Chapter 2: The Chemical Context of Life – Study Notes

Chapter 2: The Chemical Context of Life

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

Understanding the chemical basis of life is essential for studying biology. All organisms are composed of matter, which is anything that takes up space and has mass.

• Matter: Anything that occupies space and has mass.

• Element: A substance that cannot be broken down to other substances by chemical reactions.

• Compound: A substance consisting of two or more elements in a fixed ratio, exhibiting emergent properties different from its constituent elements.

• Example: Table salt (NaCl) is a compound with properties distinct from sodium and chlorine.

Elements and Compounds

• About 20–25% of the 92 natural elements are essential for life.

• Major elements: Carbon, hydrogen, oxygen, and nitrogen make up 96% of living matter.

• Minor elements: Calcium, phosphorus, potassium, and sulfur constitute most of the remaining 4%.

• Trace elements: Required in minute quantities (e.g., iron, iodine).

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

Atoms are the fundamental units of elements, and their structure determines the properties of each element.

• Atom: The smallest unit of matter that retains the properties of an element.

• Subatomic particles: Protons (positive charge), neutrons (no charge), electrons (negative charge).

• Protons and neutrons form the atomic nucleus; electrons form a cloud around the nucleus.

• Mass of protons and neutrons is nearly identical and measured in daltons.

Atomic Number and Atomic Mass

• Atomic number: Number of protons in the nucleus.

• Mass number: Sum of protons and neutrons.

• Atomic mass: Approximated by the mass number.

Isotopes and Radioactivity

• Isotopes: Atoms of the same element with different numbers of neutrons.

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

• Applications: Used as tracers in medicine (e.g., PET scans) and for radiometric dating of fossils and rocks.

• Half-life: Time required for half of the isotope to decay.

The Energy Levels of Electrons

Electrons possess potential energy based on their position relative to the nucleus. Their arrangement in shells determines chemical behavior.

• Potential energy: Energy due to location or structure.

• Electrons occupy energy levels (shells) at characteristic distances from the nucleus.

• Changes in electron energy occur in fixed steps.

• Valence electrons: Electrons in the outermost shell, crucial for chemical reactivity.

• Elements with full valence shells are chemically inert.

Electron Orbitals

• Orbital: Three-dimensional space where an electron is found 90% of the time.

• Each shell contains a specific number of orbitals; each orbital holds up to 2 electrons.

• Atoms interact to complete their valence shells.

Concept 2.3: Chemical Bonding Between Atoms

Atoms with incomplete valence shells can share or transfer electrons, forming chemical bonds that hold atoms together in molecules and compounds.

• Chemical bond: Attraction holding atoms together, representing stored potential energy.

Covalent Bonds

• Covalent bond: Sharing of a pair of valence electrons between two atoms.

• Can be single, double, or triple bonds (sharing one, two, or three pairs of electrons).

• Bonding capacity (valence): Number of covalent bonds an atom can form.

• Electronegativity: Atom’s ability to attract electrons in a covalent bond.

• Nonpolar covalent bond: Electrons shared equally.

• Polar covalent bond: Electrons shared unequally, resulting in partial charges.

• Example: Water (H2O) has polar covalent bonds.

Ionic Bonds

• Formed when atoms transfer electrons, resulting in oppositely charged ions (cations and anions).

• Ionic bond: Attraction between cation and anion.

• Ionic compounds (salts): Form crystalline structures (e.g., NaCl).

• Most salts are stable when dry but dissociate in water.

• Electronegativity difference > 1.9 (Pauling scale) typically leads to ionic bonding.

Weak Chemical Interactions

• Hydrogen bonds: Attraction between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom (usually O or N).

• Van der Waals interactions: Weak attractions due to transient local charges.

• Weak bonds are reversible and help maintain the structure of large biological molecules.

Molecular Shape and Function

• Molecular shape is determined by the positions of atoms’ valence orbitals.

• Shape is crucial for biological recognition and function (e.g., opiates mimic endorphins).

• Molecules with similar shapes often have similar functions.

Concept 2.4: Chemical Reactions Make and Break Chemical Bonds

Chemical reactions involve the making and breaking of chemical bonds, transforming reactants into products.

• Reactants: Starting molecules in a reaction.

• Products: Resulting molecules.

• Photosynthesis: Sunlight powers the conversion of CO2 and H2O to glucose and O2.

• All chemical reactions are reversible; equilibrium is reached when forward and reverse reactions occur at the same rate.

Key Equations

• Photosynthesis: $6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2$

• Chemical equilibrium: $\mathrm{A} + \mathrm{B} \rightleftharpoons \mathrm{C} + \mathrm{D}$

Table: Elements in the Human Body

Element	Symbol	Approximate % of Body Mass	Function
Oxygen	O	65%	Component of water and organic molecules; cellular respiration
Carbon	C	18.5%	Backbone of organic molecules
Hydrogen	H	9.5%	Component of water and organic molecules
Nitrogen	N	3.3%	Component of proteins and nucleic acids
Calcium	Ca	1.5%	Bone structure, signaling
Phosphorus	P	1.0%	Component of nucleic acids, ATP
Potassium	K	0.4%	Nerve function
Sulfur	S	0.3%	Component of proteins
Trace elements	Various	<0.01%	Enzyme cofactors, hormone production

Additional info: The study of chemical elements and their interactions forms the foundation for understanding biological molecules and processes, which is essential for all subsequent topics in biology.