Why life is still “watery”

Water is a molecular compound made of H₂O molecules that is liquid over much of Earth’s surface temperature range. In a liquid, particles can move, collide and react. That matters a lot. Life is chemistry in motion, and water provides the medium where that motion happens.

The first cells are thought to have originated in water. Picture an early cell 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 keep 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 survive in a dormant state, but full metabolic activity needs liquid water, so molecules can stay mobile enough for reactions to continue.

That’s why searches for life beyond Earth begin with water, especially liquid water. Not because water is “magic”, but because 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 sharing isn’t equal: 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, the oxygen atom carries a partial negative charge, written δ−, while each hydrogen atom carries a partial positive charge, written δ+. The molecule is bent rather than straight, placing the two δ+ hydrogen atoms on one side and the δ− oxygen atom on the other. So the whole water molecule is polar.

Students should be able to draw two or more water molecules with δ− beside oxygen and δ+ 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 δ+ hydrogen of one molecule is attracted to the δ− oxygen of a neighbouring molecule.

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

Here’s the first linking idea for this topic: intermolecular forces are not just 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 this way: 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. In xylem, water moves as continuous columns through narrow tubes. As water evaporates from leaf cell walls, it pulls on the water behind it. That pull travels down the xylem column because water molecules cohere to each other.

Tension is a pulling force that stretches a material or column of fluid. Xylem water is under tension: it’s being pulled upwards, a bit like a rope in a tug of war. The column does not normally snap because 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 attracted much more strongly to other water molecules than to air particles. The surface acts a little like a stretched film.

That film creates habitats at and near the surface. Water striders can stand and move on it 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 examples work well for the linking question about processes at or near surfaces: the water surface is not only a boundary; for some organisms, it is a habitat.

Cohesion is a molecular property with organism-level effects: 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 form attractions with them, including 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 can see it when water creeps through a paper towel. The same idea 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 can move water from wetter regions into drier regions.

Capillary action in plant cell walls

Plant cell walls contain cellulose, a polar carbohydrate. Water adheres to cellulose, so dry wall material tends to draw water into its tiny spaces. In leaves, evaporation removes water from moist cell walls. Adhesion then helps pull 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 act together in plants. Adhesion helps water cling to walls and move through microscopic spaces; cohesion helps keep the water column 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 works as an effective solvent for many charged and polar substances because its molecules are polar. The δ− oxygen end attracts positive ions, while the δ+ hydrogen ends attract negative ions. With polar molecules, water can also form hydrogen bonds. These attractions surround solute particles, pull them apart 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 problem with water; it matters in biology. Some molecules need to be hydrophobic to work properly, for example when they form 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. Dissolved reactants can move, collide with enzyme active sites and then be converted into products. If cell contents dry out, molecular movement becomes too restricted for normal metabolism.

Transport in plants and animals

Water’s solvent properties let it act 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 with surfaces that interact with water. The point is not that “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 behaves as a very different medium, and that creates both opportunities and problems.

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 animals 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 a particular depth. That’s one major advantage of aquatic life: the habitat itself partly supports the body against gravity.

Air gives animals little buoyancy because it is far less dense. A flying bird has to generate lift to stay airborne, and that takes energy. A ringed seal, Pusa hispida, floating or swimming in water gets much more support from the surrounding medium than a black-throated loon, Gavia arctica, gets when 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 creates much more drag than moving through air at the same speed. The support water provides has a cost: aquatic animals need streamlined bodies and strong movement to push through it.

A seal benefits from buoyancy, but it also has to move through a high-drag medium. A loon in flight experiences less drag from air, but it must produce lift to counteract gravity. It’s a useful comparison because both animals use water and air, while facing different main movement challenges.

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 can be a serious challenge, since body heat is lost rapidly to the surrounding water. Fat layers, trapped air in feathers, or dense fur can reduce that 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 helps in some ways and limits them in others. Compared with air, the habitat is thermally stable, so temperature swings are smaller. However, because water also conducts heat well, 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 says that many of Earth’s water molecules arrived when water-containing asteroids collided with the young Earth. The syllabus keeps this origin hypothesis to asteroids, so don’t drift into a long catalogue of other possible sources.

The early Earth was hot. Any surface water present during the hottest stages of formation would mostly have been vapour, and much of it could have escaped into space. Later asteroid bombardment gives a plausible way to add water after Earth had cooled enough for liquid water to remain.

Why Earth retained water

Earth has kept abundant water for billions of years. That gave life time to evolve. For this part, two reasons are needed.

First, Earth’s gravity is strong enough to hold onto 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 fell low enough for water vapour to condense into liquid water. Liquid water is easier to keep because it sits at the surface as oceans, lakes and groundwater instead of spreading out into space. Earth’s position relative to the Sun matters here: 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.