The Chemical Context of Life (Campbell Biology, Chapter 2)

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Chapter 2: The Chemical Context of Life

Introduction

Chemistry forms the foundation of biology, as all biological processes are governed by chemical principles. Understanding the chemical context of life is essential for studying biological systems, from the molecular to the organismal level.

Elements and Compounds

Elements Essential for Life

Element: A substance that cannot be broken down into other substances by chemical reactions.
Of the 92 naturally occurring elements, about 20-25% are essential for life.
Major elements: Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N) make up approximately 96% of living matter.
Other important elements: Calcium (Ca), Phosphorus (P), Potassium (K), and Sulfur (S) constitute most of the remaining 4%.
Trace elements: Required in minute quantities (e.g., Iron (Fe), Iodine (I), Zinc (Zn)).

Table: Elements in the Human Body

Element	Symbol	Percentage of Body Mass (including water)
Oxygen	O	65.0%
Carbon	C	18.5%
Hydrogen	H	9.5%
Nitrogen	N	3.3%
Calcium	Ca	1.5%
Phosphorus	P	1.0%
Potassium	K	0.4%
Sulfur	S	0.3%
Sodium	Na	0.2%
Chlorine	Cl	0.2%
Magnesium	Mg	0.1%
Trace elements	-	<0.01%

Compounds and Emergent Properties

Compound: A substance consisting of two or more elements combined in a fixed ratio (e.g., NaCl).
Compounds have emergent properties that are different from those of their constituent elements.
Example: Sodium (Na, a metal) and chlorine (Cl, a poisonous gas) combine to form sodium chloride (NaCl, table salt), which is edible and safe.

Atoms and Atomic Structure

Atomic Number and Mass Number

Atom: The smallest unit of matter that retains the properties of an element.
Atomic number (Z): Number of protons in the nucleus.
Mass number (A): Sum of protons and neutrons in the nucleus.
Electrons are found in electron shells surrounding the nucleus.

Electron Shells and Energy Levels

Electrons occupy different shells, each with a characteristic energy level and distance from the nucleus.
The first shell (closest to the nucleus) has the lowest energy; higher shells have higher energy.
Electron distribution determines the chemical properties of an atom.
Valence electrons: Electrons in the outermost shell (valence shell); they determine chemical reactivity.
Atoms with a full valence shell are chemically inert (e.g., noble gases).

Example: Electron Configuration of Oxygen

Oxygen atomic number = 8
Electron distribution: 2 in the first shell, 6 in the second shell, 0 in the third shell

Chemical Bonds

Covalent Bonds

Covalent bond: The sharing of a pair of valence electrons by two atoms.
Each atom counts the shared electrons as part of its valence shell.
Single covalent bond: Sharing of one pair of electrons (e.g., H2 molecule).
Double covalent bond: Sharing of two pairs of electrons (e.g., O2 molecule).

Electronegativity and Bond Polarity

Electronegativity: An atom’s attraction for the electrons in a covalent bond.
In a nonpolar covalent bond, electrons are shared equally (e.g., H2, O2).
In a polar covalent bond, one atom is more electronegative and pulls electrons closer, resulting in partial charges (e.g., H2O).

Ionic Bonds

Sometimes, atoms strip electrons from their bonding partners, forming ions.
Cation: Positively charged ion (lost electrons).
Anion: Negatively charged ion (gained electrons).
Oppositely charged ions attract, forming an ionic bond (e.g., Na+ and Cl- form NaCl).
In dry conditions, ionic bonds are strong; in aqueous solutions, covalent bonds are stronger.

Weak Chemical Interactions

Hydrogen Bonds

A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is attracted to another electronegative atom.
In living cells, the electronegative partners are usually oxygen or nitrogen.
Hydrogen bonds are weaker than covalent or ionic bonds but are crucial for the structure of biological molecules (e.g., DNA, proteins).

Van der Waals Interactions

Occur when electrons are distributed asymmetrically in molecules, creating temporary partial charges.
These weak interactions can be significant when many occur together (e.g., gecko feet adhering to surfaces).

Chemical Reactions

Reactants and Products

Chemical reaction: The making and breaking of chemical bonds, leading to changes in the composition of matter.
Reactants: Starting materials in a chemical reaction.
Products: Resulting materials after the reaction.
Example:

Photosynthesis as a Chemical Reaction

Photosynthesis is a key chemical reaction in biology:

Reactants: Carbon dioxide and water
Products: Glucose and oxygen

Chemical Equilibrium

A chemical reaction is at equilibrium when the rate of the forward reaction equals the rate of the reverse reaction.
At equilibrium, the concentrations of reactants and products remain constant.
Analogy: Like squirrels entering and leaving a room at equal rates.

Examples and Applications

Formica rufa (wood ants): Use chemistry (formic acid) as a defense mechanism against predators.
Trace elements: Essential in small amounts for proper biological function (e.g., iron for oxygen transport in blood).

Key Terms and Definitions

Element: Pure substance consisting of only one type of atom.
Compound: Substance formed from two or more elements in a fixed ratio.
Atom: Smallest unit of an element.
Isotope: Atoms of the same element with different numbers of neutrons.
Valence electrons: Electrons in the outermost shell, involved in bonding.
Covalent bond: Sharing of electron pairs between atoms.
Ionic bond: Attraction between oppositely charged ions.
Hydrogen bond: Weak attraction between a hydrogen atom and an electronegative atom.
Van der Waals interactions: Weak attractions due to transient local partial charges.
Chemical reaction: Process that changes reactants into products.
Chemical equilibrium: State where forward and reverse reaction rates are equal.

Additional info: Some content and definitions have been expanded for clarity and completeness based on standard biology textbooks.