The Chemistry of the Cell: Structure, Bonds, Water, and Macromolecules

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The Chemistry of the Cell

Introduction

This chapter explores the chemical foundations of cell biology, focusing on the unique properties of carbon, water, biological membranes, and the synthesis of macromolecules. Understanding these principles is essential for grasping how cells function at the molecular level.

Characteristics of Carbon

Valence and Bonding Properties

Carbon has a valence of 4, allowing it to form four covalent bonds with other atoms, including hydrogen, oxygen, nitrogen, and sulfur.
This versatility enables the formation of a wide variety of stable organic molecules essential for life.

Biologically important atoms and their valences

Covalent Bonds

Single bonds: One pair of electrons shared (e.g., C–H, C–C).
Double bonds: Two pairs of electrons shared (e.g., C═C, C═O).
Triple bonds: Three pairs of electrons shared (e.g., C≡C, C≡N).

Simple organic molecules with single bonds Simple molecules with double bonds Simple molecules with triple bonds

Bond Strength and Stability

Double and triple bonds are more stable and have higher bond energies than single bonds.
Typical bond energies (kcal/mol): C—C: 83, C—N: 70, C—O: 84, C—H: 99, C═C: 146, C≡C: 212
High bond energies make organic molecules stable under physiological conditions.

Covalent bond energies compared to hydrogen bonds and thermal energy

Effect of UV Light on Covalent Bonds

Ultraviolet (UV) light has enough energy to break covalent bonds, which can damage biological molecules such as DNA.
Visible and infrared light do not have sufficient energy to break these bonds.

Bond energy and electromagnetic spectrum

Hydrocarbons and Functional Groups

Hydrocarbons are chains or rings composed only of carbon and hydrogen.
Biological molecules often contain functional groups (e.g., hydroxyl, carboxyl, amino, phosphate) that confer specific chemical properties.

Examples of hydrocarbon structures Common functional groups in biological molecules

Stereoisomers

Stereoisomers are molecules with the same molecular formula but different spatial arrangements of atoms.
Asymmetric (chiral) carbons lead to nonsuperimposable mirror images (enantiomers).
The number of possible stereoisomers is 2n, where n is the number of asymmetric carbons.

Chirality and stereoisomers Examples of D- and L- amino acids and glucose

Characteristics of Water

Polarity and Hydrogen Bonding

Water is a polar molecule with a bent shape and partial charges on oxygen (δ–) and hydrogen (δ+).
Hydrogen bonds form between water molecules, giving water unique properties.

Polarity of water molecule Hydrogen bonding between water molecules

Cohesion, Surface Tension, and Temperature Stabilization

Water molecules are cohesive due to hydrogen bonding, resulting in high surface tension.
Water has a high heat of vaporization and boiling point, stabilizing temperature in cells and organisms.

Surface tension demonstrated by a water strider

Water as a Solvent

Water dissolves many substances due to its polarity, making it the universal solvent in biology.
Hydrophilic (water-loving) substances dissolve easily; hydrophobic (water-fearing) substances do not.
Ions and polar molecules are surrounded by spheres of hydration when dissolved in water.

Hydration of sodium and chloride ions

Selectively Permeable Membranes

Membrane Structure and Composition

Biological membranes are composed of phospholipids, glycolipids, membrane proteins, and sterols (cholesterol in animals, ergosterols in fungi, phytosterols in plants).
Phospholipids are amphipathic molecules with hydrophilic heads and hydrophobic tails, forming a lipid bilayer.

Phospholipid structure and amphipathic nature Lipid bilayer structure

Membrane Permeability

Nonpolar molecules (e.g., O2, CO2) can cross membranes easily.
Most polar molecules and ions are impermeable and require transport proteins.
Small uncharged molecules (e.g., H2O, ethanol) can cross to some extent.

Membrane permeability to different molecules

Transport Across Membranes

Ions and large polar molecules require transporters (channels or carriers) to cross the membrane.
Transport proteins provide hydrophilic pathways or actively move substances against gradients.

Types of membrane transport proteins

Synthesis by Polymerization of Small Molecules

Macromolecules and Polymerization

Cells synthesize macromolecules (proteins, nucleic acids, polysaccharides, lipids) by polymerizing small organic monomers.
Polymerization involves condensation reactions (removal of water) and requires energy (often from ATP).
Monomers are often activated by coupling to carrier molecules (e.g., tRNA, ADP, UDP).
Macromolecules have directionality (distinct ends).

Overview of macromolecule synthesis Pathways from small molecules to macromolecules

Types of Macromolecules

Nucleic acids: Polymers of nucleotides; store and transmit genetic information.
Proteins: Polymers of amino acids; perform a wide variety of cellular functions.
Polysaccharides: Polymers of monosaccharides; serve as energy storage and structural components.

Nucleic acid structure Protein structure

Mechanism of Polymerization

Monomers are activated (often by ATP) and attached to carrier molecules.
Condensation reactions join monomers, releasing water.
Hydrolysis reactions break polymers into monomers by adding water.

Steps in polymerization: activation, condensation, and elongation

Carrier Molecules

tRNA (for amino acids), ADP/UDP (for sugars), ATP/GTP (for nucleotides) act as carriers during biosynthesis.

Carrier molecules in biosynthesis

Self-Assembly and Protein Folding

Forces in Protein Folding

Protein folding is driven by hydrogen bonds, ionic bonds, van der Waals interactions, and hydrophobic interactions.
Hydrophobic groups cluster in the interior, while hydrophilic groups face the aqueous environment.

Forces in protein folding Ionic bond formation Van der Waals interactions Hydrophobic interactions in proteins

Protein Folding and Chaperones

Proteins can fold spontaneously into their functional conformations.
Denaturation: Unfolding of proteins, leading to loss of function.
Renaturation: Refolding into the correct structure, restoring function.
Chaperones: Specialized proteins that assist in the proper folding of other proteins.

Denaturation and renaturation of proteins Chaperone-assisted protein folding

Concept Checks and Applications

Key Questions

Amphipathic molecules are important for membrane structure because they have both hydrophilic and hydrophobic regions, allowing them to form bilayers.
Membranes are bilayers (not monolayers) to provide a stable barrier between aqueous environments inside and outside the cell.
Selectively permeable membranes allow some substances to cross while blocking others, maintaining cellular homeostasis.
To deliver a large, polar anti-cancer drug into cells, strategies such as encapsulation in liposomes, conjugation to cell-penetrating peptides, or use of transporter proteins can be considered.

Experimental Observations and Conclusions

Mixing RNA and coat protein from the same TMV strain forms infectious virions, indicating self-assembly and specificity.
Mixing RNA from strain A with coat protein from strain B yields strain A virus, showing that RNA determines viral identity.
Coat protein monomers can self-assemble into helical structures even without RNA, demonstrating the intrinsic self-assembly properties of proteins.