Biological Macromolecules: Structure, Types, and Functions

Overview of Biological Macromolecules

Biological macromolecules are large, complex molecules essential for life. The four major classes—carbohydrates, lipids, proteins, and nucleic acids—differ in their structure, monomers, bond types, and biological functions. Understanding their similarities and differences is fundamental in General Biology.

Carbohydrates

Structure

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio. They exist as monomers, dimers, and polymers.

• Monomers: Monosaccharides (e.g., glucose, fructose)

• Bond type: Glycosidic linkages (between monosaccharides)

• Polymers: Disaccharides (e.g., sucrose), Polysaccharides (e.g., starch, glycogen, cellulose)

• Elements: C, H, O (usually in a 1:2:1 ratio)

• Forms: Linear or ring structures (glucose often forms a ring in solution)

Function

• Primary energy source for cells (especially glucose)

• Short-term energy storage: Glycogen in animals, starch in plants

• Structural support: Cellulose in plant cell walls, chitin in fungi and arthropods

• Recognition and signaling: Carbohydrates on cell surfaces help with cell-cell communication

Examples

• Honey and salt combo uses glucose and fructose for fast energy

• Cellulose is fiber, not digestible by humans due to beta-glycosidic bonds

Lipids

Structure

Lipids are hydrophobic molecules with diverse structures, primarily composed of carbon, hydrogen, and less oxygen than carbohydrates.

• Elements: C, H, O (less oxygen than carbohydrates)

• Types:

  • Fats: Saturated (no double bonds, solid at room temp) vs. Unsaturated (one or more double bonds, liquid at room temp)

  • Phospholipids: Two fatty acids + phosphate group + glycerol

  • Steroids: Four fused carbon rings (e.g., cholesterol)

Function

• Long-term energy storage

• Cell membrane structure: Phospholipid bilayer

• Hormones and signaling: Steroids like estrogen and testosterone

• Insulation and protection: Fat cushions organs and retains heat

• Phospholipids: Key to membrane fluidity and function

Examples

• Phospholipids form the basis of cell membranes

• Cholesterol is a steroid important for membrane structure

Proteins

Structure

Proteins are polymers of amino acids, with diverse structures and functions. They contain carbon, hydrogen, oxygen, and nitrogen (sometimes sulfur).

• Monomers: Amino acids (20 types)

• Bond type: Peptide bonds (between amino and carboxyl groups)

• Elements: C, H, O, N (sometimes S)

• Levels of structure:

  • Primary: Amino acid sequence

  • Secondary: Alpha helices and beta sheets (hydrogen bonds)

  • Tertiary: 3D folding (R group interactions)

  • Quaternary: Multiple polypeptides (e.g., hemoglobin)

Function

• Structural: Collagen, keratin

• Transport: Hemoglobin carries oxygen

• Defense: Antibodies

• Movement: Actin and myosin in muscles

• Signaling: Hormones like insulin

Examples

• Protein shakes provide amino acids for muscle repair

• Enzymes are biological catalysts that lower activation energy

Nucleic Acids

Structure

Nucleic acids are polymers of nucleotides, which consist of a phosphate group, a sugar, and a nitrogenous base. They store and transmit genetic information.

• Monomers: Nucleotides (phosphate group + sugar + nitrogenous base)

• Bond type: Phosphodiester bonds (between sugar and phosphate)

• Elements: C, H, O, N, P

• Types: DNA and RNA

• DNA: Double-stranded, deoxyribose sugar, bases A-T, G-C

• RNA: Single-stranded, ribose sugar, bases A-U, G-C

Function

• Store genetic information: DNA

• Transmit genetic instructions: RNA

• Direct protein synthesis: mRNA, tRNA, rRNA

• Regulate gene expression

Examples

• DNA is the blueprint for building proteins

• RNA acts as messenger and builder during transcription and translation

Comparison Table: Biological Macromolecules

The following table summarizes the key differences and similarities among the four major biological macromolecules:

Molecule	Monomer	Bond Type	Key Elements	Main Functions
Carbohydrates	Monosaccharide	Glycosidic linkage	C, H, O	Energy, structure, signaling
Lipids	Fatty acid + glycerol	Ester linkage	C, H, O	Energy storage, membranes, hormones
Proteins	Amino acid	Peptide bond	C, H, O, N (S)	Enzymes, structure, transport, movement
Nucleic Acids	Nucleotide	Phosphodiester bond	C, H, O, N, P	Genetic info, protein synthesis

Tips to Remember

• Carbs = quick fuel

• Lipids = long-term fuel + membranes

• Proteins = doers of the cell

• Nucleic acids = blueprints of life

Key Equations and Concepts

• General formula for monosaccharides: $C_nH_{2n}O_n$ Example: Glucose is $C_6H_{12}O_6$

• Peptide bond formation: $\text{Amino acid}_1 + \text{Amino acid}_2 \rightarrow \text{Dipeptide} + H_2O$ Peptide bonds form via dehydration synthesis between amino and carboxyl groups.

• Phosphodiester bond in nucleic acids: $\text{Nucleotide}_1 + \text{Nucleotide}_2 \rightarrow \text{Dinucleotide} + H_2O$ Phosphodiester bonds link the 3' carbon of one sugar to the 5' phosphate of the next.