Water: Cohesion & Transport Revision Notes | A Level AQA Biology

1. Why Water Is Not Boring

Water covers most of the planet, makes up the majority of your body, and keeps every cell alive. Yet its behaviour is genuinely strange — it climbs up trees against gravity, supports insects on its surface, and stays liquid across a wide temperature range. None of this happens by accident. Every unusual property of water traces back to one structural feature: the hydrogen bond.

At A Level, you need to move beyond 'water is important for life' and explain precisely why — using the language of molecular structure, intermolecular forces, and biological function. This lesson focuses on two properties that the AQA specification highlights: cohesion and surface tension, and how they underpin water transport in plants.

2. The Structure That Makes Everything Possible

Picture a single water molecule ($H_2O$). The oxygen atom is strongly electronegative — it pulls the shared electrons in each O–H bond towards itself. This leaves each hydrogen atom with a slight positive charge ($\delta+$) and the oxygen with a slight negative charge ($\delta-$). The molecule is therefore a dipole: it has two poles of charge.

Because water is bent (the H–O–H bond angle is approximately 104.5°), the two partial positive charges do not cancel the partial negative charge. The molecule has a permanent dipole. This is the foundation of everything that follows: the $\delta+$ hydrogen of one molecule is attracted to the $\delta-$ oxygen of a neighbouring molecule, forming a hydrogen bond.

One hydrogen bond is fragile. Four hydrogen bonds per molecule, multiplied across billions of molecules in a droplet, produces a network of remarkable strength. This network is the origin of cohesion, surface tension, and the high specific heat capacity of water — all properties that matter enormously in biology.

3. Cohesion: Water Sticking to Itself

Cohesion is the tendency of water molecules to stick to one another via hydrogen bonds. Think of it like a chain: each molecule grips the ones around it. Pull one end of the chain upward and the rest follow. This is not a metaphor — it is almost exactly what happens inside the narrow tube-like transport cells of a plant's xylem.

In the xylem, water is pulled upward from the roots to the leaves by a process driven by transpiration (the evaporation of water from leaf surfaces). As water molecules evaporate from the mesophyll cells, they pull the water column below them upward. Because cohesion holds the water molecules together, the entire column moves as a continuous thread — this is the cohesion-tension mechanism.

The xylem vessels are narrow, which matters. In a very thin tube (a capillary), cohesive forces between water molecules and adhesive forces between water and the tube walls are strong enough to support a tall column of water against gravity. The narrower the tube, the higher the water can be pulled. Xylem vessels exploit this principle directly.

A useful way to visualise this: imagine a column of water inside a xylem vessel as a rope made of millions of tiny magnets. Each magnet clings to the ones above and below it. When the top of the rope is pulled (by transpiration), the whole rope moves upward without snapping — provided the pulling force does not exceed the strength of the magnetic links. In water, those links are hydrogen bonds.

This is why cohesion is described in the AQA specification as supporting 'columns of water in the tube-like transport cells of plants'. The key exam phrase to use is: cohesion allows water to be pulled up the xylem as a continuous column under tension, without the column breaking.

4. Surface Tension: Where Water Meets Air

Now consider what happens at the surface of a body of water — where liquid water meets air. A water molecule in the bulk of the liquid is surrounded on all sides by other water molecules, so it is pulled equally in every direction by hydrogen bonds. The net force on it is zero.

A molecule at the surface is different. There are far fewer hydrogen-bonding partners above the surface molecule than below, resulting in a net inward force. So it is pulled downward and sideways by its neighbours, but not upward. The result is a net inward force that pulls surface molecules tightly together, creating a thin, taut 'skin' at the water's surface. This is surface tension.

Surface tension is high enough in water to support small organisms. The pond skater (Gerris species) walks on water not because it is light, but because its legs spread its weight across the surface film without breaking the hydrogen bond network. The surface deforms slightly under the insect's weight but does not rupture. This is a direct biological consequence of cohesion at the air–water interface.

