What you need to know
- 1.1Water as a solventA liquid in which other substances (solutes) dissolve. Water is a good solvent for ions and polar molecules. in transport, explained using its dipoleAn uneven spread of charge across a molecule. In water, oxygen is partially negative (δ−) and the hydrogens are partially positive (δ+). nature.
Specification 1.1 focuses on water as a solvent in transport. The optional extension below explores other properties without adding them to International A Level recall.
By the end, you should be able to…
- Describe the partial charges on a water molecule.
- Explain how water interacts with ions and polarHaving an uneven distribution of charge, so the molecule has partially charged ends. Polar molecules dissolve in water. molecules.
- Link solubility to transport in an organism.
Build your understanding
Where the charges are
A water molecule contains two hydrogen atoms joined to one oxygen atom by covalent bonds. Oxygen attracts the shared electrons more strongly than hydrogen does, so those electrons sit closer to oxygen. Oxygen carries a partial negative chargeA small negative charge (δ−) on an atom, such as oxygen in water, that attracts shared electrons more strongly. (δ−) and each hydrogen carries a partial positive chargeA small positive charge (δ+) on an atom, such as hydrogen in water, whose shared electrons are pulled away from it. (δ+). The molecule is bent, so the two δ+ ends do not cancel the δ−. Water is a dipoleAn uneven spread of charge across a molecule. In water, oxygen is partially negative (δ−) and the hydrogens are partially positive (δ+).: partial charges inside one overall neutral molecule. It is not a hydrogen ion joined to an oxide ion.
Where the charges are (continued)
A hydrogen bondA weak attraction between a partially positive hydrogen atom and a partially negative atom such as oxygen or nitrogen. forms between molecules, not inside one water molecule. It is the attraction between a δ+ hydrogen on one molecule and a δ− oxygen, or sometimes a δ− nitrogen, on a different molecule. Between two water molecules it runs from a hydrogen of the first to the oxygen of the second. It is weaker than the covalent O–H bond, so it can form and break. Water molecules can stay associated without becoming one giant covalent molecule.
Which molecules interact with water
A sodium ion, Na⁺, has a full positive charge, so the δ− oxygen ends of water face it. A chloride ion, Cl⁻, has a full negative charge, so the δ+ hydrogens face it. Those attractions are between an ion and the dipole. They separate the ions from one another and keep them dispersed. Do not call that attraction a covalent bond, and do not call the water molecule itself an ion.
Which molecules interact with water (continued)
glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate. dissolves because it has several hydroxyl (–OH) groups. A hydrogen bond can form between the δ+ hydrogen of water and the oxygen of an –OH, or between the δ− oxygen of water and the hydrogen of an –OH. Many amino acidThe monomer of proteins: a central carbon with an amine group, a carboxyl group, a hydrogen and an R group. dissolve for the same reason: polarHaving an uneven distribution of charge, so the molecule has partially charged ends. Polar molecules dissolve in water. or charged groups on the molecule attract water. ureaThe main nitrogenous waste product made in the liver from ammonia, and excreted in urine. is another small polar molecule that stays in solution. A triglycerideA lipid made of one glycerol and three fatty acids joined by ester bonds. It is an energy store. does not. Its long hydrocarbon chains have no matching partial charges, so water does not hydrogen-bond to them and the lipid does not dissolve readily.
Why this matters for transport
Once a substance is dissolved, it can be carried in a moving aqueous fluid. Blood plasmaThe liquid part of blood: water carrying dissolved substances such as glucose, ions, hormones and proteins. carries dissolved glucose, amino acids, mineral ions such as Na⁺ and Cl⁻, and urea. Each dissolves because of the attractions above: a hydrogen bond to a polar group, or the δ− and δ+ ends of water facing an ion. Moving plasma then carries the dissolved substance from one region to another.
Do not assume that every substance transported in blood is freely dissolved in its water. Most oxygen is carried bound to haemoglobinA globular protein in red blood cells with four polypeptide chains, each with a haem group containing Fe²⁺ that binds one O₂.; triglycerides also need different transport arrangements. The solventA liquid in which other substances (solutes) dissolve. Water is a good solvent for ions and polar molecules. explanation applies to the soluble examples, not to every molecule in the blood.
