💧 IB Biology SL - Theme A: Unity & Diversity
A1.1 - Water: The Molecule of Life
Essential Concepts: Polarity, Hydrogen Bonding, Properties & Biological Significance
🌊 Water: The Medium of Life
Why Water is Essential for Life:
Water (H2O) is the most abundant molecule in living organisms, making up 60-70% of the human body and serving as the universal solvent in biological systems. Life as we know it cannot exist without water.
Key Facts About Water:
- Chemical Formula: H2O (two hydrogen atoms, one oxygen atom)
- Molecular Mass: 18.015 g/mol
- Bond Angle: 104.5° between the two O-H bonds
- Abundance: Covers 71% of Earth's surface
- Biological Role: Solvent, reactant, temperature regulator, transport medium
Search for: "water molecule structure diagram" or "H2O molecular geometry"
Free sources: Wikimedia Commons, OpenStax Biology
⚡ Polarity of Water
What is Polarity?
Water is a polar molecule, meaning it has an uneven distribution of electrical charge. This creates a partial positive charge (δ+) on one end and a partial negative charge (δ-) on the other.
Why Water is Polar:
Electronegativity Difference:
- Oxygen (O): Electronegativity = 3.44 (highly electronegative)
- Hydrogen (H): Electronegativity = 2.20 (less electronegative)
- Difference: 1.24 (creates polar covalent bonds)
Because oxygen is more electronegative, it pulls the shared electrons closer to itself, creating a partial negative charge (δ-) on the oxygen atom and partial positive charges (δ+) on the hydrogen atoms.
Molecular Geometry:
- Water has a bent (V-shaped) molecular geometry
- Bond angle: 104.5° (not 180° like a linear molecule)
- Two lone pairs of electrons on oxygen push the hydrogen atoms closer together
- The bent shape prevents charges from canceling out, maintaining polarity
Search for: "water polarity diagram" or "polar water molecule charges"
Free sources: Wikimedia Commons, Khan Academy
Significance of Polarity:
- Enables hydrogen bonding between water molecules
- Allows water to dissolve ionic and polar substances
- Creates cohesion and adhesion properties
- Responsible for water's unique physical properties
🔗 Hydrogen Bonding
What is a Hydrogen Bond?
A hydrogen bond is a weak electrostatic attraction between a partially positive hydrogen atom (δ+) of one water molecule and a partially negative oxygen atom (δ-) of another water molecule.
Characteristics of Hydrogen Bonds:
- Type: Intermolecular force (between molecules, not within)
- Strength: Weak individually (~20 kJ/mol) compared to covalent bonds (~460 kJ/mol)
- Collective Strength: Many hydrogen bonds together create significant forces
- Dynamics: Constantly breaking and reforming (each lasts ~1-20 picoseconds)
- Number: Each water molecule can form up to 4 hydrogen bonds (2 as donor, 2 as acceptor)
Hydrogen Bonding Pattern:
H
\
O---H···O
/ \
H H
(dotted line = hydrogen bond)
The dotted line (···) represents the hydrogen bond between molecules
Search for: "hydrogen bonding in water" or "water molecules hydrogen bonds diagram"
Free sources: OpenStax, Biology LibreTexts
Formation of Hydrogen Bonds:
- The δ+ hydrogen of one water molecule is attracted to the δ- oxygen of another
- The molecules align so that hydrogen and oxygen are close together
- Electrostatic attraction creates the hydrogen bond
- In liquid water, each molecule forms an average of 3.4 hydrogen bonds
- In ice, each molecule forms exactly 4 hydrogen bonds in a crystalline structure
Biological Importance of Hydrogen Bonds:
- Stabilize protein structure (α-helices and β-sheets)
- Hold DNA double helix together (between complementary base pairs)
- Determine water's unique properties
- Enable water to act as a temperature buffer
🧲 Cohesion and Adhesion
Cohesion:
Cohesion Definition:
Cohesion is the attraction between molecules of the same substance. In water, cohesion results from hydrogen bonding between water molecules.
Examples of Cohesion:
- Surface Tension: Water molecules at the surface are pulled inward, creating a "skin" that allows insects to walk on water
- Water Droplets: Cohesion pulls water into spherical shapes (minimizes surface area)
- Column of Water: Cohesion allows water to be pulled up tall trees in xylem vessels without breaking
Adhesion:
Adhesion Definition:
Adhesion is the attraction between molecules of different substances. Water adheres to other polar or charged surfaces.
