Wize AP Biology Textbook > Energy & Metabolism
Coupled Reactions
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Coupled Reactions
In our cells, some reactions can happen spontaneously (exergonic) while others do not (endergonic). However, we often need both types of reactions to happen for basic cells processes. How can we get those non-spontaneous reactions to occur?
- Two reactions that happen together: one helps the other.
- An energetically unfavorable reaction (endergonic, positive ΔG) is coupled with a favorable reaction (exergonic, negative ΔG), making the overall process favorable. Example: In the first reaction of glycolysis, glucose becomes phosphorylated (1), and ATP loses a phosphate (2). ATP hydrolysis into ADP always releases energy. Addition of phosphate to glucose requires energy.
Think of ATP as a hiker who wants to come down from the top of a mountain. He can jump onto a seesaw and propel another hiker up.
Wize Concept
ATP is considered the "energy currency" of the cell because it is so often used to drive other reactions forward.
- Each individual reaction has its own value of ΔG.
- Coupled reactions have a NET change in energy that is still exergonic (negative ΔG and favorable).
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Example: Endergonic or Exergonic?
The ΔG for the following equation is +34 kcal.
- Is this reaction exergonic or endergonic?
- Which direction should the arrow go? (will the reaction proceed to the left or right?)
CH4 + 2 O2 ------- CO2 + 2 H2O
- Endergonic
- Towards the left ( <––– ) endergonic reactions are spontaneous in the backwards direction. Aka under natural circumstances the reverse reaction will take place, not the forward one.
Practice: Biosynthesis Coupling
Endergonic biosynthesis reactions become spontaneous when couple with:
Practice: Reactions that Require Coupling
Which of the following reactions would likely requires ATP coupling to occur?