Cellular energy, thermodynamics and conserved metabolic pathways

What the exam asks

• Multiple Choice (Section I): Stimulus sets often show an unfamiliar metabolic pathway with numbered intermediates and enzymes. You will be asked to predict the effect of an enzyme inhibitor, identify how ATP couples reactions, or explain why a pathway is evidence of common ancestry.
• Free Response (Section II): Expect this topic in Q4 (Conceptual Analysis) or Q5 (Analyze Model or Visual Representation), where you must predict how disrupting an energy source or an enzyme affects downstream products and organismal survival.

Core ideas

1. Thermodynamics and cellular order

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed (e.g., light energy to chemical energy in photosynthesis).
• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe, as some energy is lost as unusable thermal energy (heat).
• Living organisms do not violate the Second Law. Cells are open systems that maintain high internal order (low entropy) by taking in free energy from the environment and releasing heat and disordered waste products (such as $\text{CO}_2$ and $\text{H}_2\text{O}$), which increases the total entropy of the surroundings.
• Energy balance: Energy input must always exceed energy loss to maintain homeostasis, grow, and reproduce. A loss of energy flow results in cellular death.

2. Energy coupling and ATP

Cells cannot use heat to do work. Instead, they couple cellular processes that release energy (exergonic/catabolic) to processes that require energy (endergonic/anabolic), typically using ATP (adenosine triphosphate) as the energy shuttle.