Carbon and Organic Molecules

Introduction to Carbon in Biology

Carbon is the fundamental building block of life, forming the backbone of organic molecules found in all living organisms. Its unique chemical properties allow it to form stable covalent bonds with a variety of other atoms, enabling the diversity of biological molecules.

• Versatility: Carbon can form four covalent bonds, allowing for complex molecular structures.

• Bond Stability: Covalent bonds formed by carbon are stable across a wide range of temperatures, making them ideal for biological systems.

• Bond Diversity: Carbon bonds with hydrogen, oxygen, nitrogen, phosphorus, and sulfur to create a variety of molecules.

Additional info: The electron configuration of carbon (2 electrons in the first shell, 4 in the second) explains its tetravalency.

Polar and Nonpolar Covalent Bonds

The type of covalent bond formed by carbon depends on the electronegativity of the atoms involved. This determines whether a molecule is hydrophilic (water-loving) or hydrophobic (water-fearing).

• Nonpolar Bonds: Formed when carbon bonds with atoms of similar electronegativity (e.g., C-H, C-C). These molecules are typically hydrophobic and do not dissolve well in water.

• Polar Bonds: Formed when carbon bonds with more electronegative atoms (e.g., C-O, C-N, C-P, C-S). These molecules are hydrophilic and dissolve in water.

Functional Groups

Overview of Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic properties and chemical reactivity. They are critical in determining the behavior and function of organic molecules in biological systems.

• Functional groups participate in characteristic reactions and impart specific chemistry to molecules.

• Knowing the structure and properties of functional groups is essential for understanding biological molecules.

Amino Group (–NH2)

The amino group is weakly basic and can accept a proton at physiological pH, becoming positively charged. All amino acids contain an amino group.

• Polarity: Polar

• Biological importance: Found in amino acids, the building blocks of proteins.

Carbonyl Group (Ketone and Aldehyde)

The carbonyl group consists of a carbon atom double-bonded to an oxygen atom. Its properties depend on its position within the molecule:

• Ketone: Carbonyl group is bonded to two other carbons (e.g., acetone).

• Aldehyde: Carbonyl group is bonded to one carbon and one hydrogen (e.g., glucose).

• Polarity: Polar, highly reactive, forms hydrogen bonds.

Carboxyl Group (–COOH)

The carboxyl group is both a carbonyl and a hydroxyl group attached to the same carbon. It acts as an acid, donating a proton at physiological pH.

• Polarity: Polar

• Biological importance: Present in all amino acids and fatty acids.

Hydroxyl Group (–OH)

The hydroxyl group is characteristic of alcohols and is polar, allowing molecules to form hydrogen bonds with water.

• Polarity: Polar

• Biological importance: Found in carbohydrates, proteins, and nucleic acids.

Methyl Group (–CH3)

The methyl group is nonpolar and hydrophobic. Methylation of DNA and proteins can affect gene expression and protein function.

• Polarity: Nonpolar

• Biological importance: Involved in epigenetic regulation.

Phosphate Group (–PO42–)

The phosphate group is weakly acidic and negatively charged at physiological pH. It is essential in energy transfer and nucleic acid structure.

• Polarity: Polar

• Biological importance: Found in ATP, DNA, and phospholipids.

Sulfate Group (–SO4–)

The sulfate group is negatively charged at physiological pH and may be attached to carbohydrates, proteins, and lipids.

• Polarity: Polar

• Biological importance: Found in some detergents and biological molecules.

Sulfhydryl Group (–SH)

The sulfhydryl group can form disulfide bridges, which are important for protein structure and stability.

• Polarity: Polar

• Biological importance: Found in the amino acid cysteine.

Functional Groups and Biological Function

Small changes in functional groups can have significant effects on the function of biological molecules. For example, estrogen and testosterone differ by only two functional groups, yet have vastly different biological effects.

Isomers

Types of Isomers

Isomers are molecules with the same molecular formula but different structures and properties. There are several types of isomers:

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

• Stereoisomers: Same covalent arrangement but differ in spatial orientation.

• Enantiomers: Mirror images of each other, often with different biological activities.

• Cis-Trans Isomers: Differ in the arrangement around a double bond (cis: same side, trans: opposite sides).

Molecules and Macromolecules

Monomers and Polymers

Biological macromolecules are large molecules composed of repeating subunits called monomers. The process of linking monomers to form polymers is essential for building the complex molecules necessary for life.

• Monomer: A single building block (e.g., amino acid, nucleotide, monosaccharide, fatty acid).

• Polymer: A large molecule made by linking many monomers (e.g., protein, nucleic acid, polysaccharide, lipid).

Polymer Formation and Breakdown

• Dehydration Reaction: Monomers are joined by covalent bonds through the removal of a water molecule.

• Hydrolysis Reaction: Polymers are broken down into monomers by the addition of water.

The Four Classes of Biological Molecules

Overview

All living organisms are composed of four major classes of organic molecules, each with distinct monomers and polymers:

Additional info: Each class of macromolecule has unique functions in cells, such as energy storage, structural support, catalysis, and information storage.