Explain water as a biological solvent
Connect dissolved substances with digestion, excretion and transport.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall cell cytoplasm and the meaning of a solution.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
In a salt solution, what is the water?
What you will learn
- Connect dissolved substances with digestion, excretion and transport.
- Apply the idea to a supplied biological example or investigation.
Distinguish solvent from solute
A solvent is the liquid in which a solute dissolves to form a solution. Water acts as a solvent for many biological substances. Dissolved particles can be carried in an aqueous solution; this does not mean every biological material dissolves in water.
Digestion occurs in watery conditions, and many small soluble products of digestion can be transported in solution. Dissolving and digestion are different: dissolving disperses a solute in solvent, whereas digestion chemically breaks large food molecules into smaller ones.
Link the solvent to removal and carriage
Excretion removes metabolic wastes and excess substances. Water carries dissolved substances such as urea in blood plasma and urine. Water is also the solvent in fluids transporting dissolved nutrients, ions and other substances around an organism.
Blood carries oxygen mainly bound to haemoglobin in red blood cells, so do not describe all oxygen carriage as oxygen dissolved in plasma. The solvent role concerns dissolved substances; different substances have different transport arrangements.
Sources for this lesson
Pause and explain
Which pair best distinguishes digestion from dissolving?
Worked example
Explain why water helps transport urea to the kidneys.
Show the worked solution
- Urea is a dissolved metabolic waste in the aqueous part of blood.
- Water acts as its solvent.
- The flowing solution carries the dissolved urea.
Answer Water dissolves and carries urea in solution for excretion.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Identify solvent and solute in water containing dissolved glucose.
Give me a hint
Name what does the dissolving.
Compare my reasoning
- Water is the liquid solvent.
- Glucose is the dissolved solute.
Water: solvent; glucose: solute.
Look for these in your work
- Give both roles.
Give one solvent role of water in excretion.
Give me a hint
Choose a dissolved metabolic waste.
Compare my reasoning
- Urea dissolves in aqueous body fluids.
- It is carried and removed in urine.
Water carries dissolved urea for removal.
Look for these in your work
- Relate the example to excretion.
Correct the claim that water transports every substance only by dissolving it.
Give me a hint
Consider oxygen carriage and insoluble material.
Compare my reasoning
- Many substances dissolve, but not all.
- Most blood oxygen is bound to haemoglobin.
Water is a solvent for many substances, but transport is not universally by dissolution.
Look for these in your work
- Give a valid counterexample.
Common mistakes
- Dissolving is not chemical digestion.
- Not every transported substance is mainly dissolved in water.
What can you explain now?
Which pair best distinguishes digestion from dissolving?
Compare with the explanation
Chemical breakdown versus dispersion in a solvent.
These are different processes.
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Make it stick
Give separate solvent examples for digestion, excretion and transport.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.