IB Biology—Topic A1.1 Water

Question–and–answer flashcards covering IB Biology topic A1.1 on water: polarity and hydrogen bonding, cohesion and adhesion, solvent properties, physical properties relevant to organisms, extraplanetary origin, and the search for extraterrestrial life.

What substance served as the medium in which the first cells originated?

Water

Name two reasons why water is a substance on which life depends.

It provides a stable thermal environment (high specific heat capacity) and acts as a universal solvent supporting metabolism and transport.

Why are covalent bonds in a water molecule considered polar?

Electrons are shared unequally; oxygen is more electronegative than hydrogen, giving oxygen a partial negative charge and hydrogens partial positive charges.

Between which atoms do hydrogen bonds form in adjacent water molecules?

Between the partial positive hydrogen of one molecule and the partial negative oxygen of another molecule.

How many hydrogen bonds can a single water molecule form simultaneously?

Up to four hydrogen bonds.

Define cohesion in the context of water.

Attraction between water molecules due to hydrogen bonding.

What property of water creates surface tension that allows insects to walk on water?

Cohesion among water molecules at the surface.

Which plant tissue relies on cohesion to transport water under tension?

Xylem

Define adhesion in the context of water.

Attraction of water molecules to polar or charged surfaces such as cellulose or soil particles.

What process in plants combines adhesion and cohesion to draw water upward against gravity?

Capillary action

Explain capillary action in soil.

Water adheres to charged soil particles and, with cohesion, moves through pores toward plant roots.

Why is water called a universal solvent?

Its polarity allows it to dissolve many ionic and polar substances.

Give one example of a hydrophilic molecule transported in blood plasma.

Glucose (other acceptable answers: amino acids, NaCl, antibodies).

Give one example of a hydrophobic substance whose transport in blood requires a carrier complex.

Cholesterol (or fatty acids transported in lipoproteins).

Why is O₂ only sparingly soluble in water?

Oxygen is non-polar and therefore hydrophobic.

What property of water buffers cell temperature and protects enzyme activity?

High specific heat capacity

Define specific heat capacity.

The energy required to raise the temperature of 1 kg of a substance by 1 °C.

Why does water have a high specific heat capacity?

A large amount of energy is needed to break extensive hydrogen bonding between molecules.

State one benefit to aquatic life of water’s high specific heat capacity.

Water temperature remains relatively stable, providing a consistent habitat.

What is meant by thermal conductivity?

A material’s ability to transfer heat.

Give one adaptation of the ringed seal related to water’s high thermal conductivity.

Thick blubber insulates against heat loss.

Define viscosity.

A measure of a fluid’s resistance to flow.

How do hydrophobic preen oils help the black-throated loon cope with water’s viscosity?

They reduce drag by creating a hydrodynamic surface on feathers.

Explain buoyancy in water.

Upward force exerted by water that can counteract the weight of immersed objects.

Why do ringed seals float easily?

High water buoyancy combined with the seal’s body fat provides lift.

Why does ice float on liquid water?

Ice is less dense because hydrogen bonds arrange water molecules into an open lattice.

How does floating ice benefit aquatic organisms in winter?

It insulates underlying water, allowing life to persist below.

Outline the asteroid hypothesis for the origin of Earth’s water.

Water condensed on distant asteroids; impacts delivered water, which was retained by Earth’s gravity and suitable temperatures.

Name two reasons water was retained on early Earth.

Sufficient gravity to hold water vapour and surface temperatures low enough for condensation into liquid water.

Why is the presence of water fundamental in the search for extraterrestrial life?

All known life requires water as a solvent and medium for biochemical reactions.

Define the Goldilocks zone.

The orbital distance from a star where temperatures allow liquid water to exist on a planet’s surface.

What technology can detect atmospheric water on exoplanets?

Transit spectrophotometry

During transit spectrophotometry, what indicates the presence of water?

Characteristic absorption wavelengths corresponding to water molecules.

How does water drive the folding of proteins?

Hydrophobic amino acid side chains avoid water, causing the chain to fold so hydrophilic residues remain exposed.

Describe water’s role in membrane formation.

Phospholipid heads interact with water while tails avoid it, causing spontaneous bilayer formation.

Why are most enzymatic reactions aqueous?

Enzymes require an aqueous environment for proper conformation and substrate interaction.

State one example of a molecule whose function depends on being hydrophobic and insoluble.

Cell membrane phospholipid tails (or cholesterol regulating membrane fluidity).

Compare viscosity of air, water, and blood.

Air is least viscous, water has moderate viscosity, and blood (due to cells and proteins) is more viscous than water.

What causes water’s polarity?

Unequal electron sharing between oxygen and hydrogen atoms.

Outline how hydrogen bonding leads to water’s high surface tension.

Molecules at the surface form stronger cohesive hydrogen bonds with neighbours, creating a taut surface film.

What term describes water molecules surrounding an ion in solution?

Hydration shell

Identify the solvent and solute in a saline solution of NaCl in water.

Solvent: water; Solute: sodium and chloride ions

Explain why warm water holds less dissolved oxygen than cold water.

Higher temperatures increase kinetic energy, reducing gas solubility in water.

What intermolecular force underlies both cohesion and adhesion in water?

Hydrogen bonding

State one consequence of water’s higher density compared to air for aquatic animals.

Greater buoyant support, affecting body structures (e.g., loons have denser bones to dive).

What feature allows black-throated loons to minimize heat loss in water?

Air trapped between feathers and hydrodynamic body shape.

How does capillary action aid movement of water from soil to plant roots?

Water adheres to soil particles and coheres to itself, climbing narrow spaces toward lower water potential at roots.

Why do enzymes have optimal temperature ranges?

Temperature affects molecular motion and hydrogen bonding, influencing enzyme conformation and activity.

What is meant by a logarithmic scale on a graph?

Each equal interval represents a ten-fold (or other constant base) change in value rather than an additive change.