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Cell Biology & Energy (BIO 101) · Module 2 of 8

Bioenergetics Basics

The thermodynamics, ATP chemistry, and redox reactions underlying every metabolic pathway.

In this module
  1. Free Energy & Spontaneity
  2. ATP Structure & Function
  3. Enzymes & Metabolic Pathways
  4. Oxidation-Reduction (Redox) Reactions
  5. Check yourself

Free Energy & Spontaneity

  • Gibbs free energy (ΔG) determines whether a reaction is spontaneous: negative ΔG (exergonic) releases energy and occurs spontaneously; positive ΔG (endergonic) requires energy input.
  • Cells couple exergonic reactions, like ATP hydrolysis, to endergonic reactions to drive processes that wouldn't happen spontaneously on their own.
An exergonic reaction
reaction progressfree energy GΔG < 0reactantsproducts
Products have less free energy than reactants, so ΔG < 0 and the reaction is spontaneous. It still needs activation energy to get started, which is what enzymes lower.
Worked example

How does ATP hydrolysis drive an otherwise unfavorable (endergonic) reaction in the cell?

Solution. The exergonic hydrolysis of ATP, releasing free energy, is coupled to the endergonic reaction, often via a shared phosphorylated intermediate, so the combined reaction has a net negative ΔG and proceeds.

ATP Structure & Function

  • ATP consists of adenine, ribose, and three phosphate groups; hydrolyzing the bond to the terminal phosphate releases a relatively large amount of free energy from reduced charge repulsion and increased resonance stability in the products.
  • Cells maintain a very high ATP:ADP ratio, continuously regenerating ATP from ADP + Pi as fast as it's used.
Worked example

Why does breaking the terminal phosphate bond in ATP release so much usable energy?

Solution. The three phosphate groups are all negatively charged and repel each other; removing one relieves this electrostatic strain, and the products (ADP and Pi) are more stable through resonance, both of which release energy.

Enzymes & Metabolic Pathways

  • Enzymes lower activation energy without changing a reaction's overall ΔG, dramatically speeding up reaction rates.
  • Metabolic pathways are often regulated by feedback inhibition, where the pathway's end product inhibits an earlier enzyme (often the first committed step) to prevent overproduction.
Worked example

What is feedback inhibition, and why is it metabolically efficient?

Solution. It's when the end product of a pathway inhibits an enzyme earlier in that same pathway, often the first committed step, preventing the cell from wasting resources continuing to produce a product that's already abundant.

Oxidation-Reduction (Redox) Reactions

  • Cellular respiration is fundamentally a series of redox reactions: glucose is oxidized (loses electrons) while oxygen is reduced (gains electrons) to form water.
  • Electron carriers NAD⁺/NADH and FAD/FADH₂ shuttle electrons between reactions, becoming reduced (NADH, FADH₂) when they pick up electrons and reoxidized when they release them.
Worked example

In cellular respiration, is glucose oxidized or reduced, and what happens to the electrons it loses?

Solution. Glucose is oxidized (it loses electrons and hydrogens); those electrons are picked up by electron carriers like NAD⁺ (forming NADH), which later deliver them to the electron transport chain.

Check yourself

2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.

1. What does a negative ΔG indicate about a reaction?

2. Why does ATP hydrolysis release a large amount of free energy?

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