Why life is still “watery”

Water is a molecular compound made of $H_2O$ molecules that stays liquid across much of the temperature range found on Earth’s surface. That single property matters a lot. In a liquid, particles can move around, collide and react. Life is chemistry in motion, and water provides the moving medium.

The first cells are thought to have originated in water. One simple way to imagine an early cell is as a tiny membrane-bound volume of watery solution, with dissolved substances inside, reactions taking place between them, and a boundary separating that chemistry from the outside. Modern cells still work on that basic pattern. Cytoplasm is water-based, blood plasma is water-based, plant sap is water-based, and most enzyme-catalysed reactions occur in water.

A medium is a surrounding substance that allows other substances or processes to occur within it. Water acts as the medium for life because it lets biologically important molecules dissolve, move, meet and react. Dry biological material can sometimes remain alive in a dormant state, but full metabolic activity needs liquid water so that molecules are mobile enough for reactions to continue.

That is why searches for life beyond Earth usually start with water, especially liquid water. Water isn’t “magic”; all known living systems use it as the reaction medium for most processes of life.

Polar covalent bonds inside water

A covalent bond is a chemical bond in which atoms share a pair of electrons. In a water molecule, oxygen forms covalent bonds with two hydrogen atoms. The electrons aren’t shared evenly: oxygen pulls the shared electrons more strongly than hydrogen.

A polar covalent bond is a covalent bond in which electrons are shared unequally, producing partial charges at different ends of the bond. In water, oxygen carries a partial negative charge, written $\delta^-$, while each hydrogen carries a partial positive charge, written $\delta^+$. The molecule is bent rather than straight, so the two $\delta^+$ hydrogen atoms sit on one side and the $\delta^-$ oxygen atom sits on the other. As a result, the whole water molecule is polar.

Students should be able to draw two or more water molecules with $\delta^-$ beside oxygen and $\delta^+$ beside hydrogen. Use solid lines for covalent bonds within each water molecule, and dashed or dotted lines for hydrogen bonds between molecules.

Hydrogen bonds between water molecules

A hydrogen bond is an intermolecular attraction in which a partially positive hydrogen atom in one polar molecule is attracted to a partially negative atom in another polar molecule. In water, the $\delta^+$ hydrogen in one molecule is attracted to the $\delta^-$ oxygen in a neighbouring molecule.

Don’t mix up the two bond types. The O—H covalent bonds inside a water molecule are strong chemical bonds. Hydrogen bonds between water molecules are weaker attractions. The numbers matter here: one hydrogen bond is weak, but liquid water has huge numbers of them, continually breaking and reforming. Acting together, they explain many of water’s biological properties.

Here’s the first linking idea for this topic: intermolecular forces are more than chemistry vocabulary. In water, they act as biological forces, shaping transport in plants, surface habitats, solubility, and even thermal stability.

Cohesion: water sticking to water

Cohesion is an attraction between molecules of the same substance that holds them together. Water shows cohesion because neighbouring water molecules form hydrogen bonds with one another. In class I usually put it like this: water molecules “hold hands” for a moment, let go, then hold hands again with new neighbours. These repeated attractions make water surprisingly hard to pull apart.

Water under tension in xylem

Xylem is plant transport tissue that carries water and dissolved mineral ions from roots towards leaves. Water moves through xylem as continuous columns in narrow tubes. As water evaporates from leaf cell walls, it pulls on the water behind it. Because water molecules cohere to each other, that pull passes down the xylem column.

Tension is a pulling force that stretches a material or column of fluid. Water in xylem is under tension: it is being pulled upwards, rather like a rope in a tug of war. The column usually does not snap, since many hydrogen bonds would have to break at the same place at the same time. Cohesion helps explain how tall plants move water upwards without a pump.

Surface tension and surface habitats

Surface tension is the tendency of a liquid surface to resist being stretched or broken because molecules at the surface are attracted inwards. At the surface of water, water molecules are far more attracted to other water molecules than to air particles. The surface acts a little like a stretched film.

This forms habitats at and near the surface. Water striders can stand and move on the surface because their legs spread their weight and do not easily break through the hydrogen-bonded surface film. Mosquito larvae can hang just below the surface using a breathing siphon. These work well as examples for the linking question about processes at or near surfaces: the water surface is not just a boundary; for some organisms it is a habitat.

Cohesion is a molecular property with organism-level consequences: it helps plants transport water under tension and lets small organisms exploit the water surface.

Adhesion: water sticking to other surfaces

Adhesion is an attraction between molecules of different substances that holds them together. Water sticks well to polar or charged surfaces because its partial charges can attract those surfaces, including through hydrogen bonds.

Capillary action is the movement of liquid through narrow spaces due to adhesion between the liquid and the walls of the space, often helped by cohesion within the liquid. You’ve seen this when water creeps through paper towel. The same principle matters in soil and in plant cell walls.

Capillary action in soil

Soil has tiny spaces between mineral particles and organic matter. Many of these surfaces are charged or polar, so water adheres to them. In fine pores, adhesion can pull water through soil against gravity. It helps water spread through porous soil and move from wetter regions into drier regions.

Capillary action in plant cell walls

Plant cell walls contain cellulose, a polar carbohydrate. Because water adheres to cellulose, dry wall material tends to pull water into its tiny spaces. In leaves, evaporation removes water from moist cell walls; adhesion then helps draw replacement water from nearby xylem. This keeps the cell walls wet enough for gases such as carbon dioxide to dissolve before diffusing into photosynthetic cells.

Adhesion and cohesion work together in plants. Adhesion helps water cling to walls and move through microscopic spaces; cohesion helps the water column stay continuous. That’s the linking question on intermolecular forces in miniature: different attractions combine to produce a whole-plant transport system.

