Overview
This lecture reviews key concepts in cellular energetics, including the nature of energy in biological systems, misconceptions about energy, and differences between exergonic and endergonic reactions relevant to AP Biology Unit 3.
Energy in Living Systems
- All living organisms require a constant input of energy to survive.
- Metabolic reactions can either store energy or release it, but all require activation energy to start.
Common Misconceptions About Energy
- Breaking chemical bonds requires energy; energy is not released by breaking bonds.
- The energy difference between reactants and products is the same for catalyzed and uncatalyzed reactions.
- Energy is not lost as heat; it is transferred according to the laws of thermodynamics.
- Entropy (disorder) in a system always increases, and energy cannot be created or destroyed.
Exergonic vs. Endergonic Reactions
- Exergonic reactions (e.g., ATP hydrolysis, cellular respiration) release energy, with reactants at a higher energy level than products.
- Endergonic reactions (e.g., ATP synthesis, photosynthesis) require an input of energy; reactants have lower energy than the products.
Sources of Input Energy
- ATP synthesis often relies on chemiosmosis via ATP synthase.
- Photosynthesis uses light as its input energy source.
Key Terms & Definitions
- Exergonic Reaction — Reaction where energy exits the system; products have less energy than reactants.
- Endergonic Reaction — Reaction that requires energy input; products have more energy than reactants.
- Activation Energy — Minimum energy required to start a chemical reaction.
- Chemiosmosis — Process where ATP is produced using energy from a proton gradient across a membrane.
- Entropy — A measure of disorder in a system; always increases in natural processes.
Action Items / Next Steps
- Review the unit 3 summary video in the Ultimate Review Packet.
- Study the definitions and examples of exergonic and endergonic reactions.
- Complete any assigned practice questions or readings on cellular energetics.