Biological Macromolecules: Structure, Function, and Synthesis

Biological Macromolecules

Introduction

Biological macromolecules are large, complex molecules essential for life. They are constructed from smaller organic molecules and play critical roles in cell structure, function, and information storage. The four main classes of macromolecules are carbohydrates, lipids, proteins, and nucleic acids.

Synthesis and Breakdown of Polymers

Polymer Formation and Degradation

• Polymers are long molecules consisting of many similar or identical building blocks called monomers.

• Cells use two main chemical reactions to assemble and disassemble polymers:

  • Dehydration (Condensation) Reaction: Monomers are covalently bonded together with the removal of a water molecule.

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

• These mechanisms are common to all classes of macromolecules.

Carbohydrates

Monosaccharides, Disaccharides, and Polysaccharides

• Carbohydrates serve as fuel and building material.

• The simplest carbohydrates are monosaccharides (simple sugars).

• Disaccharides are composed of two monosaccharides joined by a glycosidic linkage.

• Polysaccharides are polymers of many monosaccharides joined by glycosidic linkages.

• Monosaccharides are classified by the number of carbons (e.g., triose, pentose, hexose) and the location of the carbonyl group (aldose or ketose).

• In aqueous solutions, many sugars form ring structures.

Polysaccharide Structure and Function

• Polysaccharides have two main roles:

  • Storage: Starch (plants), Glycogen (animals)

  • Structure: Cellulose (plants), Chitin (arthropods and fungi)

• Starch is a storage polysaccharide in plants, composed entirely of glucose monomers joined mainly by α(1→4) glycosidic linkages.

• Two forms of starch:

  • Amylose: Unbranched, helical structure

  • Amylopectin: Branched, with α(1→6) linkages at branch points

• Glycogen is the storage polysaccharide in animals, similar to amylopectin but more highly branched. Stored mainly in the liver and muscles.

• Cellulose is a structural polysaccharide in plant cell walls, composed of glucose monomers joined by β(1→4) linkages, resulting in straight, unbranched chains that form strong fibers.

• Chitin is a structural polysaccharide found in the exoskeletons of arthropods and fungal cell walls. It is similar to cellulose but has a nitrogen-containing appendage on each glucose monomer.

Comparison of Major Polysaccharides

Polysaccharide	Monomer	Linkage	Function	Organism
Starch	Glucose (α)	α(1→4), α(1→6)	Energy storage	Plants
Glycogen	Glucose (α)	α(1→4), α(1→6) (more branched)	Energy storage	Animals
Cellulose	Glucose (β)	β(1→4)	Structural (cell wall)	Plants
Chitin	Glucosamine (modified glucose)	β(1→4)	Structural (exoskeleton, cell wall)	Arthropods, fungi

Lipids

General Properties and Types

• Lipids are a diverse group of hydrophobic molecules that do not form true polymers.

• Main types of lipids:

  • Fats (triglycerides)

  • Phospholipids

  • Steroids

Fats (Triglycerides)

• Constructed from glycerol and fatty acids.

• Three fatty acids are joined to glycerol by ester linkages to form a triacylglycerol (triglyceride).

• Saturated fatty acids: No double bonds between carbon atoms; usually solid at room temperature.

• Unsaturated fatty acids: One or more double bonds; usually liquid at room temperature. Double bonds cause kinks in the hydrocarbon chain.

Phospholipids

• Major components of cell membranes.

• Composed of two fatty acids and a phosphate group attached to glycerol.

• Amphipathic: hydrophilic (phosphate head) and hydrophobic (fatty acid tails) regions.

• Form bilayers in aqueous environments, fundamental to membrane structure.

Steroids

• Lipids with a carbon skeleton consisting of four fused rings.

• Examples: Cholesterol (component of animal cell membranes, precursor to other steroids), steroid hormones (e.g., testosterone, estrogen).

Proteins

Amino Acids and Peptide Bonds

• Proteins are polymers of amino acids, accounting for more than 50% of the dry mass of most cells.

• Each amino acid has a central (α) carbon attached to:

  • Amino group (—NH2)

  • Carboxyl group (—COOH)

  • Hydrogen atom

  • R group (side chain, variable)

• Amino acids are classified by the properties of their R groups: nonpolar (hydrophobic), polar (hydrophilic), or charged (acidic/basic).

• Amino acids are joined by peptide bonds formed via dehydration reactions, creating polypeptide chains.

• Polypeptides have directionality: an N-terminus (free amino group) and a C-terminus (free carboxyl group).

Levels of Protein Structure

• Primary structure: Unique sequence of amino acids.

• Secondary structure: Coiling or folding due to hydrogen bonds between backbone atoms. Main types:

  • α-helix

  • β-pleated sheet

• Tertiary structure: Overall 3D shape stabilized by interactions among R groups (hydrophobic interactions, hydrogen bonds, ionic bonds, disulfide bridges).

• Quaternary structure: Association of two or more polypeptide chains.

Protein Structure and Environmental Effects

• Protein structure depends on physical and chemical conditions (pH, salt concentration, temperature, solvents).

• Alterations can cause denaturation, leading to loss of function.

Nucleic Acids

Structure and Function

• Two types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• Nucleic acids store, transmit, and express hereditary information.

• Organisms inherit DNA from their parents; genes direct synthesis of messenger RNA (mRNA), which controls protein synthesis.

• The flow of genetic information: DNA → RNA → Protein

Nucleotide Structure

• Nucleic acids are polymers of nucleotides (polynucleotides).

• Each nucleotide consists of:

  • Nitrogenous base (purine or pyrimidine)

  • Pentose sugar (ribose in RNA, deoxyribose in DNA)

  • Phosphate group

• Nitrogenous bases:

  • Pyrimidines: Cytosine (C), Thymine (T, DNA only), Uracil (U, RNA only)

  • Purines: Adenine (A), Guanine (G)

• Nucleotides are joined by phosphodiester linkages between the 3' hydroxyl and 5' phosphate of adjacent nucleotides.

• Polynucleotides have directionality: 5' end (phosphate) and 3' end (hydroxyl).

DNA and RNA Structure

• DNA: Double helix with two antiparallel strands held together by hydrogen bonds between complementary bases (A-T, G-C).

• RNA: Usually single-stranded.

• Base pairing enables accurate replication and transmission of genetic information.

Summary Table: DNA vs. RNA

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

Key Equations and Concepts

• Dehydration Reaction (General):

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

• Hydrolysis Reaction (General):

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

• Peptide Bond Formation:

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

• Phosphodiester Bond Formation:

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

Conclusion

Understanding the structure and function of biological macromolecules is fundamental to biology. Their diverse forms and interactions underpin all cellular processes, from energy storage and structural support to information storage and transmission.