Structure and Function of Macromolecules: Foundations of Cell Biology

Structure and Function of Macromolecules

Introduction

Macromolecules are large, complex molecules essential for life, including proteins, carbohydrates, nucleic acids, and lipids. Their structure and function underpin cellular processes and biological systems. Understanding their chemical foundations is crucial for cell biology.

Carbon - The Backbone of Life

Importance of Carbon in Biological Molecules

Carbon is the central element in organic molecules due to its unique chemical properties.

• Abundance: While not the most abundant element in Earth's crust, carbon is predominant in living organisms.

• Organic vs. Inorganic Compounds: Organic compounds contain carbon-hydrogen (C-H) bonds, whereas inorganic compounds may lack these bonds (e.g., carbon dioxide).

• Versatility: Carbon forms large, complex, and diverse molecules, serving as the backbone for proteins, DNA, carbohydrates, and lipids.

Properties of Carbon

• Bonding: Carbon has four valence electrons, allowing it to form single, double, and triple bonds with other atoms.

• Molecular Diversity: Carbon can create a vast array of molecules by varying the length, branching, and ring structure of its skeleton.

• Polarity: Carbon forms both polar and nonpolar bonds, contributing to molecular diversity.

Examples of Carbon Molecules

• Diamond: Each carbon atom is bonded to four others in a tetrahedral structure, resulting in a hard, crystalline material.

• Graphite: Carbon atoms are arranged in layers of hexagonal rings, making it soft and slippery.

• Fullerenes and Nanotubes: Carbon can form spherical or tubular structures with unique properties.

Functional Groups in Organic Molecules

Role of Functional Groups

Functional groups are specific groups of atoms attached to the carbon skeleton that confer distinct chemical properties and reactivity to organic molecules.

• Key Functional Groups: Hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, phosphate, and methyl groups.

• Biological Impact: Small changes in functional groups can lead to significant differences in biological activity (e.g., estrogen vs. testosterone).

Macromolecules: Polymers and Monomers

Definition and Formation

Macromolecules are polymers, large molecules made by joining smaller units called monomers.

• Monomers: The basic molecular units (e.g., glucose, amino acids, nucleotides).

• Polymerization: Monomers are linked by condensation (dehydration) reactions, which release water.

• Enzymes: Biological catalysts that speed up polymerization and hydrolysis reactions.

Breakdown of Polymers

• Hydrolysis: Polymers are disassembled into monomers by adding water, essentially the reverse of dehydration synthesis.

Major Classes of Macromolecules

Overview

There are four major groups of macromolecules in cells:

• Proteins

• Carbohydrates

• Nucleic acids

• Lipids

Three of these (proteins, carbohydrates, nucleic acids) are true polymers; lipids are not polymers but are large biomolecules.

Carbohydrates

Structure and Function

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically in a ratio of 1:2:1 (CH2O).

• Monosaccharides: The simplest carbohydrates (e.g., glucose, fructose).

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

• Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).

Biological Roles

• Energy Source: Carbohydrates are broken down into glucose, which is used to produce ATP.

• Storage: Glucose is stored as glycogen in animals and starch in plants.

• Structural: Cellulose provides structural support in plant cell walls.

Comparison of Polysaccharides

Polysaccharide	Monomer	Linkage	Function
Starch	Glucose	α-1,4 and α-1,6	Energy storage in plants
Glycogen	Glucose	α-1,4 and α-1,6	Energy storage in animals
Cellulose	Glucose	β-1,4	Structural support in plants

Lipids

Structure and Function

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

• Fats (Triglycerides): Composed of glycerol and three fatty acids joined by ester bonds.

• Phospholipids: Major component of cell membranes; consist of two fatty acids, a phosphate group, and glycerol. Amphipathic (hydrophilic head, hydrophobic tails).

• Steroids: Lipids with four fused rings (e.g., cholesterol, hormones).

Saturated vs. Unsaturated Fats

Type	Structure	State at Room Temp	Source
Saturated	No double bonds	Solid	Animal fats
Unsaturated	One or more double bonds	Liquid	Plant and fish fats

Phospholipids and Membranes

• Phospholipid Bilayer: Forms the basic structure of cell membranes, with hydrophilic heads facing outward and hydrophobic tails inward.

• Fluid Mosaic Model: Membranes are dynamic structures with proteins embedded in a phospholipid bilayer.

Cholesterol

• Role: Modulates membrane fluidity and stability.

• Structure: Four fused rings; precursor for steroid hormones.

• Homeostasis: Plants lack cholesterol but regulate membrane fluidity by altering fatty acid composition.

Proteins

Structure and Function

Proteins are polymers of amino acids and account for more than 50% of the dry mass of most cells. They perform a wide range of functions, including catalysis, transport, structural support, and regulation.

• Amino Acids: Organic molecules with amino and carboxyl groups; 20 types found in proteins.

• Polypeptides: Chains of amino acids linked by peptide bonds.

Levels of Protein Structure

• Primary Structure: Sequence of amino acids.

• Secondary Structure: Local folding into α-helices and β-sheets via hydrogen bonding.

• Tertiary Structure: Overall 3D shape due to interactions among R groups (hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bridges).

• Quaternary Structure: Association of multiple polypeptide chains (e.g., hemoglobin, collagen).

Protein Function and Disease

• Conformation: Determines protein function.

• Genetic Mutations: Changes in primary structure can lead to diseases (e.g., sickle-cell anemia caused by a single amino acid substitution).

Nucleic Acids

Structure and Function

Nucleic acids (DNA and RNA) store and transmit genetic information. They are polymers of nucleotides.

• Nucleotide Structure: Each nucleotide consists of a phosphate group, a pentose sugar (ribose or deoxyribose), and a nitrogenous base.

• Nitrogenous Bases:

  • Pyrimidines: Cytosine, Thymine (DNA), Uracil (RNA)

  • Purines: Adenine, Guanine

• DNA: Double-stranded helix, antiparallel strands, complementary base pairing (A-T, C-G).

• RNA: Single-stranded, contains uracil instead of thymine.

Complementary Base Pairing

Base	Pairs With
Adenine (A)	Thymine (T) [DNA] / Uracil (U) [RNA]
Cytosine (C)	Guanine (G)

DNA Double Helix

• Antiparallel Structure: Strands run in opposite directions (5' to 3' and 3' to 5').

• Helix Dimensions: Diameter ≈ 2 nm; pitch per turn ≈ 3.4 nm.

• Hydrogen Bonds: Hold complementary bases together.

Functions of Nucleic Acids

• DNA: Stores genetic information, directs its own replication, and controls protein synthesis via mRNA.

• RNA: Functions in gene expression, regulation, and as part of the ribosome (rRNA).

• ATP: Adenosine triphosphate is a special nucleotide that stores energy in phosphate bonds. $\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Summary Table: Major Macromolecules

Macromolecule	Monomer	Bond Type	Main Functions
Carbohydrates	Monosaccharide	Glycosidic	Energy, structure
Lipids	Fatty acid, glycerol	Ester	Energy storage, membranes
Proteins	Amino acid	Peptide	Catalysis, structure, transport
Nucleic acids	Nucleotide	Phosphodiester	Genetic information

Additional info: Some explanations and tables have been expanded for clarity and completeness based on standard cell biology knowledge.