BIOL- 102 6/23 (unit 1)

The Importance of Water for Life

Water and Life

• It's generally accepted that all living things require water to survive, but it's also important to consider "all known" living things, as there may be exceptions that we have yet to discover.

Unique Properties of Water

• Water is critical due to its unique properties, primarily its polarity.

• Water is a polar covalent molecule, and it's one of the smallest molecules with this level of polarity.

• The combination of a special trait (polarity) and small size makes water abundant in living things.

Hydrogen Bonds

• Polarity causes water to form many strong hydrogen bonds.

• These hydrogen bonds hold water molecules together.

• Water forms hydrogen bonds with other polar molecules, making it a good solvent (hydrophilic).

• Water repels nonpolar molecules (hydrophobic), like oils and fats, which do not dissolve in water.

Temperature and Water

• Temperature is a measure of the movement of molecules. Faster movement means bigger collisions, which we feel as heat or warmth.

• Increased energy (temperature) disrupts hydrogen bonds in water.

• At higher temperatures, molecules move faster, making it harder to maintain hydrogen bonds.

Evaporation

• Water is a liquid at room temperature due to its polarity and hydrogen bonds, which hold the molecules together.

• Without these bonds, water molecules would move fast enough to be airborne at room temperature.

High Heat Capacity

• Water has a high heat capacity, meaning it takes a lot of energy to increase its temperature because the energy goes into breaking hydrogen bonds.

• It also takes a long time to cool water down, as it has to release a lot of energy.

Surface Tension

• Water has high surface tension due to the molecules on the surface holding hands via hydrogen bonds.

• Evaporation requires enough energy to break these hydrogen bonds and allow molecules to escape into the air.

• Evaporation is effective at cooling because the water absorbs heat to evaporate.

• Sweating cools the body by using the heat to evaporate water.

• Water draws heat from your body when you're in cold water (e.g. a pool at 75 degrees Fahrenheit, or 23.8 degrees Celsius).

Solid State of Water (Ice)

• Water releases a lot of energy when cooling down to become ice.

• Water forms a crystalline structure when it freezes, which is less dense than liquid water.

• Most substances become denser when they solidify, but water expands.

Implications of Ice Floating

• If ice sank, lakes would freeze from the bottom up, killing aquatic life.

• Instead, ice floats, forming an insulating layer on top of the water, preventing the rest of the lake from freezing.

Moderating Temperature

• Water moderates temperature because of its high heat capacity.

• In San Francisco, the water keeps the city warmer in the winter and colder in the summer.

• In the summer, the cold water surrounding San Francisco sucks heat out of the air, keeping it cool.

• In the winter, the water releases heat into the air, keeping the city warmer.

• The closer you are to the water, the more the temperature is moderated, which is why coastal areas have different microclimates than inland areas.

• Weather patterns generally move from west to east - weather moves from on top of the ocean.