IB Biology · Theme A Unity and diversity · Molecules
A1.1 Water
Water is a small polar molecule, and hydrogen bonds between molecules explain almost everything it does. Cohesion, adhesion, solvent action and its thermal properties all follow from those bonds. Those properties are why life is water-based on Earth, and why we look for water when we look for life elsewhere.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
· How these pages are made
Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
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A1.1.1 Life happens in water: cells are aqueous, wherever they live
The first cells arose in water, and water is still the medium of life.
Cytoplasm is mostly water; most metabolic reactions happen dissolved in it.
Substances move around cells and bodies dissolved in water.
True on land and in the air too — cells carry their water inside.
Students often think water is passive filler between a cell's working parts. In fact most of the chemistry of life takes place in the aqueous cytoplasm, not around it.
Students often think water is the medium only for aquatic organisms. In fact a bird's cells are as aqueous as a fish's; habitat and medium are different things.
A1.1.2 Polar bonds inside the molecule; hydrogen bonds between molecules
From 2028 this is supplied in the Biology data booklet — you need to recognise and interpret it, not reproduce it from memory.
In each O–H bond the electrons are shared unequally: oxygen pulls them closer.
So oxygen carries a partial negative charge (δ−) and each hydrogen a partial positive charge (δ+).
A hydrogen bond is the attraction between the δ+ hydrogen of one molecule and the δ− oxygen of another.
Each hydrogen bond is weak and short-lived; many of them together are strong.
Students often think the hydrogen bond is the bond between H and O inside a water molecule. In fact that is a polar covalent bond; the hydrogen bond is between molecules.
Students often think δ+ and δ− are full charges, making water ionic. In fact they are partial charges on a neutral molecule.
A1.1.3 Cohesion: water sticks to water
Cohesion is water molecules holding to each other through hydrogen bonds.
In xylem, cohesion lets a continuous column of water be pulled up under tension as water evaporates from leaves.
At a surface, cohesion produces surface tension, strong enough for insects to stand and move on it.
Students often think water is pumped up the xylem from below. In fact the column is pulled from above and held together by cohesion; it is under tension, not pressure.
Students often think a pond skater floats because it is less dense than water. In fact it rests on the surface film; surface tension, not buoyancy, holds it up.
A1.1.4 Adhesion: water sticks to polar or charged surfaces
Adhesion is water bonding to other polar or charged materials, such as cellulose in cell walls.
Adhesion plus cohesion gives capillary action: water creeps up narrow spaces against gravity.
This moves water through soil to roots and through cell walls in leaves.
Students often use cohesion and adhesion as one word for "stickiness". In fact cohesion is water to water; adhesion is water to something else.
Students often think capillary action lifts water to the top of a tall tree. In fact it works over short distances only; the long haul in xylem is transpiration pull.
A1.1.5 Water is the solvent for metabolism and transport
Polar and charged (hydrophilic) substances dissolve: water molecules surround and separate them.
Most enzymes work in solution, so metabolism needs water.
Dissolved substances are what blood and sap carry.
Some molecules work because they are hydrophobic: membrane lipids form a barrier precisely because they do not dissolve.
Students often think only ionic substances dissolve in water. In fact uncharged polar molecules such as glucose dissolve too, by hydrogen bonding to water.
Students often think anything insoluble in water is useless to a cell. In fact membranes depend on molecules that stay out of solution.
A1.1.6 Water's physical properties shape life in it
Buoyancy: water supports an animal's weight far more than air does, so aquatic animals need less skeletal support.
Viscosity: water resists movement more than air, so swimmers are streamlined.
Thermal conductivity: water draws heat from a body faster than air, so aquatic mammals need thick insulation.
Specific heat capacity: water's temperature changes slowly, so aquatic habitats are thermally stable.
Compare an animal of water and air: the ringed seal (Pusa hispida) is buoyed and streamlined but heavily insulated; the black-throated loon (Gavia arctica) must fly in low-viscosity air yet dive in dense, cold water.
Students often think a high heat capacity makes water warm. In fact it makes water slow to change temperature, in either direction.
Students often think air has no viscosity. In fact it has, just far less than water; flight and swimming both work against fluid resistance.
