Chapter 2: The Chemistry of the Cell

Bonding Properties of the Carbon Atom

Carbon is the fundamental element in biological molecules due to its unique bonding properties. Its ability to form four covalent bonds allows for a vast diversity of stable organic compounds.

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

• Bonding Partners: Carbon commonly bonds with hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S).

• Structural Diversity: Carbon can form chains, branched molecules, and rings, providing the backbone for complex biological molecules.

• Example: Methane (CH4) is the simplest organic molecule, with carbon bonded to four hydrogens.

Covalent Bonding of Carbon Atoms

Covalent bonds are the strongest type of chemical bond in biological systems, involving the sharing of electron pairs between atoms.

• Covalent Bond: A bond formed by the sharing of a pair of electrons between two atoms.

• Single Bond: Sharing one pair of electrons forms a single covalent bond.

• Example: The carbon-carbon single bond in ethane (C2H6).

Double and Triple Bonds in Simple Molecules

Carbon atoms can also form double and triple bonds, which involve the sharing of two or three pairs of electrons, respectively. These bonds affect the geometry and reactivity of molecules.

• Double Bond: Two pairs of electrons are shared (e.g., ethylene, C2H4).

• Triple Bond: Three pairs of electrons are shared (e.g., acetylene, C2H2).

• Bond Strength: Double and triple bonds are stronger and shorter than single bonds.

Stability of Carbon-Containing Molecules

Carbon-containing molecules are stable due to the strength of their covalent bonds. The energy required to break these bonds is significant, contributing to the stability of biological molecules.

• Bond Energies: The energy required to break a bond (measured in kilocalories per mole, kcal/mol).

• Examples of Bond Energies:

Strong Covalent Bonds Necessary for Life

Strong covalent bonds are essential for the stability of biological molecules, especially in the presence of solar radiation. The energy of visible light is not sufficient to break these bonds, protecting biomolecules from damage.

• Inverse Relationship: Wavelength and bond energy are inversely related.

• Protection: Strong covalent bonds prevent breakdown by visible and UV light.

Diversity of Carbon-Containing Molecules

Hydrocarbons, composed only of carbon and hydrogen, can form chains or rings. They serve as the foundation for more complex molecules when functional groups are added.

• Hydrocarbons: Molecules with only C and H atoms.

• Structural Variability: Linear, branched, or cyclic forms.

Functional Groups in Biological Molecules

Functional groups are specific groups of atoms within molecules that confer characteristic chemical properties and reactivity.

• Common Functional Groups: Hydroxyl (-OH), carboxyl (-COOH), amino (-NH2), phosphate (-PO4), carbonyl (C=O), and sulfhydryl (-SH).

• Role: Determine the chemical behavior of molecules.

Stereoisomers of Biological Molecules

Stereoisomers are molecules with the same molecular formula and sequence of bonded atoms, but different three-dimensional orientations. This is especially important in biology, as many biomolecules are chiral.

• Asymmetric Carbon: A carbon atom bonded to four different groups, leading to chirality.

• Enantiomers: Non-superimposable mirror images.

• Biological Specificity: Only one enantiomer is usually biologically active.

The Importance of Water in Biology

The Importance of Water

Water is the most abundant component of cells and organisms, playing a critical role in their structure and function.

• Polarity: Water's polarity is its most important attribute, leading to unique properties.

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

Polarity of Water Molecules

Water molecules have an unequal distribution of electrons, resulting in a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms.

• Dipole Moment: The molecule is bent, with a net dipole moment.

• Hydrogen Bonding: The polarity allows water molecules to form hydrogen bonds with each other and with other polar molecules.

Cohesion of Water Molecules

Water molecules are cohesive due to hydrogen bonding, which gives rise to surface tension and other unique properties.

• Hydrogen Bonds: Weak associations between the hydrogen atom of one water molecule and the oxygen atom of another.

• Biological Importance: Cohesion is critical for processes like water transport in plants.

Hydrogen Bonds and Cohesiveness

The collective effect of many hydrogen bonds accounts for water's high surface tension, boiling point, specific heat, and heat of vaporization.