5. Water as a Major Component of Cells

Cohesion and surface tension are two properties that emerge from hydrogen bonding, but the AQA specification also requires you to appreciate that water is a major component of cells and that its properties are important in biology more broadly. Typically, water makes up 70–95% of the mass of a living cell. This is not incidental — water's properties make it the ideal medium for life.

Water is an excellent solvent for ionic and polar substances. Because it is a dipole, water molecules cluster around ions and polar molecules, separating them and keeping them in solution. This means that metabolic reactions — which require reactants to be dissolved and mobile — can take place inside cells. Glucose, amino acids, mineral ions, and gases such as $CO_2$ and $O_2$ all move through cells dissolved in water.

Water also has a high specific heat capacity: it takes a large amount of energy to raise the temperature of water by 1°C. This is because much of the energy input goes into breaking hydrogen bonds rather than increasing kinetic energy. For organisms, this means that the water inside cells buffers against rapid temperature changes, maintaining a stable internal environment for enzyme-controlled reactions.

Water is also a reactant and product in many metabolic reactions. Hydrolysis reactions — which break down polymers such as polysaccharides, proteins, and nucleic acids — use water molecules to cleave bonds. Condensation reactions, which build these polymers, release water. Without water as a component of cells, these fundamental biochemical processes could not occur.

6. Linking Cohesion to Xylem Transport: A Closer Look

Earlier, we established that cohesion holds water molecules together in a continuous column. Now it is worth tracing the full sequence of events in xylem transport, because exam questions often ask you to explain the process step by step — and each step must be linked to a specific property of water.

Worked example: Explaining Water Transport in the Xylem Using Cohesion

Explain how water moves from the soil to the leaf in a plant, with reference to the properties of water. [5 marks]

1. Water evaporates from mesophyll cells in the leaf by transpiration, lowering the water potential of those cells. — [B1] State that transpiration creates a water potential gradient. Examiners want you to name the driving force before explaining the mechanism.
2. As water evaporates, it is replaced by water drawn from the xylem vessels in the leaf. This creates a tension (negative pressure) in the water column. — [B1] The word 'tension' is key here. Water is being pulled, not pushed. Many students forget to state that the column is under tension.
3. Cohesion — the attraction between water molecules via hydrogen bonds — holds the water column together as a continuous thread. The tension is transmitted down the entire column. — [B1] Name cohesion explicitly and link it to hydrogen bonds. The column does not break because cohesive forces are strong enough to withstand the tension.
4. Water enters root hair cells from the soil by osmosis (moving down a water potential gradient) and passes into the xylem. — [B1] Osmosis is the mechanism of entry at the root. You do not need to detail the apoplast/symplast pathways unless asked, but mentioning water potential gradient is expected.
5. The result is a continuous flow of water from soil → root → stem → leaf, driven by transpiration and maintained by cohesion. — [B1] A clear summary sentence that ties the mechanism together earns the final mark. Use the phrase 'continuous column' to signal understanding of cohesion's role.

Answer: Transpiration lowers water potential in leaf cells, drawing water from the xylem under tension. Cohesion (hydrogen bonds between water molecules) maintains the water column as a continuous thread, transmitting tension down to the roots. Water enters from the soil by osmosis and replaces what is lost, producing a continuous flow from soil to leaf.

Notice how each step in that walkthrough names a property and then explains its consequence. That structure — property → mechanism → biological outcome — is exactly what AQA mark schemes reward in 'explain' questions about water transport.

7. Exam Technique and Common Mistakes

Questions on water properties appear regularly in AQA Biology Paper 1, often as 4–6 mark 'explain' or 'evaluate' questions. The marks are almost always awarded for linking a named property to a specific biological consequence — not for describing the property in isolation.

That model answer earns all six marks because every sentence either names a molecular property, explains its mechanism, or states its biological consequence. Practise writing answers in that three-part structure — property, mechanism, outcome — and you will consistently hit the top of the mark scheme on water questions.