💧 Water: The Molecule of Life

Water is the most abundant molecule in living organisms, making up 60-70% of your body mass. Its unique properties arise from its polar covalent bonds and hydrogen bonding.

Why Water Matters

• All known life requires water
• Biochemical reactions occur in aqueous solutions
• Water's properties directly enable biological processes
• Understanding water is fundamental to understanding biology

Concept Check 🎯

## Polarity & Hydrogen Bonding

Water ($H_2O$) has a bent molecular geometry with a bond angle of approximately 104.5°.

Hydrogen Bonds

The partial positive charge on hydrogen atoms attracts the partial negative charge on oxygen atoms of neighboring water molecules. Each water molecule can form up to 4 hydrogen bonds.

• Hydrogen bonds are weak individually (~5% the strength of covalent bonds)
• But collectively powerful — they give water its extraordinary properties
• They are constantly breaking and reforming (about every picosecond)

Concept Check 🎯

Fill in the Blanks 🔍

## Cohesion & Adhesion

Cohesion

Cohesion is the attraction between molecules of the same substance. In water, hydrogen bonds create strong cohesion.

• Water molecules "stick" to each other
• Creates surface tension — a measure of how hard it is to break the surface
• Allows insects like water striders to walk on water

Adhesion

Adhesion is the attraction between molecules of different substances.

• Water adheres to glass, cell walls, and xylem tubes
• Combined with cohesion, creates capillary action
• Essential for water transport in plants

Capillary Action in Plants

Water moves up through narrow xylem vessels because:

1. Adhesion — water molecules cling to the xylem walls
2. Cohesion — water molecules pull neighboring molecules upward
3. Transpiration pull — evaporation at the leaves creates negative pressure

Concept Check 🎯

## Surface Tension & Biological Significance

Surface tension is the result of cohesive forces at the air-water interface.

Surface tension of water is higher than almost any other liquid because of the extensive hydrogen bonding network.

Concept Check 🎯

Fill in the Blanks 🔍

## High Specific Heat

Water has an unusually high specific heat capacity: 4.184 J/(g·°C).

This means water resists temperature change — it absorbs or releases a large amount of heat with only a small change in temperature.

Why?

• Hydrogen bonds must be broken before kinetic energy (temperature) increases
• The extensive hydrogen bond network acts as a thermal buffer

Biological Significance

Concept Check 🎯

## Evaporative Cooling & Heat of Vaporization

Water has a high heat of vaporization (2,260 J/g) — it takes a lot of energy to convert liquid water to gas.

Evaporative Cooling

When water evaporates, the highest-energy molecules escape first, leaving cooler molecules behind. This cools the remaining liquid.

• Sweating cools the body through evaporative cooling
• Transpiration cools plant leaves
• Panting cools animals without sweat glands

Without this property, organisms would overheat during metabolic activity.

Concept Check 🎯

Fill in the Blanks 🔍

## The Universal Solvent

Water is called the universal solvent because it dissolves more substances than any other liquid. This is due to its polarity.

How Water Dissolves Ionic Compounds

When $NaCl$ dissolves:

1. The δ⁻ oxygen of water surrounds $Na^+$ ions
2. The δ⁺ hydrogen of water surrounds $Cl^-$ ions
3. The ions are pulled apart and surrounded by water — they are hydrated

Hydrophilic vs. Hydrophobic

Hydrophobic molecules are excluded from the aqueous environment, which is critical for membrane formation.

Concept Check 🎯

## pH and Buffers

The pH Scale

$pH = -log[H^+]$

Each pH unit represents a 10-fold change in $[H^+]$.

Biological Buffers

Buffers resist changes in pH by accepting or donating $H^+$ ions.

• Carbonic acid buffer in blood: $H_2CO_3 ightleftharpoons H^+ + HCO_3^-$
• Maintains blood pH at 7.35-7.45
• Even small deviations can be fatal
• Enzymes function within narrow pH ranges

Concept Check 🎯

Fill in the Blanks 🔍

## Ice Floats — And That's Critical

Unlike most substances, water is less dense as a solid than as a liquid.

Why Ice Floats

• In liquid water, hydrogen bonds constantly break and reform
• When water freezes, hydrogen bonds become fixed in a crystalline lattice
• This lattice is more spread out (less dense) than liquid water
• Ice density: 0.917 g/mL vs. liquid water: 1.00 g/mL

Biological Significance

1. Insulation — Ice forms on the surface of lakes and ponds, insulating the liquid water below
2. Aquatic survival — Fish and other organisms survive winter beneath the ice
3. Prevents solid freezing — If ice sank, bodies of water would freeze from the bottom up, killing most aquatic life
4. Seasonal mixing — Ice melting in spring creates currents that distribute nutrients

Concept Check 🎯

## Summary of Water's Properties

All of these properties trace back to one feature: hydrogen bonding between polar water molecules.

Concept Check 🎯

Fill in the Blanks 🔍

## Problem-Solving Workshop: Water Properties

Let's apply what you've learned about water's properties to solve AP Biology-style problems.

Strategy for Water Property Questions

1. Identify the property being tested (cohesion, adhesion, specific heat, etc.)
2. Connect to hydrogen bonding — almost every water property question traces back to H-bonds
3. Link to biological significance — the AP exam emphasizes biological applications
4. Eliminate wrong answers by checking if the property matches the phenomenon

Concept Check 🎯

## Practice Scenarios

Scenario 1: Desert Organisms

Desert lizards are ectotherms that regulate body temperature behaviorally. During the hottest part of the day, they retreat to burrows where the soil retains moisture.

Key concept: Water's high specific heat moderates temperature underground.

Scenario 2: Transpiration Stream

Water moves from roots to leaves in tall trees (some over 100 m tall) without any pump mechanism.

Key concept: Cohesion-tension theory — transpiration creates negative pressure, and cohesion/adhesion pull water upward through xylem.

Concept Check 🎯

Fill in the Blanks 🔍

## Synthesis: Water Properties in AP Biology

Big Ideas Connected

• Big Idea 1 (Evolution): Water's properties created the aqueous environment where life evolved
• Big Idea 2 (Energy): Water's thermal properties are essential for metabolic regulation
• Big Idea 3 (Information): pH affects enzyme shape and function (protein structure)
• Big Idea 4 (Systems): Water's solvent properties enable transport and homeostasis

AP Exam Tips

1. Free-response questions often ask you to explain a property AND its biological significance
2. Always connect back to hydrogen bonding as the underlying mechanism
3. Be prepared to explain how disrupting water's properties would affect organisms
4. Know the difference between cohesion, adhesion, and surface tension

Concept Check 🎯

## Key Terms Review

Concept Check 🎯

Fill in the Blanks 🔍