Macromolecules and Their Biological Functions: Study Guide

Macromolecules in Biology

Introduction

Macromolecules are large, complex molecules essential for life. The four major classes—carbohydrates, proteins, lipids, and nucleic acids—play critical roles in cell structure and function. Understanding their structure, properties, and interactions is fundamental to General Biology.

Hydrocarbons and Functional Groups

Hydrocarbons and Chemical Modifications

• Hydrocarbons are molecules consisting entirely of carbon and hydrogen. They are typically nonpolar and hydrophobic.

• To make hydrocarbons biologically active, functional groups are added, such as hydroxyl (–OH), carbonyl (–COOH), amino (–NH2), or carboxyl (–COOH) groups.

• These modifications increase solubility and reactivity, allowing hydrocarbons to participate in biochemical reactions.

Example: Ethanol (C2H5OH) is a hydrocarbon with a hydroxyl group, making it soluble in water.

Polymers and Hydrolysis

Polymer Formation and Breakdown

• Polymers are long chains of repeating subunits (monomers) joined by covalent bonds.

• Dehydration synthesis (condensation reaction) forms polymers by removing water molecules.

• Hydrolysis breaks polymers into monomers by adding water.

Equation for Dehydration Synthesis:

$\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + H_2O$

Equation for Hydrolysis:

$\text{Polymer} + H_2O \rightarrow \text{Monomer}_1 + \text{Monomer}_2$

Carbohydrates

Monosaccharides and Their Properties

• Monosaccharides are the simplest carbohydrates (simple sugars).

• General formula: $C_nH_{2n}O_n$ (e.g., glucose: $C_6H_{12}O_6$).

• Contain a carbonyl group (aldehyde or ketone) and multiple hydroxyl groups.

• Monosaccharides differ by number of carbons and arrangement of functional groups.

• In aqueous solution, monosaccharides often form rings.

Example: Glucose and fructose are both hexoses (6-carbon sugars) but differ in the position of the carbonyl group.

Disaccharides and Polysaccharides

• Disaccharides are formed by joining two monosaccharides via a glycosidic bond (dehydration synthesis).

• Polysaccharides are long chains of monosaccharides; they serve as energy storage (e.g., starch, glycogen) or structural components (e.g., cellulose).

Table: Major Polysaccharides and Their Functions

Polysaccharide	Monomer	Function	Location
Starch	Glucose	Energy storage	Plants
Glycogen	Glucose	Energy storage	Animals
Cellulose	Glucose	Structural	Plant cell walls

Proteins

Amino Acids and Peptide Bonds

• Amino acids are the monomers of proteins. Each has a central carbon, amino group (–NH2), carboxyl group (–COOH), hydrogen atom, and variable R group (side chain).

• There are 20 different amino acids, classified by the properties of their side chains: nonpolar (hydrophobic), polar (hydrophilic), and electrically charged (acidic or basic).

• Peptide bonds form between the amino group of one amino acid and the carboxyl group of another via dehydration synthesis.

Equation for Peptide Bond Formation:

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

Protein Structure

• Primary structure: Linear sequence of amino acids.

• Secondary structure: Local folding into alpha helices and beta pleated sheets, stabilized by hydrogen bonds.

• Tertiary structure: Overall 3D shape of a polypeptide, determined by interactions among side chains (hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bridges).

• Quaternary structure: Association of multiple polypeptide chains (subunits) into a functional protein.

Example: Hemoglobin has quaternary structure, consisting of four polypeptide subunits.

Lipids

Types and Functions of Lipids

• Lipids are hydrophobic molecules, not true polymers. Major types include triglycerides (fats), phospholipids, and steroids.

• Triglycerides are composed of glycerol and three fatty acids, joined by ester bonds.

• Phospholipids have a glycerol backbone, two fatty acids, and a phosphate group; they are amphipathic (hydrophilic head, hydrophobic tails).

• Steroids are lipids with a carbon skeleton of four fused rings (e.g., cholesterol).

Table: Comparison of Major Lipid Types

Lipid Type	Structure	Main Function
Triglyceride	Glycerol + 3 fatty acids	Energy storage
Phospholipid	Glycerol + 2 fatty acids + phosphate	Membrane structure
Steroid	Four fused carbon rings	Hormones, membrane component

Nucleic Acids

Nucleotides and Nucleic Acid Structure

• Nucleic acids (DNA and RNA) are polymers of nucleotides.

• Nucleotide structure: nitrogenous base (purine or pyrimidine), five-carbon sugar (ribose or deoxyribose), and phosphate group.

• Purines: Adenine (A), Guanine (G); Pyrimidines: Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA).

• DNA is double-stranded, with complementary base pairing (A–T, G–C); RNA is usually single-stranded.

Table: Comparison of DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Strands	Double	Single
Main Function	Genetic information storage	Gene expression, catalysis

Base Pairing and DNA Replication

• Base pairing: A–T (2 hydrogen bonds), G–C (3 hydrogen bonds).

• DNA replication is semi-conservative: each new DNA molecule has one old and one new strand.

• Directionality: DNA strands run antiparallel (5' to 3' and 3' to 5').

Equation for Nucleotide Addition:

$\text{Nucleotide}_{n} + \text{Nucleotide}_{n+1} \rightarrow \text{Dinucleotide} + H_2O$

Key Terms to Know

• Hydrocarbon

• Functional group

• Carbohydrate, Monosaccharide, Disaccharide, Polysaccharide

• Starch, Glycogen, Cellulose

• Amino acid, Peptide bond, Protein structure (primary, secondary, tertiary, quaternary)

• Lipid, Fat, Saturated/Unsaturated fat, Triglyceride, Steroid, Phospholipid, Amphipathic

• Nucleotide, Nucleic acid, Nitrogenous base, Purine, Pyrimidine, Phosphate group, Ribose, Deoxyribose

Additional info:

• Some explanations and tables have been expanded for clarity and completeness.

• Key terms list is included for exam preparation.