Water as a solvent

A solvent is a liquid that dissolves another substance to form a solution. A solute is a substance that is dissolved in a solvent. A solution is a homogeneous mixture in which solute particles are dispersed among solvent particles.

Water dissolves many charged and polar substances well because its molecules are polar. The $\delta^-$ oxygen end is attracted to positive ions, while the $\delta^+$ hydrogen ends are attracted to negative ions. Water molecules can also form hydrogen bonds with polar molecules. Together, these attractions surround solute particles, separate them, and keep them dispersed.

Hydrophilic and hydrophobic substances

A hydrophilic substance is a substance that is attracted to water because it is charged or polar. Many ions, sugars and amino acids are hydrophilic, so they dissolve in water or interact strongly with it.

A hydrophobic substance is a substance that is not attracted to water because it is non-polar and uncharged. Hydrophobic substances are insoluble or only very slightly soluble in water. Lipids are the standard biological example. That isn’t a failure of water; it’s essential biology. Some molecules need to be hydrophobic to do their job, for example in forming membrane interiors that separate watery compartments.

Medium for metabolism

Metabolism is the total set of enzyme-catalysed chemical reactions occurring in a living organism or cell. Most enzymes catalyse reactions in aqueous solution. In water, dissolved reactants can move around, collide with enzyme active sites and be converted into products. If the cell contents dry out, molecular movement becomes too restricted for normal metabolism.

Transport in plants and animals

Water’s solvent properties make it useful as a transport medium. In plants, xylem sap carries dissolved mineral ions, while phloem sap carries dissolved sugars and other soluble products of photosynthesis. In animals, blood plasma carries dissolved ions, glucose, amino acids and many other hydrophilic substances.

Some biologically important substances dissolve poorly in water. Oxygen is non-polar and only sparingly soluble, so many animals use haemoglobin to increase oxygen transport in blood. Fats are hydrophobic, so they cannot simply dissolve in plasma; instead, they travel in droplets or particles whose surfaces interact with water. The key point is not “water dissolves everything” — it does not. Water dissolves many hydrophilic substances, while the insolubility of hydrophobic molecules is itself biologically useful.

Physical properties that matter in habitats

A physical property is a measurable characteristic of a substance that can be observed without changing its chemical composition. For animals, water is not just “wet air”. It’s a completely different medium, and that difference brings both advantages and difficulties.

Buoyancy

Buoyancy is the upward force exerted by a fluid on an object immersed in it. Because water is much denser than air, it gives far more buoyant support. Many aquatic animals have body densities close to that of water, so they spend relatively little energy avoiding sinking or holding their depth. That is a major opportunity in aquatic life: the habitat itself helps support the animal against gravity.

Air gives animals very little buoyancy, since it is far less dense. A flying bird has to generate lift to remain airborne, and that takes energy. A ringed seal, Pusa hispida, floating or swimming in water receives much more support from the surrounding medium than a black-throated loon, Gavia arctica, does while flying in air.

Viscosity

Viscosity is a measure of a fluid’s resistance to flow caused by internal friction within the fluid. Water is far more viscous than air. Moving through water therefore produces much more drag than moving through air at the same speed. The support water provides has a cost: aquatic animals need streamlined bodies and powerful movement to push through it.

A seal gains from buoyancy, but it has to move through a high-drag medium. A loon in flight faces less drag from air, although it must keep generating lift to counteract gravity. The comparison works well because both animals use water and air, but the main movement challenge is different for each one.

Thermal conductivity

Thermal conductivity is the rate at which heat passes through a material. Water conducts heat much faster than air. For warm-blooded aquatic animals, that creates a real problem: body heat can be lost rapidly to the surrounding water. Fat layers, trapped air in feathers, or dense fur can act as insulation and reduce heat loss.

Specific heat capacity

Specific heat capacity is the amount of thermal energy needed to raise the temperature of one kilogram of a substance by one kelvin. Water has a high specific heat capacity, mainly because energy is needed to disrupt hydrogen bonding before molecular motion increases much. Bodies of water therefore warm and cool slowly.

For aquatic animals, this can help and restrict them. Compared with air, the habitat is thermally stable, so temperature changes are smaller. Even so, water conducts heat well, so an animal warmer than the water may lose heat quickly unless it has effective insulation and circulation control.

Comparison of water and air as habitats for animals, linking physical properties to movement, support and heat exchange.

The syllabus note on examples is worth remembering kindly: either the common name or the scientific name is acceptable when referring to an organism. Use whichever you know accurately.

Where Earth’s water may have come from

An asteroid is a small rocky or metallic body that orbits the Sun and is smaller than a planet. A major hypothesis is that water-containing asteroids collided with the young Earth and brought much of its water. For this syllabus point, keep the origin hypothesis to asteroids; don’t drift into a long list of other possible sources.

The early Earth was hot. During the hottest stages of formation, any water at the surface would mostly have existed as vapour, and some could have escaped into space. Later asteroid bombardment gives a plausible way for water to be added after Earth had cooled enough for liquid water to persist.

Why Earth retained water

Earth has kept abundant water for billions of years. That gave life time to evolve. You need two reasons here.

First, Earth’s gravity is strong enough to hold on to oceans and an atmosphere. Very light gases escape more easily, but Earth loses water vapour far less readily than a smaller body with weaker gravity would.

Second, Earth’s temperatures became low enough for water vapour to condense into liquid water. Liquid water is easier to retain than vapour because it stays at the surface as oceans, lakes and groundwater instead of dispersing into space. So Earth’s position relative to the Sun matters: warm enough for liquid water, but not so hot that all water boils away.

Put simply: asteroid delivery may explain how water arrived; gravity and suitable temperatures help explain why it stayed.