Water and Life: Polarity, Bonding, and Properties

Water and It's Importance

• Earth consists of mostly water, covering 75% of its surface.
• Life on Earth originated in water and remained aquatic for three billion years prior to colonizing land.
• Terrestrial life forms remain dependent on water.
• Cells are composed of 70-95% water.

Connection Between Water and Life

• The polarity of water molecules and hydrogen bonding's results.
• Four emergent properties of water contribute to Earth's fitness for life.
• Acidic and basic conditions affect living organisms.

Polarity and Hydrogen Bonding

• Polarity is defined as a lack of symmetry.
• Many of water's properties that are suitable for life are a result of hydrogen bonding.

Emergent Properties of Water

• An emergent property is a new property that emerges at each step in the biological hierarchy due to the arrangement and interaction of parts.

Cohesive Behavior

• Cohesion, the binding together of like molecules, contributes to the transport of water and nutrients against gravity in plants.
• Surface tension, a measure of how difficult it is to stretch or break the surface of a liquid, is related to cohesion.

Moderation of Temperature

• Temperature measures average kinetic energy, which is the energy of motion.
• Heat is a form of kinetic energy measured by volume.
• Water has a high specific heat, requiring one calorie to change the temperature of one gram of water by 1°C.
• Hydrogen bonds contribute to water's high specific heat: as temperatures rise, heat is absorbed to break hydrogen bonds, and as temperatures fall, heat is released when hydrogen bonds form.
• Bodies of water impact climate and allow organisms to resist temperature changes.

Expansion Upon Freezing

• Hydrogen bonds becomes stable in ice.
• Water expands upon freezing due to hydrogen bonds.

Versatility As A Solvent

• A solvent is a dissolving agent that dissolves solutes, creating solutions.
• Water's polarity makes it a versatile solvent.
• Water dissolves hydrophilic substances, including salts and other polar molecules.
• Water does not dissolve hydrophobic substances, such as non-polar molecules like fats and oils.
• Biochemical reactions occur in water

Acidic and Basic Conditions

• A water molecule occasionally dissociates.
• 1 in every 554 million water molecules dissociate at any given time, where [H+] = 10^-7 M.
• [OH-] will be the same.
• H+ and OH- are very reactive, making their concentrations important.
• Concentrations change as acids and bases are added to water. An acid increases the [H+] of a solution, for example, HCI turning into H+ + CL-.
• A base reduces the [H+] of a solution, as seen with NaOH becoming Na+ + OH-, where OH- binds H+, forming water.
• The concentrations of H+ and OH- are measured on a pH scale, which ranges from 0-14, with a neutral pH of 7.
• pH is the negative log of [H+].
• As [H+] increases, pH decreases, and as [H+] decreases, pH increases.
• Most biological activities occur within a pH range of 6-8.
• Buffers help biological systems stay within this range.
• A buffer is a substance, typically a weak acid, that minimizes changes in [H+] and [OH-], for example, it accepts H+ when abundant and releases them when low.