AP Bio Unit 1 AP Prep

Polar molecule

A molecule with an uneven distribution of electrical charge (partial positive and partial negative regions) but no net charge overall; water is polar.

Bent (V-shaped) geometry

The shape of a water molecule; because it is not linear, oxygen’s pull on shared electrons does not cancel out, contributing to water’s polarity.

Covalent bond

A strong chemical bond formed when atoms share electron pairs; oxygen forms two covalent bonds with hydrogen in H₂O.

Electronegativity

An atom’s ability to attract shared electrons in a covalent bond; oxygen is more electronegative than hydrogen.

Partial charges (δ− and δ+)

Slight charges created by unequal electron sharing in polar covalent bonds; in water, oxygen is δ− and hydrogens are δ+.

Hydrogen bond

A weak attraction between a δ+ hydrogen (bonded to an electronegative atom like O or N) and a nearby electronegative atom with a partial negative charge.

Cohesion

Attraction between molecules of the same substance; in water, cohesion results from hydrogen bonding between water molecules.

Adhesion

Attraction between molecules of different substances; water adheres to other polar surfaces (e.g., cellulose in plant cell walls).

Capillary action

The movement of water up narrow spaces (like tubes) due to the combined effects of cohesion and adhesion.

Surface tension

A “skin-like” effect at water’s surface caused by cohesive hydrogen bonding among surface molecules, making the surface resistant to breaking.

Specific heat

The amount of energy required to raise a substance’s temperature; water’s high specific heat helps buffer temperature changes because heat is used to disrupt hydrogen bonds first.

Heat of vaporization

The energy required to convert a liquid to a gas; water’s relatively high value is due to the energy needed to break hydrogen bonds.

Evaporative cooling

Cooling that occurs when high-energy molecules evaporate from a liquid surface, lowering the average kinetic energy (temperature) of the remaining liquid (e.g., sweating).

Density anomaly of ice

Water’s unusual property that solid ice is less dense than liquid water because hydrogen bonds form an ordered lattice that spaces molecules farther apart, causing ice to float.

Solution

A homogeneous mixture in which one substance is dissolved in another (e.g., saltwater).

Solvent

The dissolving agent in a solution; in biology, water is the most common solvent.

Solute

The substance dissolved in a solution (e.g., NaCl in saltwater).

Hydration shell

A layer of water molecules surrounding an ion in solution; water’s partial charges orient to stabilize separated cations and anions.

Hydrophobic interaction

The clustering of nonpolar molecules in water because it allows water to maintain hydrogen bonding with itself; important in membrane formation and protein folding.

Element

A pure substance made of only one kind of atom; living organisms are primarily built from a limited set of elements.

Atom

The smallest unit of an element that retains that element’s properties; composed of protons and neutrons in the nucleus with electrons around it.

Isotope

Atoms of the same element with different numbers of neutrons; isotopes can be stable or radioactive and are used as tracers in biology/medicine.

Valence electrons

Electrons in the outermost energy level of an atom; they largely determine bonding behavior and reactivity.

Ionic bond

An electrostatic attraction between oppositely charged ions formed after electron transfer; in water, many ionic compounds dissociate into ions.

CHNOPS

Mnemonic for major biologically important elements: Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur—common in macromolecules like proteins, nucleic acids, carbohydrates, and lipids.

Biological macromolecules

Large, carbon-based molecules used by organisms to store energy, build cellular structures, transmit information, and carry out chemical reactions (carbohydrates, lipids, proteins, nucleic acids).

Structure determines function

Principle that a macromolecule’s specific arrangement of atoms (its structure) determines its chemical behavior and biological role.

Carbon backbone (of life)

Carbon’s ability to form four covalent bonds allows long chains, branches, and rings, creating diverse biological molecules with different shapes and properties.

Monomer

A small molecular subunit that can be linked to other subunits (e.g., an amino acid).

Polymer

A large molecule made of many repeating monomer units linked together (e.g., a polypeptide).

Dehydration synthesis (condensation reaction)

Builds polymers by forming a covalent bond between monomers while removing a molecule of water.

Hydrolysis

Breaks polymers into monomers by adding water to help break a covalent bond.

Functional group

A specific cluster of atoms that behaves in consistent ways and helps determine a molecule’s properties (polarity, acidity/basicity, reactivity).

Hydroxyl group (-OH)

Functional group that often increases polarity and water solubility; common in sugars.

Carboxyl group (-COOH)

Functional group that can donate H+ (acidic); found in amino acids and fatty acids.

Amino group (-NH2)

Functional group that can accept H+ (basic); found in amino acids.

Phosphate group (-PO4)

Functional group that often contributes negative charge and is important in energy transfer and nucleic acids.

Hydrophilic

Describes polar or charged regions that interact well with water.

Hydrophobic

Describes nonpolar regions that do not interact well with water and tend to cluster away from it.

Amphipathic

Having both hydrophilic and hydrophobic regions (e.g., phospholipids with a polar head and nonpolar tails).

