BIO 121 Lecture Notes - Lecture 5: Body Fluid, Evaporation, Sap
Document Summary
Bio121 week 3 lecture 5 (continued) 75% of earth"s surface is covered by water. Water is the only common molecule to exist in all three states of matter. Life originated in water and evolved there for 3 billion years before moving onto land. Oxygen is very electronegative: electronegativity an atom"s attraction for the shared electrons of a molecule"s covalent bonds. Water is a polar molecule: water"s oxygen atom, because of its electronegativity, pulls electrons slightly closer to itself than do water"s hydrogen atoms. Electrons aren"t shared perfectly equally: this gives the oxygen atom in a water molecule a slight negative charge and the hydrogen atoms slight positive charges. Nonpolar molecules are those in which the atoms share electrons perfectly equally: the atoms have equal electronegativities. The polarity of water molecules and the hydrogen bonds between water molecules are responsible for water"s life- supporting properties: cohesion and adhesion, heat capacity, ice is less dense than liquid water, aqueous solutions.
