Chapter 3 - bio 1201

Chapter 3: Water and Life

Polar Covalent Bonds in Water Molecules and Hydrogen Bonding

Water's unique properties stem from its molecular structure and the presence of polar covalent bonds, which enable hydrogen bonding between molecules.

• Polar covalent bond: A type of chemical bond where electrons are shared unequally between atoms, resulting in partial charges.

• Structure of water molecule: Consists of two hydrogen atoms covalently bonded to one oxygen atom. The oxygen atom is more electronegative, creating a partial negative charge, while the hydrogen atoms have partial positive charges.

• Hydrogen bond: A weak bond formed between the partially positive hydrogen atom of one water molecule and the partially negative oxygen atom of another.

Example: Hydrogen bonds are responsible for water's high boiling point compared to other molecules of similar size.

Emergent Properties of Water Contributing to Earth's Suitability for Life

Water exhibits several emergent properties due to hydrogen bonding, making it essential for life on Earth.

• Cohesion: Water molecules stick together due to hydrogen bonding, aiding in the transport of water in plants.

• Adhesion: Water molecules can also stick to other substances, which helps in processes like capillary action.

• Moderation of temperature: Water has a high specific heat, meaning it can absorb or release large amounts of heat with only slight temperature changes. This stabilizes climates and environments.

• Expansion upon freezing: Water is less dense as a solid than as a liquid, allowing ice to float and insulate aquatic environments.

• Versatility as a solvent: Water's polarity allows it to dissolve many substances, facilitating chemical reactions in living organisms.

Example: Water moderates temperature in coastal regions and inside living cells.

Additional info: Water's ability to dissolve salts, sugars, acids, and bases is crucial for biological processes.

Acidic and Basic Conditions Affecting Living Organisms

The concentration of hydrogen ions (H+) and hydroxide ions (OH-) in water determines its acidity or basicity, which can impact biological systems.

• Acids: Substances that increase the concentration of H+ ions in a solution.

• Bases: Substances that reduce the concentration of H+ ions, often by increasing OH- ions.

• pH scale: Measures the concentration of H+ ions; lower pH indicates higher acidity, higher pH indicates higher basicity.

• Buffers: Substances that minimize changes in pH by accepting or donating H+ ions.

Equation:

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

Example: Human blood is buffered to maintain a pH close to 7.4, which is vital for proper cellular function.

Additional info: Changes in environmental pH can affect enzyme activity and metabolic processes in organisms.

Property	Description	Biological Importance
Cohesion	Water molecules stick together	Transport in plants
Adhesion	Water molecules stick to other surfaces	Capillary action
High Specific Heat	Resists temperature change	Climate stability
Expansion upon Freezing	Ice is less dense than liquid water	Insulates aquatic life
Versatility as Solvent	Dissolves many substances	Facilitates reactions