Chemistry of Life: every key term you need (+ practice quiz)

More AP Biology guides

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Elements of Life (CHNOPS)

Carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur make up ~96% of living matter. Their bonding versatility builds all biological molecules.

Why Carbon?

Carbon has four valence electrons, forming four stable covalent bonds. This lets it build long chains, branches, and rings — the backbones of all organic molecules.

Covalent Bond

Atoms share electron pairs. Strong and stable, covalent bonds hold biological molecules together (e.g., the C–C and C–H bonds of organic compounds).

Polar Covalent Bond

Electrons are shared unequally because one atom is more electronegative (e.g., O in water), creating partial charges (δ+ and δ−).

Ionic Bond

One atom transfers electrons to another, creating oppositely charged ions that attract. Weaker in water, which surrounds and separates the ions.

Hydrogen Bond

A weak attraction between a slightly positive hydrogen (bonded to O or N) and a slightly negative atom nearby. Individually weak, collectively powerful — they shape water and macromolecules.

Electronegativity

An atom's pull on shared electrons. Oxygen and nitrogen are highly electronegative, making the bonds they form polar.

Water Is Polar

Oxygen hogs the shared electrons, giving it a partial negative charge and the hydrogens partial positive charges. This polarity underlies nearly all of water's life-supporting properties.

Cohesion

Water molecules stick to each other via hydrogen bonds. Cohesion pulls water columns up plant xylem and creates surface tension.

Adhesion

Water sticks to other polar/charged surfaces. With cohesion, adhesion helps water climb narrow tubes (capillary action) against gravity.

Surface Tension

Cohesion at the water's surface creates a 'skin' strong enough for insects to walk on — hydrogen bonds pulling surface molecules inward.

High Specific Heat

Water resists temperature change because hydrogen bonds absorb heat before molecules speed up. This stabilizes organisms and climates.

High Heat of Vaporization

Much energy is needed to break hydrogen bonds and turn water to vapor. As sweat evaporates it carries heat away — evaporative cooling.

Ice Floats (Less Dense Solid)

In ice, hydrogen bonds lock molecules into a spacious crystal lattice, making solid water less dense than liquid. Floating ice insulates the water below, letting life survive winter.

Universal Solvent

Water dissolves any polar or ionic (hydrophilic) substance, forming hydration shells around solutes. Most cellular chemistry happens in aqueous solution.

Hydrophilic

'Water-loving' — polar or charged substances that dissolve in or associate with water (e.g., sugars, salts).

Hydrophobic

'Water-fearing' — nonpolar substances (e.g., oils, fats) that don't dissolve in water and cluster together to minimize contact.

Solute, Solvent, Solution

The solute dissolves in the solvent to form a solution. In cells, water is the solvent; ions, sugars, and gases are common solutes.

Acid

A substance that increases the hydrogen-ion (H+) concentration of a solution. Acids have a pH below 7.

Base

A substance that decreases H+ concentration (or adds OH−). Bases have a pH above 7.

pH Scale

A logarithmic measure of H+ concentration from 0–14. Each unit is a 10× change: pH 4 has 10× more H+ than pH 5. Neutral is 7.

Buffers

Substances that resist pH change by absorbing or releasing H+. They keep body fluids (like blood, ~7.4) stable enough for enzymes to work.

Organic Molecule

A carbon-based molecule made by living things. The four major classes are carbohydrates, lipids, proteins, and nucleic acids.

Functional Groups

Specific atom clusters attached to carbon skeletons that give molecules characteristic chemical properties and reactivity.

Hydroxyl Group (–OH)

Polar; makes molecules soluble (found in alcohols and sugars). Forms hydrogen bonds with water.

Carbonyl Group (C=O)

A carbon double-bonded to oxygen. Defines aldehydes (end of chain) and ketones (within chain); common in sugars.

Carboxyl Group (–COOH)

Acts as an acid by donating H+. Found in amino acids and fatty acids; gives them acidic character.

