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A1.1

Water

Theme A · Unity and diversity · Molecules · SL and HL · plus additional higher level

Water is the first topic in the course for a reason: almost every later topic — membranes, transport in xylem, enzymes, osmosis, thermoregulation — quietly depends on one fact about the water molecule. It is polar, so water molecules stick to each other and to other polar things. Learn that fact properly here and a surprising amount of the course becomes consequence rather than content.

🎯What you need to be able to do

  • Explain why life began in water and why water is still the medium for most life processes.
  • Explain the polarity of the water molecule and draw hydrogen bonds between two or more water molecules, with partial charges shown.
  • Link cohesion and adhesion to their consequences for organisms: water under tension in xylem, surface tension as a habitat, capillary action in soil and cell walls.
  • Explain why water is a good solvent for hydrophilic substances, and why some biological molecules must be hydrophobic.
  • Compare the buoyancy, viscosity, thermal conductivity and specific heat capacity of water and air, and relate them to animals that live in each.
  • AHL Outline the hypothesis that Earth’s water arrived from asteroids, why it stayed, and why the search for extraterrestrial life is a search for liquid water.

📚The biology

Water as the medium for life

The first cells arose in water, and every cell alive today is still mostly water: the cytoplasm is an aqueous solution in which almost all metabolic reactions take place. Water carries substances round organisms (blood plasma, xylem sap, phloem sap), it is a reactant in hydrolysis and in photosynthesis, and it is a product of condensation reactions and of aerobic respiration. The reason it can do all of this comes down to its structure.

Polarity and hydrogen bonds

A water molecule is one oxygen atom covalently bonded to two hydrogen atoms. The two atoms in each bond do not share the electrons equally: oxygen is far more electronegative and pulls the shared electrons towards itself. That makes the oxygen end of the molecule slightly negative (δ−) and each hydrogen slightly positive (δ+). Because the molecule is bent rather than straight, these partial charges do not cancel out, so the whole molecule is polar.

The δ+ hydrogen of one water molecule is attracted to the δ− oxygen of a neighbour. This attraction is a hydrogen bond. Each one is weak — roughly a twentieth of the strength of a covalent bond — and in liquid water hydrogen bonds are constantly breaking and reforming. But there are an enormous number of them, and collectively they give water its unusual properties.

How to draw it
Draw each molecule as O with two H atoms at an angle. Label O as δ− and each H as δ+. Join an H of one molecule to the O of the next with a dashed line.
What the dashed line means
A hydrogen bond is an attraction between molecules. The solid lines inside a molecule are covalent bonds. Never draw a hydrogen bond as a solid line.
“Water contains hydrogen bonds” is not what an examiner wants. The hydrogen bonds are between water molecules, not within one. Inside the molecule the bonds are polar covalent bonds. The polarity of those covalent bonds is the cause; hydrogen bonding between molecules is the consequence. Answers that mix the two up lose the mark even when every other word is right.

Cohesion

Cohesion is the attraction of water molecules to each other, caused by hydrogen bonding. It has two consequences the syllabus asks for.

  • Water can be pulled under tension. In xylem, water is drawn up the plant as a continuous column because evaporation from leaves pulls on it (B3.2). A column of water can be pulled without breaking because the molecules hold together. A rope of sand would fall apart; a rope of water, in a narrow enough tube, does not.
  • Surface tension. At a water surface, molecules are pulled inwards and sideways by their neighbours but not upwards, so the surface behaves like a stretched elastic film. Small animals such as pond skaters and water striders can stand on it, spreading their weight over long legs so the surface is not broken.

Adhesion

Adhesion is the attraction of water to other substances that are polar or charged. Cellulose, the main component of plant cell walls, has many hydroxyl groups and is strongly hydrophilic, so water adheres to it; so do soil particles such as clay, which carry charges.

  • Capillary action is movement of water through narrow spaces caused by adhesion to the walls, with cohesion dragging the rest of the water along. In soil, it moves water through the pores between particles towards roots. In plant cell walls, it draws water through the mesh of cellulose fibres, which is how the walls of leaf cells stay wet and how water is drawn out of xylem vessels into the leaf.

Water as a solvent

Because it is polar, water dissolves a wide range of substances. Positive ions are surrounded by the δ− oxygen ends of water molecules and negative ions by the δ+ hydrogen ends; polar molecules such as glucose and amino acids form hydrogen bonds with water. Substances that dissolve readily are hydrophilic. (The details of how solutes are surrounded by water are in D2.3.)

This matters in two ways. Metabolism: most enzymes catalyse reactions between substances dissolved in the cytoplasm, and the substrates can only collide with the active site if they are free to move in solution. Transport: glucose, amino acids, ions, urea and many hormones are carried dissolved in blood plasma; sucrose and amino acids are carried dissolved in phloem sap; mineral ions travel dissolved in xylem sap.

