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Internal energy (E) is the total energy contained within a system, including chemical bond energy and particle motion. Enthalpy (H) is a thermodynamic function defined as internal energy plus the product of pressure and volume. In most biochemical reactions where no work occurs, the change in internal energy equals the change in enthalpy, making enthalpy the more practical measure for cellular processes.

An exothermic process releases heat to surroundings and has negative enthalpy change (ΔH < 0). An endothermic process absorbs heat and has positive enthalpy change (ΔH > 0). Living cells undergo both types of processes as open systems exchanging energy with their environment, following the first law of thermodynamics.

Enthalpy is a state function defined by the equation H = U + PV, combining internal energy with pressure-volume terms. Since absolute values of these components cannot be determined independently, only enthalpy changes (ΔH) for specific chemical or physical processes can be measured. This limitation applies universally to enthalpy calculations in chemistry and biology.

Enthalpy (ΔH) is a key component of the Gibbs Helmholtz equation: ΔG = ΔH − TΔS, which relates enthalpy change, temperature, and entropy change to determine free energy. This relationship helps predict whether endergonic and exergonic reactions in the cell will occur spontaneously under specific conditions.

Standard conditions for biological systems are defined as pH 7.0, 25 degrees Celsius, and 100 kilopascals pressure. However, actual cellular conditions vary considerably from these standards, causing calculated standard ΔG values to differ from real cellular values. This discrepancy is important when predicting reaction spontaneity inside living cells.

Living cells are open systems that exchange both matter and energy with surroundings through exothermic and endothermic processes. Exothermic reactions release energy as heat (negative ΔH), while endothermic reactions absorb energy (positive ΔH). This energy exchange follows the first law of thermodynamics and enables cells to maintain life processes.

At constant pressure, the enthalpy change (ΔH) equals the change in internal energy (ΔU) plus pressure times volume change (PΔV): ΔH = ΔU + PΔV. Most biochemical reactions occur at atmospheric pressure, making this relationship essential for calculating heat released or absorbed in cellular processes like combustion reactions.