Water

• Universal Solvent

  • Many substances dissolve in water due to polarity or dissociation to give polar ions
  • Essential transport medium for polar molecules and ions; main part of blood, tissue fluid, urine, and phloem sap
  • Facilitates metabolic reactions; soluble reactants move freely in aqueous cell cytoplasm
• Key water information

  • Most common molecule
  • Photosynthesis reactant
  • Important habitat for many (covers most of Earth’s surface)
  • ~70% body mass for most organisms (mammals ~65%)
  • Aqueous media: blood plasma, tears, urine, and cell sap
  • Small
  • 2 covalent bonds
  • Formula H_2O
  • Oxygen electronegative, slight negative charge (δ-) + hydrogen slightly positively charged (δ+)
  • Uneven charge distribution makes water molecule polar (dipolar; slight positive, other slight negative)
  • Unequal electron sharing: Oxygen has more protons, so a greater attraction for electrons than hydrogen
  • H bonds
  • Polar so attracts itself (positive end attracted to negative end of other)
  • Form hydrogen bond (H-bond) w/ weak intermolecular forces
  • Gives water many special properties
• State Changes

  High Boiling Point (100C)

  • Lots of energy needed to break H bonds
  • Remains liquid at room temperature
  • Provide aquatic habitat for many organisms + liquid cytoplasm

  Freezing + Density

  • Density change cause currents; circulate nutrients
  • Density lowers as temperatures fall below 4°C (max density)
  • Pond bottom safe non-frozen habitat
  • Ice formed at 0°C has lower density; floats
  • Water expands when freezing
  • Allow max H-bonds to form, pushing molecules apart
  • Makes a crystal lattice
  • Floating ice good; thermally insulates water (also habitat itself)
  • Large energy loss needed to change to ice; less likely
• Cohesion + Adhesion

  Adhesion

  • Water's attraction to other materials

  Cohesion

  • Water attraction to itself
  • Strong cohesive forces allow water to move up xylem vessels as continuous column in transpiration stream
  • Cohesion on pond surface makes it behave like skin; surface tension
  • Strong enoguh to support creatures like pond skaters
• Coolant

  • During chemical reactions it buffers temperature changes
  • Due to large amounts of energy needed to break H bonds
  • Internal constant temperature cellular
  • Important: enzymes are often only active in narrow temperature range
  • Keeps it thermostable