13.3 Water, Organization of Matter & Acids/Bases/Salts

Water, Matter, and pH in Biological Systems

Life is aqueous chemistry. The PA-CAT Bulletin of Information, rev. 20240815 includes Water, Organization of Matter, and Acids/Bases/Salts among General Biology fundamentals (Table 6). Understanding why water is the universal solvent and how pH shapes enzyme function underpins most physiology questions.

Organization of Matter

Matter is anything with mass and volume. The hierarchy:

• Element: a pure substance of one atom type (C, H, O, N make up about 96% of body mass).
• Atom: the smallest unit retaining element identity; a nucleus (protons + neutrons) orbited by electrons. Atomic number = protons; mass number = protons + neutrons. Isotopes differ in neutron count.
• Molecule: two or more atoms covalently bonded (H2O, O2, CO2).
• Compound: two or more different elements chemically combined (H2O is both a molecule and a compound; O2 is a molecule but not a compound).
• Ion: a charged atom or molecule (Na+, Cl-, HCO3-). Ionic bonds form between oppositely charged ions; covalent bonds share electron pairs.

A mixture is physically combined, not chemically. Solutions have solute dissolved in solvent; colloids have larger particles that stay suspended (milk, cytosol); suspensions settle over time. Blood is a suspension of cells in plasma, with plasma itself a solution of proteins, salts, and glucose.

Properties of Water That Make Life Possible

Water's bent geometry and polar O-H bonds make it a polar molecule, and its properties dominate biology:

1. Cohesion — hydrogen bonds between water molecules create surface tension (insects walk on water) and let water columns rise in plant xylem.
2. Adhesion — water clings to other polar surfaces (capillary action, water climbing glass tubes or vessel walls).
3. High specific heat — water resists temperature change, stabilizing body and environmental temperatures (sweating cools by absorbing heat; oceans moderate climate).
4. High heat of vaporization — evaporation removes large heat loads (perspiration is an effective cooling mechanism because each gram absorbs about 580 calories).
5. Universal solvent — polar water dissolves ionic and polar solutes (NaCl, glucose); hydrophobic molecules (lipids) do not dissolve.
6. Less dense as ice — solid water floats because hydrogen bonds form a spacious crystal, insulating aquatic life in winter.
7. Participant in reactions — water directly enters hydrolysis, dehydration, and photosynthesis.

Because water is polar, it surrounds charged and polar solutes with a hydration shell, which is why NaCl dissociates into Na+ and Cl- in water. This solvation is the physical basis of blood plasma, cytosol, and urine.

Acids, Bases, and the pH Scale

An acid donates protons (H+) in solution; a base accepts them. The pH scale (0-14) is the negative log of H+ concentration: pH = -log[H+]. Pure water at 25 degrees C is pH 7 (neutral). Each pH unit is a tenfold H+ change — pH 2 has 100,000 times the H+ of pH 7. A solution at pH 4 has 1000 times the H+ of a solution at pH 7.

Biological pH is tightly regulated. Buffers resist pH change by absorbing or releasing H+. The bicarbonate buffer system maintains blood pH at 7.35-7.45: CO2 + H2O to H2CO3 to H+ + HCO3-. When blood pH drops, the equilibrium shifts to form more CO2, which is exhaled; when pH rises, CO2 is retained to generate more H+. The respiratory and renal systems cooperate with buffers. Acidosis (blood pH below 7.35) and alkalosis (pH above 7.45) are clinically dangerous because enzyme active sites denature outside narrow ranges — pepsin works at gastric pH but would denature at blood pH, and blood enzymes work only near pH 7.4.

Salts and Electrolytes

A salt forms when an acid neutralizes a base — the H+ of the acid is replaced by a metal cation (NaCl from HCl + NaOH). Salts dissociate into electrolytes in solution, conducting current and driving membrane potentials. Physiologically critical electrolytes: Na+ (extracellular osmolarity), K+ (intracellular, membrane potential), Ca2+ (muscle contraction, signaling, bone), Cl- (fluid balance), and HCO3- (pH buffer). Electrolyte imbalances — hyperkalemia (high K+ causing arrhythmias), hyponatremia (low Na+ causing seizures), hypocalcemia (tetany) — illustrate why acid-base and salt physiology appear on the PA-CAT. IV fluids restore both volume and electrolyte composition, which is why normal saline contains 0.9% NaCl.

Exam Strategy

For pH items, remember: acids raise H+ and lower pH; bases lower H+ and raise pH. Buffers do not eliminate H+; they moderate change. For compound items, ask whether two different elements are chemically bonded — H2O is, O2 is not. For water-property items, match the property to its biological consequence (cohesion to xylem transport; high specific heat to temperature stability; solvent to nutrient transport). For buffer items, identify the weak acid/conjugate base pair (carbonic acid/bicarbonate in blood).