A1.1.7 Where Earth's water came from, and why it stayed HL
Earth's water was delivered from space, mainly by asteroids early in the planet's history.
Earth kept it because its gravity is strong enough to hold water molecules.
And because surface temperatures were low enough for water vapour to condense and remain liquid.
Billions of years of liquid water gave life the time it needed to evolve.
Students often think Earth made its own water, from volcanoes or from hydrogen and oxygen reacting. In fact the syllabus hypothesis is extraplanetary delivery by asteroids.
Students often think the atmosphere acts as a lid that seals water in. In fact retention is about gravity and temperature, not a seal.
A1.1.8 Looking for life means looking for liquid water HL
Because life on Earth depends on water, the search for life elsewhere starts with the search for liquid water.
A planet in a star's Goldilocks zone is at a distance where surface temperatures allow liquid water: not too hot, not too cold.
The zone's distance depends on the star; a dimmer star has a closer zone.
Water makes life possible, not certain: the zone is where to look, not proof of anything.
Students often think any planet in the Goldilocks zone has life. In fact it has the possibility of liquid water; nothing more is implied.
Students often think ice or vapour counts the same as liquid water. In fact the search is for liquid water, the medium in which reactions can happen.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 Water is described as the medium for life. What does this mean?
Answer and reasoning
Most of the processes of life take place in aqueous solution. — The first cells originated in water, and water remains the medium in which most processes of life occur: metabolic reactions happen in the aqueous cytoplasm and substances are transported dissolved in water.
Water is inert packing between the working parts of a cell. — A student who sees water as passive packaging picks this. Water is not filler between the active parts: the aqueous cytoplasm is where most of the cell's reactions actually occur.
Every substance that a cell uses must be dissolved in water. — A student who thinks nothing useful can be insoluble picks this. Some molecules, such as the hydrophobic tails of membrane phospholipids, function precisely because they do not dissolve.
Life processes occur in water only in aquatic organisms. — A student who confuses habitat with medium picks this. Land and flying organisms carry water inside their cells, and their metabolism still takes place in aqueous solution.
2 Why is each covalent bond between oxygen and hydrogen in a water molecule polar?
Answer and reasoning
Each hydrogen atom gives up its electron completely to the oxygen atom. — A student who treats the δ charges as full ionic charges picks this. In a polar covalent bond the electrons are shared unequally, not transferred; no ions form.
The shared electrons are drawn more strongly towards the oxygen atom. — Oxygen attracts the shared electron pair more strongly than hydrogen does, so the electrons are shared unequally: oxygen becomes δ− and each hydrogen δ+. This unequal sharing is what makes the bond polar.
It is a hydrogen bond, and hydrogen bonds are polar by their nature. — A student who thinks the bond inside the molecule is a hydrogen bond picks this. The O–H bond is a polar covalent bond; hydrogen bonds form between molecules because of that polarity.
Oxygen and hydrogen atoms behave as opposite magnetic poles. — A student who reads 'polar' as magnetic picks this. Polarity here is an uneven distribution of electric charge caused by unequal sharing of electrons, not magnetism.
3 Transpiration puts the column of water in a xylem vessel under tension. Why does the column not break as it is pulled upwards?
Answer and reasoning
Root pressure pushes the column upwards from below. — A student who thinks water is pushed up from the roots picks this. The column in a transpiring plant is pulled from above and is under tension; what stops it breaking is the attraction between the water molecules.
Capillary action lifts the water inside the narrow vessel. — A student who thinks capillary rise carries water up the whole plant picks this. Capillary action moves water only short distances; the column is held together under tension by cohesion.
Hydrogen bonds within each molecule resist the pull. — A student who locates hydrogen bonds inside the water molecule picks this. The bonds within a molecule are polar covalent bonds; the hydrogen bonds that keep the column continuous are between neighbouring molecules.
Hydrogen bonds hold the molecules to each other. — Cohesion due to hydrogen bonding keeps the water molecules attached to one another, so the tension created by evaporation in the leaves is transmitted down the whole column and water is drawn up without the column separating.
4 Water rises through the narrow spaces between soil particles, against gravity. Which explanation of this movement is correct?
