Chemistry of Life and Water: every key term you need (+ practice quiz)
25 flashcard terms for General Biology I Topic 1, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 15-question quiz — free, no account needed.
A bond in which two atoms share one or more electron pairs, giving each a filled valence shell. Sharing makes covalent bonds far stronger than the noncovalent interactions that shape biological structure.
Electronegativity
The pull an atom exerts on shared electrons. Oxygen and nitrogen are strongly electronegative, carbon and hydrogen much less so, which is why C-H bonds are nonpolar and O-H bonds are strongly polar.
Polar covalent bond
A shared-electron bond between atoms of unequal electronegativity, so electron density piles up near one atom. The result is a partial negative charge on that atom and a partial positive charge on its partner.
Hydrogen bond
A weak attraction between a partially positive hydrogen already bonded to oxygen or nitrogen and a nearby electronegative atom. Individually about a twentieth the strength of a covalent bond, but collectively decisive.
Water bent geometry
Two bonding pairs and two lone pairs around oxygen force a roughly 104 degree H-O-H angle. The bent shape prevents the two bond dipoles from cancelling, so the molecule as a whole is polar.
Cohesion
The tendency of water molecules to stick to one another through hydrogen bonding. Cohesion transmits tension up a column of xylem sap, allowing transpiration to pull water from root to leaf without pumping.
Adhesion
Attraction between water and a different polar or charged surface, such as cellulose in a cell wall or glass in a capillary tube. Adhesion plus cohesion produces capillary rise in narrow tubes.
Surface tension
The resistance of a liquid surface to being stretched, caused by unbalanced cohesive forces on the topmost molecules. Water has an unusually high surface tension for a small molecule, letting insects stand on ponds.
Specific heat capacity
The heat needed to raise one gram of a substance by one degree Celsius. Water needs about one calorie per gram, several times most liquids, because added energy first breaks hydrogen bonds rather than speeding molecules.
Heat of vaporization
The energy required to convert one gram of liquid into gas at constant temperature. Water needs roughly 540 calories per gram because every hydrogen bond a molecule holds must be broken before it escapes.
Evaporative cooling
The drop in temperature of a liquid left behind when its most energetic molecules escape as vapor. Sweating and leaf transpiration exploit water's large heat of vaporization to dump heat efficiently.
Ice density anomaly
Below freezing each water molecule locks into four hydrogen bonds in an open lattice, spacing molecules farther apart than in liquid water. Ice is therefore about nine percent less dense and floats, insulating the water below.
Hydration shell
The ordered cage of water molecules that surrounds a dissolved ion, oriented so partial negative oxygens face cations and partial positive hydrogens face anions. It is why salts dissolve and stay dispersed.
Hydrophilic substance
A polar or charged substance that interacts favorably with water, either dissolving in it or, like cotton fibers, binding it tightly at the surface without going into solution.
Hydrophobic substance
A nonpolar substance such as a hydrocarbon that cannot hydrogen bond with water. It is excluded not because water repels it but because water gains entropy by releasing the ordered shell around it.
Hydrophobic effect
The entropy-driven clustering of nonpolar groups in water. Aggregation frees ordered shell water into the bulk, so the process is favorable mainly through an increase in solvent entropy rather than direct attraction.
Amphipathic molecule
A molecule carrying both a polar or charged region and a nonpolar region, such as a phospholipid or a soap. Amphipathic molecules self-assemble into micelles, bilayers and monolayers at interfaces.
Molarity
Moles of solute per liter of solution, the standard concentration unit in biochemistry. One mole is Avogadro's number of particles, and a mole of any compound weighs its molecular mass in grams.
Water autoionization
The reversible splitting of water into a hydrogen ion and a hydroxide ion. At 25 degrees Celsius the product of the two concentrations is a constant of ten to the minus fourteen, which fixes the pH scale.
pH
The negative base-ten logarithm of hydrogen ion concentration. Because the scale is logarithmic, each unit is a tenfold change, so blood at 7.4 has about a quarter the hydrogen ions of a solution at 8.
Acid
A proton donor. Strong acids such as hydrochloric acid dissociate essentially completely in water, while weak acids like carbonic acid establish an equilibrium between protonated and deprotonated forms.
Base
A proton acceptor, or equivalently a substance that raises hydroxide concentration in water. Ammonia accepts a proton directly, while sodium hydroxide raises hydroxide by dissociating.
Buffer
A weak acid paired with its conjugate base, which resists pH change by donating protons when base is added and absorbing them when acid is added. Buffering is strongest when pH is near the acid's pKa.
Bicarbonate buffer system
The main buffer of vertebrate blood, linking dissolved carbon dioxide, carbonic acid and bicarbonate. Because the lungs adjust carbon dioxide and the kidneys adjust bicarbonate, the system is open and unusually powerful.
Isotope
An atom of an element with the usual proton number but a different neutron count. Radioactive isotopes such as phosphorus-32 and sulfur-35 gave biology its earliest way to label and track specific molecules.