Key terms
A dipoleAn uneven spread of charge across a molecule. In water, oxygen is partially negative (δ−) and the hydrogens are partially positive (δ+). is the separation of partial charges inside one water molecule. A hydrogen bondA weak attraction between a partially positive hydrogen atom and a partially negative atom such as oxygen or nitrogen. is the attraction between the δ+ hydrogen of one molecule and a δ− oxygen or nitrogen on a neighbour. In water, that neighbour’s oxygen is the usual partner. The hydrogen bond is not the covalent bond inside the molecule, and water is not a pair of ions. A solventA liquid in which other substances (solutes) dissolve. Water is a good solvent for ions and polar molecules. is the liquid; the dissolved substance is the soluteA substance that is dissolved in a solvent..
cohesionThe attraction of water molecules to one another through hydrogen bonds., the attraction of water molecules to one another through those hydrogen bonds, is a different property. It is not the specification 1.1 explanation of how plasmaThe liquid part of blood: water carrying dissolved substances such as glucose, ions, hormones and proteins. carries glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate.. For solvent transport, name the attraction between water and the dissolved substance: oxygen facing Na⁺, hydrogen facing Cl⁻, or a hydrogen bond to an –OH group on glucose.
NOT: water is not H⁺ joined to O₂−. A hydrogen bond is not the covalent O–H bond.
- Partial charges, not full ionic charges
- Polar solutes dissolve; non-polar lipids do not
- Dissolved glucose can be carried in plasma
- Most oxygen is on haemoglobin, not free in water
Two traps this diagram blocks
- Do not say water transports everything. Name a soluble example such as glucose, and keep most oxygen with haemoglobin.
- Do not call the attraction between water molecules the bond inside one water molecule.
From the diagram alone
Question
Explain how the dipole nature of water allows glucose to be transported in blood plasma. [3]
Show a model answer
Oxygen in water is δ− and the hydrogens are δ+, so water is polar. Polar groups on glucose are attracted to those partial charges, including by hydrogen bonding, so glucose dissolves. Movement of the plasma then carries the dissolved glucose.
OPTIONAL EXTENSIONMore properties of waterNot International A-level content
Outside this course specification: International A Level Biology point 1.1 asks you to explain water as a solvent in transport using its dipole nature. The properties below are for wider biological understanding, not an extra list to memorise for that point.
IB Diploma Biology HL-style depth. IB topic A1.1 includes cohesion, adhesion and aquatic-habitat properties in content shared by SL and HL. The final section below identifies the two ideas that are specifically additional higher level. Other related properties broaden the comparison; this is not a substitute for either qualification’s specification.
Start at the molecule
Oxygen attracts shared electrons more strongly than hydrogen. Each O–H covalent bond within a water molecule is polar: oxygen is partially negative (δ−), and each hydrogen is partially positive (δ+). The attraction between a δ+ hydrogen on one molecule and a δ− oxygen on another is a hydrogen bond between molecules. These intermolecular bonds continually break and reform in liquid water; they are not the covalent O–H bonds.
Properties and biological consequences
High specific heat capacity
Specific heat capacity is the energy needed to raise 1 kg of a substance by 1 °C. Water has a high value: some incoming energy disrupts the changing network of hydrogen bonds rather than immediately increasing molecular motion and temperature.
Why it matters in biology: A pond warms and cools more slowly than the surrounding air, so aquatic organisms experience smaller short-term temperature swings. Water-rich tissues also resist rapid temperature change, helping enzymes work within a suitable range.
High latent heat of vaporisation
Latent heat of vaporisation is the energy required to turn liquid water into vapour without raising its temperature during the phase change. Molecules must escape attractions, including hydrogen bonding. At ordinary temperatures, the more energetic surface molecules evaporate preferentially.
Why it matters in biology: Sweat uses heat from the skin to evaporate; the remaining liquid and skin cool. Transpiration can cool a leaf in the same way. This is different from high specific heat capacity, which slows a temperature change while water remains liquid.
Cohesion and surface tension
Cohesion is attraction between water molecules arising largely from hydrogen bonds. Molecules at an air–water interface have fewer water neighbours above them, creating a surface that resists being stretched: surface tension.
Why it matters in biology: Evaporation from leaves creates tension in the xylem; cohesion transmits this pull along a continuous water column. A broken column can interrupt transport. At a pond surface, surface tension creates a habitat that can support small organisms such as water striders.
Adhesion
Adhesion is attraction between water and another polar or charged surface, unlike cohesion between water molecules. A narrow, wettable space lets adhesion and cohesion together draw water along it by capillary action.
Why it matters in biology: Water moves through small spaces in soil and plant cell walls. Adhesion to xylem walls helps support the column, but capillary action alone cannot lift water through a tall tree; evaporation-driven tension is essential.
