Water and Its Properties

Hydrophilic and Hydrophobic Substances

Water is a universal solvent in biological systems, and its interactions with other substances are crucial for life. Substances can be classified based on their affinity for water.

• Hydrophilic: Substances that have an affinity for water and dissolve easily in it. These are typically polar or charged molecules (e.g., salts, sugars).

• Hydrophobic: Substances that repel water and do not dissolve easily. These are usually nonpolar molecules (e.g., oils, fats).

• Example: Towels are hydrophilic, absorbing water, while waxed paper is hydrophobic and repels water.

Importance of Water in Biological Systems

Water's unique properties make it essential for life:

• Cohesion: Water molecules stick together due to hydrogen bonding.

• Surface Tension: The cohesive forces at the surface of water allow it to resist external force.

• Evaporative Cooling: As water evaporates, it removes heat, helping organisms regulate temperature.

• Solvent Abilities: Water dissolves many substances, facilitating chemical reactions in cells.

Solutions, Molarity, and Ions

Molarity

Molarity is a measure of the concentration of solutes in a solution, commonly used to track the number of moles of a substance per liter of solution.

• Definition: Molarity (M) = moles of solute / liters of solution

• Application: Used to report concentrations in biological fluids and laboratory solutions.

Ions in Solution

Water can dissociate into ions, which are critical for many biological processes.

• Hydrogen Ion (H+): Contributes to acidity.

• Hydroxide Ion (OH-): Contributes to basicity.

• Ion Formation: Water molecules can shift protons, forming H+ and OH- ions.

• Example: Dissociation of water:

Acids, Bases, and the pH Scale

Acids and Bases

Acids and bases are substances that alter the concentration of hydrogen ions in a solution.

• Acids: Increase the concentration of H+ ions when dissolved in water (e.g., hydrochloric acid).

• Bases: Decrease the concentration of H+ ions, often by increasing OH- ions (e.g., sodium hydroxide).

• Example: Hydrochloric acid (HCl) dissociates in water:

pH Scale

The pH scale measures the concentration of hydrogen ions in a solution, indicating its acidity or basicity.

• Definition: pH = -log10[H+]

• Range: 0 (most acidic) to 14 (most basic), with 7 being neutral.

• Biological Relevance: Most living organisms maintain a near-neutral pH (around 7).

• Example: Human blood has a pH close to 7.4.

Buffers

Buffers are substances that help maintain a constant pH in biological systems by absorbing or releasing hydrogen ions.

• Function: Prevent drastic changes in pH when acids or bases are added.

• Example: Carbonic acid-bicarbonate buffer system in blood.

Carbon and Its Biological Importance

Properties of Carbon

Carbon is the backbone of organic molecules and is essential for life due to its ability to form diverse and stable bonds.

• Versatility: Can form single, double, and triple bonds; bonds with many elements.

• Complexity: Forms large, complex molecules (e.g., carbohydrates, proteins, lipids, nucleic acids).

• Example: All living organisms are primarily composed of carbon-based molecules.

Carbon Cycle

Carbon cycles through the atmosphere, living organisms, and the environment.

• Photosynthesis: Plants convert carbon dioxide into organic molecules.

• Respiration: Organisms release carbon dioxide back into the atmosphere.

• Application: Carbon is transferred from plants to animals and back to the environment.

Summary Table: Acids, Bases, and pH

Additional info:

• Hydrogen bonds are critical for water's properties, including cohesion, surface tension, and its role as a solvent.

• Buffers are essential for maintaining homeostasis in living organisms.

• Carbon's ability to form four covalent bonds makes it uniquely suited for building the complex molecules of life.