Chapter 3: Biological Molecules and the Chemistry of Life – Study Guide

Organic Molecules and Carbon Chemistry

Definition and Importance of Organic Molecules

Organic molecules are compounds primarily composed of carbon atoms bonded with hydrogen, oxygen, nitrogen, and other elements. They are fundamental to all living organisms.

• Organic Molecule: Any molecule containing carbon atoms bonded to hydrogen; typically found in living things, but some can be synthesized artificially.

• Versatility of Carbon: Carbon can form four covalent bonds, allowing for a variety of stable structures such as chains, rings, and branches.

• Structural Diversity: Carbon skeletons can vary in length, branching, double bond position, and ring formation.

Isomers

Isomers are molecules with the same molecular formula but different structures and properties.

• Structural Isomers: Differ in the covalent arrangement of atoms.

• Cis-Trans (Geometric) Isomers: Differ in spatial arrangement around double bonds.

• Enantiomers: Mirror images of each other, important in biological systems.

Steroids and Functional Groups

Steroid Structure and Function

Steroids, such as estradiol and testosterone, are lipids with a characteristic four-ring structure. Small differences in functional groups lead to significant differences in biological activity.

• Estradiol vs. Testosterone: Both have the same carbon skeleton but differ in attached functional groups, resulting in different effects in the body.

Functional Groups

Functional groups are specific groups of atoms within molecules that determine the chemical properties of those molecules.

• Examples: Hydroxyl (-OH), Carbonyl (C=O), Carboxyl (-COOH), Amino (-NH2), Sulfhydryl (-SH), Phosphate (-PO4), Methyl (-CH3).

• Role: Functional groups participate in chemical reactions and influence molecular behavior.

Carbohydrates

Monosaccharides and Disaccharides

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

• Glucose: A six-carbon monosaccharide (C6H12O6).

• Sucrose: A disaccharide formed from glucose and fructose.

• Bond Formation: Glycosidic linkage forms via dehydration synthesis (removal of water).

• Hydrolysis: The reverse reaction, breaking bonds by adding water.

Equation for Dehydration Synthesis:

$\text{Monosaccharide}_1 + \text{Monosaccharide}_2 \rightarrow \text{Disaccharide} + H_2O$

Polysaccharides

Polysaccharides are long chains of monosaccharides linked by glycosidic bonds.

• Roles: Energy storage (starch in plants, glycogen in animals) and structural support (cellulose in plants, chitin in fungi and arthropods).

• Cellulose Digestion: Most animals cannot digest cellulose due to lack of appropriate enzymes; some, like cows, rely on symbiotic bacteria.

Lipids

Fats and Fatty Acids

Lipids are hydrophobic molecules, including fats, phospholipids, and steroids.

• Fat Molecule: Composed of glycerol and three fatty acids.

• Saturated Fatty Acid: No double bonds between carbon atoms; solid at room temperature.

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

Phospholipids

Phospholipids are major components of cell membranes, consisting of two fatty acids, a glycerol, and a phosphate group.

• Structure: Hydrophilic (water-attracting) head and hydrophobic (water-repelling) tails.

• Location: Found in all living cell membranes, forming bilayers.

Proteins

Amino Acids and Protein Structure

Proteins are polymers of amino acids, each with a central carbon, amino group, carboxyl group, hydrogen atom, and variable R group (side chain).

• Amino Acid Structure: Central carbon (α-carbon), amino group (-NH2), carboxyl group (-COOH), hydrogen, and R group.

• Hydrophobic vs. Hydrophilic Side Chains: Hydrophobic side chains are nonpolar; hydrophilic side chains are polar or charged.

Levels of Protein Structure

• Primary Structure: Sequence of amino acids in a polypeptide chain.

• Secondary Structure: Local folding into α-helices and β-pleated sheets, stabilized by hydrogen bonds.

• Tertiary Structure: Overall 3D shape of a polypeptide, determined by interactions among R groups.

• Quaternary Structure: Association of multiple polypeptide chains.

Protein Function and Denaturation

• Major Roles: Enzymes, structural support, transport, signaling, defense, movement.

• Denaturation: Loss of protein's native structure due to heat, pH, or chemicals, resulting in loss of function.

• Mutations: Changes in amino acid sequence can alter protein shape and function.

Chemical Bonds in Proteins

• Peptide Bonds: Link amino acids in primary structure.

• Hydrogen Bonds: Stabilize secondary structure.

• Disulfide Bridges, Ionic Bonds, Hydrophobic Interactions: Stabilize tertiary and quaternary structures.

Nucleic Acids

DNA, RNA, and Nucleotides

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

• Nucleotide Structure: Consists of a phosphate group, a five-carbon sugar (deoxyribose or ribose), and a nitrogenous base.

• Flow of Genetic Information: DNA → RNA → Protein (the "central dogma" of molecular biology).

Diagram of a Nucleotide: (Phosphate group) — (Sugar) — (Nitrogenous base)

Summary Table: Types of Biological Molecules

Type	Monomer	Polymer	Main Functions
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structure
Lipids	Fatty acid, glycerol	Triglyceride, phospholipid	Energy storage, membranes, signaling
Proteins	Amino acid	Polypeptide	Enzymes, structure, transport, signaling
Nucleic Acids	Nucleotide	DNA, RNA	Genetic information storage and transfer

Additional info: These study notes expand on the question prompts by providing definitions, explanations, and context for each major topic in biological molecules, as would be expected in a General Biology college course.