The Chemistry of the Cell

Introduction

This chapter explores the fundamental chemical principles underlying cell biology, focusing on the properties of carbon, water, biological membranes, and macromolecules. Understanding these concepts is essential for grasping the molecular basis of cellular structure and function.

Bonding Properties of the Carbon Atom

Valence and Bonding Capacity

• Carbon has a valence of 4, allowing it to form four covalent bonds with other atoms.

• Carbon atoms can form stable covalent bonds with other carbon atoms and with oxygen (O), hydrogen (H), nitrogen (N), and sulfur (S).

Example: Carbon forms the backbone of organic molecules such as carbohydrates, proteins, lipids, and nucleic acids.

Covalent Bonding of Carbon Atoms

• Covalent bonds involve the sharing of a pair of electrons between two atoms.

• Single, double, and triple bonds can form between carbon atoms, affecting molecular shape and reactivity.

Example: Methane (CH4) has single bonds, while ethylene (C2H4) has a double bond between carbons.

Double and Triple Bonds

• Double and triple bonds involve two or three pairs of electrons shared between atoms, respectively.

• These bonds are shorter and stronger than single bonds.

Example: Ethylene (C2H4) and acetylene (C2H2).

Stability of Carbon-Containing Molecules

• Carbon-carbon single bond: ~83 kcal/mol

• Carbon-carbon double bond: ~146 kcal/mol

• Carbon-hydrogen bond: ~99 kcal/mol

• These high bond energies contribute to the stability of organic molecules.

Strong Covalent Bonds and Life

• Strong covalent bonds are necessary to withstand the energy of solar radiation.

• Bond strength is inversely related to bond length; shorter bonds are stronger.

• High bond energies protect biomolecules from being broken by visible or UV light.

Carbon-Containing Molecules

Diversity of Hydrocarbons

• Hydrocarbons are chains or rings composed only of carbon and hydrogen.

• They serve as the framework for more complex biological molecules.

Functional Groups in Biological Molecules

• Functional groups are specific groupings of atoms within molecules that confer characteristic chemical properties.

• Common functional groups include hydroxyl (-OH), carboxyl (-COOH), amino (-NH2), phosphate (-PO4), and sulfhydryl (-SH).

Stereoisomers of Biological Molecules

• An asymmetric carbon atom is attached to four different atoms or groups, leading to stereoisomerism.

• Enantiomers are mirror-image isomers; only one form is usually biologically active.

Example: L-amino acids are used in proteins, while D-amino acids are rare in nature.

The Importance of Water

Water as the Most Abundant Cellular Component

• Water is the most abundant molecule in cells and organisms.

• Its most critical attribute is polarity, which underlies its unique properties.

• Key properties: cohesiveness, temperature-stabilizing capacity, and solvent properties.

Polarity of Water Molecules

• Water has an unequal distribution of electrons, making it a polar molecule.

• Partial negative charge near the oxygen atom, partial positive charge near the hydrogen atoms.

Cohesion of Water Molecules

• Water molecules form hydrogen bonds with each other due to polarity.

• Hydrogen bonds are weak individually but strong collectively, leading to high surface tension and boiling point.

Hydrogen Bonds and Cohesiveness

• Hydrogen bonding gives water its high surface tension, boiling point, specific heat, and heat of vaporization.

Water as a Universal Solvent

• Water dissolves a wide variety of substances due to its polarity.

• It forms hydration shells around ions and polar molecules, facilitating their dissolution.

Biological Membranes

Selective Permeability of Membranes

• Membranes create a barrier between the cell and its environment.

• They are selectively permeable, allowing some substances to cross while restricting others.

Amphipathic Nature of Membrane Phospholipids

• Phospholipids have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

• This amphipathic nature drives the formation of the lipid bilayer.

Lipid Bilayer as the Basis of Membrane

• The hydrophobic tails of phospholipids face inward, away from water, while hydrophilic heads face outward.

• This arrangement forms the lipid bilayer, the fundamental structure of biological membranes.

Selective Permeability of Lipid Bilayers

• Lipid bilayers allow small, nonpolar molecules to pass freely, while restricting ions and large polar molecules.

• Transport proteins facilitate the movement of specific substances across the membrane.

Macromolecules in Cells

Critical Role of Macromolecules

• Macromolecules are essential for cellular structure and function.

• They include proteins, nucleic acids, and polysaccharides.

Synthesis of Biological Macromolecules

• Macromolecules are synthesized by the stepwise polymerization of monomers.

• Each type of macromolecule is composed of specific monomers: amino acids (proteins), nucleotides (nucleic acids), monosaccharides (polysaccharides).

Types of Macromolecular Polymers

• Cells contain three major types of macromolecular polymers: proteins, nucleic acids, and polysaccharides.

• Proteins and nucleic acids have a wide variety of monomers, while polysaccharides have fewer types.

Stepwise Polymerization of Monomers

• Monomers are added one at a time to a growing polymer chain through condensation (dehydration) reactions.

• This process is catalyzed by specific enzymes.

Self-Assembly of Macromolecules

• Many macromolecules spontaneously fold into their functional forms without external guidance, a process called self-assembly.

• Proteins called molecular chaperones may assist in proper folding.

Example: The tobacco mosaic virus (TMV) assembles spontaneously from its RNA and protein subunits.

Noncovalent Bonds in Macromolecular Folding

• Noncovalent interactions, including hydrogen bonds, ionic bonds, van der Waals interactions, and hydrophobic interactions, are crucial for the folding and stability of macromolecules.

Spontaneity of Protein Folding

• Protein folding is driven by the sequence of amino acids and stabilized by noncovalent interactions.

• Proper folding is essential for biological activity.