Biology 1 - Lesson 3: The Molecules of Life

Overview

Living organisms are composed of countless molecules that collaborate to sustain life processes. In this lesson, we explore the four major classes of biological macromolecules—carbohydrates, lipids, proteins, and nucleic acids—and delve into how their structures relate to their diverse functions. These core molecules are the building blocks for everything from cellular energy to genetic information.

1. Carbohydrates

1.1 Definition and Classification

Carbohydrates are organic compounds composed primarily of carbon (C), hydrogen (H), and oxygen (O), usually in a 1:2:1 ratio. They generally serve as sources of quick energy and structural materials in cells.

• Monosaccharides: Simple sugars (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides linked together (e.g., sucrose = glucose + fructose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).

Carbohydrates

Monosaccharides

• Glucose
• Fructose
• Galactose

Disaccharides

• Sucrose
• Lactose
• Maltose

Polysaccharides

• Starch
• Cellulose
• Glycogen

1.2 Functions and Examples

1. Energy Storage:

   • Starch in plants

   • Glycogen in animals (stored primarily in liver and muscle cells)

2. Structural Support:

   • Cellulose in plant cell walls

   • Chitin in the exoskeletons of insects and crustaceans

1.3 Real-Life Applications

• Diet and Nutrition: Athletes often “carb-load” to increase glycogen stores for prolonged energy.

• Biofuels: Researchers explore converting cellulose into sustainable energy sources.

2. Lipids

2.1 Types of Lipids

Lipids are hydrophobic (water-fearing) molecules with diverse structures and functions. They generally contain long hydrocarbon chains or fused ring structures. Major types include:

1. Fats (Triglycerides)

   • Glycerol backbone + three fatty acids

   • Saturated fats have no double bonds (solid at room temperature, e.g., butter).

   • Unsaturated fats have one or more double bonds (liquid at room temperature, e.g., olive oil).

2. Phospholipids

   • Glycerol + two fatty acids + phosphate group

   • Amphipathic nature (hydrophilic head, hydrophobic tail) makes them key components of cell membranes.

3. Steroids

   • Four fused carbon rings (e.g., cholesterol)

   • Cholesterol is crucial for membrane fluidity and is a precursor to steroid hormones (e.g., estrogen, testosterone).

2.2 Biological Importance of Lipids

1. Energy Storage

   • Lipids store more energy per gram than carbohydrates, making them efficient long-term energy reserves.

2. Insulation and Protection

   • Fat in adipose tissue helps organisms maintain body temperature and protect vital organs.

3. Cell Membrane Structure

   • Phospholipid bilayers form the structural basis of cellular membranes, crucial for compartmentalization and selective permeability.

4. Cell Signaling

   • Steroid hormones regulate many physiological processes, including metabolism and reproductive functions.

2.3 Real-Life Applications

• Medical Considerations: High blood cholesterol levels are associated with cardiovascular disease. Dietary balance of saturated vs. unsaturated fats is an important public health topic.

• Industrial Uses: Vegetable oils (unsaturated fats) are used in biofuel production; some lipids serve as lubricants or in cosmetic formulations.

3. Proteins

3.1 Amino Acids: The Building Blocks

Proteins are polymers made of amino acids. Each amino acid consists of:

• An amino group (–NH₂)

• A carboxyl group (–COOH)

• A unique side chain (R-group) that determines its chemical properties

Twenty common amino acids combine to form an incredible diversity of proteins.

3.2 Protein Structure

Protein structure can be broken down into four levels:

1. Primary Structure: Linear sequence of amino acids.

2. Secondary Structure: Localized folding (e.g., α-helices, β-sheets) due to hydrogen bonding.

3. Tertiary Structure: Overall 3D shape formed by interactions among R-groups (e.g., hydrophobic packing, ionic bonds, disulfide bridges).

4. Quaternary Structure: Assembly of multiple polypeptide chains into a functional protein (e.g., hemoglobin).

Four Levels of Protein Structure

Primary Structure

The linear sequence of amino acids in the polypeptide chain.

Secondary Structure

Local folding of the polypeptide backbone into α-helices and β-sheets.

α-Helix

β-Sheet

Tertiary Structure

The overall three-dimensional shape of a polypeptide, stabilized by various interactions.

Quaternary Structure

The arrangement or interaction of multiple polypeptide subunits.

3.3 Protein Functions

1. Enzymes

   • Biological catalysts that speed up chemical reactions (e.g., lactase breaks down lactose).

2. Transport

   • Hemoglobin carries oxygen in the blood.

