• The subcomponents of biological molecules and their sequences determine the properties of that molecule

Properties of Water

  • Water: 2 hydrogen and 1 oxygen bonded together with (Polar) COVALENT bonds bcuz oxygen is more electronegative, so electrons pulled closer to it
    • Water is POLAR- overall charge is unevenly distributed (+ & -), allows molecules to form HB
  • The hydrogen bonds between water molecules result in cohesion, adhesion, and high surface tension, self-regulation, expansion upon freezing, versatility as a solvent
  • Cohesion: water molecules attached to each other (bcuz of HB)
    • Makes water “sticky”
    • Ex: water molecules evaporate from leaf by pulling on neighboring water which then draw up molecules behind them
  • Adhesion: water molecules attached to OTHER things
    • Positively charged part attract negatively charged ends of other polar compounds and vice versa
  • Cohesion + Adhesion allows for capillary action: transport of water up roots & against gravity
    • Water pulls each other up and can adhere to narrow tubing during transpiration
  • Surface tensions: “Tendency of molecules to be pulled from the surface to the interior of a liquid”
    • Water has high surface tension bcuz of increased HB forces at surface that resist being stretched/broken → allows organisms to walk on water


  • Water has high specific heat because lots of energy is absorbed when bonds break and released when hydrogen bonds form
    • So body of water can absorb lots of heat and resist chemical change; needs lots of energy to freeze
  • Cool air when it is warm and release heat in the winter
  • Stabilizes ocean temperature, climate, and organisms (bcuz they are mostly made up of water)
  • When water changes physical state, energy is absorbed but water temp remains constant → absorbed energy just changes HB

Evaporative Cooling

  • As water evaporates, cools the surface of the earth & organisms bcuz molecules with more heat energy evaporate quicker
    • Hydrogen bonds need to break for water to vaporize

Expansion Upon Freezing

  • Water is less dense/expands as a solid bcuz weak HB stabilize/crystalize, keeping molecules too far apart (also slow) to break them
  • So ice floats and insulates water, reducing heat loss from water below → warmer water that is less likely to freeze
    • If sank then all water would freeze over → no aquatic life

Versatility as a Solvent

  • Solvent: dissolving agent Solute: substance dissolved
  • Water versatile solvent because polar molecules are attracted to other polar molecules and form HB (adhesive)
    • (+) and (-) attraction
  • Ionic compounds are soluble because poles of polar water molecules interact with them and separate into ions
  • Water helps cell transport substances throughout the body (ex. nutrients, antibiotics)
  • Hydrophilic: affinity for water, polar Hydrophobic: repel water, nonpolar

Acids and Bases 

  • Water molecule can transfer H+ to another water forming OH (hydroxide) and hydronium ion
    • Water is an acid and base
  • Hydrogen Ion: Single proton, highly reactive/energy/unstable
  • Acids and bases cause imbalance in H and OH
  • Acids: decrease pH, ione in water to increase H+, less hydronium ion. 0-6
  • Bases: increase pH decreases H+, more OH; 8-14
    • Bases break into OH which can combine with H+ to create water (accept H+)
  • Buffer solutions: maintain a ~constant pH when either acids or bases are added
    • Resist shifts in pH by donating H⁺ to a solution when bases are added, and accepting H⁺ when acids are added
  • pH Scale: measures the concentration of H+ → Formula:  pH = -Log[h+]
    • Change of one pH number represents a tenfold change in hydrogen concentration
      • Ex: pH of 3 is ten times more acidic as pH of 4

Water Potential

  • “Measure of potential energy of water & eagerness to go from high potential to low”
  • Water potential = pressure potential + solute potential
    • High water potential = more water molecules (higher concentration) & will move somewhere else
    • More solutes = increases osmolarity and decreases water potential