Organic Molecules in Biology

Organic vs Inorganic Compounds

Organic molecules are essential to life and are primarily composed of carbon, hydrogen, and oxygen. They are distinguished from inorganic compounds by their structure and function in living organisms.

• Organic Compounds: Contain carbon, hydrogen, and usually oxygen. They serve as the actual building blocks of cells.

• Inorganic Compounds: Do not contain carbon-based structures (with few exceptions, e.g., CO2). They are not part of the cell's structure but are involved in cellular functions.

Carbon Chemistry and Bonding

Carbon-Based Molecules

Carbon atoms have four valence electrons, allowing them to form covalent bonds with a variety of other atoms, resulting in a wide diversity of organic molecules.

• Carbon atoms can form straight chains, branched chains, or rings.

• Carbon forms the "skeletons" of organic molecules.

• Single bonds: Formed when two atoms share one pair of electrons.

• Double/triple bonds: Formed when atoms share two or three pairs of electrons, respectively.

Functional Groups

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

• Hydroxyl group (-OH): Found in alcohols; makes molecules hydrophilic (water-soluble).

• Carboxyl group (-COOH): Found in organic acids.

• Amino group (-NH2): Found in amino acids.

• Phosphate group (-PO4): Found in nucleic acids and ATP.

Macromolecules: Structure and Function

Large Carbon Molecules

Macromolecules are large molecules formed by the joining of smaller units called monomers. When monomers join, they form polymers.

• Monomers: Simple, repeated units.

• Polymers: Chains of monomers.

Condensation (Dehydration Synthesis) and Hydrolysis Reactions

• Condensation Reaction: Monomers are joined together by removing a molecule of water. This process is also called dehydration synthesis.

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

Energy Currency: ATP

Adenosine triphosphate (ATP) is the primary energy carrier in cells. It stores and releases energy as needed for cellular processes.

• ATP contains high-energy phosphate bonds. Breaking these bonds releases energy.

The Four Major Types of Organic Molecules

Carbohydrates

Structure and Types

Carbohydrates are composed of carbon, hydrogen, and oxygen, usually in a 1:2:1 ratio. They are classified by the number of sugar units:

• Monosaccharides: Simple sugars (e.g., glucose). Major source of short-term energy.

• Disaccharides: Two monosaccharides joined together (e.g., sucrose).

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

Functions

• Provide energy for metabolism.

• Serve as structural components (e.g., cellulose in plant cell walls).

Examples

• Starch: Energy storage in plants.

• Glycogen: Energy storage in animals.

• Cellulose: Structural component in plants.

Lipids

Structure and Types

Lipids are nonpolar molecules that are insoluble in water. They are composed of fatty acids and glycerol.

• Saturated fatty acids: All single carbon-carbon bonds; straight chains; solid at room temperature (e.g., butter, animal fat).

• Unsaturated fatty acids: One or more double carbon-carbon bonds; bent chains; liquid at room temperature (e.g., vegetable oils).

Functions

• Long-term energy storage.

• Insulation and protection.

• Major component of cell membranes (phospholipids).

Examples

• Fats, oils, waxes, steroids.

• Phospholipids: Make up the bilayer of cell membranes.

Proteins

Structure

Proteins are polymers of amino acids. Each amino acid has a central carbon, an amino group, a carboxyl group, a hydrogen atom, and a variable R group.

• There are 20 different amino acids.

• Amino acids are linked by peptide bonds to form polypeptides.

• The sequence of amino acids determines the protein's structure and function.

Functions

• Structural support (e.g., muscle, skin, hair).

• Transport of nutrients and molecules.

• Enzymes: Speed up chemical reactions.

• Defense (antibodies).

Enzymes

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy. They are specific to their substrates and are not consumed in the reaction.

• Enzyme activity is affected by pH, temperature, and concentration.

• Enzymes help maintain homeostasis.

Factors Affecting Enzyme Activity

• Temperature: High temperatures can denature enzymes, changing their shape and function.

• pH: Each enzyme works best at a specific pH.

• Concentration: Increasing enzyme concentration increases reaction rate up to a point.

Nucleic Acids

Structure

Nucleic acids (DNA and RNA) are polymers of nucleotides. Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base.

• DNA: Deoxyribonucleic acid; double-stranded; uses deoxyribose sugar; bases: adenine, thymine, guanine, cytosine.

• RNA: Ribonucleic acid; single-stranded; uses ribose sugar; bases: adenine, uracil, guanine, cytosine.

Functions

• Store and transmit genetic information.

• Direct the synthesis of proteins.

Comparison of DNA and RNA

Key Terms and Definitions

• Monomer: A small molecule that can join with others to form a polymer.

• Polymer: A large molecule made up of repeating monomer units.

• Peptide bond: The bond formed between amino acids in a protein.

• Activation energy: The minimum energy required to start a chemical reaction.

• Hydrolysis: The chemical breakdown of a compound due to reaction with water.

• Dehydration synthesis: The process of joining two molecules by removing water.

Important Equations

• General formula for carbohydrates:

• ATP hydrolysis:

• Peptide bond formation (dehydration synthesis):

Summary Table: Macromolecules and Their Monomers

Additional info: Some explanations and definitions have been expanded for clarity and completeness based on standard General Biology curriculum.