A1.1 Water and D2.3 Water Potential

Life originated in water, and most biochemical processes occur in aqueous solutions. About 70% of organisms are water. Water dissolves solutes for metabolism, transports substances, and supports chemical reactions.

Oxygen is more electronegative than hydrogen, so electrons in the covalent bonds are unequally shared. Oxygen gains a partial negative charge (δ–), hydrogens partial positive charges (δ+), making water polar.

Weak intermolecular attractions form between the δ+ hydrogen of one water molecule and the δ– oxygen of another. Individually weak, but collectively strong, these bonds give water its unique properties.

Cohesion = water molecules stick to each other via H-bonds. • Allows surface tension → insects “walk on water.” • Enables water to move in unbroken columns under tension in plant xylem.

Adhesion = attraction between water and other polar molecules. • Allows capillary action (water climbing narrow spaces like xylem walls). • Creates meniscus in tubes. • Helps distribute water in plant tissues and soil.

Water dissolves polar and ionic substances by surrounding ions with oppositely charged poles (solvation). • Transports glucose, amino acids, ions in blood plasma. • Dissolves reactants for metabolism (enzymes act in aqueous solution). • Non-polar substances (lipids, oils) do not dissolve → critical for membrane formation.

• High specific heat capacity • High latent heat of vaporization • High heat of fusion • Thermal conductivity

Resists rapid temp changes → stable aquatic habitats.

Evaporation removes large amounts of heat → sweating/panting cools organisms.

Absorbs much energy before freezing → aquatic organisms survive cold seasons.

Quickly transfers heat → marine mammals need insulation (blubber).

At low temps, H-bonds form a lattice that spaces molecules apart, lowering density. Ice floats, insulating underlying water, allowing aquatic life to survive below frozen surfaces.

• Buoyancy: fish use swim bladders; diving birds compress lungs. • Thermal conductivity: seals rely on blubber for insulation. • Viscosity: affects swimming efficiency. • Specific heat: stabilizes body temperature in aquatic environments.

Water is essential as a solvent and medium for metabolism. Scientists search for exoplanets in the “Goldilocks zone,” where liquid water can exist. Hypotheses suggest Earth’s water came from asteroids and was retained due to gravity and suitable temperatures.

The tendency of water to move into a solution due to solute concentration. Higher solute concentration = higher osmotic potential.

• Isotonic: Equal solute conc. → no net water movement (dynamic equilibrium). • Hypotonic: Lower solute conc. outside → water enters cell. • Hypertonic: Higher solute conc. outside → water leaves cell.

Hypotonic: water enters → swelling, possible lysis (bursting). Hypertonic: water leaves → shrinkage (crenation). Isotonic: normal shape maintained.

Hypotonic: water enters → turgid, provides support. Hypertonic: water leaves → plasmolysis, cell shrinks away from wall. Isotonic: flaccid, no pressure on cell wall.

To prevent harmful osmotic effects on cells: • Hypotonic IV → swelling and bursting of RBCs. • Hypertonic IV → shrinkage of RBCs. • Isotonic IV → maintains equilibrium, safe for tissues and organs in transplant preparation.

Freshwater protozoa use contractile vacuoles to expel excess water. Rate of contraction depends on osmolarity of environment.

The irreversible loss of turgor when plant cells lose water in a hypertonic solution, causing the plasma membrane to detach from the cell wall.

Measure tissue length/mass in solutions of varying osmolarities. At isotonic concentration, no net change occurs. Reliability can be improved using repeats, calculating standard deviation, and showing error bars.

((Final Value - Initial Value) / Initial Value) x 100