The Structure and Function of Large Biological Molecules

Biological Molecules

Introduction

Large biological molecules, also known as macromolecules, are essential for life. They include carbohydrates, lipids, proteins, and nucleic acids. Each class of molecule has unique structures and functions that are critical for cellular processes.

Major Classes of Biological Molecules

Polymers and Monomers

• Polymers are large molecules made by joining many smaller units called monomers through covalent bonds.

• Dehydration reactions build polymers by removing water, while hydrolysis breaks them down by adding water.

Four Classes of Biological Molecules

• Carbohydrates

• Lipids

• Proteins

• Nucleic acids

Carbohydrates

Overview

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically in a ratio of 1:2:1. They serve as energy sources and structural materials.

• Monosaccharides: Simple sugars (e.g., glucose, fructose) with the general formula (CH2O)n.

• Disaccharides: Formed by joining two monosaccharides via a glycosidic linkage (e.g., sucrose, lactose).

• Polysaccharides: Long chains of monosaccharides; serve as energy storage (e.g., starch in plants, glycogen in animals) or structural support (e.g., cellulose in plants).

Example: Starch is a polysaccharide used by plants to store energy.

Lipids

Overview

Lipids are hydrophobic molecules that include fats, phospholipids, and steroids. They are not true polymers but are grouped together due to their insolubility in water.

• Fats (triglycerides): Composed of glycerol and three fatty acids. Used for energy storage.

• Saturated fats: No double bonds between carbon atoms; solid at room temperature.

• Unsaturated fats: One or more double bonds; liquid at room temperature.

• Phospholipids: Major component of cell membranes; have hydrophilic heads and hydrophobic tails.

• Steroids: Lipids with a carbon skeleton of four fused rings (e.g., cholesterol, testosterone, estradiol).

Example: Cholesterol is a steroid important for membrane structure and as a precursor for hormones.

Proteins

Overview

Proteins are polymers of amino acids and perform a vast array of functions, including catalysis, structure, transport, and signaling.

• Amino acids: Building blocks of proteins; 20 different types, each with a unique side chain (R group).

• Peptide bond: Covalent bond linking amino acids in a protein.

• Levels of protein structure:

  • Primary: Sequence of amino acids.

  • Secondary: Local folding (α-helix, β-sheet) stabilized by hydrogen bonds.

  • Tertiary: Overall 3D shape of a polypeptide.

  • Quaternary: Association of multiple polypeptide chains.

• Protein denaturation: Loss of structure and function due to environmental changes (e.g., temperature, pH).

Example: Hemoglobin is a protein with quaternary structure that transports oxygen in blood.

Nucleic Acids

Overview

Nucleic acids store and transmit genetic information. The two main types are DNA and RNA.

• Nucleotide: Monomer of nucleic acids, consisting of a phosphate group, a five-carbon sugar (deoxyribose or ribose), and a nitrogenous base.

• Nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C), and Uracil (U; in RNA only).

• DNA: Double-stranded helix; stores genetic information.

• RNA: Single-stranded; involved in protein synthesis.

• Functions:

  • DNA replication

  • Directing protein synthesis (transcription and translation)

Example: mRNA carries genetic instructions from DNA to ribosomes for protein synthesis.

Key Terms and Concepts

• Dehydration reaction: Chemical reaction that joins monomers by removing water.

• Hydrolysis: Chemical reaction that breaks polymers into monomers by adding water.

• Glycosidic linkage: Covalent bond joining two monosaccharides.

• Phospholipid bilayer: Structure forming the basic framework of cell membranes.

• Peptide bond: Covalent bond between amino acids in a protein.

• Phosphodiester bond: Covalent bond linking nucleotides in nucleic acids.

• Antiparallel: Refers to the opposite orientation of the two strands in DNA.

• Double helix: The structure of DNA, consisting of two intertwined strands.

Comparison Table: Major Classes of Biological Molecules

Class	Monomer	Polymer	Main Functions	Examples
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structure	Starch, glycogen, cellulose
Lipids	Fatty acids, glycerol	Not true polymers	Energy storage, membranes, hormones	Fats, phospholipids, steroids
Proteins	Amino acid	Polypeptide	Catalysis, structure, transport, signaling	Enzymes, hemoglobin, antibodies
Nucleic acids	Nucleotide	DNA, RNA	Genetic information storage and transfer	DNA, RNA

Key Equations and Structures

• General formula for monosaccharides:

  • $C_nH_{2n}O_n$

• Peptide bond formation:

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

• Phosphodiester bond in nucleic acids:

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

Additional info:

• Protein structure is determined by the sequence of amino acids and the interactions between their side chains.

• DNA and RNA differ in their sugars (deoxyribose vs. ribose) and in one nitrogenous base (thymine in DNA, uracil in RNA).