A1.1 Water

cohesion

the attraction of water molecules to one another, caused by hydrogen bonds

applications of cohesion

• transport of water under tension in xylem

• use of water surfaces as habitats due to surface tension

water molecules at the surface cannot form hydrogen bonds above them, and there is a relatively stronger attractions between the molecules to the the side and below them. The surface therefore becomes more difficult to break (surface tension)

adhesion

the tendency of water molecules to be attracted to and stick to different substances, rather than to themselves

applications of adhesion

• enables water to adhere to the cellulose wall (contains glucose which has a slight polar imbalance) - helps to keep water from dropping down due to gravity

• adheres to polar molecules within soil particles - helps soil retain water even in dry conditions 

gaps between soil particles allow channels for water to move through capillary action 

capillary action

the upward or inward movement of water through narrow spaces, driven by cohesion (water molecules sticking to each other) and adhesion (water molecules sticking to other substances) against the opposing force of gravity

solvent properties

Water surrounds and separates other polar molecules and ions from each other (because of water’s polarity, it is attracted to hydrophillic/polar molecules and forms a hydration shell around them - dissolving them by forming hydrogen bonds)

why is water a good solvent

• Transport: blood plasma and xylem/phloem carry nutrients, gases, waste dissolved in water.

• Reactions: metabolic reactions (enzyme activity, photosynthesis, respiration) occur in aqueous solutions.

• Excretion: urine dissolves wastes like urea.

polar molecule

molecule with an unequal distribution of electron density, leading to partial positive and partial negative charges across different regions.

electronegativity

The ability of an atom to attract shared electrons in a covalent bond; oxygen is highly electronegative in biological molecules.

hydrogen bonds

Weak intermolecular attractions between a hydrogen atom covalently bonded to an electronegative atom (eg; O or N) and another electronegative atom.

surface tension

A measure of how difficult it is to break the surface of a liquid; water’s high surface tension is due to cohesive hydrogen bonds.

Solute + example

A substance that is dissolved in a solvent to form a solution (e.g., glucose in blood plasma).

solvent

A liquid that dissolves other substances; water is the universal solvent in biology due to its polarity.

hydrophilic + example

Molecules attracted to water (water loving) and able to dissolve in it, typically polar or ionic (e.g; glucose)

hydrophobic + example

Molecules that repel water (water hating) and do not dissolve, typically nonpolar (e.g., lipids)

Buoyancy

The upward force exerted by a fluid that allows organisms to float; in water, aided by the density difference between body structures and water.

density

Mass per unit volume; water’s maximum density at 4°C allows ice to float, providing insulation for aquatic life.

thermal conductivity

The ability of a substance to transfer heat; water’s relatively high thermal conductivity allows efficient heat distribution in organisms.

specific heat capacity

The amount of energy needed to raise the temperature of 1 kg of a substance by 1°C; water’s high value stabilises environmental and organismal temperatures.

viscosity

A fluid’s resistance to flow; water’s low viscosity allows efficient circulation in blood and transport in xylem.

goldilocks zone

The orbital region around a star where conditions (temperature, liquid water availability) are suitable for life.

Asteroid

A rocky or metallic body orbiting the Sun; impacts from asteroids may influence Earth’s climate and evolution of life.