Amino Group (–NH2)

Acts as a base by accepting H+. Found in amino acids; central to protein and nitrogen chemistry.

Phosphate Group (–PO4)

Carries negative charge and stores energy in its bonds. Central to ATP, DNA/RNA backbones, and phospholipids.

Sulfhydryl Group (–SH)

Found in some amino acids (cysteine); two can form disulfide bonds that stabilize protein tertiary structure.

Methyl Group (–CH3)

Nonpolar; often affects gene expression when added to DNA or proteins (methylation) and shape/function of molecules.

Monomer

A single small subunit that links with others to form a polymer (e.g., a glucose molecule or an amino acid).

Polymer

A large molecule built from repeating monomers (e.g., starch from glucose, protein from amino acids).

Dehydration Synthesis

Monomers are joined by removing a water molecule (one loses –OH, the other –H), forming a covalent bond. Builds all polymers; requires energy.

Hydrolysis

A polymer is broken apart by adding water, which splits the bond between monomers. Digestion is hydrolysis.

Carbohydrates

Sugars and their polymers (CH2O ratio). Functions: quick energy, energy storage, and structural support. Monomers are monosaccharides.

Monosaccharide

A single sugar (e.g., glucose, fructose, galactose), the monomer of carbohydrates and a cell's main quick energy source.

Glucose

The key monosaccharide (C6H12O6); fuel for cellular respiration and the building block of starch, glycogen, and cellulose.

Disaccharide

Two monosaccharides joined by dehydration synthesis (e.g., sucrose = glucose + fructose; lactose; maltose). Linked by a glycosidic bond.

Polysaccharide

Many monosaccharides linked into a polymer — for storage (starch, glycogen) or structure (cellulose, chitin).

Starch

A plant storage polysaccharide of alpha-glucose. Easily hydrolyzed by animals for energy; stored in roots and seeds.

Glycogen

The animal storage polysaccharide of glucose, highly branched, stored in liver and muscle for rapid energy mobilization.

Cellulose

A structural polysaccharide of beta-glucose forming plant cell walls. Its bonds resist most animals' enzymes — it's dietary fiber.

Chitin

A structural polysaccharide (with nitrogen) forming arthropod exoskeletons and fungal cell walls.

Alpha vs. Beta Glucose

Same formula, different –OH orientation. Alpha links (starch, glycogen) are digestible; beta links (cellulose) resist digestion — structure dictates function.

Lipids

Diverse hydrophobic molecules (fats, phospholipids, steroids). Not true polymers. Functions: long-term energy storage, membranes, insulation, signaling.

Fats (Triglycerides)

A glycerol bonded to three fatty acids by dehydration synthesis (ester linkages). Store more than twice the energy per gram of carbohydrates.

Fatty Acid

A long hydrocarbon chain with a carboxyl group. The chain's hydrophobic; saturation determines whether the fat is solid or liquid.

Saturated Fat

Fatty acids with no C=C double bonds — straight chains that pack tightly, so they're solid at room temperature (animal fats).

Unsaturated Fat

Fatty acids with one or more C=C double bonds that kink the chain, preventing tight packing — liquid oils at room temperature (plant fats).

Phospholipid

A glycerol with two fatty acids and a phosphate group: a hydrophilic 'head' and two hydrophobic 'tails.' The amphipathic basis of all cell membranes.

Amphipathic

Having both hydrophilic and hydrophobic regions — the property that makes phospholipids self-assemble into bilayers in water.

Phospholipid Bilayer

In water, phospholipids arrange with hydrophilic heads facing out and hydrophobic tails inward — the fundamental structure of membranes.

Steroids

Lipids with four fused carbon rings (e.g., cholesterol, testosterone, estrogen). Cholesterol stabilizes membranes and is a hormone precursor.

Proteins

Polymers of amino acids that do most cellular work: enzymes, structure, transport, signaling, defense, and movement. The most diverse macromolecules.

