自由エネルギー

Free energy, abbreviated as G for scientist Gibbs, measures useful energy extractable from a reaction to do work. It represents energy available after accounting for entropy. Cells must maintain free energy flow to stay alive; at equilibrium with no net change in G, cells would die because they lack leftover energy for essential functions.

Endergonic reactions require energy input, making the change in free energy (ΔG) positive; energy enters the system. Exergonic reactions release energy, making ΔG negative; energy exits the system. In photosynthesis, plants perform endergonic reactions using sunlight to build glucose. In cellular respiration, animals perform exergonic reactions breaking down glucose to release stored energy.

When a system reaches equilibrium, there is no net change in free energy, and reactions stop flowing in either direction. Cells would die without energy to perform work. To survive, cells constantly change concentrations of reactants and products, maintaining disequilibrium and allowing metabolism to continue running continuously.

Glucose molecules store chemical energy in their bonds created during photosynthesis. Plants build glucose from carbon dioxide and water using sunlight energy in an endergonic reaction. This stored energy can later be released when organisms break down glucose during cellular respiration, providing energy for cellular work and maintaining life processes.

Photosynthesis converts carbon dioxide and water into glucose and oxygen using sunlight energy. This reaction requires energy input from the sun, making it endergonic with a positive ΔG. The energy from sunlight is stored in the chemical bonds of glucose molecules, creating energy storage for plants and organisms that consume them.

Cellular respiration breaks down glucose and oxygen to produce carbon dioxide and water, releasing the energy stored in glucose bonds. This exergonic reaction has a negative ΔG because energy exits the system. The released energy powers cellular functions, making respiration the primary way animals extract usable energy from food.

Most chemical reactions are reversible and can proceed in both directions. By adjusting concentrations of reactants and products, cells shift reaction direction to prevent equilibrium. This dynamic control allows cells to maintain the free energy gradients necessary for metabolism catabolism and anabolism, ensuring reactions continue flowing to support life.