Water

In a nutshell

Water makes up most of a cell, and almost every property that makes it so useful comes from one fact: a water molecule is a dipole that forms hydrogen bonds with its neighbours.

This subtopic is about five of those properties, why hydrogen bonding causes each one, and the biological job each one does. The exam almost always asks you to link a property to a function, so learn them as pairs, not as a list.

Assumed knowledge: Monomers and polymers (condensation and hydrolysis).

Core content

The water molecule: a dipole that hydrogen bonds

Everything below follows from the structure of one water molecule, so start here.

In H₂O the oxygen atom pulls the shared electrons more strongly than the hydrogen atoms do. This leaves the oxygen slightly negative (δ^-) and each hydrogen slightly positive (δ⁺).

A molecule with this uneven charge distribution is a polar molecule (a dipole).

The δ⁺ hydrogen of one water molecule is attracted to the δ^- oxygen of a neighbouring water molecule. This weak attraction is a hydrogen bond.

Each hydrogen bond is weak on its own, but there are a huge number of them, and it is the sheer number that gives water its properties.

Water as a metabolite

A metabolite is a substance that takes part in a metabolic reaction, as a reactant or a product.

Water is a metabolite in two reaction types you already know:

• Condensation: two molecules are joined by a new bond, and a molecule of water is released. Building polymers (for example a disaccharide, a triglyceride, a polypeptide) works this way.
• Hydrolysis: a bond between two molecules is broken by adding a molecule of water. Digesting large molecules works this way.

So water is not just the background liquid, it is chemically used up and produced inside the cell.

Water as a solvent

Because water is polar, it attracts and surrounds other charged or polar particles, so they separate and dissolve. Water is the main solvent in cells and body fluids.

This matters for two linked reasons:

• Most metabolic reactions happen in solution, and dissolved substances can move and collide, so reactions occur faster than they would in a solid.
• Dissolved substances can be transported: for example glucose, amino acids and ions carried in blood plasma, or mineral ions carried in xylem.

The AQA chain of reasoning for the marks is: polar molecule, so acts as a solvent, so metabolic reactions can occur in solution.

Relatively high (specific) heat capacity

The specific heat capacity of a substance is the energy needed to raise the temperature of 1 g of it by 1 °C.

Water has a relatively high specific heat capacity: it takes a lot of energy to change its temperature. Some of the energy added goes into breaking hydrogen bonds rather than making the molecules move faster, so the temperature rises slowly.

You do not need to memorise these numbers (the exact values are not credited). The point the chart makes is the word relatively: water sits well above other common substances.

Biological importance: water can gain or lose a lot of energy with only a small temperature change, so it buffers changes in temperature. This keeps the temperature of cells, and of large bodies of water, relatively stable, which keeps enzymes working near their optimum.

Still don't get it? · why "relatively high heat capacity" comes from hydrogen bonds

Picture a room full of people all holding hands with the people next to them. "Temperature" is how fast they are jigging about on the spot. If you feed energy into the room, some of it goes into people jigging faster, but a lot of it is spent tugging joined hands apart first. So for the same push of energy, the jigging speeds up only slowly.

Now the biology. The joined hands are the hydrogen bonds between water molecules. Heating water adds energy, but a big share of that energy goes into breaking hydrogen bonds instead of making the molecules move faster. Since temperature is really just how fast the molecules are moving, the temperature climbs slowly. That is what "high heat capacity" means: a lot of energy in for a small temperature rise.

In the exam: say that water has a relatively high (specific) heat capacity, so it can gain or lose a lot of energy without much change in temperature, and that this buffers temperature changes. Do not write that the hydrogen bonds are "strong", and do not merge this with latent heat of vaporisation. The idea is that there are many hydrogen bonds to break, and breaking them takes energy.

Relatively large latent heat of vaporisation

The latent heat of vaporisation is the energy needed to change a liquid into a gas (to evaporate it).

For water this is relatively large, because evaporating water means breaking the hydrogen bonds holding the molecules together, which takes a lot of energy.

Biological importance: because so much energy is carried away when only a small mass of water evaporates, evaporation is an efficient way to lose heat. Organisms get a cooling effect with little loss of water. Examples are sweating in mammals and transpiration in plants.

Cohesion and surface tension

Cohesion is the attraction between molecules of the same type. Water molecules cohere strongly because they hydrogen-bond to one another.

Two consequences you must be able to explain:

• Columns of water in transport cells. Water molecules stick together in a continuous column, so when water is pulled up the narrow, tube-like xylem cells of a plant, the whole column moves up together without breaking apart.
• Surface tension. Where water meets air, the surface molecules are pulled inwards and sideways by cohesion (there is no water above to pull them up). The surface behaves like a thin skin, which is strong enough to support small organisms such as pond skaters.

