AP BIO CHPATER 3

Hydrogen in molecules

Hydrogen is partially positive when bound to highly electronegative atoms (F, O, N)

Hydrogen bonds

Formed when H is attracted to F, O, or N in another molecule

Water molecule polarity

Polar covalent bonds in water cause an uneven electron distribution; electrons spend more time near oxygen

Polarity effects

Allows water molecules to form hydrogen bonds with each other

Emergent properties of water

Four properties that support life: cohesion, moderation of temperature, expansion upon freezing, versatility as a solvent

Cohesion

Hydrogen bonds hold water molecules together; contributes to high surface tension and transport in plants

Surface tension

Measure of how difficult it is to stretch or break the surface of a liquid; high in water due to cohesion

Adhesion

Attraction between water and other substances (e.g., plant cell walls); helps counter gravity

Water transport in plants

Transpiration pulls water upward through plants

Moderation of temperature

Water absorbs heat from warm air and releases it to cooler air; large heat capacity with slight temperature change

Temperature

Average kinetic energy of molecules

Kinetic energy

Energy of motion; thermal energy is kinetic energy from random motion of atoms/molecules

Heat

Transfer of thermal energy between bodies

Calorie (cal)

Heat required to raise 1 g of water by 1ºC; also heat released when 1 g cools by 1ºC

Kilocalorie (kcal)

1000 calories; used on food labels

Joule (J)

Unit of energy; 1 J = 0.239 cal, 1 cal = 4.184 J

Specific heat

Amount of heat absorbed or lost to change 1 g of substance by 1ºC; water = 1 cal/(g ºC)

Water’s high specific heat

Water resists temperature change due to hydrogen bonding; heat absorbed when bonds break, released when bonds form

Heat transfer

Occurs via conduction, convection, radiation

Temperature moderation

Large bodies of water absorb/store heat during day/summer and release it gradually at night/winter; moderates coastal climates

Evaporation

Transformation of liquid to gas; heat of vaporization is heat absorbed to convert 1 g to gas

Evaporative cooling

Surface cools as liquid evaporates; stabilizes temperatures in organisms and bodies of water

Ice density

Water is less dense as a solid due to crystalline lattice; ~10% less dense than liquid water

Water density

Maximum density at 4ºC; ice floats, preventing freezing of entire bodies of water

Environmental impact of ice

Glacial and Arctic ice loss threatens organisms relying on ice; global warming concerns

Solution

Solution is a homogeneous liquid mixture of substances

Solvent

Substance that dissolves solutes

Solute

Substance dissolved in a solvent

Aqueous solution

Water is the solvent

Water as solvent

Water dissolves ionic compounds (hydration shell) and polar molecules; can dissolve large molecules like proteins with ionic/polar regions

Hydrophilic

Substance that has affinity for water

Hydrophobic

Substance that does not have affinity for water; oils are hydrophobic; major components of cell membranes

Molecular mass

Sum of all atom masses in a molecule

Moles

1 mole (mol) = 6.02×10^23 molecules; Avogadro’s number

Molarity (M)

Number of moles of solute per liter of solution

Search for life on other planets

Focus on planets with water; Mars has water; some exoplanets show evidence of water vapor

Hydrogen ion in water

A hydrogen atom in a hydrogen bond can transfer as a proton (H+); molecule losing proton becomes OH–; molecule gaining proton becomes H3O+

Water dynamic equilibrium

Water molecules dissociate and reform at the same rate; H+ and OH– concentrations equal in pure water

H+ and OH– reactivity

H+ and OH– are highly reactive; small concentration changes affect cellular chemistry

Strong vs weak acids/bases

Strong dissociate completely in water; weak acids/bases reversibly release/accept H+ but shift balance

pH definition

pH = –log[H+]; neutral solution has pH 7

Acidic solution

pH < 7

Basic solution

pH > 7

Biological pH range

Most biological fluids have pH 6–8

Buffers

Substances that minimize changes in H+ and OH– concentrations; usually weak acid + corresponding base

Ocean acidification

Human CO2 absorbed by oceans forms carbonic acid; reduces carbonate ions, affecting calcifying organisms like corals

Environmental impact

Ocean acidification can cause ecosystem-wide changes; human action and awareness can improve water quality