Examples of Adhesion:
- Capillary Action: Water climbs up narrow tubes against gravity
- Meniscus: Water curves upward in a glass tube due to adhesion to glass
- Plant Transport: Water adheres to cellulose in cell walls, aiding upward movement
Search for: "capillary action water" or "water meniscus diagram"
Free sources: Wikimedia Commons, Physics educational sites
Capillary Action Explained:
Capillary action occurs when adhesion > cohesion:
- Water molecules adhere to the walls of a narrow tube (e.g., xylem vessel)
- These molecules pull other water molecules up through cohesion
- Water continues to rise until the weight of the water column equals the adhesive and cohesive forces
- The narrower the tube, the higher water can rise
Biological Significance:
- Transpiration-Cohesion-Tension Theory: Explains how water moves from roots to leaves in tall trees (up to 100+ meters)
- Continuous Water Column: Cohesion prevents the water column from breaking under tension
- Soil Water Absorption: Adhesion helps water move through soil to plant roots
- Cell Wall Hydration: Water adheres to cellulose, maintaining cell structure
Search for: "transpiration cohesion tension mechanism" or "water transport in xylem"
Free sources: OpenStax Biology, Khan Academy
🧪 Solvent Properties of Water
Water: The Universal Solvent
Water is called the "universal solvent" because it dissolves more substances than any other liquid. This property is crucial for biological systems.
What Water Can Dissolve:
Hydrophilic Substances (Water-loving):
- Ionic Compounds: Salts like NaCl, KCl, CaCl2
- Polar Molecules: Sugars (glucose, sucrose), amino acids, small alcohols
- Charged Molecules: Acids, bases, proteins with charged regions
Hydrophobic Substances (Water-fearing):
- Non-polar Molecules: Lipids, fats, oils, waxes
- Non-polar Gases: O2, N2, CO2 (slightly soluble)
- Hydrocarbons: Alkanes, benzene
How Water Dissolves Ionic Compounds:
Dissolving NaCl (Table Salt):
- Dissociation: NaCl crystal separates into Na+ and Cl- ions
- Hydration Shell Formation:
- Water's δ- oxygen surrounds Na+ ions (positive attracts negative)
- Water's δ+ hydrogens surround Cl- ions (negative attracts positive)
- Stabilization: Hydration shells prevent ions from recombining
- Dispersion: Ions spread throughout the solution
Search for: "hydration shell diagram" or "ionic dissolution in water"
Free sources: Chemistry LibreTexts, OpenStax Chemistry
How Water Dissolves Polar Molecules:
Dissolving Glucose (C6H12O6):
- Glucose has multiple -OH (hydroxyl) groups that are polar
- Water molecules form hydrogen bonds with these -OH groups
- Hydrogen bonding overcomes glucose-glucose attractions
- Glucose molecules disperse throughout the solution
Biological Importance of Water as Solvent:
- Metabolic Reactions: Most biochemical reactions occur in aqueous solution
- Nutrient Transport: Glucose, amino acids, minerals dissolved in blood plasma
- Waste Removal: Urea, CO2, excess ions dissolved for excretion
- Cell Communication: Hormones and signaling molecules travel in solution
- Digestion: Enzymes function in aqueous environment of digestive system
- Osmosis: Water movement across membranes depends on solute concentration
🌡️ Physical Properties of Water
Unique Properties Due to Hydrogen Bonding
Water's physical properties are unusual compared to other molecules of similar size, all due to extensive hydrogen bonding.
1. High Specific Heat Capacity
Definition & Value:
Specific heat capacity is the amount of energy needed to raise the temperature of 1 gram of a substance by 1°C.
Water's specific heat capacity: 4.18 J g-1 °C-1 (very high!)
Why is it High?
- Heat energy must break hydrogen bonds before molecules can move faster (increase temperature)
- Much of the added energy goes into breaking bonds, not increasing kinetic energy
- Therefore, water requires a lot of energy for temperature to rise
Biological Consequences:
- Temperature Stability: Organisms maintain stable internal temperature despite external fluctuations
- Aquatic Habitats: Oceans and lakes resist temperature changes, providing stable environments
- Climate Moderation: Coastal areas have milder climates (water absorbs/releases heat slowly)
- Body Temperature: Human body (70% water) resists temperature changes
2. High Heat of Vaporization
Definition & Value:
Heat of vaporization is the energy required to convert 1 gram of liquid to gas at boiling point.
Water's heat of vaporization: 2,260 J g-1 (exceptionally high!)
Why is it High?
- ALL hydrogen bonds must be broken for water to evaporate
- Requires significant energy input
- Water molecules in vapor have no hydrogen bonds between them
Biological Consequences:
- Evaporative Cooling: Sweating cools the body as water evaporates, removing heat
- Transpiration: Plants cool leaves through water evaporation
- Prevention of Overheating: Minimal water loss causes maximum cooling effect
- Panting: Animals cool via water evaporation from lungs and tongue
3. High Boiling Point
Comparison:
| Molecule | Molecular Mass | Boiling Point |
|---|---|---|
| H2O (Water) | 18 g/mol | 100°C |
| H2S (Hydrogen sulfide) | 34 g/mol | -60°C |
| NH3 (Ammonia) | 17 g/mol | -33°C |
| CH4 (Methane) | 16 g/mol | -162°C |
Biological Consequences:
- Liquid at Body Temperature: Water remains liquid in the temperature range of most organisms (0-100°C)
- Wide Temperature Range: Provides suitable environment for life on Earth
- Prevents Evaporation: Water doesn't easily evaporate at normal body temperatures
4. Ice is Less Dense than Liquid Water
Unusual Property:
- Density of ice: 0.92 g cm-3
- Density of liquid water (4°C): 1.00 g cm-3
- Result: Ice floats on water (unusual for solids!)
