Properties of Water

Bonding in Water Molecules

Water molecules exhibit unique bonding properties that are essential for life. The structure and polarity of water contribute to its chemical behavior and interactions.

• Covalent Bond: The bond formed by sharing electrons between the oxygen and hydrogen atoms within a water molecule.

• Polarity: Water is a polar molecule due to the uneven distribution of electrons, resulting in a partial negative charge (δ-) on oxygen and a partial positive charge (δ+) on hydrogens.

• Hydrogen Bond: A weak attraction between the δ+ hydrogen of one water molecule and the δ- oxygen of another. These bonds are responsible for many of water's unique properties.

• Bent Molecular Shape: Water has a bond angle of approximately 105°, contributing to its polarity.

Example: The diagram shows water molecules forming hydrogen bonds, with the oxygen atom attracting electrons more strongly than hydrogen.

Solvency

Water's ability to dissolve substances is crucial for biological processes, including metabolic reactions and transport.

• Solvency: The ability of a solvent to dissolve matter.

• Hydrophilic Substances: Charged or polar substances that dissolve easily in water (e.g., salts, sugars).

• Hydrophobic Substances: Nonpolar substances that do not dissolve easily in water (e.g., oils, fats).

• Universal Solvent: Water can dissolve a wide variety of substances, making it essential for life.

Example: Water dissolves ionic compounds like NaCl by surrounding ions and separating them.

Water as a Solvent

The polarity of water enables it to dissolve many substances, facilitating chemical reactions and transport in cells.

• Ion Dissolution: Water molecules surround and separate ions due to their partial charges.

• Solute Orientation: The partially negative oxygen faces cations (e.g., Na+), while the partially positive hydrogens face anions (e.g., Cl-).

• Bond Angle: The 105° angle between hydrogens in water contributes to its effectiveness as a solvent.

Example: Water dissolves table salt (NaCl) by separating Na+ and Cl- ions.

Cohesion and Adhesion

Water molecules interact with each other and other substances through cohesive and adhesive forces, impacting biological systems.

• Adhesion: Attraction between water molecules and other substances (e.g., water sticking to glass).

• Cohesion: Attraction between water molecules themselves, due to hydrogen bonding.

• Capillary Action: Water moves up narrow tubes against gravity, important for plant water transport.

• Surface Tension: The elastic film at the water's surface caused by cohesive forces, allowing small objects to rest on water.

Example: Capillary action enables water to move up plant stems; surface tension allows insects to walk on water.

Chemical Reactivity of Water

Water participates in and facilitates chemical reactions essential for life.

• Ionization: Water can ionize acids and salts, aiding in chemical reactions.

• Hydrolysis: Water breaks down polymers into monomers by adding a water molecule.

• Condensation (Dehydration Synthesis): Water is removed to join monomers into polymers.

Example: Digestion involves hydrolysis reactions, breaking down macromolecules.

Thermal Stability of Water

Water's thermal properties help regulate temperature in living organisms.

• Heat Capacity: Amount of heat required to raise the temperature of a substance by 1°C.

• High Heat Capacity: Water absorbs and retains heat, stabilizing internal body temperature.

• Heat of Vaporization: Energy required to convert water from liquid to vapor ().

• Evaporative Cooling: As water evaporates, it removes heat, cooling surfaces (e.g., perspiration).

Example: Sweating cools the body as water evaporates from the skin.

Mixtures and Solutions

Water forms various mixtures, each with distinct properties relevant to cell biology.

• Solution: Homogeneous mixture of solute dissolved in solvent (e.g., saltwater).

• Colloid: Mixture with larger solutes that remain suspended, often cloudy (e.g., milk).

• Suspension: Mixture with large solutes that settle out over time (e.g., blood cells in plasma).

Organic Molecules: Carbon and Functional Groups

Carbon Structure and Bonding

Carbon is the backbone of organic molecules due to its versatile bonding properties.

• Bonding: Carbon forms four covalent bonds, allowing for rings and long chains.

• Common Partners: Hydrogen, oxygen, nitrogen, sulfur.

• Backbone: Carbon chains serve as the framework for organic molecules.

Example: Glucose and amino acids have carbon backbones.

Functional Groups

Functional groups are specific groups of atoms attached to the carbon backbone, determining the properties and reactivity of organic molecules.

• Amino Group (-NH2): Found in amino acids and proteins.

• Carboxyl Group (-COOH): Found in organic acids.

• Other Groups: Hydroxyl (-OH), phosphate (-PO4), methyl (-CH3).

Monomers and Polymers

Macromolecules are formed by joining smaller units called monomers into polymers.

• Monomer: A single subunit (e.g., nucleotide, amino acid).

• Polymer: A chain of monomers bonded together.

• Polymerization: The process of bonding monomers to form polymers, often via dehydration synthesis.

Example: DNA is a polymer of nucleotides; proteins are polymers of amino acids.

Dehydration Synthesis and Hydrolysis

Cells build and break down macromolecules through dehydration synthesis and hydrolysis reactions.

• Dehydration Synthesis: Monomers join to form polymers, releasing a water molecule.

• Hydrolysis: Polymers are split into monomers by adding water.

Equations:

• Dehydration synthesis:

• Hydrolysis:

Example: Digestion involves hydrolysis of dietary polymers into absorbable monomers.

Additional info: Expanded explanations and tables were added for completeness and clarity.