Still don't get it? · how cohesion holds a column of water together in xylem

Think of a long train of wagons linked by couplings. Pull the front wagon and the whole train follows, because each coupling drags the wagon behind it. Cut one coupling and the train splits.

A column of water in a xylem vessel is that train. Each water molecule is a wagon, and the hydrogen bonds between molecules are the couplings. As water evaporates from the leaves at the top, water is pulled up, and because the molecules are hydrogen-bonded to one another, the whole column is dragged upwards as one continuous thread rather than snapping.

In the exam: the creditable idea is strong cohesion between water molecules (from hydrogen bonding), which supports a continuous column of water in the tube-like transport (xylem) cells of a plant. The same cohesion, at a water-air surface, produces surface tension.

Summary: each property linked to its role

Every property traces back to hydrogen bonding, and every property earns marks only when you attach it to a function.

Worked examples

Model 4-mark answer, "Explain how the high heat capacity of water is useful to living organisms."

The lesson here is that a 4-mark "explain" needs four linked points in a causal order, not four facts about water.

1. Water has a relatively high (specific) heat capacity, because a lot of energy is needed to break the hydrogen bonds between water molecules.
2. So water can gain or lose a large amount of energy with only a small change in temperature.
3. This buffers (resists) changes in temperature inside cells and in bodies of water such as lakes.
4. A stable temperature keeps enzymes near their optimum and provides a stable habitat for aquatic organisms.

Model 3-mark answer, "Explain why water is a good solvent for metabolic reactions."

1. Water is a polar molecule (a dipole with δ⁺ and δ^- regions).
2. So it attracts and surrounds charged or polar solutes, which dissolve.
3. Metabolic reactions can then take place in solution, where the reactants can move and collide.

Common exam mistakes

• Confusing high heat capacity with latent heat of vaporisation, or inventing the phrase "latent heat capacity". Heat capacity is about raising temperature; latent heat of vaporisation is about evaporating. They are different properties with different jobs (buffering temperature vs cooling).
• Saying the high heat capacity is caused by "strong" hydrogen bonds. Individual hydrogen bonds are weak. The point is that there are many of them and breaking them takes energy.
• Writing that hydrogen bonds form "between H⁺ and O^2- ions in water". Water molecules are not split into ions here. Hydrogen bonds form between the δ⁺ hydrogen of one water molecule and the δ^- oxygen of another molecule.
• Stating a property with no biological function attached (or a function with no property). Almost every question wants the property and the linked use, so always answer in the pair.
• Confusing cohesion (attraction between water molecules) with adhesion (attraction between water and a different surface). The spec point is cohesion; do not switch the word.
• Being vague about water as a metabolite ("water is used in reactions"). Name it: a reactant in hydrolysis and a product of condensation.
• For the cooling point, forgetting the "little water lost" half: the value is that a large amount of heat is removed by evaporating only a small mass of water.

Key definitions

• Metabolite: a substance that takes part in a metabolic reaction, either as a reactant or as a product.
• Condensation reaction: a reaction that joins two molecules together with the formation of a chemical bond and the elimination of a molecule of water.
• Hydrolysis: the breaking of a chemical bond between two molecules involving the use of a water molecule.
• Polar molecule (dipole): a molecule with an uneven distribution of charge, having a slightly positive (δ⁺) region and a slightly negative (δ^-) region.
• Hydrogen bond: a weak attraction between the δ⁺ region of one molecule and the δ^- region of a neighbouring molecule.
• Solvent: a liquid in which a solute dissolves.
• Specific heat capacity: the energy required to raise the temperature of 1 g of a substance by 1 °C.
• Latent heat of vaporisation: the energy required to change a substance from a liquid to a gas (to evaporate it).
• Cohesion: the attraction between molecules of the same type, holding them together (in water, due to hydrogen bonding).
• Surface tension: the effect at a water-air surface where cohesion between water molecules makes the surface behave like a skin.

Specification

• I can explain that water is a metabolite in many metabolic reactions, including condensation and hydrolysis reactions.
• I can explain that water is an important solvent in which metabolic reactions occur.
• I can explain that water has a relatively high heat capacity, which buffers changes in temperature.
• I can explain that water has a relatively large latent heat of vaporisation, which provides a cooling effect with little loss of water through evaporation.
• I can explain that water has strong cohesion between its molecules, which supports columns of water in the tube-like transport cells of plants and produces surface tension where water meets air.
• I can trace each of these properties back to water being a polar molecule that forms hydrogen bonds.

Related notes

• Monomers and polymers
• Inorganic ions
• Mass transport in plants