Water, Energy, and the Chemistry of Life: Foundations for General Biology

Water, Energy, and the Chemistry of Life

Introduction

This section introduces the foundational chemical principles underlying biological systems, focusing on the role of water, energy, and organic molecules in life. Understanding these concepts is essential for studying cell structure, function, and the molecular basis of life.

Structure Defines Function

Central Theme in Biology

• Structure defines function: The shape and composition of biological molecules determine their roles in cells and organisms.

• Water defines structure: Water's unique properties influence the structure and behavior of biomolecules.

The Chemical Evolution Hypothesis

Origin of Life

• Chemical evolution: Life began with the formation of increasingly complex carbon-based molecules from simple precursors.

• Self-replicating systems: Once a chemical system could replicate itself, biological evolution could begin.

• Prerequisites for life: The emergence of self-replicating molecules and compartmentalization were key steps.

The Importance of Organic Molecules

Definition and Examples

• Organic molecules: Molecules containing carbon atoms bonded to other elements (often hydrogen, oxygen, nitrogen), arranged in chains or rings.

• Examples: Octane (C8H18), Glucose (C6H12O6).

• Significance: Most molecules in living organisms are organic, with carbon as a central element due to its ability to form four covalent bonds.

Functional Groups in Organic Molecules

Key Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic properties and chemical reactivity. They are critical in determining the behavior of organic molecules in biological systems.

Functional Group	Family of Molecules	Properties	Example
Amines (-NH2)	Amines	Acts as a base; tends to attract a proton	Glycine (an amino acid)
Carboxyl (-COOH)	Carboxylic acids	Acts as an acid; tends to lose a proton in solution	Acetic acid
Aldehyde (-CHO)	Aldehydes	Can react with certain compounds to produce larger molecules	Acetaldehyde
Ketone (C=O within chain)	Ketones	Can react with certain compounds to produce larger molecules	Acetone

• Linking names and groups: Recognizing functional groups helps predict molecular behavior and reactivity.

Major Classes of Biological Macromolecules

Polymers and Monomers

• Proteins: Polymers of amino acids.

• Nucleic acids: Polymers of nucleotides.

• Carbohydrates: Polymers of simple sugars (monosaccharides).

• Lipids: Not true polymers; form noncovalent assemblies due to hydrophobic interactions.

• Polymerization: Monomers are joined by covalent bonds (except lipids), often via condensation reactions.

Condensation and Hydrolysis Reactions

• Condensation (dehydration) reaction: Joins two monomers, releasing a molecule of water. Requires energy input.

• Hydrolysis: Breaks a covalent bond by adding water, releasing monomers. Releases energy.

Equation:

$[\text{Monomer}] + [\text{Monomer}] \xrightarrow{\text{condensation}} [\text{Polymer}] + \text{H}_2\text{O}$ $[\text{Polymer}] + \text{H}_2\text{O} \xrightarrow{\text{hydrolysis}} [\text{Monomer}] + [\text{Monomer}]$

• Residues: Monomers incorporated into a polymer (e.g., amino acid residue).

Atomic Structure and the Periodic Table

Atoms and Isotopes

• Atoms: Consist of protons, neutrons, and electrons.

• Atomic number: Number of protons in the nucleus.

• Mass number: Number of protons plus neutrons.

• Isotopes: Atoms of the same element with different numbers of neutrons. Some are radioactive and useful in biological research (e.g., C-14 dating, medical imaging).

Chemical Bonds

Covalent Bonds

• Covalent bond: Atoms share pairs of electrons to fill their valence shells.

• Single, double, triple bonds: Number of shared electron pairs; more shared pairs = stronger bond.

• Molecules: Groups of atoms held together by covalent bonds.

• Compounds: Molecules containing atoms of different elements.

Polar vs. Nonpolar Covalent Bonds

• Electronegativity: Tendency of an atom to attract electrons. Increases across a period and up a group in the periodic table.

• Nonpolar covalent bond: Electrons are shared equally (e.g., H2).

• Polar covalent bond: Electrons are shared unequally, leading to partial charges (e.g., H2O).

Example: In water, oxygen is more electronegative than hydrogen, resulting in a partial negative charge on O and partial positive charges on H.

Ionic Bonds

• Ionic bond: Electrons are transferred from one atom to another, creating oppositely charged ions that attract each other.

• Cation: Positively charged ion (loses electron).

• Anion: Negatively charged ion (gains electron).

Water: Properties and Importance

Polarity and Hydrogen Bonding

• Water is polar: The molecule has a partial negative charge near the oxygen and partial positive charges near the hydrogens.

• Hydrogen bonds: Weak attractions between the partial positive charge of hydrogen in one molecule and the partial negative charge of oxygen or nitrogen in another.

Unique Properties of Water

• Cohesion: Water molecules stick to each other, leading to high surface tension.

• Adhesion: Water molecules stick to other polar substances, enabling capillary action.

• High specific heat: Water absorbs a lot of energy before changing temperature.

• High heat of vaporization: Water requires significant energy to evaporate.

• Density anomaly: Ice is less dense than liquid water due to hydrogen bonding, allowing it to float.

Water as a Solvent

• "Like dissolves like": Polar and charged substances (hydrophilic) dissolve in water; nonpolar substances (hydrophobic) do not.

• Hydrophobic effect: Nonpolar molecules aggregate in water, increasing the entropy of the system.

Acids, Bases, and pH

Definitions

• Acid: Proton (H+) donor.

• Base: Proton acceptor or hydroxide (OH-) donor.

• Water dissociation: $\text{H}_2\text{O} \rightleftharpoons \text{H}^+ + \text{OH}^-$

• pH scale: $\text{pH} = -\log[\text{H}^+]$

• Neutral pH: $[\text{H}^+] = 1.0 \times 10^{-7} \text{M}$, so pH = 7.0

Buffers

• Buffer: Substance that minimizes changes in pH by absorbing or releasing protons.

• Biological importance: Buffers maintain stable pH in cells, blood, and ecosystems (e.g., bicarbonate buffer system).

Example: The bicarbonate buffer system in blood and oceans helps regulate pH and maintain homeostasis.

Summary Table: Functional Groups

Group	Structure	Property	Example
Amino	-NH2	Basic, forms hydrogen bonds	Glycine
Carboxyl	-COOH	Acidic, forms hydrogen bonds	Acetic acid
Hydroxyl	-OH	Polar, forms hydrogen bonds	Ethanol
Phosphate	-PO4	Acidic, energy transfer	ATP
Sulfhydryl	-SH	Forms disulfide bonds	Cysteine

Key Vocabulary

• Potential energy

• Chemical energy

• Kinetic energy

• Thermal energy

• Temperature

• Heat

• Enthalpy (total heat content of a system)

• Entropy (measure of disorder)

Learning Outcomes

• Use key vocabulary related to chemistry of life

• Identify elements and isotopes based on atomic structure

• Describe types of chemical bonds and their strengths

• Relate molecular structure and electrons to chemical and potential energy

• Understand the molecular logic of polymers, condensation, and hydrolysis

• Classify molecules by polarity and solubility

• Describe properties of water and their biological significance

• Explain the relationship between water dissociation, pH, and buffers

• Compare the bicarbonate buffering system in ocean and blood