1. Water
2.2

2. Essential Idea: Water is the medium of life.
2.2
Water
Understandings:
Water molecules are polar and hydrogen bonds form between them.
Hydrogen bonding and dipolarity explain the cohesive, adhesive, thermal and solvent properties of water
Substances can be hydrophilic or hydrophobic
Applications:
Compare the thermal properties of water with those of methane
Use of water as a coolant in sweat
Modes of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in blood in relation to their solubility in water

3. Quick Review: Covalent Bonding
Atoms share electrons
Can have single, double, triple bonds
Relatively strong bonds
Many different types for specific kinds of molecules (ex. peptide, glycosidic, phosphodiester)

4. WATER
Polarity: Unequal sharing of electrons leads to uneven distribution of charges around a molecule
Hydrogen side of water molecule being slightly positive while oxygen side is slightly negative
Attraction between positive side of one water molecule and negative side of next = Hydrogen bond

5. One hydrogen bond is weak, but a drop of water contains millions and millions, making water very strong

6. Cohesion Water molecules are attracted to other water molecules
Water forms into droplets (which join together when close)
Water has strong surface tension (remember penny lab?)
Water can move as a column in plants
(1:55)

7. Adhesion
Attraction between water molecules and other types of molecules
Also at play in movement of water in vascular plants (capillary action)
Once adhesion moves water up the vascular tissue, adhesion keeps it from dropping back down

8. Thermal Properties
High Specific Heat – water can absorb a lot of heat without changing temperature
 Moderate climate near large bodies of water
 Living things contain large amounts of water – helps stabilize body temp
High Heat of Vaporization – have to add a lot of heat to get water to evaporate
Coolant – When you sweat, the sweat absorbs your body heat in order to evaporate
Water
Methane
Polarity
Polar
Non-polar
Hydrogen Bonds
Yes No
Boiling Point
100 degrees C
-162 degrees C
Freezing Point
0 degrees C
-183 degrees C

9. Solvent Capable of dissolving an abundance of other molecules
Makes it extremely valuable in transporting molecules in your body
Water being transported by cohesion and adhesion in plants is carrying dissolved substances everywhere it goes
Blood is composed mainly of water – carries dissolved substances around your body (glucose, ions, amino acids)
Also makes it an excellent medium for metabolic reactions

10. Like likes like
Hydrophilic molecules – molecules that dissolve in water
Generally polar (polar solvent-water-dissolves polar solutes)
Ex. glucose and other carbohydrates, ions such as sodium (Na+, Ca++, Cl-)
Hydrophobic molecules – don’t dissolve in water
Generally non-polar (charges evenly distributed around molecule, atoms share electrons equally)
Ex. Lipids, methane
Hydrophobic Interactions - There is often a slight
attraction between non-polar molecules, and they
avoid water together

11. ***Whether a molecule can be dissolved by water or not determines how easily it can be transported in the body***