A1

Water molecule (H₂O)

Consists of one oxygen atom covalently bonded to two hydrogen atoms.

Bond angle in water

Approximately 104.5°, deviating from the ideal tetrahedral angle due to lone pairs on the oxygen.

Electronegativity

The ability of an atom to attract shared electrons in a covalent bond.

Polarity of water

Water is a polar molecule due to the unequal sharing of electrons, with δ⁻ on oxygen and δ⁺ on hydrogens.

Hydrogen bond

A weak electrostatic attraction between a hydrogen atom covalently bonded to an electronegative atom and a lone pair on another electronegative atom.

Cohesion

The attraction between molecules of the same substance, primarily due to hydrogen bonding in water.

Adhesion

The attraction between molecules of different substances, often occurring when water molecules are attracted to polar or charged surfaces.

Specific heat capacity of water

Amount of energy required to raise the temperature of 1 gram of water by 1°C, which is 4.18 J/g/°C.

Latent heat of vaporisation

Energy required to convert 1 gram of liquid water to gas at constant temperature, measured at 2260 J/g.

Latent heat of fusion

Energy required to convert 1 gram of solid water (ice) to liquid at constant temperature, measured at 334 J/g.

Solvent properties of water

Water's ability to dissolve more substances than any other common liquid, making it essential for various chemical and biological processes.

Density of ice vs liquid water

Ice is less dense than liquid water, allowing it to float due to a rigid crystalline lattice structure.

Transparency of water

Water allows sunlight penetration, facilitating photosynthesis in aquatic ecosystems.

Incompressibility of water

Water's volume does not significantly decrease under pressure, allowing for hydrostatic skeletons and turgor pressure in plants.

Amphoteric nature of water

Water acts as both an acid (proton donor) and a base (proton acceptor), crucial for maintaining biological pH.

Two guiding questions for A1.1 Water

1) What physical and chemical properties of water make it essential for life? 2) What are the challenges and opportunities of water as a habitat?

A1.1.1 (learning objective)

Water as the medium for life

A1.1.2 (learning objective)

Hydrogen bonds as a consequence of the polar covalent bonds within water molecules

A1.1.3 (learning objective)

Cohesion of water molecules due to hydrogen bonding and consequences for organisms

A1.1.4 (learning objective)

Adhesion of water to materials that are polar or charged and impacts for organisms

A1.1.5 (learning objective)

Solvent properties of water linked to its role as a medium for metabolism and for transport in plants and animals

A1.1.6 (learning objective)

Physical properties of water and the consequences for animals in aquatic habitats

Why is water unique on Earth (states of matter)?

Water is the only substance on Earth that naturally exists in three states (solid/ice, liquid, gas/vapor) within the range of natural temperatures, helping regulate climate and support life

Define 'medium' (in biology context)

The substance or environment where something happens (e.g. water is the medium for fish)

Why is water considered the medium for life?

Life on Earth depends on water; no known life can exist without it. Most cells are ~70-80% water

Approximately what % of a cell is water?

70-80%

Three things water provided that allowed life to begin

Stable conditions, a medium for chemical reactions, and protection from UV radiation

What % of Earth's water is in oceans (saltwater) vs freshwater?

97% saltwater (oceans), 3% freshwater

What % of Earth's surface is covered by water?

71% (making it the 'blue planet')

How long has water existed on Earth?

At least 3.8 billion years

How does ocean depth compare to mountain height?

Deep oceans exceed the height of the tallest mountains

Why is water a polar molecule?

Electrons are shared unequally between oxygen and hydrogen in the polar covalent bonds

Which atom in water is more electronegative?

Oxygen — it pulls the shared electrons closer to itself

What partial charge does oxygen carry in water?

Partial negative (δ⁻)

What partial charge do the hydrogens carry in water?

Partial positive (δ⁺)

What does water's polarity allow it to do?

Form weak associations with other polar molecules or charged ions; δ⁻ poles attract δ⁺ poles of other molecules and vice versa

Are hydrogen bonds intermolecular or intramolecular?

Intermolecular (between molecules)

How does a hydrogen bond form?

When a δ⁺ hydrogen atom is attracted to a δ⁻ fluorine, oxygen, or nitrogen atom of another molecule

Why are hydrogen bonds stronger than standard polar associations?

Due to the particularly high electronegativity of F, O and N

Which three atoms can form hydrogen bonds with hydrogen?

Fluorine (F), Oxygen (O), Nitrogen (N)

What unique properties of water does hydrogen bonding cause?

Thermal, cohesive, and solvent properties

Exam tip: when drawing water molecules, how should hydrogen bonds be drawn?

As a dashed line

Exam tip: how should covalent bonds in a water molecule be drawn?

As solid lines (between O and H)

Define cohesion (water)

The attraction of water molecules to each other due to hydrogen bonding

Define surface tension

The property of a substance to resist an external force

Why does water have high surface tension?

Hydrogen bonding between water molecules allows the liquid to resist low levels of external force

Biological consequence of water's high surface tension

Makes water sufficiently dense for certain smaller organisms (e.g. water striders) to move along its surface

What are xylem vessels?

Thin narrow tubes that transport water (and dissolved minerals) from the roots to the leaves of plants

How does cohesion help plants transport water?

Cohesion of polar water molecules allows water to form a continuous column moving up the xylem/stem

In what direction does xylem move water?

