Energy and Metabolism

Core Concepts

Energy and its forms

Energy exists as kinetic energy (energy of motion) and potential energy (stored energy, such as energy in chemical bonds). Chemical energy is potential energy held in the arrangement of atoms and bonds; when bonds rearrange in a reaction, energy can be released. Heat is the most disordered form of energy and, for cells, is mostly lost — cells cannot harness heat to do work the way a steam engine can, which is why organisms constantly need new energy from food.

The two laws of thermodynamics

• First law: energy cannot be created or destroyed, only converted from one form to another. Cells don't make energy; they convert chemical energy from food into ATP and, ultimately, heat and mechanical work.
• Second law: every energy conversion increases entropy (disorder) in the universe. No process is 100% efficient — some energy is always lost as heat. This is why organisms must keep taking in energy: maintaining the order of a living body costs constant energy input.

Free energy and spontaneity

Chemists use free energy (G) to describe the energy available to do work. The change in free energy, ΔG, tells you whether a reaction can proceed on its own:

• Exergonic reaction (ΔG < 0): releases energy; thermodynamically favorable; can occur spontaneously. Cellular respiration and ATP hydrolysis are exergonic.
• Endergonic reaction (ΔG > 0): requires energy input; not favorable on its own. Building proteins and synthesizing ATP are endergonic.

"Spontaneous" in this context means energetically favorable, not fast. A sugar cube is thermodynamically poised to burn, but without a spark (activation energy) it sits there — and inside cells, without enzymes, reactions would be far too slow to support life.

Enzymes: the catalysts that make life fast

Enzymes are proteins (or RNAs, in some cases) that speed up reactions by lowering the activation energy — the energy needed to get a reaction started. They work by binding specific reactant molecules (substrates) at an active site, orienting them so the reaction can proceed. Key properties:

• Enzymes are not consumed by the reactions they catalyze; one enzyme molecule can process many substrate molecules.
• They are specific — each enzyme typically handles one substrate or one class of reactions.
• Their activity depends on conditions: temperature, pH, and the presence of inhibitors or activators. High fever or extreme pH can denature an enzyme and stop its reaction.

ATP: the energy currency

ATP consists of adenine (a nitrogenous base), ribose (a 5-carbon sugar), and three phosphate groups. The bonds between the phosphate groups are high-energy: when the terminal phosphate is removed, ATP becomes ADP (adenosine diphosphate) + inorganic phosphate (Pᵢ), releasing energy that cells use to drive work. Commonly taught reference figure: ATP hydrolysis releases roughly 7.3 kcal/mol (about 30.5 kJ/mol); exact values in textbooks vary slightly, so verify against your current text.

Cells regenerate ATP from ADP + Pᵢ using energy from catabolism — this is what cellular respiration (topics 2–4 of this chapter) does. ATP powers three types of cellular work: chemical work (building molecules), transport work (pumping substances across membranes), and mechanical work (muscle contraction, chromosome movement).

Redox reactions: moving electrons

Many metabolic reactions are oxidation–reduction (redox) reactions, in which electrons are transferred. Oxidation is the loss of electrons; reduction is the gain of electrons (remember "LEO says GER" or "OIL RIG"). In cellular respiration:

• Glucose is oxidized — it loses electrons (and hydrogen).
• Electron carriers NAD⁺ and FAD are reduced — they gain electrons and become NADH and FADH₂.
• These loaded carriers deliver electrons to the machinery that makes ATP.

Worked Example: Coupling an Endergonic Reaction to ATP

Cells need to trap glucose inside by adding a phosphate group: glucose + Pᵢ → glucose-6-phosphate. This reaction is endergonic — it will not happen on its own. The cell couples it to ATP hydrolysis:

1. ATP hydrolyzes: ATP + H₂O → ADP + Pᵢ (exergonic, releases energy).
2. The enzyme hexokinase binds both ATP and glucose, transferring the phosphate directly onto glucose.
3. Net result: glucose-6-phosphate (trapped in the cell) is formed, and the energy cost is paid by ATP.

This is a pattern you'll see everywhere in metabolism: an energetically unfavorable reaction is hitched to a favorable one, usually ATP hydrolysis, with an enzyme as the coupling device. The same logic explains how muscles contract (ATP powers myosin), how pumps transport ions (ATP powers the pump), and how ribosomes build proteins (ATP and GTP pay for each added amino acid).

Common Confusions