Water and Life

Water is essential for all known forms of life. Life started in water; all organisms rely on water for biochemical processes.
Earth is a “water planet”: almost 75% of the surface is covered by water; most of Earth's water is saltwater in the oceans.
Water is a simple molecule (H₂O) with a polar structure that enables unique properties essential for life.
Water’s properties arise from hydrogen bonding and polarity: cohesion, adhesion, and capillary action support transport in plants and animals.
Water is involved in many biochemical reactions and serves as a solvent for many substances.
Water’s high boiling point and expansion on freezing have major ecological and physiological implications (ice floats, protecting aquatic life in winter).

Water molecule composition: two hydrogen atoms and one oxygen atom:
$ext{H}_2 ext{O}$
Oxygen is more electronegative, giving water a partial negative charge on O and partial positive charges on the H atoms.
This charge separation makes water a polar molecule and supports hydrogen bonding between neighboring water molecules.
Water dissolves many substances, including ionic compounds like table salt (NaCl), by stabilizing ions (Na⁺, Cl⁻) in solution.
Hydrogen bonds form between molecules, are numerous, and collectively stronger as a group than covalent bonds within a molecule.

Hydrogen bonds: temporary attractions between the positive H end of one molecule and the negative O end of another.
Cohesion: water molecules stick to each other, leading to droplet formation and surface tension.
Adhesion: water molecules stick to other substances (e.g., plant tissues), enabling movement along surfaces.
Capillary action: adhesion + cohesion allow water to move against gravity in narrow spaces (e.g., in plant stems, capillaries, or a straw).
Water has a relatively high boiling point due to extensive hydrogen bonding, keeping it liquid over a wide temperature range on Earth.
Ice expands when water freezes, giving ice a lower density than liquid water, so ice floats on water.

Water is involved in key biochemical reactions:
- Photosynthesis (water is a reactant):
 $6\,CO2 + 6\,H2O + \text{Energy} \rightarrow C6H{12}O6 + 6\,O2$
- Cellular respiration (water is produced):
 $C6H{12}O6 + 6\,O2 \rightarrow 6\,CO2 + 6\,H2O + \text{Energy}$
Water serves as a solvent for many biochemical reactions and helps transport nutrients.

Water transport in plants relies on cohesion and adhesion, moving water up through roots and stems.
Evaporation and transpiration contribute to the movement of water within plants.

Where is most of Earth’s water found? (Ocean/ Saltwater)
What percent of Earth’s water is freshwater? (Refer to course content; freshwater is far less than total water due to oceans being the major reservoir.)
What is polarity? Describe the polarity of water. (Water is polar; oxygen end is slightly negative, hydrogen ends are slightly positive.)
How could you demonstrate to a child that solid water is less dense than liquid water? (Ice floats on liquid water.)
Explain how water’s polarity is related to its boiling point. (Polarity enables hydrogen bonding; extensive hydrogen bonding raises the boiling point.)
Explain why metabolism in organisms depends on water. (Water acts as solvent and reactant/product in key biochemical reactions; many metabolic processes require water.)