BIOB 160 Principles of Living Systems · Unit 2: Energy and the cell · on Exam 2
2.1 Metabolism and energy
Study notes for Metabolism and energy in BIOB 160 Principles of Living Systems at Montana State University: the big ideas, key formulas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in StudyCat.
Big ideas
- Metabolism is all of an organism’s chemical reactions. Catabolic pathways break molecules down and release energy (cellular respiration); anabolic pathways build molecules and consume energy (protein synthesis).
- Kinetic energy is energy of motion (heat is the kinetic energy of molecules); potential energy is stored by position or structure, including chemical energy in bonds.
- The first law of thermodynamics: energy is neither created nor destroyed, only transferred or transformed. The second law: every energy transfer increases the entropy (disorder) of the universe, and some energy is lost as heat.
- Free energy ΔG = ΔH − TΔS predicts whether a reaction is spontaneous. Exergonic reactions release free energy (ΔG < 0, spontaneous); endergonic reactions absorb it (ΔG > 0, not spontaneous).
- Cells are open systems that never reach equilibrium; a cell at equilibrium is dead. Metabolic pathways keep reactions moving by removing products.
Key formulas
Free energy change
$$\Delta G = \Delta H - T\Delta S$$
Worked example
Reaction X has ΔG = +5 kcal/mol and reaction Y has ΔG = −12 kcal/mol. Which is spontaneous? If the cell couples them, what is the overall ΔG?
Show the solution
Y is exergonic (ΔG < 0) and spontaneous; X is endergonic. Coupled, ΔG = +5 + (−12) = −7 kcal/mol, so the combined process can proceed.
Common mistakes
- Thinking “spontaneous” means fast. It means the reaction can proceed without added energy; it can be very slow.
- Believing living things violate the second law. They stay organized by increasing entropy in their surroundings.
- Mixing exergonic (releases energy, ΔG negative) and endergonic (requires energy, ΔG positive).
Exam tipReading: Campbell ch. 8 (pp. 143–150). Not on Exam 1; it starts the Exam 2 material. Know the sign conventions cold.
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