Water
Guiding Questions
- What physical and chemical properties of water make it essential for life?
- What are the challenges and opportunities of water as a habitat?
SL and HL Content
Learning Objective: A1.1.1 - Water as the medium for life
Water & the Origin of Life
Life on Earth depends on water; no known life can exist without it.
The first cells originated in water, likely in the oceans.
Water shielded early life from harmful ultraviolet radiation.
The Earth initially lacked liquid water due to extreme heat.
As the planet cooled, water formed and the water cycle began.
Oceans provided the solvent necessary for the complex biochemical reactions that support life.
The cell membrane evolved to separate intracellular water (cytoplasm) from ocean water.
Syllabus Link: B2.1 – Membranes & Membrane Transport
Water: The Essential Component of Life
Water is the primary solvent for biological processes.
Water is the main component of living things.
Most cells are approximately 70-80% water.
Water provides the environment in which the biochemical reactions of life can occur.
Water also takes part in and is produced by many reactions.
Water is present in various biological fluids, including:
Cytoplasm (site of cellular reactions)
Organelle fluids (e.g., mitochondria, nucleus)
Tissue fluid (between cells in multicellular organisms)
Blood and lymph (essential for transport and immune responses)
Water is also a habitat for aquatic life - ocean, lakes, and rivers.
Water as a Universal Solvent
Solute: The substance that is dissolved in a liquid.
Example: Salt in saltwater.
Solvent: The liquid in which the solute dissolves.
Example: Water in saltwater.
Solution: A homogeneous mixture of a solute dissolved in a solvent.
Example: Saltwater (a solution of salt and water)
Aqueous solutions (water-based solutions) are essential for life.
Water allows:
Transport of substances in and out of cells.
Dissolving of enzymes for biochemical reactions.
Efficient diffusion of molecules in biological systems.
71% of Earth’s surface is covered by water, making it a “blue planet.”
Learning Objective: A1.1.2 - Hydrogen bonds as a consequence of the polar covalent bonds within water molecules
The Molecular Structure of Water
A water molecule (H2O) has one oxygen (O) atom and two hydrogen (H) atoms covalently bonded together.
The bonds are polar covalent, meaning the electrons are shared unequally.
Each hydrogen atom shares a pair of electrons with the oxygen atom.
Why Water is a Polar Molecule
Water is a polar molecule; electrons are shared unequally.
Oxygen is more electronegative than hydrogen
therefore it pulls the shared electrons closer to itself
Oxygen’s higher electronegativity causes it to gain a partial negative charge (δ⁻)
The hydrogen atoms carry a partial positive charge (δ⁺)
Hydrogen Bonds: Intermolecular Forces in Water
A hydrogen bond is a weak electrostatic attraction between the δ⁺ hydrogen of one water molecule and the δ⁻ oxygen of another.
These bonds are intermolecular forces.
Hydrogen bonds are weak individually but collectively strong.
Hydrogen bonds are temporary and constantly forming and breaking in liquid water.
Exam Tip: Practice drawing and labeling water molecules, including hydrogen bonds and polarity (δ⁺, δ⁻).
Properties of Water
The polarity of water and hydrogen bonding is key to its unique properties.
These properties of water are crucial for biological processes such as nutrient transport, temperature regulation, and providing habitats for aquatic organisms.
Learning Objective: A1.1.3 - Cohesion of water molecules due to hydrogen bonding and consequences for organisms
Cohesion of Water Molecules
Cohesion is the attraction of water molecules to each other due to hydrogen bonding.
Hydrogen bonds are weak individually but strong collectively, creating significant cohesive forces in water.
Key consequence: Water molecules move together as a continuous column, important in biological processes.
Surface Tension as a Result of Cohesion
Surface tension is the property of water that allows it to resist an external force.
Cause: Water molecules at the surface experience stronger cohesion because they lack neighboring molecules above.
This inward force creates a “skin” on the water surface, making it difficult to break.
Surface Tension in Nature
Enables movement on water:
Some insects and small creatures move across water without sinking due to surface tension.
Many organisms depend on surface tension for movement and survival.
Helps create stable habitats on the water’s surface.
Examples: Water Striders and Basilisk Lizards use surface tension to stay afloat and move efficiently.
Explains droplet formation:
Water molecules at the surface pull inward, creating spherical droplets.
This inward force makes it harder to break the water’s surface, requiring high energy.
Learning Objective: A1.1.4 - Adhesion of water to materials that are polar or charged and impacts for organisms
Adhesion of Water Molecules to Other Substances
Adhesion is the attraction between water molecules and other polar or charged surfaces due to hydrogen bonding.
Materials water adheres to:
Cellulose in plant cell walls (e.g., xylem, mesophyll)
Glass and capillary tubes (demonstrated in lab experiments)
Soil particles (essential for water absorption by plants)
Importance of Cohesive & Adhesive Forces in Plant Transport
Water moves upward in plants due to cohesion (water molecules sticking together) and adhesion (water molecules sticking to xylem walls).
