Unit 1: Chemistry of Life

Element

A pure substance made of only one type of atom that cannot be broken down into simpler substances by chemical means.

Atom

The smallest unit of an element that still has that element’s properties; the basic building block of matter.

Trace element

An element required by organisms only in very small amounts (e.g., Fe, I, Cu).

Proton

A positively charged subatomic particle found in the atomic nucleus.

Neutron

An uncharged subatomic particle found in the atomic nucleus.

Electron

A negatively charged subatomic particle that occupies regions around the nucleus; electron behavior drives bonding.

Isotope

Atoms of the same element (same number of protons) that differ in their number of neutrons.

Valence electrons

Electrons in the outermost energy level of an atom; the electrons directly involved in chemical bonding.

Electronegativity

A measure of how strongly an atom attracts shared electrons in a covalent bond; differences create bond polarity.

Covalent bond

A strong chemical bond formed when atoms share one or more pairs of electrons.

Polar covalent bond

A covalent bond in which electrons are shared unequally, creating partial positive and partial negative charges.

Ionic bond

An attraction between oppositely charged ions formed after electrons are transferred from one atom to another.

Cation

A positively charged ion formed when an atom loses one or more electrons.

Anion

A negatively charged ion formed when an atom gains one or more electrons.

Hydrogen bond

A weak attraction between a partially positive hydrogen (bonded to an electronegative atom like O or N) and a partially negative atom nearby.

Chemical reaction

A process that rearranges atoms by breaking and forming chemical bonds; atoms are conserved in ordinary biological reactions.

Water polarity

Water has polar covalent O–H bonds and a bent shape, giving it a partially negative oxygen end and partially positive hydrogen ends.

Cohesion

The tendency of molecules of the same kind to stick together; in water, cohesion is due to hydrogen bonding.

Hydrophobic effect

The tendency of nonpolar molecules to aggregate in water because minimizing nonpolar surface area helps water maintain a stable hydrogen-bond network.

Acid

A substance that increases hydrogen ion concentration in solution by donating (releasing) H⁺.

Base

A substance that decreases hydrogen ion concentration by accepting H⁺ and/or increasing hydroxide (OH⁻) in solution.

pH

A logarithmic measure of hydrogen ion concentration: pH = −log[H⁺].

Buffer

A solution (often a weak acid and its conjugate base) that resists changes in pH when acids or bases are added.

Organic molecule (biology)

In AP Biology usage, a molecule that contains carbon (a helpful rule of thumb, with some chemistry exceptions).

Carbon (tetravalent)

Carbon has four valence electrons and can form four covalent bonds, enabling chains, branches, rings, and diverse biological molecules.

Hydroxyl group (–OH)

A polar functional group that increases a molecule’s ability to form hydrogen bonds; common in sugars and alcohols.

Carboxyl group (–COOH)

An acidic functional group that can donate H⁺ and become negatively charged (–COO⁻).

Amino group (–NH₂)

A basic functional group that can accept H⁺ and become positively charged (–NH₃⁺).

Phosphate group (–PO₄²⁻ and related)

A functional group often carrying negative charge; important in nucleic acids and energy transfer (e.g., ATP).

Monomer

A small molecular subunit that can be linked with others to form a larger molecule.

Polymer

A large molecule made of many repeating monomer units (e.g., proteins, nucleic acids, many carbohydrates).

Dehydration synthesis (condensation)

A reaction that builds polymers by forming a covalent bond between monomers while producing water.

Hydrolysis

A reaction that breaks polymers into monomers by adding water to help break covalent bonds (common in digestion).

Monosaccharide

A single sugar molecule that can serve as quick energy and as a building block for larger carbohydrates.

Glycosidic linkage

The covalent bond that joins sugars (monosaccharides) together, formed by dehydration synthesis.

Starch

A plant energy-storage polysaccharide made of alpha-linked glucose.

Glycogen

An animal energy-storage polysaccharide made of alpha-linked glucose; typically more highly branched than starch for rapid glucose access.

Cellulose

A plant structural polysaccharide of beta-linked glucose; straight chains hydrogen-bond into strong fibers in cell walls.

Triglyceride (triacylglycerol)

A fat molecule composed of one glycerol backbone bonded to three fatty acids; used for long-term energy storage (e.g., in adipocytes).

Phospholipid

An amphipathic lipid with a hydrophilic phosphate-containing head and two hydrophobic fatty acid tails; self-assembles into bilayers in water.

Lipid saturation

A description of how many C=C double bonds are in fatty acid tails: saturated = none (straight, packs tightly); unsaturated = one or more (kinks, packs less tightly).

Cholesterol

A steroid lipid in animal membranes that helps modulate membrane fluidity and serves as a precursor for some hormones and vitamin D.

Amino acid

The monomer of proteins; has a central carbon bonded to an amino group, carboxyl group, hydrogen, and an R group (side chain).

Peptide bond

The covalent bond formed by dehydration synthesis between the carboxyl group of one amino acid and the amino group of another.

Denaturation

Loss of a protein’s native shape (often disrupting secondary/tertiary/quaternary structure) due to changes such as temperature, pH, or salt; usually reduces function.

Nucleotide

The monomer of nucleic acids, consisting of a phosphate group, a five-carbon sugar, and a nitrogenous base.

DNA (deoxyribonucleic acid)

A nucleic acid with deoxyribose sugar and bases A, T, C, G; usually double-stranded and used for long-term information storage.

RNA (ribonucleic acid)

A nucleic acid with ribose sugar and bases A, U, C, G; usually single-stranded and essential in protein synthesis and gene expression/regulation.

Complementary base pairing

Specific pairing of bases via hydrogen bonds in DNA: A pairs with T, and C pairs with G (supporting double-helix stability and information copying).

ATP (adenosine triphosphate)

A nucleotide-related molecule used for cellular energy transfer; its phosphate-rich structure makes ATP hydrolysis useful for coupling reactions in cells.