Just as important is what does not dissolve. Non-polar substances such as lipids are hydrophobic, and several functions depend on exactly that:

  • membranes are made of phospholipids whose hydrophobic tails form a barrier that water-soluble substances cannot easily cross (B2.1);
  • triglycerides can be stored in large amounts without affecting the water balance of a cell;
  • waxy cuticles on leaves reduce water loss because water does not pass through them.

Hydrophobic substances that must travel in blood need help: cholesterol and fats are carried inside lipoprotein particles, and oxygen, which is only sparingly soluble, is mostly carried bound to haemoglobin.

Physical properties: water compared with air

Animals that live in water face a very different physical world from animals that live in air. The syllabus names four properties to compare.

Buoyancy
Water is about 800 times denser than air, so it provides an upward force that supports an animal’s body. Aquatic animals need less skeletal support; in air, the skeleton carries the full weight.
Viscosity
Water is roughly 50 times more viscous than air. Moving through it takes much more energy, which favours streamlined bodies. Air offers little resistance but also little support.
Thermal conductivity
Water conducts heat around 20 times faster than air. An animal in cold water loses body heat far faster than in cold air at the same temperature.
Specific heat capacity
Water needs a lot of energy to change temperature: about 4.2 J g−1 °C−1, four times more than air per gram. Bodies of water therefore change temperature slowly and are thermally stable habitats.

The high specific heat capacity is itself a consequence of hydrogen bonding: energy supplied to water goes first into breaking hydrogen bonds, not into making the molecules move faster, so the temperature rises only slowly.

Two examples show how animals cope with living across both media.

  • The black-throated loon (Gavia arctica) is a diving bird. It is streamlined, and its legs sit far back on its body, which makes it a powerful swimmer against the viscosity of water but so clumsy on land that it rarely walks. Unlike most birds it has solid, dense bones, which reduce buoyancy and let it dive. Dense, oily plumage traps a layer of air that insulates against the high thermal conductivity of cold water.
  • The ringed seal (Pusa hispida) lives in Arctic seas under ice. It is streamlined, with flippers for propulsion. Its main insulation is a thick layer of blubber: fur insulates well in air but poorly when wet, because water conducts heat away so effectively. Buoyancy supports its heavy body in water, while on the ice it moves awkwardly.

Where Earth’s water came from AHL

Earth formed hot, and much of the water on or near its surface at the time would have been lost. The leading hypothesis for why Earth has oceans today is that water was delivered from space by collisions with asteroids (and possibly comets) rich in water ice. One line of evidence is that the ratio of deuterium to ordinary hydrogen in the water of some asteroid-derived meteorites closely matches that of Earth’s oceans.

Arriving was not enough: the water also had to stay. Two conditions allowed that. Earth is massive enough for its gravity to retain water vapour in the atmosphere rather than letting it escape into space, and once the planet cooled, temperatures were low enough for water vapour to condense into liquid oceans. The persistence of liquid water for billions of years is what gave life time to evolve.

Water and the search for life elsewhere AHL

Every living thing we know needs liquid water, so the search for life beyond Earth is in practice a search for liquid water. The Goldilocks zone (the habitable zone) is the range of distances from a star at which a planet would be neither too hot, so that water boils away, nor too cold, so that it freezes permanently, but instead can have liquid water on its surface.

Being in the zone is not sufficient — a planet also needs enough mass to hold an atmosphere — and being outside it does not rule water out: moons such as Europa and Enceladus, far beyond the Sun’s Goldilocks zone, are thought to have liquid oceans under ice, kept warm by tidal heating.

✏️Worked example

A small pond contains 1.0 m3 of water (mass 1000 kg). On a sunny morning it absorbs 50 kJ of energy from the Sun. The same amount of energy is absorbed by 1.0 m3 of air (mass 1.2 kg) above it.
Specific heat capacity: water 4180 J kg−1 °C−1; air 1005 J kg−1 °C−1.
(a) Calculate the temperature rise of the water and of the air, using \( \Delta T = \dfrac{E}{mc} \).
(b) Explain the difference in terms of the molecular structure of water.
(c) Suggest one consequence for organisms living in the pond.

(a) For the water:

\[ \Delta T = \frac{50\,000}{1000 \times 4180} = 0.012\ ^{\circ}\mathrm{C} \]

For the air:

\[ \Delta T = \frac{50\,000}{1.2 \times 1005} = 41\ ^{\circ}\mathrm{C} \]

The same energy warms the air about 3500 times more than the water. Two things combine to cause this: water’s specific heat capacity is about four times higher, and a cubic metre of water has over 800 times the mass of a cubic metre of air.

