Polar covalent bond
A covalent bond in which electrons are shared unequally because the two atoms differ in electronegativity. The O–H bonds in water are polar covalent.
Water PropertiesPolarity
A property of molecules with regions of partial positive and partial negative charge. Water's polarity comes from the unequal sharing of electrons in its O–H bonds.
Water PropertiesHydrogen bond
A weak attraction between the slightly positive hydrogen of one polar molecule and a slightly negative atom (usually O or N) of another. Individually weak; collectively, they shape water, DNA, and proteins.
Water PropertiesCohesion
The tendency of water molecules to stick to each other through hydrogen bonding. Cohesion is what pulls a column of water up the xylem of a tree.
Water PropertiesAdhesion
The attraction between water molecules and other polar substances. Adhesion helps water cling to the inner walls of plant xylem.
Water PropertiesSurface tension
The "skin-like" property of water's surface caused by hydrogen bonding between surface water molecules. It allows small insects to walk on water.
Water PropertiesSpecific heat capacity
The amount of heat needed to raise 1 gram of a substance by 1°C. Water's specific heat is unusually high, so it resists temperature changes — helping organisms maintain homeostatic body temperatures.
Water PropertiesHeat of vaporization
The energy required to convert 1 gram of a liquid to a gas. Water's heat of vaporization is high, which is why sweating cools the body — heat is carried away as water evaporates.
Water PropertiesHydrophilic
"Water-loving" — describing polar or charged molecules that interact well with water.
Water PropertiesHydrophobic
"Water-fearing" — describing nonpolar molecules (like fats and oils) that don't mix with water.
Water PropertiesCHNOPS
The six elements most prevalent in living things: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. C, H, and O are by far the most abundant.
ElementsCarbon's bonding
Carbon has four valence electrons, allowing it to form four covalent bonds. This versatility lets carbon build long chains, branches, and rings — the skeletons of every macromolecule.
ElementsMonomer
A small repeating subunit that joins with others to form a polymer. Examples: amino acids (proteins), nucleotides (nucleic acids), monosaccharides (carbohydrates).
MacromoleculesPolymer
A long chain built from monomers linked by covalent bonds. Proteins, nucleic acids, and complex carbohydrates are polymers.
MacromoleculesDehydration synthesis
A reaction that joins two monomers by removing the equivalent of a water molecule. Also called condensation. Used to build all polymers; requires energy.
MacromoleculesHydrolysis
A reaction that breaks the covalent bond between two monomers by adding water. The reverse of dehydration synthesis; releases energy.
MacromoleculesCarbohydrate
A macromolecule made of carbon, hydrogen, and oxygen (usually in a 1:2:1 ratio). Functions: short-term energy storage, structural support (cellulose), cell recognition.
CarbohydratesMonosaccharide
A single simple sugar; the monomer of carbohydrates. Glucose is the most common example.
CarbohydratesPolysaccharide
A long chain of monosaccharides — can be linear or branched. Examples: starch, glycogen, cellulose.
CarbohydratesStarch
A branched polysaccharide of glucose used by plants for energy storage. Broken down by hydrolysis when energy is needed.
CarbohydratesGlycogen
A highly branched polysaccharide of glucose used by animals (in liver and muscle) for energy storage. The animal equivalent of starch.
CarbohydratesCellulose
A linear polysaccharide of glucose with a structural (not storage) role — it's the main component of plant cell walls. Humans can't digest cellulose.
CarbohydratesLipid
A nonpolar, hydrophobic macromolecule. Lipids aren't true polymers — they don't form from a single repeating monomer. Functions: long-term energy storage, membranes, signaling.
LipidsFatty acid
A long hydrocarbon chain ending in a carboxyl group. Joins with glycerol to form fats. Can be saturated (no double bonds) or unsaturated (one or more double bonds).
LipidsSaturated vs. unsaturated
Saturated fatty acids have only single C–C bonds, pack tightly, and are solid at room temperature (e.g., butter). Unsaturated fatty acids have double bonds that kink the chain, preventing tight packing — they stay liquid (e.g., olive oil).
LipidsPhospholipid
A lipid with a hydrophilic phosphate "head" and two hydrophobic fatty acid "tails." This amphipathic structure causes phospholipids to spontaneously self-assemble into bilayers — the basis of all cell membranes.
LipidsNucleic acid
A polymer of nucleotides that stores or transmits genetic information. The two types are DNA and RNA.
Nucleic AcidsNucleotide
The monomer of nucleic acids. Three parts: a five-carbon sugar (deoxyribose or ribose), a phosphate group, and a nitrogenous base (A, T, G, C, or U).
Nucleic AcidsAntiparallel double helix
The structure of DNA — two complementary nucleotide strands twisted around each other, running in opposite 5′→3′ directions.
Nucleic AcidsBase pairing (A–T, G–C, A–U)
The hydrogen-bonding rules between nitrogenous bases. In DNA: adenine pairs with thymine; guanine pairs with cytosine. In RNA: adenine pairs with uracil instead of thymine.
Nucleic AcidsPurine vs. pyrimidine
Purines (A and G) have a double-ring structure; pyrimidines (C, T, U) have a single ring. A purine always pairs with a pyrimidine, keeping the helix a uniform width.
Nucleic AcidsDNA vs. RNA
DNA: deoxyribose sugar, uses thymine, usually double-stranded. RNA: ribose sugar, uses uracil, usually single-stranded.
Nucleic AcidsProtein
A polymer of amino acids that does most of the work of the cell — enzymes, transport, structure, signaling, defense.
ProteinsAmino acid
The monomer of proteins. A central carbon with four attachments: an amine group (–NH₂), a carboxyl group (–COOH), a hydrogen, and a variable R group (side chain) that gives each amino acid its unique chemistry.
ProteinsR group (side chain)
The variable portion of an amino acid that distinguishes one from another. R groups can be hydrophobic, hydrophilic, or charged — and their interactions drive protein folding.
ProteinsPeptide bond
The covalent bond formed by dehydration synthesis between the carboxyl group of one amino acid and the amine group of the next.
ProteinsPrimary structure
The linear sequence of amino acids in a polypeptide. Determined by DNA. Ultimately determines all higher levels of structure.
ProteinsSecondary structure
Local folding patterns (alpha-helices and beta-pleated sheets) formed by hydrogen bonding between atoms of the polypeptide backbone — not the R groups.
ProteinsTertiary structure
The full 3D shape of a polypeptide. Formed by interactions between R groups: hydrogen bonds, ionic bonds, hydrophobic clustering, and disulfide bridges.
ProteinsQuaternary structure
The arrangement of two or more polypeptide subunits into a single functional protein. Hemoglobin (four subunits) is the classic example.
ProteinsDenaturation
Loss of a protein's normal 3D shape due to extreme heat, pH change, or chemicals. Disrupts function — and is usually irreversible.
Proteins