Chapter 2: The Chemical Context of Life – Study Notes

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Chapter 2: The Chemical Context of Life

Introduction

This chapter explores the fundamental chemical principles that underlie biological processes. Understanding the chemical context of life is essential for studying how living organisms function at the molecular level.

Concept 2.1: Matter Consists of Chemical Elements in Pure Form and in Combinations Called Compounds

Definition of Matter

Matter is anything that takes up space and has mass.
All organisms are composed of matter.

Elements and Compounds

An element is a substance that cannot be broken down to other substances by chemical reactions.
A compound is a substance consisting of two or more elements combined in a fixed ratio.
Compounds have emergent properties that are different from those of their constituent elements.
Example: Sodium (Na) and chlorine (Cl) are both dangerous in pure form, but together they form sodium chloride (NaCl), or table salt, which is safe to eat.

Elements of Life

About 20-25% of the 92 natural elements are essential for life.
Carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) make up about 96% of living matter.
The remaining 4% is mostly calcium (Ca), phosphorus (P), potassium (K), and sulfur (S).
Trace elements are required by organisms in minute quantities (e.g., iron, iodine, zinc).

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%

Adaptation to Toxic Elements

Some elements can be toxic to organisms.
Certain species can adapt to environments containing toxic elements (e.g., plants adapted to serpentine soils).

Concept 2.2: An Element’s Properties Depend on the Structure of Its Atoms

Atomic Structure

An atom is the smallest unit of matter that retains the properties of an element.
Atoms are composed of subatomic particles:
- Neutrons (no charge)
- Protons (positive charge)
- Electrons (negative charge)
Protons and neutrons form the atomic nucleus; electrons form a cloud around the nucleus.
Proton and neutron mass is approximately 1 dalton; electron mass is negligible.

Atomic Number and Atomic Mass

The atomic number is the number of protons in an atom’s nucleus.
The mass number is the sum of protons and neutrons.
Atomic mass is the atom’s total mass, approximately equal to the mass number.

Isotopes

Atoms of the same element with different numbers of neutrons are called isotopes.
Radioactive isotopes decay spontaneously, releasing particles and energy.
Radioactive isotopes are used as diagnostic tools in medicine (e.g., PET scans).

Radiometric Dating

Radioactive decay occurs at a fixed rate, expressed as the half-life of the isotope.
Radiometric dating measures the ratio of different isotopes to estimate the age of fossils or rocks.

Energy Levels of Electrons

Energy is the capacity to cause change.
Potential energy is energy due to location or structure.
Electrons have potential energy based on their distance from the nucleus; electrons are found in electron shells with characteristic energy levels.
Electrons can move between shells by absorbing or releasing energy.

Electron Distribution and Chemical Properties

The chemical behavior of an atom is determined by the distribution of electrons in its electron shells.
The valence shell is the outermost electron shell; electrons in this shell are called valence electrons.
Atoms with a full valence shell are chemically inert (unreactive).

Electron Orbitals

An orbital is a three-dimensional space where an electron is found 90% of the time.
Each electron shell consists of a specific number of orbitals.
No more than two electrons can occupy a single orbital.

Concept 2.3: The Formation and Function of Molecules and Ionic Compounds Depend on Chemical Bonding Between Atoms

Chemical Bonds

Atoms with incomplete valence shells can share or transfer valence electrons, resulting in chemical bonds.
The main types of chemical bonds are covalent bonds and ionic bonds.

Covalent Bonds

A covalent bond is the sharing of a pair of valence electrons by two atoms.
A single covalent bond involves one pair of shared electrons; a double covalent bond involves two pairs.
The valence of an atom is its bonding capacity, usually equal to the number of unpaired electrons in its valence shell.
Covalent bonds can form between atoms of the same or different elements.

Electronegativity and Bond Polarity

Electronegativity is an atom’s attraction for electrons in a covalent bond.
In a nonpolar covalent bond, electrons are shared equally.
In a polar covalent bond, one atom is more electronegative, causing unequal sharing and partial charges.
Example: In water (), oxygen is more electronegative than hydrogen, resulting in a polar molecule.

Ionic Bonds

Atoms sometimes strip electrons from their bonding partners, forming ions (charged atoms or molecules).
A cation is a positively charged ion; an anion is a negatively charged ion.
Oppositely charged ions attract each other, forming an ionic bond.
Compounds formed by ionic bonds are called ionic compounds or salts (e.g., sodium chloride, NaCl).

Weak Chemical Interactions

Most strong bonds in organisms are covalent, but weak interactions are also important.
Hydrogen bonds form when a hydrogen atom covalently bonded to one electronegative atom is attracted to another electronegative atom (often oxygen or nitrogen).
Van der Waals interactions are weak attractions between molecules or parts of molecules that result from transient local partial charges.

Molecular Shape and Function

A molecule’s size and shape are key to its function.
Shape is determined by the positions of atoms’ orbitals and the types of bonds formed.
Molecular shape determines how biological molecules recognize and respond to one another (e.g., morphine and endorphins binding to brain receptors).

Concept 2.4: Chemical Reactions Make and Break Chemical Bonds

Chemical Reactions

Chemical reactions are the making and breaking of chemical bonds.
Reactants are the starting molecules; products are the resulting molecules.
Example:
Photosynthesis:
Chemical reactions are reversible; products of the forward reaction become reactants for the reverse reaction.
Chemical equilibrium is reached when the forward and reverse reactions occur at the same rate, and the concentrations of reactants and products remain constant.

Additional info: These notes are based on the introductory section of Chapter 2 from Campbell Biology, focusing on the chemical basis of life, atomic structure, chemical bonding, and the nature of chemical reactions in biological systems.