Biological Macromolecules: Properties, Structures, and Functions

Biological Macromolecules

Introduction

Biological macromolecules are large, complex molecules essential for life. They include carbohydrates, proteins, nucleic acids, and lipids, each with unique properties, structures, and functions. Understanding these molecules is fundamental to General Biology.

Carbohydrates

Overview of Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen. They serve as energy sources and structural components in living organisms.

• Definition: Carbohydrates are sugars and polymers of sugars containing a carbonyl group and multiple hydroxyl groups.

• General Formula: Most carbohydrates have molecular formulas that are multiples of $CH_2O$.

• Function: They provide nutrients and fuel for cells, and can serve as building blocks for amino acids or as monomers for larger carbohydrates.

• Example: Glucose is the most common monosaccharide and is used in cellular respiration.

Types of Carbohydrates

• Monosaccharides: Simple sugars (e.g., glucose, fructose) with the general formula $C_nH_{2n}O_n$.

• Disaccharides: Two monosaccharides joined by covalent bonds (e.g., sucrose = glucose + fructose).

• Polysaccharides: Polymers with many sugars joined via dehydration reactions.

Storage and Structural Polysaccharides

• Storage Polysaccharides:

  • Starch: Polymer of glucose stored in plants.

  • Glycogen: Polymer of glucose stored in animals (liver and muscle cells).

• Structural Polysaccharides:

  • Cellulose: Forms plant cell walls; provides rigidity.

  • Chitin: Forms exoskeleton of arthropods.

Practice Example

• Question: Identify a monosaccharide with 4 carbons.

• Answer: Formula: $C_4H_8O_4$, Name: Threose

Proteins

Overview of Proteins

Proteins are macromolecules made of polypeptides, which are polymers of amino acids. Their three-dimensional shape determines their function in biological systems.

• Composition: Proteins are comprised of carbon, hydrogen, oxygen, nitrogen, and sulfur.

• Structure: Proteins fold into specific 3D shapes; shape determines function.

Amino Acids and Peptide Bonds

• Amino Acids: Each has a unique side chain (R group) that determines its properties.

• Side Chain Properties:

  • Nonpolar (hydrophobic)

  • Polar (hydrophilic)

  • Charged/ionic (hydrophilic)

• Peptide Bond Formation: The carboxyl group of one amino acid bonds to the amino group of another via a dehydration reaction, forming a peptide bond.

Polypeptides and Protein Structure

• Polypeptides: Chains of amino acids linked by peptide bonds; each has a unique sequence and directionality (N-terminus and C-terminus).

• Protein Folding: Polypeptides twist and fold due to R group interactions, forming functional proteins.

Levels of Protein Structure

• Primary Structure: Sequence of amino acids determined by genes.

• Secondary Structure: Coils and folds due to hydrogen bonding (e.g., alpha helix, beta pleated sheet).

• Tertiary Structure: Folding due to interactions between side chains (hydrophobic interactions, disulfide bridges).

• Quaternary Structure: Association of two or more polypeptides (found in some proteins).

Functions of Proteins

• Antibody: Protects the body from disease.

• Enzyme: Catalyzes chemical reactions.

• Messenger: Transmits signals (e.g., hormones).

• Structural: Provides structure and support.

• Transport/Storage: Binds and carries small molecules.

Practice FRQ Example

• Question: How can deletion of three nucleotides in the CFTR gene result in cystic fibrosis?

• Explanation: The deletion alters the primary structure of the protein, affecting its folding and function, which disrupts ion channel regulation in epithelial tissue.

Nucleic Acids

Overview of Nucleic Acids

Nucleic acids are polymers made of nucleotide monomers. They store, transmit, and express hereditary information.

• Types: Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA)

Structure of Nucleotides

• Components:

  • Nitrogenous base (pyrimidines: cytosine, thymine, uracil; purines: adenine, guanine)

  • Five-carbon sugar (deoxyribose in DNA, ribose in RNA)

  • Phosphate group

• Nucleoside: Portion without phosphate group.

Polynucleotides and DNA/RNA Structure

• Phosphodiester Linkage: Connects adjacent nucleotides, forming the sugar-phosphate backbone.

• Directionality: 5' to 3' end.

• DNA: Double helix of two antiparallel polynucleotide strands held by hydrogen bonds (A-T, G-C).

• RNA: Single-stranded, variable in shape; base pairing (A-U, G-C).

Practice Example

• Question: Given DNA segment 5'-CATGTCAAC-3', what is the complementary strand?

• Answer: 3'-GTACAGTTG-5'

Lipids

Overview of Lipids

Lipids are hydrophobic molecules that do not form true polymers. They are important for energy storage, membrane structure, and signaling.

• Types: Fats, phospholipids, steroids

• Properties: Nonpolar, hydrophobic

Fats

• Composition: Glycerol (an alcohol) and fatty acids (long carbon chains with carboxyl group)

• Bond: Fatty acids join to glycerol via ester linkage (bond between hydroxyl and carboxyl group)

• Saturated Fatty Acids: No double bonds; saturated with hydrogen

• Unsaturated Fatty Acids: One or more double bonds

Phospholipids

• Structure: Two fatty acids, glycerol, and a phosphate group

• Function: Major component of cell membranes; assemble as a bilayer in water (hydrophobic tails, hydrophilic heads)

Steroids

• Structure: Four fused rings with unique groups attached

• Example: Testosterone

Summary Table: Macromolecule Comparison

Macromolecule	Elements Present	Monomer	Polymer
Carbohydrates	Carbon, hydrogen, oxygen	Monosaccharide	Polysaccharide
Proteins	Carbon, hydrogen, oxygen, nitrogen, sulfur	Amino acid	Polypeptide
Lipids	Carbon, hydrogen, oxygen (sometimes phosphorus for phospholipids)	Glycerol and fatty acids	Does not form true polymers
Nucleic Acids	Carbon, hydrogen, oxygen, nitrogen, phosphorus	Nucleotide	DNA, RNA

Additional info: Academic context and explanations have been expanded for clarity and completeness. All major macromolecule classes relevant to General Biology are covered.