Biological Importance of Water

Specific heat is a measure of thermal inertia. Water's specific heat is defined as 1 calorie per gram per degree Celsius. The high value is due to the energy needed to first disrupt hydrogen bonds before molecular kinetic energy (and thus temperature) can increase.

When ions or polar molecules dissolve, water molecules orient themselves according to the charge. The δ⁺ hydrogens face an anion, and the δ⁻ oxygen faces a cation. This layer of tightly bound water is the hydration shell, which isolates and stabilizes the solute in solution.

The latent heat of fusion is the heat energy released when water freezes. Because this value is high for water, the freezing process releases heat, slowing the rate of ice crystal formation. This protects cell contents from lethal intracellular freezing in organisms exposed to sub-zero temperatures.

Water moves from higher water potential to lower water potential. If the external solution has a lower (more negative) water potential (hypertonic), water leaves the cell by exosmosis. The protoplast shrinks and detaches from the rigid cell wall, a process called plasmolysis.

In the Krebs cycle, the enzyme fumarase catalyzes the addition of a water molecule across the double bond of fumarate. This hydration reaction converts fumarate to malate. Water is a direct reactant in this specific step, not just a solvent.

Osmosis is the net passive diffusion of water across a selectively permeable membrane from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration). It does not require energy and is central to water relations in cells.

In ice, each water molecule forms a maximum of four hydrogen bonds in a rigid, crystalline hexagonal lattice. This structure holds molecules farther apart on average than in the liquid state, reducing the density. This lattice is an open, ordered arrangement, maximizing H-bonding.

Large bodies of water absorb solar energy during the day and release it slowly at night without large temperature swings. This property, due to the extensive hydrogen bonding network, protects aquatic organisms from thermal shock and provides a stable environment.

The reaction CO₂ + H₂O ⇌ H₂CO₃ is catalyzed by carbonic anhydrase. Here, water is not just a solvent but a substrate that chemically participates in the reaction by combining directly with carbon dioxide.

Water has a V-shaped bent geometry (104.5° bond angle). Oxygen's higher electronegativity pulls shared electrons closer, creating a partial negative charge (δ⁻) on oxygen and partial positive charges (δ⁺) on hydrogens. The bonds are polar covalent, not ionic, and the unequal sharing creates a molecular dipole.

Water dissolves polar and ionic solutes by forming hydration shells. The partial charges of water molecules interact electrostatically with ions or polar groups, and hydrogen bonding stabilizes the dissolved state. Low molecular weight and high specific heat are separate properties.

Hydrolysis (hydro = water, lysis = splitting) uses water as a reactant. The bond in the polymer is broken, and the components of water (H and OH) are added to the resulting monomers. Enzymes catalyze this reaction, but water is a substrate, not a catalyst.

Water's high specific heat (1 cal/g°C) means it absorbs considerable heat energy for a small temperature increase. This property, due to hydrogen bonding, provides thermal stability to organisms and large bodies of water, protecting protoplasm from drastic temperature shifts.

Enzymatic digestion of starch is a hydrolysis reaction. Water molecules are used to break the α-1,4 glycosidic bonds between glucose monomers. The H from water attaches to one glucose, and the OH attaches to the adjacent glucose.

Cohesion creates a strong network of hydrogen bonds at the water-air interface, generating surface tension. This film-like layer is resistant to external force, supporting objects denser than water if they do not break the surface layer.

At 4°C, water is densest. Below 4°C, it expands, and ice (0°C) is ~9% less dense, so it floats. This surface ice layer insulates the liquid water below, maintaining a temperature above freezing and allowing aquatic life to survive winter.

Transpiration pull creates negative pressure in xylem. Due to strong cohesion (H-bonds between water molecules), the continuous water column is pulled upwards. Adhesion to xylem walls also assists, but the tensile strength of the water column is a direct result of cohesion.

During the light-dependent reactions, water undergoes photolysis. Water is split (oxidized) by the oxygen-evolving complex, providing replacement electrons to P680 (Photosystem II) and releasing protons (H⁺) and molecular oxygen (O₂) as a byproduct.

When sweat evaporates, the phase change from liquid to gas requires a large amount of heat energy (latent heat of vaporization). This heat is absorbed from the skin's surface, effectively cooling the body. The cooling is due to heat removal, not heat release.