Summary
The minimum energy needed to start a reaction, lowered by enzymes without changing the overall energy change.
In depth
Reactants must overcome an energy barrier to reach a transition state, even when the reaction is thermodynamically favourable. Enzymes stabilise the transition state and offer an alternative pathway with a lower barrier, so many more molecules have enough energy at body temperature.
Heat also increases reaction rate by adding energy, but would damage cells.
Key idea: enzymes change the rate, not the final equilibrium.
Heat can raise reaction rates by increasing the fraction of molecules with enough energy, but would damage cells, so enzymes lower the barrier instead; a drop of only a few tens of kilojoules per mole can speed a reaction by a factor of billions.
Enzymes do not change the free energy difference between reactants and products, so they cannot make an unfavourable reaction go and do not change the equilibrium; they only reach it faster. The transition state is held more tightly than the substrate, and transition-state analogues make powerful inhibitors.
Frequently asked questions
What is Activation Energy?
The minimum energy needed to start a reaction, lowered by enzymes without changing the overall energy change.
Which area of biology covers Activation Energy, and how advanced is it?
Activation Energy is one of the 51 topics in Biomolecules & Biochemistry. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Enzymes, Free Energy in Biology, Factors Affecting Enzymes.
Can you give an example of Activation Energy?
Hydrogen peroxide breaks down slowly on its own, but catalase in liver tissue produces rapid fizzing of oxygen.
What should I learn before Activation Energy?
Work through the Beginner topics in Biomolecules & Biochemistry first, such as Water Properties, Carbon Chemistry, Carbohydrates, then return to Activation Energy. Intermediate topics assume the Beginner vocabulary.
What topics are related to Activation Energy?
The closest topics to Activation Energy are Enzymes, Free Energy in Biology, Factors Affecting Enzymes, ATP Structure and Function. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Activation Energy?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Activation Energy. To practise the whole of Biomolecules & Biochemistry, use the Biomolecules & Biochemistry quiz.
How is Activation Energy different from Enzymes?
Activation Energy: The minimum energy needed to start a reaction, lowered by enzymes without changing the overall energy change. Enzymes: Biological catalysts, mostly proteins, that speed up reactions by lowering the activation energy. Read both side by side and note what each one explains.
How should I study Activation Energy step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Enzymes and explain the difference aloud. Finally, take the Biomolecules & Biochemistry quiz and come back to this page for any question you miss.