Answer and reasoning
Cohesion between the water and the soil particles draws the water upwards. — A student who uses 'cohesion' for any sticking that water does picks this. Attraction between water and a different material is adhesion; cohesion is the attraction between water molecules.
Water adheres to the polar or charged surfaces of the soil particles. — Soil particle surfaces are polar or charged, so water molecules are attracted to them (adhesion). Together with cohesion between water molecules, this pulls water through the narrow spaces: capillary action.
Water adheres to any solid surface, whatever the surface is made of. — A student who thinks water sticks to everything picks this. Adhesion needs a polar or charged surface; water does not adhere to non-polar surfaces such as wax.
Air pressure in the soil pushes the water up into the spaces. — A student who explains capillary rise by suction or pressure picks this. No external pressure is involved; the rise is caused by adhesion to the particle surfaces combined with cohesion.
5 Why do so many different substances dissolve in water?
Answer and reasoning
Water splits every solute into positive and negative ions. — A student who thinks dissolving always means forming ions picks this. Ionic compounds do separate into ions, but polar molecules such as glucose dissolve intact by hydrogen bonding to water.
Water breaks the covalent bonds within the solute molecules. — A student who thinks dissolving is a chemical breakdown picks this. Dissolving is a physical change: the solute's molecules stay whole and become surrounded by water molecules.
Water molecules are magnetic and so attract other substances. — A student who reads 'polar' as magnetic picks this. Water's polarity is an uneven distribution of electric charge, and it attracts solutes electrically, not magnetically.
Polar water molecules attract polar and charged solutes. — The δ+ and δ− regions of water molecules are attracted to charged particles and to the polar groups of molecules, so a wide variety of hydrophilic substances dissolve. This is why water is the medium for metabolism and transport.
6 Which statement correctly describes a consequence of the physical properties of water for a ringed seal (Pusa hispida)?
Answer and reasoning
Because it can sink, the water gives its body no support. — A student who thinks upthrust acts only on floating objects picks this. Upthrust acts on any immersed body; the seal's density is close to that of water, so most of its weight is supported.
It loses heat slowly because water is a poor conductor of heat. — A student who merges specific heat capacity with conductivity picks this. Water conducts heat about 25 times faster than air, so the seal loses heat rapidly and needs thick blubber.
Upthrust from the sea water supports most of its body weight. — Water is dense, so the upthrust on the seal's body is close to its weight (buoyancy). The seal needs far less skeletal support than a land animal, which receives negligible upthrust from air.
The sea stays warm because of water's high specific heat capacity. — A student who reads 'high heat capacity' as 'warm' picks this. High specific heat capacity makes the sea's temperature stable, not warm; Arctic sea water is close to 0 °C.
7 Where is most of the water on Earth thought to have come from? HL
Answer and reasoning
From hydrogen and oxygen gases that reacted in the early atmosphere — A student who remembers hydrogen burning in oxygen picks this. The early atmosphere did not contain free hydrogen and oxygen in the amounts needed; the hypothesis studied is extraplanetary delivery.
From water formed inside the Earth and released by volcanoes — A student who recalls the GCSE story of volcanic outgassing picks this. Volcanoes moved water to the surface but did not make it; the water itself is thought to have arrived from beyond the planet.
From water-bearing asteroids that collided with the early Earth — The hypothesis in the guide is that Earth's water is of extraplanetary origin, delivered by water-bearing asteroids that struck the early Earth. Its abundance since then has allowed life to evolve.
From nowhere: planets in the Goldilocks zone are bound to hold liquid water — A student who thinks being in the Goldilocks zone guarantees water picks this. The zone only means temperatures could allow liquid water to persist; it does not supply water, which is thought to have been delivered by asteroids.
8 Astronomers find a rocky planet orbiting within the Goldilocks zone of its star. Which conclusion is justified? HL
Answer and reasoning
Liquid water and living organisms must both exist on the planet's surface. — A student who treats the zone as proof of life picks this. The zone only means temperatures could allow liquid water; the Moon is in the Sun's zone yet has no liquid surface water and no life.
It orbits at the same distance from its star as Earth does from the Sun. — A student who thinks the zone is a fixed distance picks this. The zone depends on the star's energy output; around a dim star it is much closer in, around a bright star further out.
