A1.1 Water

Water is the medium in which most life processes occur. The first cells originated in water, and modern cells still carry out most reactions in aqueous cytoplasm.

Water keeps many ions and polar molecules dissolved, so reactants can diffuse, collide and be acted on by enzymes. It also transports dissolved substances within organisms.

When explaining water as a medium, link three ideas: dissolved substances, movement or mixing, and a biological process such as metabolism or transport.

In cytoplasm, dissolved glucose and enzymes can move and interact, allowing enzyme-catalysed reactions to occur.

A medium is the environment in which a process occurs. Water does not have to be a reactant in every reaction that takes place in it.

Each O-H bond in water is polar covalent because oxygen attracts the shared electrons more strongly than hydrogen. Oxygen is partially negative (delta-) and each hydrogen is partially positive (delta+).

Because a water molecule is V-shaped, its bond dipoles do not cancel. The delta+ hydrogen of one molecule is attracted to the delta- oxygen of another, forming an intermolecular hydrogen bond.

To represent two water molecules: draw solid O-H covalent bonds within each V-shaped molecule, label O as delta- and H as delta+, then draw a dotted line from H(delta+) on one molecule to O(delta-) on the other and label it hydrogen bond.

Many individually weak hydrogen bonds act together. This collective attraction helps explain cohesion and the relatively high energy needed to separate water molecules.

Do not draw a hydrogen bond between O and H within the same water molecule: that connection is a polar covalent bond.

Cohesion is attraction between molecules of the same substance; in water it supports continuous columns and surface tension.

Hydrogen bonding resists separation. This allows water to transmit tension through xylem and lets the surface support small organisms.

Distinguish cohesion from adhesion before explaining a transport or surface effect.; separate property, mechanism and biological use

A continuous water column in a plant xylem vessel can be pulled upward as leaves lose water, because water molecules cohere. This gives a concrete prediction from the stated water condition.

Cohesion alone does not lift water without transpiration, adhesion and a continuous pathway. Interpret the result within the stated evidence and scale limits.

Water dissolves many ionic and polar substances because its partial charges attract ions and polar regions of molecules.

Water molecules form hydration shells around ions and hydrogen bonds with many polar solutes. Dissolved particles remain mobile, so they can be transported in blood, xylem or phloem and can take part in enzyme-catalysed reactions in aqueous solution.

Hydrophilic substances interact with water and tend to dissolve. Hydrophobic substances are non-polar and tend to remain separate; this insolubility is important for structures such as lipid membranes.

When sodium chloride dissolves, the oxygen side of water faces Na+ and the hydrogen sides face Cl-, producing hydration shells that keep the ions dispersed.

Water is not a literally universal solvent. Many non-polar substances dissolve poorly, and solubility does not mean that a substance is harmless.

Compared with air, water is denser and more viscous, conducts heat more rapidly, and has a higher specific heat capacity. Each difference has a distinct consequence for aquatic animals.

A streamlined ringed seal experiences less drag in viscous water, while blubber reduces heat transfer to the water.

Thermal conductivity is the rate of heat transfer; specific heat capacity is the energy needed to change temperature. Do not use the terms interchangeably.

An aquatic adaptation should be explained by naming the water property, the challenge it creates, and how the feature improves survival or movement.

A complete explanation is: water has higher thermal conductivity than air, so a seal loses heat rapidly; its blubber slows this heat transfer.

Do not claim that blubber is mainly an adaptation to high specific heat capacity. The direct challenge is water's higher thermal conductivity.

Within the IB syllabus, the extraplanetary hypothesis is that water-rich asteroids delivered water to Earth. Its long-term presence helped provide conditions in which life could evolve.

Two conditions explain retention: temperatures became low enough for water vapour to condense, and Earth's gravity was strong enough to retain water over geological time.

Use the chain: asteroid delivery -> cooling and condensation -> gravitational retention -> persistent water available during the evolution of life.

If surface and atmospheric temperatures allow water vapour to condense, water can collect as liquid instead of remaining only as vapour; gravity helps keep that water associated with Earth.

Treat asteroid delivery as a hypothesis, not a proven statement that asteroids supplied all water. For this objective, do not expand the origin explanation to comets or internal sources.

The search for extraterrestrial life often prioritizes places where liquid water could exist because water supports the chemical interactions required by known life.

A star's Goldilocks zone is the range of distances where temperatures may allow liquid water at a planet's surface. A planet that is too close may be too hot; one too far away may be too cold.

When interpreting planetary data, first identify whether the planet lies in the Goldilocks zone, then state that this makes liquid water and habitability possible rather than certain.

If a table shows one planet at a distance compatible with surface liquid water, that planet is the best candidate for further investigation, not evidence that life is already present.

Being in the Goldilocks zone is not proof of liquid water or life; other planetary conditions also affect whether surface water can persist.