage, resulting in different properties and functions.

Nucleic Acids

Structure and Information Storage

Nucleic acids store and transmit genetic information. The two main types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• Nucleic acids are polymers of nucleotides, each consisting of a pentose sugar, a phosphate group, and a nitrogenous base.

• DNA contains deoxyribose sugar; RNA contains ribose sugar.

• DNA is typically double-stranded, with complementary base pairing (A with T, C with G) via hydrogen bonds.

• RNA is usually single-stranded and can act as a catalyst and information carrier.

Example: The sequence of nucleotides in DNA encodes the instructions for building proteins.

RNA and the Origin of Life

• RNA is hypothesized to have been the original self-replicating molecule due to its ability to store information and catalyze reactions.

Summary Table: Major Macromolecules

Key Learning Outcomes

• Describe the synthesis and breakdown of biological polymers.

• Explain the structure and function of proteins, including the four levels of protein structure.

• Compare the structures and functions of the three main classes of lipids.

• Describe the structure and roles of carbohydrates in cells.

• Explain how nucleic acid structure enables information storage and transfer, and discuss the role of RNA in early life.