Water 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

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)

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

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

Cohesion Water molecules are attracted to other water molecules Water forms into droplets (which join together when close) https://www.youtube.com/watch?v=btD84ESfTJM Water has strong surface tension (remember penny lab?) https://www.youtube.com/watch?v=OAliTzF8lyc Water can move as a column in plants https://www.youtube.com/watch?v=f8TBxPOwjvs (1:55)

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

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

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

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

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

http://www.youtube.com/watch?v=Oe3St1GgoHQ