Chemistry of Life: Atoms, Bonds, and Water in Biology

Chapter 2: Chemistry of Life

Introduction

This chapter explores the fundamental chemical principles that underpin biological processes. Understanding the chemistry of life is essential for grasping how living organisms function, from the atomic level to complex biochemical reactions.

Atoms and Elements

Basic Structure of Atoms

Atoms are the smallest units of matter that retain the properties of an element. Each atom consists of a nucleus containing protons and neutrons, surrounded by electrons in orbitals.

• Protons: Positively charged particles in the nucleus.

• Neutrons: Neutral particles in the nucleus.

• Electrons: Negatively charged particles orbiting the nucleus.

The number of protons defines the element (atomic number).

Major Elements in Biology

Four elements make up 96% of the human body’s weight:

• Oxygen (O)

• Carbon (C)

• Hydrogen (H)

• Nitrogen (N)

Trace elements, such as iodine, are required in small amounts for specific biological functions (e.g., thyroid metabolism).

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons. Some isotopes are radioactive and decay over time, emitting energy that can be detected and used in biological research.

• Stable isotopes: Do not decay.

• Radioactive isotopes: Decay spontaneously, emitting particles and energy.

Electron Configuration and Reactivity

Electron Shells

Electrons are arranged in shells around the nucleus. The outermost shell (valence shell) determines an atom’s chemical reactivity.

• Atoms are stable when their valence shell is full (usually 8 electrons).

• Atoms will gain, lose, or share electrons to achieve stability.

Periodic Table

The periodic table organizes elements by increasing atomic number and groups elements with similar chemical properties.

Chemical Bonds

Types of Chemical Bonds

Chemical bonds are energy relationships among valence electrons. They allow atoms to achieve stability and form molecules essential for life.

• Ionic Bonds: Formed when one atom donates an electron to another, creating charged ions (cations and anions) that attract each other.

• Covalent Bonds: Formed when two atoms share one or more pairs of electrons. Can be single, double, or triple bonds.

• Hydrogen Bonds: Weak attractions between a hydrogen atom (attached to an electronegative atom like oxygen or nitrogen) and another electronegative atom.

Ionic Bonds

• Involve transfer of electrons.

• Result in formation of ions: Na+ (cation), Cl- (anion).

Covalent Bonds

• Non-polar covalent bonds: Equal sharing of electrons (e.g., O2, CO2).

• Polar covalent bonds: Unequal sharing of electrons, resulting in partial charges (e.g., H2O).

Hydrogen Bonds

• Occur between polar molecules.

• Important in water, DNA, and protein structure.

Comparison of Bond Types

Bond Type	Strength	Formation	Example
Ionic	Strong (in dry conditions)	Electron transfer	NaCl
Covalent	Strong	Electron sharing	H2O, O2
Hydrogen	Weak	Attraction between partial charges	Between water molecules

Chemical Reactions

Nature of Chemical Reactions

Chemical reactions involve making and breaking bonds, transforming reactants into products. They are essential for metabolism and energy transfer in cells.

• Exergonic reactions: Release energy (e.g., cellular respiration).

• Endergonic reactions: Require energy input (e.g., photosynthesis).

Example equation for photosynthesis:

$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

Water: The Molecule that Supports Life

Properties of Water

Water is vital for life due to its unique chemical and physical properties, many of which arise from hydrogen bonding.

• Cohesion and Adhesion: Water molecules stick to each other (cohesion) and to other surfaces (adhesion).

• High Specific Heat: Water absorbs and retains heat, moderating temperature changes.

• Low Density of Ice: Ice floats because it is less dense than liquid water.

• Universal Solvent: Water dissolves many substances, facilitating chemical reactions in cells.

Emergent Properties of Water

1. Cohesive/Adhesive Behavior: Enables transport of water in plants and surface tension.

2. Moderation of Temperature: Water’s high specific heat stabilizes environments.

3. Low Density of Ice: Allows aquatic life to survive under ice.

4. Universal Solvent: Facilitates biochemical reactions.

Ocean Acidification

Chemical Causes and Biological Effects

Ocean acidification results from increased CO2 dissolving in seawater, forming carbonic acid and lowering pH. This affects marine organisms, especially those with calcium carbonate shells.

Relevant equation:

$CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow HCO_3^- + H^+$

Summary Table: Key Concepts

Concept	Definition	Example/Application
Atom	Smallest unit of an element	Hydrogen atom
Isotope	Atom with different number of neutrons	Carbon-14
Ionic Bond	Electron transfer between atoms	NaCl
Covalent Bond	Electron sharing between atoms	H2O
Hydrogen Bond	Weak attraction between polar molecules	Between water molecules
Exergonic Reaction	Releases energy	Cellular respiration
Endergonic Reaction	Requires energy	Photosynthesis
Ocean Acidification	Decrease in ocean pH due to CO2	Impact on coral reefs

Conclusion

Understanding the chemistry of life provides a foundation for studying biological processes. Atoms, elements, chemical bonds, and water’s properties are central to the structure and function of living organisms.