One direction only — upwards (roots to leaves)

Define adhesion (water)

The attraction between water molecules and other polar or charged substances/surfaces

Define capillary action

Movement of water in narrow spaces due to adhesion (water sticks to surfaces) and cohesion (water molecules pull each other)

What does capillary action allow water to do?

Move against gravity

Examples of surfaces water adheres to

Glass, plant cell walls, soil

How does tube width affect capillary action?

The thinner the space/tube, the higher the water climbs

Why can water move from cell to cell in plants by capillary action?

Cellulose in plant cell walls is porous and polar, so polar water adheres to the polar cellulose

Define pressure gradient (water transport)

A difference in pressure that causes water to move from one place to another

Mnemonic for water transport in plants

Transpiration pulls, cohesion holds, adhesion supports

Role of capillary action in soil

Responsible for movement of water through soil from the deeper water table

Why is water called the universal solvent?

Its polarity allows a wide variety of hydrophilic substances to dissolve in it

Define hydrophilic

Water-loving — substances (polar/charged) that freely associate and readily dissolve in water

Examples of hydrophilic substances

Glucose, ions (Na⁺, K⁺, Cl⁻), amino acids, proteins and enzymes

Why is glucose dissolving in water important?

Important for cellular respiration

Why is dissolving ions (Na⁺, K⁺, Cl⁻) important?

Required for nerve signaling

Define metabolism

The complex network of interdependent and interacting chemical reactions occurring in living organisms

Define enzyme

A biological catalyst (protein) that speeds up chemical reactions

Why does water act as a good metabolic medium?

It dissolves hydrophilic substances forming an aqueous solution, allowing dissolved molecules to move and collide with enzymes

Where do most enzymes catalyse reactions?

In aqueous solution

Example of an enzyme working in an aqueous medium

Amylase in saliva — breaks down starch into sugar

Define hydrophobic

Water-hating — substances (large non-polar molecules like lipids) that do not freely associate or dissolve in water

Are lipids hydrophilic or hydrophobic?

Hydrophobic (large non-polar molecules)

Difference between enzyme and catalyst (definition)

Enzyme = biological catalyst (protein). Catalyst = any substance that speeds up a reaction

Difference between enzyme and catalyst (specificity)

Enzymes are highly specific (one substrate). Catalysts are not specific

Difference between enzyme and catalyst (structure)

Enzymes are proteins with an active site. Catalysts are not necessarily proteins

Difference between enzyme and catalyst (conditions)

Enzymes work best at specific pH and temperature. Catalysts often work in wider conditions

Example of an enzyme vs a catalyst

Enzyme: amylase, catalase. Catalyst: platinum, iron

Why is cohesion important? (5 points)

1) Water molecules stick together via H-bonding. 2) Forms a continuous water column in xylem. 3) Allows water to be pulled up during transpiration. 4) Prevents the column breaking. 5) Enables transport against gravity

Why is adhesion important? (5 points)

1) Water sticks to polar surfaces (e.g. xylem walls). 2) Helps water move up narrow tubes (capillary action). 3) Supports upward movement against gravity. 4) Helps maintain the water column. 5) Enables water movement through soil to roots

What is transported in xylem vs phloem?

Xylem transports mineral ions (and water). Phloem transports dissolved nutrients

What does blood plasma transport in animals?

Dissolved solutes — amino acids, simple sugars, wastes (urea), and small amounts of gases (O₂ and CO₂)

How are lipids transported in animal blood?

Packaged with proteins to form water-soluble lipoproteins

How may lipids be transported in plants?

Mechanism not well understood, but may involve conjugation (joining) to amino acids and transport via the phloem

Four key physical properties of water (from H-bonding)

Buoyancy, viscosity, thermal conductivity, specific heat capacity

Define buoyancy

The upward force exerted by a fluid on an object, allowing objects to float

Define viscosity

The resistance of a fluid to motion

Define thermal conductivity

The ability of a substance to transfer heat

Define specific heat capacity

The amount of energy needed to raise the temperature of a substance

Compare density: water vs air

Water is denser than air, providing greater buoyancy

Compare viscosity: water vs air

Water has higher viscosity, making movement harder

Compare thermal conductivity: water vs air

Water conducts heat better, leading to higher heat loss in aquatic organisms

Compare specific heat capacity: water vs air

Water has higher specific heat capacity, stabilising temperature fluctuations

When does an object float vs sink in water?

Floats when less dense than water, sinks when denser than water

How does the black-throated loon adapt to buoyancy?

Uses buoyancy to float, but has solid bones to increase density and reduce buoyancy for diving

How does the ringed seal adapt to buoyancy?

Blubber adds buoyancy while allowing efficient swimming; floats with only its snout above water to conserve energy

Scientific name of the black-throated loon

Gavia arctica

Scientific name of the ringed seal

Pusa hispida

How does the black-throated loon adapt to water's viscosity?

Webbed feet provide greater propulsion; streamlined body reduces drag

How does the ringed seal adapt to water's viscosity?

Uses flippers for propulsion; streamlined body shape reduces resistance

How much better does water conduct heat than air?

28 times better than air

How does the black-throated loon adapt to water's thermal conductivity?

Traps air between feathers for insulation; uses an oil gland to coat feathers and make them waterproof