Key Biological Molecules and Their Bonds

Carbohydrates

Hexoses and Pentoses

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, and are classified based on the number of carbon atoms they contain. The most common types are hexoses (6 carbons) and pentoses (5 carbons).

• Hexose: A monosaccharide with six carbon atoms. Examples include glucose and fructose.

• Pentose: A monosaccharide with five carbon atoms. Examples include ribose and deoxyribose.

Examples:

• Glucose (C6H12O6): A primary energy source in cells.

• Fructose: A sugar found in fruits; isomer of glucose.

• Ribose: A component of RNA.

• Deoxyribose: A component of DNA (lacks one oxygen atom compared to ribose).

Glycosidic Bonds: Carbohydrates can join via glycosidic bonds through a condensation (dehydration synthesis) reaction, releasing water:

• Reaction: Carbohydrate + Carbohydrate → Glycosidic bond + H2O

Equation:

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

Lipids

Fatty Acids and Triglycerides

Lipids are hydrophobic molecules that include fatty acids, triglycerides, and phospholipids. Fatty acids can be classified as cis or trans based on the configuration around the double bond.

• Fatty Acids: Long hydrocarbon chains with a carboxyl group at one end.

• Triglycerides: Formed by the condensation of three fatty acids with one glycerol molecule.

• Phospholipids: Major components of cell membranes, containing two fatty acids, a phosphate group, and glycerol.

• Cis vs. Trans: Cis fatty acids have hydrogen atoms on the same side of the double bond, causing a bend; trans fatty acids have hydrogens on opposite sides, resulting in a straighter chain.

Bonding: Fatty acids and phospholipids do not form covalent bonds with each other; instead, they aggregate via London dispersion forces and van der Waals interactions.

Proteins

Amino Acids and Peptide Bonds

Proteins are polymers of amino acids linked by peptide bonds. Each amino acid contains an amino group, a carboxyl group, and a unique side chain (R group).

• Peptide Bond: A covalent bond formed between the amino group of one amino acid and the carboxyl group of another via a condensation reaction, releasing water.

Equation:

$\text{Amino acid}_1 + \text{Amino acid}_2 \rightarrow \text{Dipeptide} + H_2O$

Nucleic Acids

Nucleotides and Phosphodiester Bonds

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

• Phosphodiester Bond: The covalent bond that links the 3' carbon atom of one sugar molecule to the 5' carbon atom of another, forming the backbone of DNA and RNA.

• Formation: Two nucleotides join via a condensation reaction, releasing water.

Equation:

$\text{Nucleotide}_1 + \text{Nucleotide}_2 \rightarrow \text{Dinucleotide} + H_2O$

Summary Table: Types of Biological Molecules and Their Bonds

Molecule Type	Monomer	Polymer	Bond Type	Example
Carbohydrate	Monosaccharide	Polysaccharide	Glycosidic bond	Starch, Cellulose
Protein	Amino acid	Polypeptide	Peptide bond	Hemoglobin, Enzymes
Nucleic Acid	Nucleotide	DNA/RNA	Phosphodiester bond	DNA, RNA
Lipid	Fatty acid, Glycerol	Triglyceride, Phospholipid	Ester bond (within molecule); London dispersion/van der Waals (between molecules)	Fats, Oils, Membranes

Key Points

• Condensation (Dehydration Synthesis) Reactions: Join monomers by removing water.

• Hydrolysis: Breaks polymers into monomers by adding water (not shown in original notes, but important for context).

• Non-covalent Interactions: Lipids aggregate via weak forces rather than covalent bonds.

Additional info: Hydrolysis is the reverse of condensation and is essential for digestion and metabolism of biological macromolecules.