Transpiration (evaporation from leaves) creates tension, pulling water upward from roots to leaves
this is called the cohesion-tension theory.
Cohesion keeps the water column unbroken, while adhesion prevents it from falling back down.
Key Points
Ensures water availability in soil:
Helps plants access water even in dry conditions.
Supports water movement in plants:
Prevents xylem from drying out
Enables continuous hydration of plant cells
Aids transpiration and gas exchange in leaves:
Keeps mesophyll cells moist for CO₂ absorption.
Contributes to the water cycle by promoting evaporation.
Adhesion & Capillary Action
Capillary action is the movement of water through narrow spaces due to adhesion and cohesion.
Capillary action allows water to move against gravity without external force.
Why is Capillary Action Important?
It plays a crucial role in water movement in soil and water transport in plants.
Helps sustain plant hydration and enables nutrient uptake.
Capillary Action in Soil
How it works in soil:
Soil contains microscopic channels that act like capillary tubes.
Water molecules adhere to soil particles, creating a network of water movement.
Root hairs absorb water from these channels, allowing plants to take in water from the soil
Capillary Action in Plant Cell Walls
How it works in plant cell walls:
Cellulose fibers in plant walls are hydrophilic (slightly polar).
Water adheres to cellulose, allowing capillary action within plant tissues.
Enables continuous water flow within plant tissue.
Learning Objective: A1.1.5 - Solvent properties of water linked to its role as a medium for metabolism and for transport in plants and animals
Why Water is an Excellent Solvent
Polarity of water molecules allows them to surround and dissolve hydrophilic (water-loving) substances.
Hydrogen bonding enables the breakdown of ionic and polar covalent compounds.
Water allows diffusion of solutes, making them more chemically reactive.
Hydrophilic Molecules and Water’s Role in Metabolism
Hydrophilic substances dissolve readily in water (water-loving).
Examples of Hydrophilic Substances:
Glucose (important for cellular respiration).
Ions (e.g., Na⁺, K⁺, Cl⁻) required for nerve signaling and osmotic balance.
Amino acids (building blocks of proteins).
Proteins and enzymes (biological catalysts that facilitate reactions).
Most enzymes function in aqueous solutions:
Water maintains enzyme structure and stability.
Hydrogen bonds form between enzyme active sites and substrates.
Metabolic reactions occur efficiently in water.
Water as a Transport Medium in Plants
Xylem Transport:
Water in xylem vessels is not pure; it is an aqueous solution of inorganic ions.
Ions like calcium (Ca²⁺), potassium (K⁺), and sodium (Na⁺) dissolve in water and are transported from roots to leaves.
Phloem Transport:
Water carries sugars (e.g., sucrose) and other solutes in phloem sap.
Mass flow of water drives nutrient transport to growing tissues.
Water as a Transport Medium in Animals
Blood plasma is an aqueous solution containing:
Ions (e.g., Na⁺, Cl⁻, Ca²⁺) for nerve function and muscle contraction.
Glucose and amino acids for energy and protein synthesis.
Proteins (e.g., albumin, fibrinogen) for clotting and immune function.
Oxygen transport:
Oxygen is poorly soluble in water.
Hemoglobin in red blood cells increases oxygen transport efficiency.
Carbon dioxide transport:
Moderately soluble in water.
Forms carbonic acid (H₂CO₃), which dissociates into H⁺ and HCO₃⁻ (bicarbonate ions), helping maintain blood pH balance.
Hydrophobic Molecules and Their Biological Importance
Hydrophobic substances do not dissolve in water (water-fearing).
Examples:
Steroid hormones (e.g., testosterone, oestradiol):
Pass directly through cell membranes because they are non-polar.
Membrane-bound proteins:
Contain hydrophobic regions that anchor them into cell membranes.
Phospholipid bilayer of cell membranes:
Hydrophobic fatty acid tails form the inner membrane layer.
Hydrophilic phosphate heads face outward, interacting with water.
Leaf wax cuticle:
Hydrophobic waxy layer prevents water loss by evaporation.
Learning Objective: A1.1.6 - Physical properties of water and the consequences for animals in aquatic habitats
Physical Properties of Water
Buoyancy
Viscosity
Thermal Conductivity
Specific Heat Capacity
The unique properties of water result from hydrogen bonding.
Contrast of water with air:
Water is denser than air, providing greater buoyancy.
Water has higher viscosity, making movement harder compared to air.
Water conducts heat better, leading to higher heat loss in aquatic organisms.
Water has higher specific heat capacity, stabilizing temperature fluctuations.
Buoyancy and Its Effects on Animals
Buoyancy is the ability of a fluid to exert an upward force on an object.
How it works:
Objects float when less dense than water and sink when denser.
The buoyant force in water is greater than in air, allowing animals to remain suspended.