• Surface Tension: Water molecules at the surface are pulled inward, creating a 'skin'.

• High Specific Heat: Water can absorb a lot of heat before changing temperature.

Water as a Universal Solvent

Water's polarity allows it to dissolve a wide variety of substances, making it the universal solvent in biological systems.

• Solvation: Water surrounds ions and polar molecules, facilitating chemical reactions.

• Example: Dissolving salts and sugars in the cytoplasm.

The Importance of Selectively Permeable Membranes

Cell membranes create a physical barrier between the cell's interior and the external environment, controlling the movement of substances in and out of the cell.

• Selective Permeability: Allows some molecules to pass while restricting others.

• Function: Maintains homeostasis and protects cellular contents.

Membrane Structure and Function

The Amphipathic Nature of Membrane Phospholipids

Phospholipids are the main component of cellular membranes, possessing both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

• Structure: Consist of a hydrophilic head and two hydrophobic fatty acid tails.

• Amphipathic: Molecules with both polar and nonpolar regions.

The Lipid Bilayer as the Basis of Membrane

Phospholipids spontaneously arrange into a bilayer in aqueous environments, with hydrophobic tails facing inward and hydrophilic heads facing outward.

• Lipid Bilayer: The fundamental structure of all biological membranes.

• Fluid Mosaic Model: Describes the dynamic and flexible nature of the membrane.

Lipid Bilayers Are Selectively Permeable

The lipid bilayer allows selective passage of certain molecules while blocking others, based on size, polarity, and charge.

• Permeable: Small, nonpolar molecules (e.g., O2, CO2).

• Impermeable: Ions and large polar molecules (e.g., glucose, Na+).

Macromolecules: Structure and Assembly

Macromolecules Are Critical for Cellular Form and Function

Cells are composed of large macromolecules that determine their structure and function. These include proteins, nucleic acids, and polysaccharides.

• Hierarchical Structure: Macromolecules assemble into larger complexes and organelles.

The Synthesis of Biological Macromolecules

Macromolecules are synthesized by the stepwise polymerization of monomers, a process that is highly regulated and specific.

• Monomers: Building blocks such as amino acids, nucleotides, and monosaccharides.

• Polymerization: Monomers are joined by covalent bonds to form polymers.

Three Kinds of Macromolecular Polymers in Cells

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

• Proteins: Polymers of amino acids, perform a wide variety of functions.

• Nucleic Acids: Polymers of nucleotides, store and transmit genetic information.

• Polysaccharides: Polymers of sugars, serve as energy storage and structural components.

Stepwise Polymerization of Monomers

Macromolecules are synthesized by the sequential addition of monomers, usually involving the removal of a water molecule (condensation reaction).

• Directionality: Polymers have a defined direction (e.g., N-terminus to C-terminus in proteins).

• Enzymatic Control: Enzymes catalyze the polymerization process.

The Importance of Self-Assembly

Many biological structures form spontaneously by self-assembly, where the information for folding and assembly is inherent in the molecules themselves.

• Self-Assembly: The process by which molecules adopt a defined arrangement without guidance or management from an outside source.

• Example: Protein folding and virus assembly.

Case Study: The Tobacco Mosaic Virus (TMV) and Self-Assembly

The TMV is a classic example of self-assembly, where the viral RNA and protein subunits spontaneously form a functional virus particle.

• Spontaneous Assembly: Occurs without external direction, driven by the properties of the components.

Noncovalent Bonds and Interactions in Macromolecules

Noncovalent interactions are crucial for the folding and stability of macromolecules, including proteins and nucleic acids.

• Types of Noncovalent Interactions:

  • Hydrogen bonds

  • Ionic bonds

  • Van der Waals interactions

  • Hydrophobic interactions

• Role: Stabilize three-dimensional structures and enable molecular recognition.

The Spontaneity of Protein Folding

Protein folding is a spontaneous process driven by the chemical properties of amino acids and the surrounding environment, resulting in a functional three-dimensional structure.

• Folding Pathways: Determined by the sequence of amino acids (primary structure).

• Chaperones: Some proteins require molecular chaperones to fold correctly.