The Chemical Context of Life (Campbell Biology, Ch. 2) - Study Notes

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Chapter 2: The Chemical Context of Life

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

All living organisms are composed of matter, which exists in various forms and combinations. Understanding the nature of matter is fundamental to biology, as it underpins the structure and function of all biological molecules.

Matter: Anything that takes up 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. Compounds have emergent properties distinct from their constituent elements.

Example: Table salt (NaCl) is a compound with properties different from sodium (a reactive metal) and chlorine (a poisonous gas).

The Elements of Life

Of the 92 naturally occurring elements, about 20–25% are essential for life.
Major elements: Carbon, hydrogen, oxygen, and nitrogen make up about 96% of living matter.
Other important elements: Calcium, phosphorus, potassium, and sulfur account for most of the remaining 4%.
Trace elements: Required in minute quantities (e.g., iron, iodine).

Element	Approximate % of Human Body Mass
Oxygen (O)	65%
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%
Other elements	0.5%

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

Atoms are the smallest units of matter that retain the properties of an element. The structure of atoms determines the chemical behavior of elements.

Atom: Composed of subatomic particles: neutrons (no charge), protons (positive charge), and electrons (negative charge).
Neutrons and protons form the atomic nucleus; electrons form a cloud around the nucleus.
Proton and neutron mass ≈ 1 dalton; electron mass is negligible.

Atomic Number and Atomic Mass

Atomic number: Number of protons in the nucleus (defines the element).
Mass number: Sum of protons and neutrons.
Atomic mass: Total mass of an atom, approximately equal to the mass number.

Isotopes

Atoms of the same element with different numbers of neutrons.
Radioactive isotopes: Unstable; decay spontaneously, emitting particles and energy.

The Energy Levels of Electrons

Energy: Capacity to cause change.
Potential energy: Energy due to location or structure.
Electrons have potential energy based on their distance from the nucleus; found in electron shells.
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, especially in the outermost shell (valence shell).
Elements with a full valence shell are chemically inert.

Electron Orbitals

Orbital: 3D space where an electron is found 90% of the time.
Each shell has a specific number of orbitals; each orbital holds up to 2 electrons.
Atoms interact to complete their valence shells.

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

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

Covalent Bonds

Covalent bond: Sharing of a pair of valence electrons between two atoms.
Single bond: Sharing of one pair of electrons.
Double bond: Sharing of two pairs of electrons.
Valence: Bonding capacity of an atom.
Electronegativity: Atom’s attraction for electrons in a covalent bond.
Nonpolar covalent bond: Electrons shared equally.
Polar covalent bond: Electrons shared unequally, resulting in partial charges.

Ionic Bonds

Formed when one atom strips electrons from another, creating ions.
Cation: Positively charged ion.
Anion: Negatively charged ion.
Ionic bond: Attraction between cation and anion.
Ionic compounds (salts): Often form crystals; dissociate easily in water.

Weak Chemical Interactions

Most strong bonds in organisms are covalent, but weak bonds (hydrogen bonds, van der Waals interactions) are crucial for biological structure and function.
Weak bonds are reversible, allowing dynamic molecular interactions.

Hydrogen Bonds

Form when a hydrogen atom covalently bonded to one electronegative atom is attracted to another electronegative atom (usually oxygen or nitrogen).

Van der Waals Interactions

Occur when transiently positive and negative regions of molecules attract each other.
Individually weak, but collectively significant (e.g., gecko’s toe hairs adhering to surfaces).

Molecular Shape and Function

Molecular shape is determined by the positions of atoms’ orbitals and is key to function.
Shape determines how molecules recognize and respond to each other (e.g., opiates and endorphins binding to brain receptors).

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 chemical reaction.
Products: Resulting molecules after the reaction.
All chemical reactions are reversible; the direction depends on the relative concentrations of reactants and products.
Chemical equilibrium: Point at which forward and reverse reactions occur at the same rate; concentrations of reactants and products remain constant.

Example: Photosynthesis

Sunlight powers the conversion of carbon dioxide and water into glucose and oxygen.
Equation:

Additional info: Understanding chemical principles is foundational for studying biological molecules, cellular processes, and the interactions that sustain life.