Buoyancy
A submerged body displaces water and receives an upward buoyant force equal to the weight of the displaced water. Because water is far denser than air, this support is much greater in water than on land.
Why it matters in biology: Buoyancy reduces the weight an aquatic animal must support, but it does not eliminate the need to control depth. Fish can adjust a gas-filled swim bladder; seals must still swim or use their body composition to manage buoyancy.
Viscosity and drag
Viscosity describes resistance to flow. Water is more viscous and much denser than air, so a moving animal encounters substantial drag. Drag rises strongly with speed and depends on body shape.
Why it matters in biology: Streamlined bodies and smooth surfaces reduce the energy needed to swim. The same density that provides buoyancy therefore also makes movement through water more demanding.
Thermal conductivity
Thermal conductivity describes how readily heat passes through a material. Water conducts heat more effectively than still air, so an animal can lose heat quickly to cold water.
Why it matters in biology: A ringed seal’s blubber slows heat loss. This is a different issue from water’s high specific heat capacity: conductivity concerns the rate of heat transfer, while heat capacity concerns the energy needed to change temperature.
Incompressibility
Liquid water changes volume very little when pressure is applied: it is nearly incompressible, rather than perfectly incompressible. Pressure can therefore be transmitted through water-filled compartments.
Why it matters in biology: Water entering a plant cell by osmosis presses the contents against the cell wall; the wall resists expansion and turgor develops. Fluid pressure can also support a hydrostatic skeleton, such as in an earthworm.
Ice floats; liquid water is densest near 4 °C
As water freezes, hydrogen bonds organise molecules into a more open lattice. The solid occupies more volume than the same mass of liquid, so ice floats. Pure freshwater reaches its maximum density near 4 °C at ordinary pressure.
Why it matters in biology: Cooling surface water sinks until it is near 4 °C; colder water remains above it and can freeze at the surface. The ice slows further heat loss, leaving liquid water beneath as a winter habitat.
Water on Earth and beyond
How did Earth get and keep water? One hypothesis is that water-bearing asteroids delivered some of it to the early Earth. Earth’s gravity helped retain water and water vapour; temperatures low enough for condensation allowed liquid water to accumulate. The amount delivered, lost and recycled is still an area of research, so asteroid delivery should be presented as a hypothesis rather than a proven single origin.
Why look for liquid water elsewhere? Life as we know it relies on water as a medium for chemical reactions. The “Goldilocks zone” is a range of distances from a star where surface temperatures could permit liquid water, given a suitable atmosphere. Being in that zone does not prove that a planet has water or life; atmospheric pressure and composition also matter.
A runner cools as sweat evaporates, yet a pond warms slowly on a sunny day. Identify the different thermal property involved in each case and explain how hydrogen bonding contributes.
Show reasoning
Sweat cooling involves high latent heat of vaporisation: separating water molecules during evaporation requires energy drawn from the skin. Slow pond warming involves high specific heat capacity: adding energy causes only a relatively small temperature rise because the hydrogen-bond network takes up some energy. One concerns a change of state; the other concerns a change in temperature.
For comparison, Pearson UK Biology A (9BN0) point 1.2 specifies water as a solvent in transport. The separate Pearson UK Biology B AS specification, point 1.7, explicitly names high specific heat capacity, surface tension, incompressibility and maximum density at 4 °C. Latent heat of vaporisation, cohesion and adhesion are included here as related extension ideas, not as an International A Level recall list.
Sources: Pearson UK Biology A, point 1.2 · Pearson UK Biology B AS, point 1.7 · IB Biology guide, A1.1 (publicly hosted copy) · OpenStax: water properties
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate. is carried in plasmaThe liquid part of blood: water carrying dissolved substances such as glucose, ions, hormones and proteins.. Y: an –OH group on glucose hydrogen-bonds with water, so glucose dissolves. Z: moving plasma carries the dissolved glucose. Incomplete: “Water is a good solventA liquid in which other substances (solutes) dissolve. Water is a good solvent for ions and polar molecules. because it is polarHaving an uneven distribution of charge, so the molecule has partially charged ends. Polar molecules dissolve in water..” Improved: “A δ+ hydrogen of water is attracted to the oxygen of a glucose –OH group, so glucose dissolves and plasma can carry it.” The improvement names where the hydrogen bondA weak attraction between a partially positive hydrogen atom and a partially negative atom such as oxygen or nitrogen. forms and the transport result. Other correct wording is fine if those links stay.
