A1.1

Electronegativity

• An atom's ability to attract electrons in a bond.

Covalent Bonds

• Form when atoms share electrons

• In a water molecule sharing is unequal because of oxygen being more electronegative than hydrogen.

Structure of a Water Molecule

• Polar molecule with two H atoms that have a partial + charge

• One O atom that has a partial - charge

Why Water is Polar

• Uneven charge distribution makes water a polar ‍molecule,

• Has a positive end (hydrogen side) and a ‍negative end (oxygen side).

Hydrogen Bonds

• Polarity is what allows hydrogen bonds to form between water molecules

• The δ+ hydrogen ‍of one molecule is electrostatically attracted to the δ− oxygen of neighbouring molecule

• Since opposite charges attract, these polar regions create ‌weak intermolecular attractions called hydrogen bonds

Cohesion

• Attraction between ‌water molecules due to hydrogen bonding

• Each water molecule can form hydrogen bonds ‍with neighbouring water molecules

How Cohesion Enables Xylem Transport

• Water evaporates from leaves through stomata (transpiration)

• This ‌creates tension (negative pressure) that pulls water ‍up through the xylem

• Hydrogen bonds between water ‌molecules allow them to be pulled as a ‍continuous column from roots to leaves

• Matters because hydrogen ‌bonds hold water molecules together, preventing the ‌column from snapping and allows water to move up against gravity

Surface Tension

• Water molecules at the surface are ‍pulled inward by hydrogen bonds with molecules below and ‍beside them

• This creates a "skin" ‌or elastic-like layer at the water's surface

• ‍Small organisms can walk on water due to surface ‍tension

Transpiration

• Movement of water from roots to leaves via xylem

Transpiration Steam

• As water leaves leaf, it is ‍replaced by water drawn up from the roots through the xylem.

• This ‍continuous flow from roots to leaves is the transpiration stream.

• So ‍a cost of gas exchange also delivers water and dissolved minerals to ‌the whole plant.

Water as a Medium for Life

• Comes from polar nature

• Has unique liquid properties important to life on Earth

Water's Unique Liquid Properties Important to Life on Earth

• Cohesion

• Buoyancy

• Adhesion

• Density

• Specific Heat

• Thermal Conductivity

• Viscosity

Goldilocks Zone

• Search for extraterrestrial life is focused on finding planets within Goldilocks Zone, where conditions are just right for liquid water to exist.

Factors of Goldilocks Zone

• Temperature (heat vs distance of the sun)

• Size of the Planet (enough gravity to hold water onto water and atmosphere to trap water vapor)

Goldilocks zone

• Region around a star where conditions are “just right” for liquid water to exist on a planet’s surface

Temperature Determines Water'‍s State

• Too ‍close to the star: Temperatures too high ‍→ water evaporates → exists only as vapor.

• Too ‍far from the star: Temperatures too low ‌→ water freezes → exists only as ice.‌

• Just right (Goldilocks zone): Temperatures allow ‍water to remain liquid.

Finding ⁣Water Means Finding Life

• Water is ‌the medium for metabolism and biochemical reactions

• All known life on ‍Earth requires liquid water to function.

• Without liquid ‍water, the complex chemical processes needed for ‌life as we know it cannot occur.

Life and Water

• Cohesion

• Adhesion

• Surface tension

• Solvent

• Viscosity

• Thermal Conductivity

• Specific Heat

Three Hypotheses About Origins of Water

1. Always here

2. Formed here

3. Arrived with asteroid (within planetary eclipses)

Why Early ⁣Earth Was Too Hot for Water

• During Earth's formation, temperatures ‍were extremely high.

• Any water present would have evaporated and been ‌lost to space.

• Therefore, water had to come from an extraplanetary ‍source, beyond Earth itself.

How Asteroids Delivered Earth's Water

• During Earth'‍s early history, there was a period of intense asteroid bombardment.‍

• Many of these asteroids contained water (as ice or hydrated minerals)‌.

• When asteroids collided with Earth, they delivered water to the ‍planet's surface.

Evidence Asteroids Delivered Earth’s Water

• The isotopic composition of water in some asteroids ‍matches Earth's water

Buoyancy

• How well an object can float

Buoyancy in Water

• Water = more dense

• Less energy needed to remain afloat

Buoyancy in Air

• Air = less dense

• More energy required to stay afloat

Viscosity

• Measure of a fluids resistance to flow, determined by the internal friction between its molecules

• High Viscosity = thicker, slower flowing liquid (like honey)

• Low Viscosity = thinner, faster flowing liquid (like water)

Viscosity in Water

• Water = more viscous

• Aquatic ‍organisms experience high drag forces while moving.

• Must overcome significant resistance to swim.

Viscosity in Air

• Air = low viscous

• Creates minimal resistance

Thermal Conductivity

• Rate at which heat transfers through a substance

Thermal Conductivity in Water

• Water = high thermal conductivity

• Conducts heat away from the body efficiently.

Thermal Conductivity in Air

• Air = Low Thermal Conductivity

• Poor conductor of heat

• Animals retain body heat more easily

Specific Heat Capacity

• Amount of energy required to raise the temperature of 1 gram of ‌a substance by 1°C

Specific Heat Capacity of Water

• Specific Heat Capacity = High

• Resists temperature changes

Specific Heat Capacity of Air

• Specific Heat Capacity = Low

• Air temperature changes rapidly with environmental conditions

Water as a Powerful Solvent

• Water is often called the "universal solvent" because ‌it dissolves a wide variety of substances, due to its polarity.

• When polar or ‍charged substances enter water, water molecules surround them, forming hydration shells.

• The δ− ‍oxygen attracts positive charges; the δ+ hydrogens attract negative charges.

• This separates and ‍suspends the particles, keeping them dissolved.

Hydrophilic

• “Water loving”

• Polar molecules and charged ions are hydrophilic

• Dissolves easily ‍in water

• Ex. Glucose and amino acids

Hydrophobic

• "Water-fearing"

• Non-polar molecules are hydrophobic.‌

• Do not dissolve in water.

• Dissolves in non-polar solvents instead.

• Ex. Fats, ‍oils, cholesterol

Solvent

• Medium that holds solute

Solute

• Particle found in the medium

Water as a Universal Solvent

• Like dissolves like

• Polar molecules dissolve in water

• Many important compounds for life are polar

Metabolism

• Sum of all chemical reactions occurring in an organism to ‍maintain life

• Including reactions that break down molecules for energy (catabolism) and reactions ‌that build molecules for growth and repair (anabolism)

Adhesion

• Attraction between water molecules and other surfaces, enabling water to cling to them and move against gravity

Capillary Action

• The movement of water through narrow spaces, even against gravity

• Requires both adhesion and cohesion

Cell Walls Stay Hydrated by Adhesion

• Water molecules adhere to cellulose in the cell wall ‌through hydrogen bonding.

• Cohesion between water molecules creates a continuous network of water throughout the ‍cell wall

• Together, adhesion and cohesion produce capillary ‌action within the cell wall, drawing water through the narrow spaces of the cellulose ‍mesh.

• As water evaporates from leaf surfaces during transpiration, adhesive forces pull more water ‍from the xylem into the cell walls.

Pleural Cavity

Maintains lungs and prevents them from collapsing

Diffusion

Movement of particles from a region of high concentration to a region of low concentration