Water dissolves substances, carries them around, and provides the medium for cell reactions. DNA stores the instructions for making proteins, and RNA helps to use them.
This lesson closes the SPM Biology chemical composition of cells chapter. It builds on comparing carbohydrates, proteins and lipids.
What does water do?
| Role | Example |
|---|---|
| Solvent | Dissolves glucose, salts and gases so they can be used |
| Transport medium | Carries substances in blood and plant sap |
| Reaction medium | Chemical reactions in cells happen in water |
| Cooling | Evaporation of sweat takes heat from the skin |
Give the role and the example together, because a bare role gains fewer marks.
Worked example: sweating on a hot day
A student runs in the afternoon and starts to sweat. The question asks why this helps.
- The body gains heat from the exercise.
- Sweat is mostly water, and it spreads over the skin.
- The water evaporates from the skin, and evaporation takes heat energy from the body.
- The body loses heat, so the body temperature does not rise too far.
The answer uses the property of water (evaporation) and links it to the job (cooling).
What do DNA and RNA do?
DNA is a nucleic acid that stores the genetic information in a cell. A section of DNA carries the instructions for making one protein. RNA is a second nucleic acid that helps to carry those instructions out, so the cell can make the protein.
Think of DNA as the master recipe kept safe in the nucleus, and RNA as a working copy used in the kitchen.
The mistake that costs marks
The common slip is to write that DNA “makes proteins” or that water “gives energy”. DNA holds the instructions, and cells make the proteins. Water is essential but supplies no energy.
| Wrong | Right | |
|---|---|---|
| DNA | Makes proteins | Stores the instructions for making proteins |
| Water | Gives energy | Dissolves, transports, and acts as a medium |
| Sweating | Water is lost, so you cool | Evaporation takes heat from the skin |
Check yourself
State two roles of water in the human body and the job of DNA.
Answer
Water is a solvent that dissolves substances, and a transport medium in the blood. Evaporation of sweat also cools the body.
DNA stores the genetic information that carries the instructions for making proteins.
What to study next
Test the whole chapter in the chemical composition of cells practice set. Practise with the biological data interpretation trainer, and log slips in the mistake log and paper-error review.
If you would like a teacher to sharpen your answers, see online one-to-one Biology tuition.