Any water there, whether ice, liquid or vapour, would support life equally. — A student who counts water in any state picks this. Life needs liquid water as its medium for dissolving, transporting and reacting; ice and vapour cannot serve.
Liquid water could exist on its surface, so it is worth investigating. — The Goldilocks zone is the range of distances at which surface temperatures could allow liquid water. That makes the planet a candidate for the search for life, but the presence of water and of life must be established separately.
9 During transpiration, water molecules leave the liquid on the surface of a cell wall and enter the air. Which bonds are broken as each molecule leaves?
Answer and reasoning
Polar covalent bonds between its oxygen and hydrogen atoms — A student who thinks a physical change breaks the molecule apart picks this. Evaporation leaves both O–H covalent bonds intact; the molecule escapes whole.
Hydrogen bonds between the oxygen and hydrogens inside it — A student who locates hydrogen bonds inside the molecule picks this. There are no hydrogen bonds within a water molecule; its internal bonds are polar covalent bonds, which do not break.
Hydrogen bonds between it and neighbouring molecules — Hydrogen bonds link the δ+ hydrogens of one molecule to the δ− oxygens of its neighbours. These weak intermolecular bonds must be broken for the molecule to leave the liquid; the covalent bonds inside it are unaffected.
Ionic bonds between its oxygen and hydrogen ions — A student who reads δ+ and δ− as full charges picks this. Water is not made of ions; the partial charges give rise to hydrogen bonds, not ionic bonds.
10 A pond skater rests on a pond. Its legs make small dents in the water surface but do not pass through it. What allows the surface to support the insect?
Answer and reasoning
Cohesion between surface molecules makes the surface act like an elastic film. — Molecules at the surface are held to their neighbours by hydrogen bonds and resist being separated, so the surface behaves like a stretched film. This surface tension, a consequence of cohesion, lets small animals use the water surface as a habitat.
The insect is less dense than water, so upthrust holds it at the surface. — A student who treats the insect as a floating object picks this. A pond skater is denser than water and sinks if its legs pass through the surface; it is supported by surface tension, not by upthrust.
Water adheres to the insect's legs and so holds them up on the surface. — A student who thinks water sticks to any surface picks this. The legs are water-repellent; if water adhered to them they would be wetted and pulled through the surface.
The strong hydrogen bonds at the surface form a rigid, solid-like layer. — A student who thinks hydrogen bonds are strong and permanent picks this. Individual hydrogen bonds are weak and constantly re-forming; the surface is a flexible film that dents, not a rigid layer.
Read the ones marked not yet in Learn, then Verify.
Verify confirm before you go
8 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 In a leaf, water evaporates from the cellulose cell walls of spongy mesophyll cells. How does adhesion contribute to water transport?
Answer and reasoning
It pushes water out of the xylem into the cell walls under root pressure. — A student who thinks water is pushed from the roots picks this. Adhesion pulls water into the cell walls; it does not push, and root pressure is not the force involved in a transpiring leaf.
It holds water against the waxy cuticle so that less of it evaporates. — A student who thinks water adheres to any surface picks this. Wax is non-polar, so water does not adhere to the cuticle; adhesion acts on the polar cellulose of cell walls.
It draws water into the cell walls to replace water that evaporates. — Water adheres to the polar cellulose of the cell walls, so as water evaporates it is replaced by water drawn through the walls by capillary action. This pull is transmitted to the xylem, generating the tension that moves water up the plant.
It alone lifts water the whole way from the roots up to the leaf. — A student who believes capillary action moves water up the whole plant picks this. Adhesion acts in the cell walls over short distances; long-distance transport is by tension transmitted through the cohesive water column.
2 Which statement about the hydrophobic hydrocarbon tails of the phospholipids in a cell membrane is correct?
Answer and reasoning
Their insolubility in water is what holds the bilayer together as a barrier. — Because the tails are non-polar and insoluble, they are excluded from water and cluster together in the centre of the bilayer, with the hydrophilic heads facing the water. The membrane's function depends on the tails being hydrophobic.
