Water, Acids, Bases, and Solutions in Biology

Water: Structure and Properties

Importance of Water in Biology

Water is a vital molecule for life, as most biochemical reactions in cells occur in aqueous environments. Its unique chemical and physical properties make it essential for biological systems.

• Water is a polar covalent molecule: The chemical formula is H2O. The oxygen atom is more electronegative than hydrogen, resulting in an uneven distribution of electrons.

• Partial charges: Oxygen has a partial negative charge (δ−), and hydrogens have partial positive charges (δ+).

• Hydrogen bonding: The polarity allows water molecules to form hydrogen bonds with each other and with other polar molecules.

Structure of the Water Molecule

• Oxygen atom: Has a higher electronegativity, attracting electrons more strongly.

• Hydrogen atoms: Have a partial positive charge due to electron deficit.

• Hydrogen bonds: Weak attractions between the hydrogen atom of one water molecule and the oxygen atom of another.

Key Properties of Water

• Cohesion: Water molecules stick to each other due to hydrogen bonding. This property allows for surface tension, enabling small insects to walk on water.

• Adhesion: Water molecules stick to other polar substances, aiding in capillary action (movement of water up plant stems).

• High specific heat: Water can absorb or release large amounts of heat with little temperature change, helping organisms maintain stable internal temperatures.

• High heat of vaporization: A large amount of energy is required to convert water from liquid to gas, which helps with cooling (e.g., sweating).

Examples and Applications

• Surface tension: Water striders can walk on water due to cohesion.

• Temperature regulation: Organisms use water to buffer against rapid temperature changes.

Solutions, Solvents, and Solutes

Definitions

• Solution: A homogeneous mixture of two or more substances.

• Solvent: The substance present in the greatest amount; in biology, water is the most common solvent.

• Solute: The substance dissolved in the solvent.

Types of Solutions

• Aqueous solution: A solution in which water is the solvent.

• Dissociation of salts: Ionic compounds like NaCl dissociate into ions (Na+ and Cl−) in water.

Hydrophilic and Hydrophobic Substances

• Hydrophilic: Substances that dissolve easily in water (e.g., salts, sugars, proteins with polar groups).

• Hydrophobic: Substances that do not dissolve in water (e.g., oils, fats, nonpolar molecules).

Property	Hydrophilic	Hydrophobic
Water solubility	High	Low
Example	Glucose, NaCl	Oils, fats
Interaction with water	Forms hydrogen bonds	Repels water

Acids, Bases, and pH

Definitions

• Acid: A substance that increases the hydrogen ion (H+) concentration in a solution.

• Base: A substance that decreases the hydrogen ion concentration, often by releasing hydroxide ions (OH−).

• pH: A measure of hydrogen ion concentration; defined as:

$pH = -\log_{10}[H^+]$

• Neutral solution: pH = 7 (equal concentrations of H+ and OH−).

• Acidic solution: pH < 7 (higher H+ concentration).

• Basic solution: pH > 7 (lower H+ concentration).

Strength of Acids and Bases

• Strong acid: Completely dissociates in water (e.g., HCl).

• Weak acid: Partially dissociates in water (e.g., CH3COOH).

• Strong base: Completely dissociates to release OH− (e.g., NaOH).

• Weak base: Partially dissociates or reacts incompletely (e.g., NH3).

Type	Name	Chemical Formula	Strength
Acid	Hydrochloric acid	HCl	Strong
Acid	Acetic acid	CH3COOH	Weak
Base	Sodium hydroxide	NaOH	Strong
Base	Ammonia	NH3	Weak

pH Scale and Logarithmic Nature

• Each unit change in pH represents a tenfold change in [H+].

• For example, a solution with pH 5 has 10 times more H+ than a solution with pH 6.

• Similarly, a solution with pH 4 has 1,000 times more H+ than a solution with pH 7.

Buffers

Buffers are substances that minimize changes in pH by absorbing or releasing H+ or OH−. They are crucial for maintaining stable pH in biological systems, such as blood, where the bicarbonate buffer system operates.

• Example: The bicarbonate buffer system involves carbonic acid (H2CO3) and bicarbonate ions (HCO3−).

Practice and Application Questions

1. Draw a molecule of water and indicate the location of dipoles.

2. Explain how molecules of water interact using hydrogen bonding. Draw a water molecule hydrogen bonding with another water molecule.

3. Contrast adhesion and cohesion. Explain how some organisms can walk on the surface of water.

4. Draw a molecule of water hydrogen bonding with an ammonia molecule.

5. How does a water molecule interact with a large protein in a cell?

6. Explain how water controls temperature in terms of specific heat and heat of vaporization.

7. Define solution, contrast solvent and solute.

8. Contrast hydrophilic and hydrophobic substances.

9. What is pH? What does a change in pH of 1 unit mean in terms of hydrogen ion concentration?

10. What are buffer systems? Why are they important in living organisms?

11. What are two ways bases affect the ions in a solution? What do bases do to pH of a solution?

12. What is the difference between a strong acid and a weak acid? What is an example of each in terms of name and chemical formula?

13. Explain the importance of buffers in an organism.

Additional info: These notes expand on the provided material with definitions, examples, and tables for clarity and completeness, as expected in a college-level General Biology course.