Lecture 4

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Organic Compounds

The Central Role of Carbon

Organic compounds are the foundation of all living organisms, and carbon is the essential element that forms the backbone of these molecules. The unique properties of carbon allow it to form a vast array of complex and diverse molecules necessary for life.

Definition: Organic compounds are molecules primarily composed of carbon atoms covalently bonded to other elements such as hydrogen, oxygen, nitrogen, and sulfur.
Abundance: Carbon is the fourth most abundant element in the universe, though not among the top ten on Earth. Despite this, all known life is carbon-based due to its chemical versatility.
Versatility: Carbon can form four covalent bonds, allowing for a wide variety of stable and complex molecular structures, including chains, rings, and branches.
Examples: Glucose (C6H12O6), dopamine, and pentane are all organic compounds.

Structure and Diversity of Organic Molecules

Covalent Bonding and Molecular Diversity

Carbon's ability to form four covalent bonds leads to a diversity of organic molecules with different shapes and functions.

Covalent Bonds: Carbon atoms can bond to other carbons, forming long chains (carbon skeletons), branched structures, or rings.
Other Elements: Organic molecules often include hydrogen, oxygen, nitrogen, and sulfur, contributing to their chemical properties.
Isomers: Molecules with the same chemical formula but different structures (e.g., glucose and fructose).

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.

Definition: A functional group is a cluster of atoms that imparts specific chemical properties to an organic molecule.
Role: Functional groups are the sites of chemical reactivity, such as dehydration and hydrolysis reactions.

Functional Group	Properties	Examples
Hydroxyl (-OH)	Polar; forms hydrogen bonds; involved in dehydration and hydrolysis reactions	Sugars, alcohols, some steroids
Carboxyl (-COOH)	Polar and acidic; can donate H+; forms carboxylic acids	Amino acids, fatty acids
Amino (-NH2)	Polar and basic; can accept H+	Amino acids, nucleic acids
Sulfhydryl (-SH)	Nonpolar; forms disulfide bonds	Cysteine (an amino acid), proteins
Phosphate (-PO4)	Polar; involved in energy transfer	ATP, nucleic acids

Macromolecules and Polymers

Monomers and Polymers

Most biological macromolecules are polymers, large molecules made by joining many smaller units called monomers.

Monomer: A small molecule that can join with other similar molecules to form a polymer (e.g., glucose, amino acids, nucleotides).
Polymer: A large molecule composed of repeating monomer units (e.g., starch, proteins, DNA).
Analogy: Monomers are like building blocks; polymers are the completed structure.

Types of Biological Macromolecules

Carbohydrates
Proteins
Nucleic Acids
Lipids

Chemical Reactions in Biology

Dehydration Synthesis and Hydrolysis

Macromolecules are assembled and disassembled by two main types of reactions:

Dehydration Synthesis (Condensation): Joins monomers by removing a water molecule, forming a covalent bond.
Hydrolysis: Breaks covalent bonds in polymers by adding a water molecule, releasing monomers.

Example Equation (Dehydration Synthesis):

Example Equation (Hydrolysis):

Carbohydrates

Structure and Function

Carbohydrates are hydrophilic molecules that serve as a primary energy source and structural material in living organisms.

Monosaccharides: Simple sugars (e.g., glucose, fructose) with the general formula (CH2O)n.
Disaccharides: Two monosaccharides joined by a glycosidic bond (e.g., sucrose = glucose + fructose, lactose = glucose + galactose).
Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose, chitin).
Functions: Energy storage (starch in plants, glycogen in animals), structural support (cellulose in plants, chitin in fungi and arthropods).

Polysaccharide	Source	Function	Structure
Starch	Plants	Energy storage	Unbranched or slightly branched glucose polymer
Glycogen	Animals	Energy storage	Highly branched glucose polymer
Cellulose	Plants	Structural support (cell wall)	Unbranched, straight glucose polymer; forms microfibrils
Chitin	Fungi, arthropods	Structural support (cell wall, exoskeleton)	Polymer of modified glucose units

Lipids

Structure and Types

Lipids are hydrophobic (water-insoluble) molecules that include fats, oils, phospholipids, and steroids. They are not true polymers but are assembled from smaller components.

Fats and Oils: Composed of glycerol and three fatty acids (triglycerides). Serve as long-term energy storage.
Fatty Acids: Long hydrocarbon chains with a carboxyl group at one end. Can be saturated (no double bonds) or unsaturated (one or more double bonds).
Phospholipids: Glycerol backbone, two fatty acids, and a phosphate group. Major component of cell membranes.
Steroids: Four fused carbon rings with various functional groups (e.g., cholesterol, hormones like estrogen and testosterone).

Type	Structure	Function	Example
Fat (Triglyceride)	Glycerol + 3 fatty acids	Energy storage, insulation	Butter, oil
Phospholipid	Glycerol + 2 fatty acids + phosphate group	Main component of cell membranes	Phosphatidylcholine
Steroid	Four fused carbon rings	Membrane structure, hormones	Cholesterol, testosterone

Saturated Fats: No double bonds in fatty acid chains; solid at room temperature.
Unsaturated Fats: One or more double bonds; liquid at room temperature.

Biological Importance of Lipids

Energy storage (fats/oils)
Structural components of cell membranes (phospholipids, cholesterol)
Signaling molecules (steroid hormones)

Example: Cholesterol is a key component of animal cell membranes and the precursor for steroid hormones such as estrogen and testosterone.

Additional info: Proteins and nucleic acids are also major classes of biological macromolecules, but detailed coverage is not included in the provided materials.