Water: Five Properties Linked to Function
Five properties of water (metabolite, solvent, high heat capacity, latent heat of vaporisation, cohesion) and how each helps organisms, all explained by hydrogen bonding. Includes the 4-mark exam question, a worked model answer and the examiner trap on confusing cohesion with adhesion.
Chapters (9)
Exam question [4 marks]
Water is a major component of cells. Explain how four properties of water are important to a land plant.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Water is a solvent, so ions / polar substances dissolve and are transported, and/or metabolic reactions occur in solution (either consequence earns the mark) | "dissolves minerals / sugars", "reactions happen in solution" | "water dissolves everything"; "water is a solvent" with no consequence |
| 2 | High heat capacity, so it buffers changes in temperature / temperature changes slowly, keeping enzymes and cells at a stable temperature | "absorbs a lot of heat energy for a small temperature rise", "stable temperature for enzymes" | "high heat capacity" alone; "water stays cold" |
| 3 | Large latent heat of vaporisation, so evaporation (transpiration) removes a lot of heat and cools the plant with little water lost | "a lot of energy needed to evaporate", "cooling effect" | "evaporation gives energy"; "water boils at a high temperature" |
| 4 | Strong cohesion (hydrogen bonds between water molecules), so water forms a continuous column in the xylem | "molecules stick together", "unbroken column / pulled up", "surface tension" (if linked to cohesion) | "adhesion" used for water sticking to water; "water sticks to xylem walls" alone |
One mark per property, so four different properties are needed; two points about the same property score once. Each mark needs the property and its importance. Water as a metabolite (used in condensation, hydrolysis and photosynthesis) is also creditable as an alternative to any one of the four above, but still only four marks in total. Turgor/support is not one of the specification properties and is not credited.
Model answer (4/4)
Water is a solvent, so minerals and sugars dissolve and can be transported, and metabolic reactions happen in solution. It has a high heat capacity, so it buffers changes in temperature and keeps enzymes at a stable temperature. It has a large latent heat of vaporisation, so evaporation from the leaf removes a lot of heat and cools the plant while losing little water. Hydrogen bonds give strong cohesion between water molecules, so water stays in a continuous column in the xylem.
Examiner trap
- Saying water has a 'high specific heat capacity' without linking to buffering temperature change
- Confusing cohesion with adhesion
- Saying evaporation 'gives energy' rather than removes heat
Key terms
- metabolite
- solvent
- heat capacity
- latent heat of vaporisation
- cohesion
- surface tension
- hydrogen bonds
- polar
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:19The polar water molecule
It all starts with the shape of one water molecule. Oxygen pulls electrons towards itself, so the oxygen end is slightly negative and the hydrogen ends are slightly positive. That makes water polar. The slightly positive hydrogen of one molecule is attracted to the slightly negative oxygen of the next. That attraction is a hydrogen bond.
Every property in this video comes from polarity and hydrogen bonds.
0:47Water as a metabolite
Property one. Water is a metabolite. That means it takes part in reactions. Condensation releases a molecule of water, and hydrolysis uses a molecule of water to break a bond. Water is also a reactant in photosynthesis.
1:04Water as a solvent
Property two. Water is a solvent. Its slightly charged ends are attracted to ions and to other polar molecules, so they surround them and they dissolve. That lets substances be carried around the plant, and it means metabolic reactions can happen in solution inside the cell.
1:23High heat capacity
Property three. Water has a relatively high heat capacity. It takes a lot of energy to raise its temperature, because energy first has to loosen the hydrogen bonds. So temperature changes slowly. Water buffers changes in temperature, and that keeps cells and enzymes at a steady temperature.
1:43Latent heat of vaporisation
Property four. Water has a relatively large latent heat of vaporisation. To evaporate, each molecule needs a lot of energy to break free of its hydrogen bonds. That energy is taken from the leaf, so evaporation removes heat. It cools the plant, and only a small amount of water is lost. Note the wording. Evaporation takes heat away. It does not give energy.
2:10Cohesion and surface tension
Property five. Water molecules show strong cohesion. They stick to each other, because of hydrogen bonds. In the tube-like transport cells of a plant, the xylem, that holds the water together in an unbroken column, so it can be pulled up the plant. Where water meets air, cohesion also produces surface tension, which is why a pond skater can stand on a pond.
3:07Examiner trap and recap
The trap. Cohesion is water sticking to water. Do not call it adhesion, which is water sticking to something else.
Recap. Polar water, hydrogen bonds, five jobs. Next, the small charged particles that dissolve in all that water. Inorganic ions.
Watch first
This lesson builds on:
- Condensation and Hydrolysis Reactions · AQA 7402 · 3.1.1
Keep going
More from 3.1 Biological molecules
- Condensation and Hydrolysis Reactions · AQA 7402 · 3.1.1
- Maltose, Sucrose, Lactose: Glycosidic Bonds · AQA 7402 · 3.1.2
- Alpha vs Beta Glucose: Starch vs Cellulose · AQA 7402 · 3.1.2
- Benedict's and Iodine Tests Explained · AQA 7402 · 3.1.2
All 21 3.1 Biological molecules lessons Whole playlist on YouTube
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-020.