EXAMPLES:
Black-throated loon (Gavia arctica)
Uses buoyancy to float on water but needs to overcome buoyancy for diving.
Has solid bones to increase density and reduce buoyancy.
Ringed seal (Pusa hispida)
Blubber adds buoyancy while allowing efficient swimming.
Floats with only its snout above water, conserving energy.
Viscosity and Its Impact on Movement
Viscosity is the resistance of a fluid to motion. is the resistance of a fluid to motion.
Adaptations for movement:
Black-throated loon (Gavia arctica)
Webbed feet provide greater propulsion in water.
Streamlined body reduces drag.
Ringed seal (Pusa hispida)
Uses flippers for propulsion.
Has a streamlined body shape to reduce resistance
Thermal Conductivity and Heat Retention
Thermal conductivity is the ability of a substance to transfer heat.
Adaptations for heat retention:
Black-throated loon (Gavia arctica)
Traps air between feathers for insulation.
Uses an oil gland to coat feathers, making them waterproof.
Ringed seal (Pusa hispida)
Has a thick blubber layer to reduce heat loss.
Insulated fur minimizes exposure to cold water.
Specific Heat Capacity and Environmental Stability
Specific heat capacity is the amount of energy needed to raise the temperature of a substance.
Implications for aquatic animals:
Aquatic habitats remain stable despite temperature changes in air.
Black-throated loon (Gavia arctica)
Experience milder temperature variations in water than in air.
Ringed seal (Pusa hispida)
Benefit from stable sea temperatures, allowing year-round survival.
Why Water is an Effective Coolant
Water has a high latent heat of vaporization, meaning it takes a lot of energy to turn liquid water into vapor.
Evaporation requires heat:
To evaporate, water must absorb large amounts of heat energy from the body to break hydrogen bonds between water molecules.
Sweating cools the body:
As sweat evaporates, it removes heat from the skin, cooling the body surface.
ADDITIONAL HIGHER LEVEL (HL)
Learning Objective: A1.1.7 - Extraplanetary origin of water on Earth and reasons for its retention
The Extraplanetary Origin of Water
Earth formed ~4.5 billion years ago in an environment too hot for liquid water.
Early Earth’s surface was molten magma, meaning water could not have been present in liquid form.
Water must have originated from extraterrestrial sources.
Asteroids as the Source of Earth’s Water
Scientists hypothesize that asteroids delivered water to Earth.
Evidence:
Hydrated minerals within asteroids contain water locked in crystal structures.
Carbonaceous chondrite meteorites contain up to 28% water.
Hydrogen isotope ratios (deuterium vs. protium) in Earth’s oceans match those in asteroids.
Ancient eucrite achondrites (meteorites from Vesta) have a similar deuterium-to-protium ratio as Earth’s water.
4.5-billion-year-old meteorites containing liquid water have been found on Earth, supporting this theory.
The Role of Gravity in Retaining Water
Earth’s size and mass create a gravitational pull strong enough to retain water molecules.
When asteroids impacted Earth, water vapor was released but trapped by gravity.
Unlike smaller celestial bodies (e.g., Mars, Moon), Earth’s gravity prevents water from escaping into space.
The Role of Temperature in Retaining Water
Early Earth was too hot for liquid water; as it cooled, water condensed into oceans.
Critical temperature factors:
Close enough to the Sun to maintain liquid water.
Far enough from the Sun that water did not evaporate into space.
Water could remain in solid, liquid, and gaseous states, supporting the water cycle.
Learning Objective: A1.1.8 - Relationship between the search for extraterrestrial life and the presence of water
The Goldilocks Zone and Its Importance
The Goldilocks Zone (also called the Habitable Zone) is the region around a star where temperatures allow liquid water to exist.
The presence of water is a key indicator when searching for extraterrestrial life.
If a planet is too close to its star, water boils and evaporates (e.g., Venus).
If a planet is too far from its star, water freezes (e.g., Mars).
Earth is perfectly positioned in the Goldilocks Zone of the Sun, enabling it to retain liquid water.
Factors Affecting a Planet’s Habitability
Stable Temperatures:
A planet must have temperatures suitable for liquid water.
Earth’s atmosphere and magnetic field help regulate temperature and protect against radiation.
Gravity:
A planet must have sufficient mass to retain an atmosphere and prevent water from escaping into space.
Type of Star:
The Sun is a G-type star, emitting the right amount of energy for Earth to be in the habitable zone.
K- and M-type stars (more common) have narrower habitable zones and emit higher radiation, which may be harmful to life.
Searching for Extraterrestrial Water
Scientists use transit spectroscopy to detect water on distant planets:
By analyzing the wavelengths of absorbed light, scientists can identify water molecules in the atmosphere.
Exoplanets and Water Signatures:
Scientists are actively searching for exoplanets (planets outside our solar system) that have water signatures.
