Let’s put our sneakers on and go for a run while listening to Episode 56!
Let’s put our sneakers on and go for a run while listening to Episode 56! Melanie begins by debunking some myths to make our chemistry and physics colleagues happy (1:30). Next, remember key activation energy concepts (2:43). Lastly, analyze two graphs to distinguish between energy storing and energy releasing reactions (4:36).
The Question of the Day asks (8:02) True or False? ΔG is constant in catalyzed and uncatalyzed reactions.
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap Activation Energy
Zoom out:
Unit 3 - Cellular Energetics
Topics 3.2-3.4
Big idea - Energetics
Sometimes I'm convinced that the hardest part of exercising is putting on your sneakers. I’m serious. Imagine you're currently on the couch, snuggled up with your dog or cat, and you need to muster up the effort to go for a run. “Its only 30 minutes”, you say to yourself, “it's easy. But this Netflix episode is also only 30 minutes sooooooo.” Once you put your sneakers on, the rest seems to happen automatically - headphones in, running playlist on, quick stretch - and you’re off. It's not that you don't want to exercise, it's just that sometimes committing to an activity is difficult. Much like you, reactions need to put in a little bit of effort and prep in order for cellular activities to occur.
Let’s Zoom in:
First - let’s debunk some myths and make our chemistry and physics colleagues happy. Myth 1 “Energy is released when bonds are broken.” Truth - breaking bonds requires energy. The emphasis needs to be on the remaining energy of the products - are they at a lower or higher energy level than reactants? Reactions require an input of energy, known as activation energy to occur. Enzymes reduce how much energy is required for a chemical reaction to continue and thus, makes metabolism more efficient. Note that the difference in energy from reactants to products remains the same in a catalyzed vs uncatalyzed reaction. Myth 2 “Energy is lost as heat.” The laws of thermodynamics state that energy cannot be created or destroyed and the entropy of a system is always increasing. Energy isn’t lost, it's just transferred. And while yes, it is difficult for biological systems to reuse and transfer heat energy into another form, it all comes out in the wash. Some chemical reactions are energy storing while others are energy releasing.
Two things to note - All molecules have energy stored in chemical bonds, with some having more energy than others. And - chemical reactions involve the forming and breaking of these bonds. So what is activation energy and how does it come into play? Activation energy (indicated as capital E subscript capital A) is the initial energy input for a chemical reaction to occur. Activation energy is typically applied by heat, which is absorbed by the reactant molecules, causing faster motion, greater collisions and more strain on chemical bonds. Even those reactions which are exergonic, energy releasing with a negative delta G, need energy to place molecules and bonds into their unstable transition state. Reactant molecules don’t stay in a transition state long, but quickly move onto the next step.
Let’s analyze two graphs to distinguish between energy storing and energy releasing reactions. The y axis is Gibbs free energy and the x axis is reaction progress. The line drawn on the graph shows how the energy level changes over time during a chemical reaction. Energy levels increase in both graphs at first, with the peak of each curve representing activation energy. This hill will be smaller in catalyzed reactions, typically done with enzymes in cells (and that's a whole other episode recap).
Exergonic reactions have energy exiting the system with reactants at a higher energy level than products. These reactions have a negative delta G (free energy value) and are spontaneous. Imagine yourself walking along the graphed line - You go up and over the hill, then land in a lower valley. Examples of exergonic reactions include the formation of ADP and the process of cellular respiration. Energy cannot be created or destroyed - just transferred. Cellular respiration transfers energy from glucose into ATP (with many many steps in between).
Endergonic reactions require additional energy beyond activation with reactants at a lower energy level than products. Energy is stored in these products. These reactions have a positive delta G and are not spontaneous. Imagine yourself walking along the graphed line - You go up and over the hill, coming down only partially before landing on a plateau. Examples of endergonic reactions include the formation of ATP and the process of photosynthesis. Where does this input of energy come from? Well for ATP - perhaps its chemiosmosis through ATP synthase. For photosynthesis? Thank you for the sunlight.
Closing out with our analogy. You still want to go for a run - with the reaction moving from the couch to the great outdoors. The activation energy required involved lacing up your sneakers with a transition state of starting your running playlist and stretching. Once in that transition state, you easily start your run! Activation energy values do vary from reaction to reaction, influencing reaction rate and spontaneity. The activation energy required for your run would have been even greater if instead of being on the couch, you were in bed asleep and in pajamas!
Time for unit connections. You’ll find activation energy in unit 3 with enzymes, biological catalysts. Alright - what about the exam? Energy and graphing analogies can definitely show up, especially in the FRQs. You need to understand the concept of energy, but the equation for Gibbs free energy is beyond the scope of the exam. Biggest association here is with activation energy and enzymes - so be prepared to associate the two and predict outcomes of a disrupted system (enzyme denaturing).
To recap……
Activation energy is the input of energy required to get “over the hump” of a chemical reaction - no matter if endergonic or exergonic. This energy is typically achieved through heat, and moves reactants to their transition state.
Coming up next on the Apsolute RecAP Biology Edition: Electron Transport Chains
Today’s question of the day is about reactions
Question of the day: True or False? ΔG is constant in catalyzed and uncatalyzed reactions.