(b) Water molecules are polar and are held together by many hydrogen bonds. Much of the energy absorbed is used to break hydrogen bonds rather than to increase the kinetic energy of the molecules, so the temperature rises only a little. Air molecules are not hydrogen bonded, so almost all the energy goes into faster molecular motion — a temperature rise.

(c) The pond is a thermally stable habitat: its temperature changes little between day and night or with short spells of weather. Aquatic organisms, many of which cannot regulate their own body temperature, are therefore not exposed to rapid changes that could slow or denature their enzymes.

Check it. An order-of-magnitude check catches most slips: 50 kJ is a modest amount of energy, a tonne of water is a lot of matter, so the water should barely change — hundredths of a degree is sensible. For air, 41 °C is large but air is very light, so a large value is expected. If your answer for water came out in whole degrees, you have probably used 4.18 (per gram) with a mass in kilograms.
Mixing up per gram and per kilogram. Specific heat capacity is quoted both as 4.18 J g−1 °C−1 and as 4180 J kg−1 °C−1. They are the same value. Match the unit of mass in your calculation to the unit in the constant, or your answer will be out by a factor of 1000.

📝Practise

Work through these on paper, then reveal the answer. Question 6 is AHL.

1. Explain why a water molecule is polar.
Oxygen is more electronegative than hydrogen, so in each O–H covalent bond the shared electrons are pulled towards the oxygen. Oxygen therefore carries a partial negative charge (δ−) and each hydrogen a partial positive charge (δ+). The molecule is bent, so the two partial positive charges are on the same side and do not cancel the partial negative charge on the oxygen — the molecule as a whole has a positive end and a negative end.
2. Distinguish between cohesion and adhesion, giving one consequence of each for a plant.
Cohesion is attraction between water molecules, due to hydrogen bonds between them. Consequence: water in xylem can be pulled up as a continuous column under tension without breaking. Adhesion is attraction between water molecules and other polar or charged substances. Consequence: water is drawn through the cellulose cell walls of leaf cells (and through soil) by capillary action, keeping the walls moist and drawing water out of xylem.
3. Glucose is carried dissolved in blood plasma, but triglycerides are not. Explain this difference and state one advantage of triglycerides being insoluble.
Glucose has many polar hydroxyl (–OH) groups that form hydrogen bonds with water molecules, so it is hydrophilic and dissolves. Triglycerides are made mostly of long non-polar hydrocarbon chains that cannot form hydrogen bonds with water, so they are hydrophobic; they must be carried in lipoprotein particles instead. Advantage of insolubility: triglycerides can be stored in large quantities in adipose tissue without dissolving in the cytoplasm, so they do not affect the osmotic balance of cells (and they stay where they are stored).
4. A ringed seal and a land mammal of similar size are both exposed to a temperature of 0 °C, the seal in seawater and the land mammal in air. Explain which animal loses heat faster, and how the seal is adapted to this.
The seal loses heat faster, because water has a much higher thermal conductivity than air (roughly twenty times), so heat is conducted away from the body surface far more rapidly at the same temperature difference. Fur is a poor insulator in water, because when wet it no longer traps an insulating layer of air. The seal is adapted with a thick layer of blubber (adipose tissue) under the skin, which insulates it whether wet or dry. Its streamlined shape also reduces surface area relative to volume.
5. Pond skaters can stand on the surface of water but sink if a drop of detergent is added. Suggest why.
The pond skater is supported by surface tension. Water molecules at the surface are held together by cohesion (hydrogen bonds to their neighbours), so the surface acts like an elastic film that resists being broken by the skater’s weight spread over its long legs. Detergent molecules come between water molecules at the surface and disrupt the hydrogen bonding, lowering the surface tension, so the surface can no longer support the skater’s weight.
6. AHL A newly discovered planet orbits within its star’s Goldilocks zone. Discuss whether this makes it likely to support life.
For: the Goldilocks zone is the range of distances where temperatures would allow liquid water on a planet’s surface, and all known life depends on liquid water as its medium for metabolism and transport. Against / not sufficient: being in the zone only means liquid water is possible. The planet also needs enough mass for its gravity to retain water vapour and an atmosphere; it needs a supply of water in the first place (on Earth thought to have come from asteroids); and liquid water is necessary but not sufficient for life, which also needs carbon compounds and an energy source. Conversely, bodies outside the zone (e.g. icy moons with subsurface oceans) may have liquid water, so the zone is a guide for where to look, not a guarantee. Conclusion: it makes the planet a reasonable target for further study, not a likely home for life.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Khan Academy — Water, acids and bases in the biology course: short, clear videos on hydrogen bonding, cohesion and adhesion, and the specific heat of water.
  • USGS Water Science School — readable explanations of surface tension, capillary action and heat capacity, with everyday examples and photographs.
  • NASA Science — “follow the water” articles on the habitable zone, Europa and Enceladus, which make the AHL understandings concrete.