Explain why living organisms need enzyme-controlled rates
Relate catalysis to reaction rates that can sustain life.
Section B5.1 · Enzymes
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 protein building blocks and the distinction between a molecule and a living cell.
- Use the supplied observations, quantities and units rather than assuming an optimum for every enzyme.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does metabolism include?
What you will learn
- Relate catalysis to reaction rates that can sustain life.
- Apply the explanation to the stated evidence and recognise the limits of the model.
Rate matters for living cells
Living cells continually carry out reactions to maintain themselves, release usable energy, build molecules and process nutrients. A reaction that is possible in principle may proceed too slowly without suitable enzyme catalysis to meet a cell’s needs.
Enzymes increase rates under conditions compatible with the organism. This is why they are important in all living organisms, rather than only in digestion. A cell needs coordinated reaction rates across its metabolism.
Use demand and supply carefully
A supplied demand model lets you compare a production rate with a stated requirement. If a reaction produces 0.2 units/min but the model requires 1.0 unit/min, it falls short; increasing the rate to 2.0 units/min meets that specific model requirement. These arbitrary units are teaching data, not a measured organism.
Catalysis alone does not guarantee life or unlimited growth. Cells still require substrates and appropriate conditions, and other processes can limit their function. Describe why a sufficient rate is necessary without claiming it is the only requirement.
Sources for this lesson
Pause and explain
Why can a possible reaction still need enzyme catalysis?
Worked example
A model cell needs 1.0 unit/min. A reaction produces 0.2 unit/min without enzyme and 2.0 units/min with enzyme. Explain the significance.
Show the worked solution
- Compare 0.2 with the 1.0 requirement.
- Compare 2.0 with the same requirement.
- Link enzyme action to a sufficiently rapid supply.
Answer The enzyme-controlled rate meets the stated demand, while the uncatalysed rate does not.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
Explain why “the reaction can happen” is insufficient to show it can support a living cell.
Give me a hint
Add the missing time dimension.
Compare my reasoning
- Cells have ongoing demands.
- A possible but very slow reaction may not supply products quickly enough.
The reaction must occur at a sufficient rate to meet cell needs.
Look for these in your work
- Include rate.
- Connect it to demand.
A model requires 3 units/min. Compare reaction rates of 1 unit/min and 4 units/min.
Give me a hint
Use the stated threshold only.
Compare my reasoning
- 1 is below 3.
- 4 is above 3.
The 1 unit/min rate falls short; the 4 units/min rate meets this model’s requirement.
Look for these in your work
- Compare both values.
- Restrict the result to the model.
A cell has an active enzyme but no supply of its substrate. Why does this challenge the claim that enzymes alone guarantee metabolic production?
Give me a hint
The catalyst cannot replace reactants.
Compare my reasoning
- The enzyme speeds conversion of suitable substrates.
- Without substrate, that conversion lacks its reactant.
Enzyme activity cannot create the missing substrate; suitable reactants and conditions are also required.
Look for these in your work
- Identify missing substrate.
- Preserve the catalyst/reactant distinction.
Common mistakes
- Do not limit enzyme importance to digestion.
- A sufficient reaction rate is not the only condition for life.
What can you explain now?
Why can a possible reaction still need enzyme catalysis?
Compare with the explanation
Its uncatalysed rate may be too low to sustain cell needs.
Possibility and speed are different.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Recall two cell needs, then explain why reaction rate matters for both.
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.