Isomer

A molecule with the same molecular formula as another molecule but a different structure, often leading to different biological function.

Covalent bond

A strong bond that holds the main backbone of polymers together (e.g., glycosidic, peptide, phosphodiester bonds).

Hydrogen bond

A weak interaction important for macromolecule shape, such as DNA base pairing and protein secondary structure (strong collectively, weak individually).

Glycosidic linkage

The covalent bond that links monosaccharides together in carbohydrates; formed by dehydration synthesis.

Triglyceride

A lipid made from glycerol and fatty acids; used for long-term energy storage and insulation due to many C-H bonds and low water attraction.

Saturated fatty acid

A fatty acid with no double bonds; straighter chains that pack tightly (often associated with more solid fats).

Unsaturated fatty acid

A fatty acid with one or more double bonds; kinked chains that pack less tightly, increasing membrane fluidity when present in phospholipids.

Peptide bond

The covalent bond linking amino acids in a polypeptide; formed by dehydration synthesis.

Nucleotide

The monomer of nucleic acids, consisting of a phosphate group, a five-carbon sugar (ribose or deoxyribose), and a nitrogenous base.

Phosphodiester bond

The covalent bond that links nucleotides, connecting the sugar of one nucleotide to the phosphate of the next to form the sugar-phosphate backbone.

Nucleic acids

Biological macromolecules that store, transmit, and help express genetic information; the information is encoded in the sequence of bases.

DNA (deoxyribonucleic acid)

A nucleic acid used primarily for long-term information storage; typically double-stranded and forms a double helix.

RNA (ribonucleic acid)

A nucleic acid often involved in information transfer and functional roles; typically single-stranded and able to fold into complex shapes.

Nucleotide

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

Nucleoside

A sugar + nitrogenous base (no phosphate group); differs from a nucleotide by lacking the phosphate.

Nitrogenous base

A ring-shaped, nitrogen-containing part of a nucleotide that carries genetic information; its order (sequence) encodes information.

Purines

Nitrogenous bases with two rings (larger); adenine (A) and guanine (G).

Pyrimidines

Nitrogenous bases with one ring (smaller); cytosine (C), thymine (T, in DNA), and uracil (U, in RNA).

Thymine (T) vs. Uracil (U)

Thymine is used in DNA and uracil is used in RNA; in RNA base pairing, adenine (A) pairs with uracil (U) instead of thymine (T).

Sugar-phosphate backbone

The repeating structural framework of a nucleic acid strand made of alternating sugars and phosphates; bases attach to the sugars and stick out from the backbone.

Phosphodiester bond

The covalent bond that links nucleotides in a strand by connecting the phosphate of one nucleotide to the 3′ carbon (OH) of the next nucleotide’s sugar.

5′ and 3′ ends

The two chemically distinct ends of a nucleic acid strand: the 5′ end has a free phosphate, and the 3′ end has a free hydroxyl (-OH) on the sugar.

Directionality (polarity)

The property that nucleic acid strands have a 5′ end and a 3′ end; many enzymes add nucleotides only to the 3′ end.

Dehydration synthesis (condensation reaction)

A polymer-forming reaction where a covalent bond forms and water is produced; nucleotides polymerize to build the sugar-phosphate backbone.

Complementary base pairing

Specific pairing between bases in double-stranded regions: A pairs with T in DNA (or with U in RNA), and G pairs with C; supports accurate copying.

Hydrogen bonds (in base pairing)

Weak bonds that hold complementary bases together across strands; A–T (or A–U) forms 2 hydrogen bonds, and G–C forms 3.

Antiparallel

Describes the orientation of two strands in double-stranded DNA (and many paired regions): one runs 5′→3′ while the other runs 3′→5′.

GC content

The proportion of G–C base pairs in a DNA region; higher GC content often correlates with greater thermal stability because G–C pairs form three hydrogen bonds.

Deoxyribose vs. ribose

Deoxyribose (in DNA) has an H at the 2′ carbon, while ribose (in RNA) has an OH at the 2′ carbon; the 2′ OH makes RNA generally less chemically stable.

Double helix

The typical shape of DNA: two antiparallel strands twisted together, with sugar-phosphate backbones on the outside and stacked bases inside.

Template strand

A DNA strand used to build a complementary nucleic acid strand (e.g., during RNA synthesis); the new strand is complementary and antiparallel to the template.

mRNA (messenger RNA)

An RNA type that carries a copy of a gene’s information from DNA to ribosomes for protein production.

rRNA (ribosomal RNA)

An RNA type that is a major structural and functional component of ribosomes involved in protein synthesis.

tRNA (transfer RNA)

An RNA type that delivers amino acids to the ribosome during protein synthesis; its shape helps it match codons to the correct amino acids.

Central dogma (DNA → RNA → protein)

A common framework for information flow in cells: DNA stores information, RNA acts as a complementary copy/functional intermediary, and proteins are produced based on that information.