A1.1 Water Unity and Diversity

The first cells originated in water, providing an internal compartment for controlled biochemical reactions.
Water is the primary solvent in the human body; aqueous solutions are the medium for most life processes and metabolic reactions.
The presence of water enabled the formation of the phospholipid bilayer in early cells.

Polar Covalent Bonds: Result from unequal sharing of electrons within a water molecule.
Partial Charges: The atom with higher electron density acquires a partial negative charge ( $\delta -$ ), while the atom with lower density acquires a partial charge ( $\delta +$ ).
Hydrogen Bonds: Intermolecular attractions occurring between the $\delta +$ hydrogen of one molecule and the $\delta -$ oxygen or nitrogen of another.
These bonds are the strongest intermolecular force and dictate water's unique properties.

Cohesion: Attraction between water molecules. It creates surface tension, allowing organisms like the basilisk lizard and water striders to move on water surfaces.
Xylem Transport: Cohesion maintains a continuous water column during transpiration.
Adhesion: Attraction between water and other polar/charged surfaces.
Capillary Action: Adhesion to xylem walls prevents the water column from breaking; adhesion to soil particles helps roots access water.
Cohesion-Tension Theory: Both forces work together to move water upwards against gravity.

Hydrophilic vs. Hydrophobic: Polar molecules (sugars, salts) dissolve in water; non-polar molecules (fats, steroid hormones) do not.
Metabolic Medium: Most enzymes catalyze reactions in aqueous solutions within the cytoplasm or organelles.
Transport: Water's polarity facilitates the transport of hydrophilic compounds in blood (animals) and xylem (plants).
Non-polar Functions: Hydrophobicity is essential for cell membrane integrity and the plant cuticle's ability to prevent dehydration.

Buoyancy: Upward force is significant in water (supports weight) but negligible in air.
Viscosity: Resistance to flow is higher in water, creating more drag for swimming organisms.
Thermal Conductivity: Water conducts heat more effectively than air.
Specific Heat Capacity: Water requires more energy to change temperature ( $1\,K / °C$ ), providing a stable thermal habitat.
Organism Adaptations: - Black-throated loon (Gavia arctica): Uses buoyancy to float; streamlined body to overcome viscosity; oil glands for insulation. - Ringed seal (Pusa hispida): Blubber reduces heat loss; paddle-like feet overcome viscosity; buoyancy allows surfacing to breathe.

Asteroid Theory: Water likely arrived via asteroids containing hydrated material.
Isotopic Evidence: The ratio of Hydrogen (1 proton, 1 electron) to Deuterium (1 proton, 1 neutron, 1 electron) in Earth’s oceans matches that found in asteroids.
Retention: Earth's strong gravitational field and cool temperatures (allowing condensation of liquid $H_2O$ ) prevented water from escaping into space.

Goldilocks Zone: The habitable region around a star where temperatures permit liquid water on a planet's surface.
Since water is the universal medium for life, liquid water is the primary indicator used when searching for life on other planets.

Define polarity. [1]
Describe the nature of hydrogen bonding. [2]
Distinguish between covalent and hydrogen bonding. [2]
Explain the consequences of cohesion on living organisms. [4]
Draw two water molecules experiencing hydrogen bonding. [4]
Explain the consequences of adhesion on living organisms. [4]
Compare the physical properties of water and air. [4]
Explain the adaptations two organisms have to the physical properties of water. [8]
Describe the chemical and physical properties of water that arise due to hydrogen bonding. [8]
Discuss the challenges and opportunities of water as a habitat. [8]
Additional Higher Level: - Define the Goldilocks zone. [1] - Outline why water was retained on Earth. [2] - Explain why the search for extraterrestrial life is linked to the presence of water. [2] - Outline the extraplanetary origin of water on Earth. [3]