Water and Life (Chapter 3): Properties, Functions, and Biological Importance

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Water and Life

The Molecule That Supports All Life

Water is essential for all living organisms, making up 70-95% of most cells and surrounding them. Its abundance is a key factor in Earth's habitability.

Key Point 1: All living organisms require water more than any other substance.
Key Point 2: Most cells are composed primarily of water.
Key Point 3: Water's presence enables life on Earth.

Polarity of Water

Molecular Structure and Hydrogen Bonding

Water molecules are polar, with an uneven distribution of charge due to polar covalent bonds between oxygen and hydrogen. This polarity allows water molecules to form hydrogen bonds with each other.

Key Point 1: Polar covalent bonds create partial positive and negative charges on hydrogen and oxygen atoms, respectively.
Key Point 2: Hydrogen bonds form between the slightly positive hydrogen of one molecule and the slightly negative oxygen of another.
Example: Water's polarity enables it to dissolve many substances.

Properties of Water

Facilitating Life

Water exhibits four key properties that support life: cohesive behavior, ability to moderate temperature, expansion upon freezing, and versatility as a solvent.

Cohesive behavior: Water molecules stick together due to hydrogen bonding.
Ability to moderate temperature: Water absorbs and releases heat slowly.
Expansion upon freezing: Ice is less dense than liquid water, allowing it to float.
Versatility as a solvent: Water dissolves a wide range of substances.

Cohesion and Adhesion

Transport and Interaction

Cohesion refers to the attraction between water molecules, while adhesion is the attraction between water and other substances. These properties are crucial for processes such as water transport in plants.

Cohesion: Hydrogen bonds hold water molecules together, creating a highly structured liquid.
Adhesion: Water molecules are attracted to other materials, such as plant cell walls.
Example: Cohesion and adhesion enable water to move upward against gravity in plants (capillary action).

Surface Tension

Resistance to Surface Disruption

Surface tension is the measure of how difficult it is to break the surface of a liquid, resulting from cohesive forces among water molecules.

Key Point: Surface tension allows small organisms, like insects, to walk on water.
Related to: Cohesion among water molecules.

Moderation of Temperature by Water

Specific Heat and Thermal Stability

Water can absorb or release large amounts of heat with minimal temperature change due to its high specific heat.

Specific heat: The amount of heat required to raise the temperature of 1 g of a substance by 1°C.
Water's specific heat: 1 cal/g/°C.
Example: Coastal climates are moderated by the ocean's high specific heat.

Heat and Temperature

Kinetic Energy and Measurement

Heat is the total kinetic energy due to molecular motion, while temperature measures the average kinetic energy of molecules.

Kinetic energy: Energy of motion.
Heat: Total kinetic energy in a substance.
Temperature: Average kinetic energy of molecules.

Water's High Specific Heat

Role of Hydrogen Bonding

Water's high specific heat is due to hydrogen bonding. Breaking hydrogen bonds requires energy, while forming them releases energy.

Key Point: Heat disrupts hydrogen bonds before increasing molecular motion.
Key Point: As temperature drops, heat is released from hydrogen bond formation.

Evaporative Cooling

Heat of Vaporization and Cooling Effects

Evaporation is the transition from liquid to gas. Water's high heat of vaporization means it absorbs much heat before evaporating, leading to cooling effects.

Heat of vaporization: Heat required for 1 g of liquid to become gas.
Evaporative cooling: As water evaporates, the surface cools because the fastest molecules leave as gas.
Example: Sweating cools the body.

Density of Water

Ice Floats on Water

Ice is less dense than liquid water because hydrogen bonds in ice are more ordered and stable, creating a lattice structure.

Key Point: Hydrogen bonds in ice keep molecules apart, making ice less dense.
Example: Aquatic life survives under floating ice in winter.

Water: The Solvent of Life

Solubility and Hydration Shells

Water dissolves many substances due to its polarity, forming hydration shells around ions and polar molecules.

Solute: Substance dissolved in a solvent.
Solution: Homogeneous mixture of solute and solvent.
Aqueous solution: Water is the solvent.
Hydration shell: Sphere of water molecules surrounding dissolved ions.
Example: NaCl dissolves in water, forming Na+ and Cl- ions surrounded by water.

Hydrophilic and Hydrophobic Substances

Affinity for Water

Hydrophilic substances have an affinity for water (polar), while hydrophobic substances do not (nonpolar).

Hydrophilic: Polar molecules, such as salts and sugars.
Hydrophobic: Nonpolar molecules, such as oils.

Solute Concentration

Molecular Mass, Moles, and Molarity

Chemical reactions depend on solute concentration. Molecular mass is the sum of atomic masses in a molecule. Molarity (M) is moles of solute per liter of solution.

Molecular mass: Measured in g/mol.
1 mole: molecules (Avogadro's number).
Molarity (M):
Example: To make 1 L of 1 M NaCl solution, use 58.44 g NaCl.

Making Solutions: Calculations

Examples of Solution Preparation

NaCl: For 1 L of 1 M solution:
NaCl: For 500 mL of 5 M solution:
Glucose (C6H12O6): For 500 mL of 1 M solution:

Acids and Bases

Ionization of Water and pH

Water ionizes slightly, producing hydronium (H3O+) and hydroxide (OH-) ions. Acids increase [H+], bases decrease it.

Neutral solution: [H+] = [OH-] = M
Acids: Proton donors, increase [H+]
Bases: Proton acceptors, decrease [H+]

pH Scale

Definition and Calculation

pH is the negative logarithm of hydrogen ion concentration.

Formula:
Neutral pH:
Acidic solutions: pH < 7
Basic solutions: pH > 7

Table: Calculating pH Values and Hydroxide Ion Concentrations

Substance	[H+]	log [H+]	pH
Gastric juice	0.01,	-2	2
Pure water, neutral solution	0.0000001,	-7	7
Household ammonia	0.00000000001,	-11	11

Biological Importance of pH

Influence on Biological Systems

pH affects molecular shape, reaction rates, binding ability, and solubility. Most living cells require a stable internal pH near 7.

Key Point: Large changes in pH can disrupt cellular processes.

Buffers

Maintaining pH Stability

Buffers minimize changes in [H+] and [OH-] by reversibly binding or releasing hydrogen ions. Most buffers are weak acid-base pairs.

Key Point: Buffers help maintain pH in biological fluids, such as blood.

Biological Buffers

Carbonic Acid-Bicarbonate System

The pH of human blood is maintained by the carbonic acid-bicarbonate buffer system.

Reaction:
Key Point: Addition of H+ shifts equilibrium to form carbonic acid; addition of OH- shifts equilibrium to form bicarbonate.
Example: Adding 0.01 mol HCl to water drops pH from 7.0 to 2.0, but in blood only from 7.4 to 7.3.

Ocean Acidification

Impact of CO2 on Marine Systems

About 25% of human-generated CO2 is absorbed by oceans, forming carbonic acid and leading to acidification. This process depletes carbonate ions, affecting marine organisms that rely on calcification.

Key Point: Ocean acidification threatens coral reefs and shell-forming organisms.

Elaborative Interrogation

Active Learning Strategy

Elaborative interrogation involves asking and answering 'how' and 'why' questions to deepen understanding of concepts.

Example Prompt: What is a covalent bond?
Follow-up Questions: How does it relate to valence electrons? Why are they important for biological systems? How do covalent bonds compare to other bonds, like ionic and hydrogen bonds? How does electronegativity affect covalent bonds?