3. Defense

   • Antibodies target pathogens in the immune system.

4. Structural Support

   • Collagen provides strength in connective tissues.

5. Signaling

   • Hormonal proteins (e.g., insulin) regulate metabolic processes.

3.4 Real-Life Applications

• Medical Diagnostics: Enzyme levels in blood (e.g., liver enzymes) can indicate disease states.

• Biotechnology: Industrial enzymes are used in detergents, food processing, and bioremediation.

4. Nucleic Acids

4.1 DNA and RNA

Nucleic acids (DNA and RNA) store and express genetic information:

• DNA (Deoxyribonucleic Acid):

  • Double-stranded helix with bases adenine (A), thymine (T), guanine (G), and cytosine (C).

  • Stores and transmits hereditary information.

• RNA (Ribonucleic Acid):

  • Single-stranded; uracil (U) replaces thymine.

  • Involved in protein synthesis (e.g., mRNA, tRNA, rRNA) and gene regulation.

4.2 Structure and Monomers

Nucleic acids are polymers of nucleotides, each composed of:

1. A pentose sugar (deoxyribose in DNA, ribose in RNA)

2. A phosphate group

3. A nitrogenous base (A, T, G, C in DNA; A, U, G, C in RNA)

4.3 Real-Life Applications

• Genetic Testing: Identifies mutations responsible for inherited diseases.

• Forensic Science: DNA profiling can link suspects to crime scenes.

• Biotechnology: Recombinant DNA technology, CRISPR gene editing, and mRNA-based vaccines.

5. Exercise: Modeling Macromolecules

Objective: Build simple 3D models of each macromolecule class to visualize their structures and bonding.

1. Materials

   • Colored clay or modeling dough

   • Toothpicks

   • Labels or sticky notes

2. Procedure

   • Carbohydrate Model: Form small spheres for monosaccharides; connect them to form disaccharides or polysaccharides.

   • Lipid Model: Create a “glycerol backbone” (one clay piece) and attach “fatty acid tails” (three elongated pieces).

   • Protein Model: Make individual amino acids with distinct “R-groups” and connect them into a polypeptide chain.

   • Nucleic Acid Model: Assemble “nucleotides” (sugar, phosphate, base) and link them into a short DNA or RNA strand.

3. Analysis

   • Observe how monomers link to form polymers.

   • Discuss how functional groups (carboxyl, amino, phosphate, etc.) determine the molecule’s properties.

6. Additional Learning Components

6.1 Historical Anecdote: Friedrich Miescher and the Discovery of Nuclein

In 1869, Swiss physician Friedrich Miescher isolated a substance from the nuclei of white blood cells (obtained from pus-soaked bandages!). He called this substance “nuclein,” which later turned out to be DNA.

6.2 Researcher Spotlight: Dorothy Crowfoot Hodgkin

Hodgkin (1910–1994) used X-ray crystallography to determine the structures of biomolecules, including cholesterol, penicillin, and vitamin B12. She was awarded the Nobel Prize in Chemistry (1964) for her pioneering work on the structure of important biochemical substances.

6.3 Notable Breakthrough: The First Protein to Be Sequenced—Insulin

Frederick Sanger (1918–2013) determined the amino acid sequence of insulin in 1955, proving that proteins have a defined chemical structure. He later contributed significantly to DNA sequencing methods.

6.4 Advanced Reading Suggestions

• “Lehninger Principles of Biochemistry” by Nelson and Cox: In-depth coverage of biochemical pathways and macromolecule functions.

• Primary Research Articles in journals like Nature or Cell focusing on new protein structures or RNA-based therapeutics.

6.5 Real-World Impact

• Medical Implications: Understanding protein structure is crucial for drug design; for instance, designing drugs that inhibit specific enzymes in pathogens.

• Agricultural Biotechnology: Genetically modified crops often involve manipulating the plant’s nucleic acids to enhance traits like pest resistance or nutritional content.

7. Recall Questions

1. Carbohydrates: How do monosaccharides, disaccharides, and polysaccharides differ in structure and function?

2. Lipids: Why are lipids generally hydrophobic, and what role do phospholipids play in cell membranes?

3. Proteins: Name and describe the four levels of protein structure, giving at least one real-world example for each level.

4. Nucleic Acids: Compare DNA and RNA in terms of structure and function. What is a nucleotide made of?

5. Exercise Reflection: If you built the 3D models, which features of the macromolecules were easier (or more challenging) to visualize?

Use these questions to test your understanding of how molecular structures form the basis of biological function, from the sweetness of simple sugars to the coding of genes in DNA.