WATER AND LIFE

Without water there is no life• Cells are 70-90% water• Most cells are surrounded by water• The abundance of water is the main reason

there is life on Earth

I. The Polarity of Water

A. Water is a polar molecule formed by polar covalent bonds• The huge difference in electronegativities

between O and H cause shared e- to spend a greater amount of time closer to O’s nucleus

• OH bonds and NH bonds are polar covalent• O side is slightly negative while H side is

slightly positive

B. Water forms hydrogen bonds• Weak attraction between positive H and negative

O between water molecules or any molecules with OH and NH

http://programs.northlandcollege.edu/biology/biology1111/animations/hydrogenbonds.html

II. Water’s 4 Emergent Properties that Contribute to Life

• Cohesive nature• Ability to moderate temperature• Expansion upon freezing• Versatile solvent

A. Cohesion and adhesion

• Cohesion: water molecules stick together thru hydrogen bonds

• Adhesion: water molecules and other polar molecules (like cellulose in cell walls)

1. Role in water transport in plants

• As water evaporates out of open stomata in plants (transpiration), cohesion and adhesion pull water up plant xylem

https://www.youtube.com/watch?v=mc9gUm1mMzc

2. Surface tension

• Due to cohesion, the surface of water is hard to break

2. Temperature Moderation

• Water absorbs heat warmer air and releases heat to cooler air without changing much in temperature because it has a high specific heat

• Specific heat: amount of heat that must be absorbed or released in order to change the temp by 1o C

• Water’s high specific heat is due to hydrogen bonding–Heat is absorbed when hydrogen bonds

break–Heat is released when hydrogen bonds

form

• Evaporative cooling and water–Water has a high heat of vaporization:

amount of heat needed to transform 1g of liquid water to water vapor•Heat increases molecular motion•Hydrogen bonds limit molecular motion•Much heat must be added to cause water

to evaporate

C. Ice Floats

• What do you notice about water molecules in liquid water and ice?

• If ice sank all bodies of water would eventually freeze solid making life impossible

D. Versatile Solvent

• Water can dissolve ionic compounds and large molecules with ionic and polar regions by forming hydration shells around them

• https://www.youtube.com/watch?v=EBfGcTAJF4o

Hydrophilic: water lovingHydrophobic: water hating

III. Acids Bases pH and Buffers

• In pure water [H+] = [OH-]• Adding acids and bases to water alters this• pH measures how acidic or basic a solution is• acid increases [H+] while base decreases [H+]• [H+][OH-] = 10-14

• What is [H+] and [OH-]?• pH = -log[H+]• in pure water, pH = -(-7) = 7

Buffers are substances that minimize changes in pH by reversibly combining with H+