Their insolubility means that they have no function in the membrane. — A student who thinks only dissolved molecules can be useful picks this. The function of some molecules depends on them being hydrophobic and insoluble; the tails form the membrane's core.
They are pushed away from water by a repulsive force from water molecules. — A student who takes 'hydrophobic' to mean actively repelled picks this. There is no repulsion; water molecules attract each other more strongly than they attract the tails, so the tails are excluded.
They are insoluble because they contain no ions that water can separate. — A student who thinks only ionic substances dissolve picks this. Glucose contains no ions yet dissolves; the tails are insoluble because they are non-polar and cannot form hydrogen bonds with water.
3 Glucose is carried dissolved in blood plasma and in phloem sap, and it is broken down by enzymes in the aqueous cytoplasm. Which property of glucose makes this possible?
Answer and reasoning
It separates into positive and negative ions in water. — A student who thinks dissolving means forming ions picks this. Glucose is a covalent molecule and does not ionise; it dissolves because it is polar.
Its many hydroxyl (–OH) groups form hydrogen bonds with water. — The polar –OH groups of glucose hydrogen-bond with water molecules, so glucose is hydrophilic and dissolves. Dissolved glucose can be transported in plasma and phloem sap and acted on by enzymes in aqueous solution.
It is a small molecule, and small molecules dissolve readily. — A student who equates small with soluble picks this. Solubility depends on polarity, not size: large polar proteins dissolve, while small non-polar molecules do not.
Water breaks its covalent bonds, splitting it into soluble fragments. — A student who thinks dissolving breaks molecules apart picks this. Dissolving is a physical change; a glucose molecule stays whole and is surrounded by water molecules.
4 Thermal conductivity: water 0.60 W m⁻¹ K⁻¹, air 0.025 W m⁻¹ K⁻¹. Specific heat capacity: water 4200 J kg⁻¹ K⁻¹, air 1000 J kg⁻¹ K⁻¹. A ringed seal in sea water at 5 °C and a black-throated loon standing in air at 5 °C have the same body temperature. Other factors being equal, which conclusion is justified?
Answer and reasoning
Heat is conducted away from the seal's skin only about 4 times faster. — A student who confuses the two properties uses the specific heat capacity ratio (4200 / 1000 ≈ 4). The rate of conduction depends on thermal conductivity, and 0.60 / 0.025 = 24.
The seal loses less heat, because water stores much more heat than air. — A student who reads heat capacity as a store of warmth picks this. Water's high specific heat capacity keeps its temperature stable; it does not reduce the rate at which heat leaves the seal.
The seal is in colder surroundings, since water at 5 °C is colder than air at 5 °C. — A student who judges temperature by how cold something feels picks this. Both are at 5 °C; water feels colder only because it conducts heat away from the skin much faster.
Heat is conducted away from the seal's skin about 24 times faster. — Rate of conduction is proportional to thermal conductivity, other factors being equal: 0.60 / 0.025 = 24. This is why a seal needs thick blubber in water no colder than the air the loon stands in.
5 A black-throated loon (Gavia arctica) flies through air and dives through water. Which statement about these two fluids is correct?
Answer and reasoning
Water is far more viscous than air, so it resists the loon's movement much more. — Water's viscosity is about 50 times that of air, so a diving loon meets far greater resistance than a flying one. Animals that move through water benefit from streamlining for this reason.
Air has no viscosity, so the loon meets no resistance when it flies. — A student who thinks only liquids are viscous picks this. Air does have viscosity, though it is low, so flight meets some resistance; it is simply far less than in water.
Water conducts heat more slowly than air, so the loon cools less when diving. — A student who merges heat capacity with conductivity picks this. Water conducts heat about 25 times faster than air, so the loon loses heat far more quickly while diving.
Water stores more heat than air, so the loon is kept warmer while it dives. — A student who reads high heat capacity as warmth picks this. Water's high specific heat capacity stabilises its temperature; it does not warm the loon, which loses heat faster in water.
6 The Moon formed close to Earth and was struck by the same water-bearing asteroids, yet its surface has almost no water today. Which explanation is correct? HL
Answer and reasoning
It has no ozone layer to act as a lid that keeps water vapour in. — A student who pictures the atmosphere as a lid picks this. No layer seals water in; it is gravity that retains water molecules, and the Moon's gravity is too weak to do so.