Be precise about charge
Use “partial positive” and “partial negative” for water. Na⁺ and Cl⁻ have full charges. Do not describe a water molecule as a positive hydrogen ion joined to a negative oxygen ion.
Avoid this misconception
The claim: Water transports everything because everything dissolves in it.
The correction: Many ions and polar molecules dissolve; non-polar lipids do not readily dissolve. Name an appropriate example such as glucose in plasma.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Explain why glucose can be transported in blood plasma.
Show suggested answer
Glucose has –OH groups. A hydrogen bond can form between those groups and water, so glucose dissolves in the water of plasma. Movement of the plasma then carries the dissolved glucose.
Practice question
Retrieval and explanation · suggested answer
Which region of a water molecule faces a dissolved sodium ion, Na⁺? Explain your choice.
Show suggested answer
The oxygen region faces Na⁺. Oxygen has a partial negative charge, so it is attracted to the positively charged sodium ion. The hydrogen ends, which are δ+, face a negative ion such as Cl⁻ instead.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A molecule contains several polar groups but a very long non-polar region. Can you conclude that it will dissolve readily in water? Justify your answer.
Show a hint
Think about the whole molecule rather than one group.
Explore the reasoning
No. Polar groups can interact with water, but the large non-polar region may make the molecule poorly soluble overall. Solubility depends on the balance of interactions across the molecule. A polar group alone is not sufficient evidence of high solubility.
Recall this lesson
Lesson recall questions
Question 1 of 5: Describe, in order, how a sodium ion becomes dissolved and then transported in plasma.
Water as a transport solvent
Practice question
Retrieval and explanation · suggested answer
Describe, in order, how a sodium ion becomes dissolved and then transported in plasma.
Lesson: 1A 1: The Chemistry of Life
Build the sequence from memory first. Then reveal the model order.
Tap the steps in the order you recall. Tap a placed step to send it back.
- 01Next step
- 02Waiting
- 03Waiting
Tap to place
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Water carries a chemical story
1A · The Chemistry of LifeEnvironmental scientists investigate substances carried in water. The same solvent behaviour that makes biological transport possible allows water to carry dissolved minerals through the environment. Imagine comparing water…Specification 1.1Read & use the scienceClose article
Why this matters
Environmental scientists investigate substances carried in water. The same solvent behaviour that makes biological transport possible allows water to carry dissolved minerals through the environment. Imagine comparing water upstream and downstream of a discharge: a transparent sample can still carry dissolved ions and polar molecules. Water’s partial charges help explain why these substances travel with it, but cannot identify which substances are present. The investigation therefore needs measurements, comparable sampling conditions and a sensible reference site. This is the useful connection to biological transport: a solvent makes movement possible without making the transported substance visible, harmless or chemically identified.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 1.1 understand the importance of water as a solvent in transport, including its dipole nature
Use the science
A clear river sample looks clean. Does its appearance show that it contains no dissolved substances?
Explore a possible response
No. Dissolved ions and many dissolved molecules are invisible. Appearance alone cannot establish composition; an appropriate chemical analysis is needed.
Explore the source
- US Geological Survey: water as a solventUS Geological Survey
Why water cannot wash away an oil spill
1A · The Chemistry of LifeAn oil-spill scientist has to predict where pollution will travel and which clean-up method might help. Water is a good solvent for many ionic and polar substances, but…Specification 1.1Read & use the scienceClose article
Why this matters
An oil-spill scientist has to predict where pollution will travel and which clean-up method might help. Water is a good solvent for many ionic and polar substances, but that does not make it a solvent for everything. Much of an oil mixture behaves differently from dissolved salts: it can form a separate phase rather than a uniform solution. Responders must also consider currents, weather and damage caused by the clean-up itself. NOAA describes cases where aggressive washing harmed shorelines. Molecular properties therefore inform an environmental decision, but cannot settle that decision without ecological evidence.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 1.1 understand the importance of water as a solvent in transport, including its dipole nature
Use the science
Why could filtering visible oil droplets from seawater fail to remove every dissolved contaminant? Use polarity and the distinction between a mixture and a solution.
Explore a possible response
Droplets are a separate phase and may be trapped; dissolved molecules pass with the water. Different components have different solubilities, so visual clarity is not evidence of chemical purity.
Explore the sources
- Oil spillsNOAA
- Oil, petroleum, N.O.S.NOAA CAMEO Chemicals