Amino Acid

A protein monomer with a central carbon bonded to an amino group, a carboxyl group, a hydrogen, and a variable R-group. Twenty kinds exist.

R-Group (Side Chain)

The variable part of an amino acid that determines its chemistry — nonpolar, polar, acidic, or basic — and thus a protein's folding and function.

Peptide Bond

The covalent bond joining amino acids, formed by dehydration synthesis between one's carboxyl and the next's amino group.

Polypeptide

A chain of amino acids linked by peptide bonds. One or more polypeptides fold into a functional protein.

Primary Structure

The unique linear sequence of amino acids in a polypeptide, encoded by a gene. It determines all higher-level folding.

Secondary Structure

Local folding — alpha helices and beta pleated sheets — stabilized by hydrogen bonds along the polypeptide backbone.

Tertiary Structure

The overall 3-D shape of a polypeptide, driven by R-group interactions: hydrophobic clustering, hydrogen and ionic bonds, and disulfide bridges.

Quaternary Structure

The arrangement of two or more polypeptide subunits into one functional protein (e.g., hemoglobin's four chains).

Disulfide Bridge

A strong covalent bond between two cysteine sulfhydryl groups that locks tertiary structure in place.

Denaturation

Loss of a protein's shape (and function) due to heat, pH change, or salts breaking the bonds that maintain folding. Structure = function, so shape loss = function loss.

Structure Determines Function

The central theme of biology: a molecule's shape, dictated by its sequence and bonds, dictates what it can do — as in enzymes and receptors.

Enzymes (as Proteins)

Protein catalysts that speed reactions by lowering activation energy. Their specific active-site shape fits specific substrates.

Nucleic Acids

Polymers of nucleotides that store and transmit hereditary information: DNA (the blueprint) and RNA (the messenger and worker).

Nucleotide

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

Nitrogenous Bases

Adenine, thymine, cytosine, guanine (DNA); uracil replaces thymine in RNA. The sequence encodes genetic information.

Purines vs. Pyrimidines

Purines (A, G) have two rings; pyrimidines (C, T, U) have one. A purine always pairs with a pyrimidine, keeping the helix uniform.

Complementary Base Pairing

A pairs with T (or U) via two hydrogen bonds; C pairs with G via three. This rule underlies DNA replication and transcription.

DNA vs. RNA

DNA: double-stranded, deoxyribose, uses thymine, stable long-term storage. RNA: single-stranded, ribose, uses uracil, short-lived worker.

Sugar-Phosphate Backbone

Alternating sugars and phosphates linked by covalent (phosphodiester) bonds form the structural sides of a nucleic-acid strand; bases project inward.

Antiparallel Strands

DNA's two strands run in opposite 5'→3' directions. This orientation is required for base pairing and directs replication.

5' and 3' Ends (Directionality)

Nucleic-acid strands have a 5' phosphate end and a 3' hydroxyl end. New nucleotides are added only to the 3' end — synthesis is 5'→3'.

ATP

Adenosine triphosphate — the cell's energy currency. Energy is released when the bond to its terminal phosphate is hydrolyzed (ATP → ADP + Pi).

Directionality of Macromolecules

Polymers have distinct ends and orientations (e.g., N- to C-terminus in proteins, 5'→3' in nucleic acids) that determine how they're built and read.

Emergent Properties

Novel properties arise at each level of organization that the parts alone lack — e.g., water's behavior emerges from H-bonding, not from lone H and O atoms.

Monomers Across Life

Organisms build the same four macromolecule classes from a shared set of monomers — evidence of common ancestry and life's unity.

Hydrophobic Interactions

Nonpolar regions cluster in water to minimize disruption of hydrogen bonding. This drives membrane formation and protein folding.

Isomers

Molecules with the same formula but different arrangements (e.g., glucose vs. fructose). Different structure means different function.