Why is Ice Less Dense?
- In liquid water: hydrogen bonds constantly break and reform, molecules pack closely
- In ice: each water molecule forms exactly 4 hydrogen bonds in a rigid crystalline structure
- The crystal structure has large spaces between molecules
- These spaces make ice less dense than liquid water
Search for: "ice crystal structure vs liquid water" or "water density ice floating"
Free sources: Chemistry educational sites, Wikimedia Commons
Biological Consequences:
- Aquatic Life Protection: Ice forms on top of lakes/ponds, insulating liquid water below where organisms survive
- Prevent Complete Freezing: Bodies of water freeze from top down, not bottom up
- Habitat Preservation: Bottom layers remain liquid even in winter
- Warning: Ice expansion can burst cells if organisms freeze (why antifreeze proteins evolved)
5. Surface Tension
Definition:
Surface tension is the cohesive force at the surface of a liquid that makes it behave like an elastic sheet.
Water has one of the highest surface tensions of any liquid (72.8 mN m-1 at 20°C).
Why Does Water Have High Surface Tension?
- Water molecules at the surface have hydrogen bonds only on sides and below
- No hydrogen bonds above (with air molecules)
- This creates a net inward force, pulling surface molecules together
- Surface acts like a stretched membrane
Biological Consequences:
- Insect Walking: Water striders and other insects can walk on water surface
- Water Droplet Formation: Water forms spherical droplets (minimizes surface area)
- Transport in Plants: Contributes to water column stability in xylem
- Alveoli Function: Surfactant reduces surface tension in lungs for breathing
Search for: "water strider surface tension" or "insect walking on water"
Free sources: Nature photography sites, Wikimedia Commons
🌟 Consequences of Water's Properties for Life
Water Makes Life Possible
The unique properties of water arising from its polarity and hydrogen bonding are essential for all known forms of life.
Summary of Biological Consequences:
| Property | Cause | Biological Consequence |
|---|---|---|
| Polarity | Electronegativity difference + bent shape | Universal solvent; enables hydrogen bonding |
| Hydrogen Bonding | Electrostatic attraction between δ+ H and δ- O | Causes all unique properties of water |
| Cohesion | H-bonds between water molecules | Water transport in plants; surface tension |
| Adhesion | H-bonds with polar surfaces | Capillary action; water movement in xylem |
| High Specific Heat | Energy needed to break H-bonds | Temperature stability in organisms and habitats |
| High Heat of Vaporization | All H-bonds must break for evaporation | Evaporative cooling (sweating, transpiration) |
| Ice Less Dense | Crystalline structure with spaces | Ice floats; aquatic life survives winter |
| Excellent Solvent | Polarity and H-bonding ability | Medium for metabolic reactions; transport |
Why Life Depends on Water:
- Metabolic Medium: All biochemical reactions occur in aqueous solution
- Transport: Nutrients, gases, and wastes transported dissolved in water
- Temperature Regulation: Maintains stable internal environment
- Structural Support: Hydrostatic pressure in cells provides shape and rigidity
- Lubrication: Synovial fluid, mucus, tears contain water
- Chemical Reactant: Participates in photosynthesis, hydrolysis, condensation reactions
- Habitat: Provides stable environment for aquatic organisms
IB Exam Key Points:
- Understand WHY water has each property (link to molecular structure)
- Be able to explain the BIOLOGICAL SIGNIFICANCE of each property
- Know examples of how organisms utilize water's properties
- Understand the relationship between polarity, hydrogen bonding, and all other properties
- Be able to compare water with other molecules (e.g., why water has higher boiling point than H₂S)
📝 Quick Review & Key Takeaways
Click here for the #1 concept to remember! 🎯
Essential Equations & Values to Memorize:
- H2O - Water molecule formula
- Bond angle = 104.5° - Bent molecular geometry
- Specific heat capacity = 4.18 J g-1 °C-1
- Heat of vaporization = 2,260 J g-1
- Density of ice = 0.92 g cm-3
- Density of water (4°C) = 1.00 g cm-3
Common IB Exam Questions:
- Explain why water is a polar molecule (include molecular geometry)
- Describe hydrogen bonding between water molecules
- Explain how water's properties make it suitable as a medium for life
- Compare cohesion and adhesion with examples
- Explain why ice floats and its biological significance
- Describe the thermal properties of water and their importance
- Explain how water acts as a solvent for polar and ionic substances
Study Tips for A1.1 Water:
- Draw and label water molecule structure repeatedly
- Create a concept map linking polarity → H-bonds → properties → biological consequences
- Practice explaining each property with molecular-level reasoning
- Make flashcards for each property and its biological significance
- Review diagrams of hydrogen bonding, ice structure, and hydration shells
- Be able to give specific biological examples for each property