Its gravity is too weak to hold water vapour, which escapes to space. — Retention of water depends on gravity strong enough to hold water molecules and on temperatures low enough for water to condense. The Moon's small mass gives it gravity too weak to retain water vapour, so water delivered by asteroids was lost.
It had no volcanoes to release water made from rock in its interior. — A student who thinks water is made inside a planet picks this. Water is not made from rock; the hypothesis studied is that water was delivered by asteroids, and the Moon received them too.
It lies outside the Goldilocks zone, so any water froze or boiled off. — A student who equates the zone with having water picks this. The Moon orbits the Sun at the same distance as Earth and is inside the zone; being there does not guarantee water.
7 Over a clear spring day and night, the air above a lake varies between 2 °C and 18 °C, but the lake water 1 m below the surface stays between 7 °C and 8 °C. Which explanation of the stable water temperature is correct?
Answer and reasoning
Water has a high specific heat capacity, so a large energy input or loss is needed to change its temperature. — Water's specific heat capacity is about 4200 J kg⁻¹ K⁻¹, roughly four times that of air per kilogram, and the lake also has a very large mass, so the energy gained by day and lost by night changes its temperature only slightly. This stability of temperature is a consequence of water's high specific heat capacity for animals in aquatic habitats.
Water has a high thermal conductivity, so heat passes through it too quickly to change its temperature. — A student who merges thermal conductivity with specific heat capacity picks this. Thermal conductivity describes how fast heat passes through water, not how much its temperature changes; it is the high specific heat capacity that makes a large energy change produce only a small temperature change.
Water stores a large amount of heat, so the lake is warmer than the air above it throughout the day. — A student who reads high heat capacity as warmth picks this. The data show the air reaching 18 °C by day while the lake stays at 7 °C to 8 °C, so the lake is not warmer; high specific heat capacity makes the temperature stable, not high.
Energy supplied to the water is used to break the covalent bonds inside its molecules rather than to raise its temperature. — A student who thinks supplying energy to water breaks its molecules apart picks this. Warming the lake is a physical change and the O–H covalent bonds stay intact; the energy raises the temperature, but a very large amount is needed because of water's high specific heat capacity.
8 Two water molecules are drawn using the notation for polarity and hydrogen bonding. Which feature of the drawing is correct?
Answer and reasoning
A dashed line joins the oxygen to one of the two hydrogens within the same water molecule. — A student who locates hydrogen bonds inside the water molecule picks this. The line between an oxygen and a hydrogen of the same molecule is a solid line for a polar covalent bond; the dashed hydrogen bond always runs between two different molecules.
A dashed line joins a hydrogen labelled δ+ on one molecule to the oxygen labelled δ− on the other. — The δ− is written beside the oxygen and δ+ beside each hydrogen, because the shared electrons are drawn towards the oxygen. A hydrogen bond is an attraction between molecules, shown as a dashed line from the δ+ hydrogen of one molecule to the δ− oxygen of its neighbour.
The oxygen is labelled with a full − and each hydrogen with a full +, because the molecule is made of ions. — A student who reads the δ symbols as full ionic charges picks this. The δ indicates a partial charge from unequal sharing of electrons; no electron is transferred, so water contains no ions and the labels are δ+ and δ−, not + and −.
A solid line joins the two molecules, because the bond between them is as strong as a covalent bond. — A student who thinks hydrogen bonds are as strong as covalent bonds picks this. The bond between molecules is a hydrogen bond, much weaker than a covalent bond and continually breaking and re-forming, which is why it is drawn as a dashed rather than a solid line.
That was your twenty minutes. Real practice on A1.1 is past-paper questions marked against the mark scheme.
What the exam asks of A1.1
Paper 1A asks you to recognise a property from a description or a consequence from a property. Paper 1B may give data on surface tension, solubility or temperature change and ask what property explains it. Paper 2 uses *outline* and *explain*: name the property, say what causes it (hydrogen bonding), then give the consequence for a named organism. At HL, expect *discuss* on the origin of water or the Goldilocks zone: give the hypothesis, its limits, and the reasoning.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
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