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

Chapter 3: Molecules of Cells

Introduction to Organic Compounds

Organic compounds are the foundation of cellular structure and function. Their diversity arises from the unique properties of carbon atoms, which can form stable covalent bonds in various configurations.

• Carbon Atom Properties: Carbon has four valence electrons, allowing it to form up to four covalent bonds with other atoms, including itself.

• Structural Diversity: Carbon chains can be straight, branched, or arranged in rings, leading to a vast array of organic molecules.

• Simplest Organic Compound: Methane (CH4) is the simplest hydrocarbon, consisting of one carbon atom bonded to four hydrogen atoms.

• Structural Representations: Organic molecules can be depicted using structural formulas, ball-and-stick models, and space-filling models.

• Isomers: Molecules with the same molecular formula but different structural arrangements (e.g., glucose and fructose).

Hydrocarbons

Hydrocarbons are organic molecules composed entirely of carbon and hydrogen. They serve as the backbone for more complex organic molecules.

• Composition: Only carbon (C) and hydrogen (H) atoms.

• Properties: Nonpolar, hydrophobic, and serve as energy sources (e.g., fossil fuels).

• Examples: Methane, ethane, propane, butane.

Functional Groups

Functional groups are specific groups of atoms within molecules that determine the chemical properties and reactivity of organic compounds.

• Key Functional Groups:

  • Hydroxyl group (-OH): Found in alcohols; increases solubility in water.

  • Carbonyl group (C=O): Found in aldehydes and ketones; affects reactivity.

  • Carboxyl group (-COOH): Found in carboxylic acids; acts as an acid.

  • Amino group (-NH2): Found in amines and amino acids; acts as a base.

  • Phosphate group (-PO4): Found in nucleic acids and ATP; involved in energy transfer.

  • Methyl group (-CH3): Nonpolar; affects gene expression.

• Example: The difference between the sex hormones testosterone and estradiol is due to the presence of different functional groups.

Chemical Group	Examples
Hydroxyl (-OH)	Alcohols (e.g., ethanol)
Carbonyl (C=O)	Aldehydes, Ketones
Carboxyl (-COOH)	Carboxylic acids (e.g., acetic acid)
Amino (-NH2)	Amines, Amino acids
Phosphate (-PO4)	Nucleic acids, ATP
Methyl (-CH3)	Methylated compounds

Macromolecules: Building Blocks of Life

Polymers and Monomers

Biological macromolecules are large polymers formed by joining smaller monomers through condensation (dehydration synthesis) reactions.

• Polymer: A long molecule consisting of many similar or identical building blocks (monomers) linked by covalent bonds.

• Monomer: The repeating unit that serves as the building block of a polymer.

• Condensation Reaction: Monomers are joined by removing a water molecule.

• Hydrolysis: Polymers are broken down into monomers by adding water.

• Four Main Classes:

  1. Carbohydrates

  2. Lipids

  3. Proteins

  4. Nucleic Acids

Carbohydrates

Carbohydrates are energy-rich organic compounds made of carbon, hydrogen, and oxygen. They serve as fuel and structural materials in cells.

• Monomer: Monosaccharide (simple sugar, e.g., glucose)

• Functional Groups: Hydroxyl (-OH) and carbonyl (C=O)

• General Formula: $C_nH_{2n}O_n$

• Types:

  • Monosaccharides: Glucose, fructose (isomers)

  • Disaccharides: Maltose, sucrose, lactose

  • Polysaccharides: Starch (plant storage), glycogen (animal storage), cellulose (plant cell walls), chitin (arthropod exoskeletons)

• Sweetness Scale: Different sugars and artificial sweeteners vary in sweetness relative to sucrose.

Compound	Sweetness Relative to Sucrose
Lactose	Not as sweet
Maltose	Same sweetness
Glucose	Slightly sweeter
Fructose	4 times sweeter
Aspartame	150 times sweeter
Saccharine	450 times sweeter
Sucralose	600 times sweeter
Neotame	8,000 times sweeter

Example: High Fructose Corn Syrup (HFCS) is made by converting glucose from corn starch into fructose, resulting in a sweeter product used in processed foods.

Lipids

Lipids are hydrophobic molecules that serve as long-term energy storage, structural components, and signaling molecules.

• Monomer: Fatty acids

• Properties: Insoluble in water, soluble in nonpolar solvents

• Types:

  • Triglycerides: Three fatty acids linked to glycerol; energy storage

  • Phospholipids: Major component of cell membranes; amphipathic

  • Steroids: Four fused carbon rings; includes cholesterol and hormones

• Saturated vs. Unsaturated Fats:

  • Saturated: No double bonds; solid at room temperature

  • Unsaturated: One or more double bonds; liquid at room temperature

• Trans Fats: Produced by partial hydrogenation; increase risk of heart disease

• Example: Cholesterol is the most abundant steroid in the body, essential for membrane structure and hormone synthesis.

Proteins

Proteins are polymers of amino acids that perform a wide range of functions, including catalysis, structure, transport, and regulation.

• Monomer: Amino acid

• Structure: Each amino acid has an amino group (-NH2), carboxyl group (-COOH), and a unique side chain (R group)

• Peptide Bond: Covalent bond linking amino acids

• Levels of Structure:

  1. Primary: Sequence of amino acids

  2. Secondary: Local folding (alpha-helix, beta-pleated sheet) stabilized by hydrogen bonds

  3. Tertiary: Overall 3D shape due to interactions among R groups

  4. Quaternary: Association of multiple polypeptide chains

• Function: Protein shape determines function; denaturation disrupts activity

• Examples: Enzymes (catalysts), structural proteins (collagen), contractile proteins (actin, myosin), hormones (insulin)

Enzymes

Enzymes are proteins that catalyze biochemical reactions, increasing reaction rates without being consumed.

• Substrate: The reactant an enzyme acts upon

• Active Site: Region where substrate binds

• Specificity: Enzymes are highly specific for their substrates

• Factors Affecting Activity: Temperature, pH, inhibitors, cofactors

• Example: Lactase digests lactose; mutations in the lactase gene can lead to lactose intolerance

Nucleic Acids

Nucleic acids store and transmit genetic information. DNA and RNA are polymers of nucleotides.

• Monomer: Nucleotide (composed of a phosphate group, pentose sugar, and nitrogenous base)

• DNA: Double helix; bases are adenine (A), thymine (T), cytosine (C), guanine (G)

• RNA: Single-stranded; uracil (U) replaces thymine

• Base Pairing:

  • A pairs with T (2 hydrogen bonds)

  • G pairs with C (3 hydrogen bonds)

• Function: DNA sequences code for proteins; mutations can alter protein structure and function

• Example: Lactase gene mutations can cause lactose intolerance by affecting enzyme production

Type	Sugar	Bases	Strands
DNA	Deoxyribose	A, T, C, G	Double
RNA	Ribose	A, U, C, G	Single

Lactose Tolerance and Evolution

Lactose tolerance is a genetic trait that allows adults to digest lactose. It is more common in populations with a history of dairy farming.

• Lactase Persistence: Continued production of lactase enzyme into adulthood

• Genetic Mutations: Multiple mutations in the human genome enable lactase persistence

• Evolutionary Advantage: Ability to utilize dairy products provided a nutritional benefit in certain populations

Additional info: The study notes include inferred details and expanded academic context for clarity and completeness.