AP Biology Chapter 2 Study Guide: The Chemical Context of Life

General chemistry concepts relevant to biology, including atomic structure, bonding types, the properties of water, and pH dynamics.

Four most abundant elements in living matter

Oxygen ($65\%$), Carbon ($18.5\%$), Hydrogen ($9.5\%$), and Nitrogen ($3.3\%$), which together make up approximately $96\%$ of living matter.

Essential elements

Elements required by an organism in relatively large amounts for normal growth and reproduction.

Trace elements

Elements required by an organism in only minute quantities.

Element

A substance that cannot be broken down chemically to other substances.

Compound

A substance consisting of two or more different elements combined in a fixed ratio.

Iodine

A trace element essential for thyroid hormone production in vertebrates; a deficiency in this element causes goiter.

Emergent properties of compounds

The phenomenon where compounds have different chemical and physical characteristics from their constituent elements, such as toxic sodium and chlorine forming edible salt.

Subatomic particles

The three main components of an atom: protons ($+1$ charge), neutrons ($0$ charge), and electrons ($-1$ charge).

Neutron calculation

The number of neutrons is determined by subtracting the atomic number from the mass number ($\text{mass number} - \text{atomic number} = \text{neutrons}$).

Atomic number

The number of protons in the nucleus of an atom, which uniquely identifies each element.

Isotopes

Atoms of the same element that have the same number of protons but different numbers of neutrons, resulting in different masses.

Radioactive isotope

An isotope with an unstable nucleus that spontaneously decays, giving off particles and energy.

Valence shell

The outermost electron shell of an atom, which determines an atom's chemical behavior based on the distribution of its electrons.

Covalent bond

A chemical bond that involves the sharing of electrons between atoms.

Ionic bond

A chemical bond involving the electrostatic attraction between oppositely charged ions.

Electronegativity

The tendency of an atom to pull electrons toward itself, creating partial charges and polar bonds when atoms have different values.

Polar molecule

A molecule with an uneven charge distribution due to polar bonds and molecular shape.

Nonpolar molecule

A molecule with an even charge distribution.

Weak chemical bonds

Attractions including hydrogen bonds, van der Waals interactions, and ionic bonds in aqueous solutions.

Hydrogen bond

A bond formed between a hydrogen atom with a partial positive charge (bonded to an electronegative atom) and another electronegative atom.

Molecular shape

A critical biological factor that determines how molecules recognize and bind to each other with specificity.

Van der Waals interactions

Weak attractions between temporarily positive and negative regions of molecules when atoms are very close together.

Chemical equilibrium

The state in a reaction where the forward and reverse reactions occur at equal rates, resulting in constant concentrations of reactants and products.

Photosynthesis equation

$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$, representing the conversion of inorganic molecules to organic glucose using solar energy.

Cohesion

The mechanism where hydrogen bonds between water molecules create a continuous column, helping plants transport water from roots to leaves.

Surface tension

A measure of the resistance to stretching or breaking a liquid's surface, caused by hydrogen bonds.

Specific heat of water

The characteristic that allows water to resist temperature change because energy is required to break hydrogen bonds before molecules can move faster.

Evaporative cooling

The process where the highest-energy molecules escape as vapor, leaving lower-energy molecules behind to cool the remaining liquid.

Density of Ice

Ice is less dense than liquid water because hydrogen bonds lock molecules into a spacious crystalline structure, allowing it to float.

Hydrophilic

Substances that have an affinity for water and can form hydrogen bonds.

Hydrophobic

Substances that are nonpolar and seem to repel water.

Acid

A substance that increases the $H^+$ concentration of a solution.

Base

A substance that decreases the $H^+$ concentration of a solution, either by accepting $H^+$ or releasing $OH^-$.

pH

A measure of hydrogen ion concentration on a logarithmic scale, calculated as $pH = -\log[H^+]$.

Neutrality

A $pH$ value of $7$, where $[H^+] = [OH^-] = 10^{-7}\,M$ at $25^{\circ}C$.

Buffer

A substance consisting of a weak acid-base pair that can donate or accept $H^+$ to resist pH changes.

Carbonic acid-bicarbonate system

The buffer system described by the reaction $H_2CO_3 \rightleftharpoons HCO_3^- + H^+$, which helps maintain blood pH around $7.4$.

Ocean acidification

The lowering of ocean pH caused by $CO_2$ from burning fossil fuels dissolving in seawater and forming carbonic acid.

Molarity

A measure of concentration in aqueous solutions, calculated as moles of solute per liter of solution ($M = mol/L$).