6. Metabolism

Definition of metabolism

Metabolism refers to all the chemical reactions that happen inside your body to keep you alive. These reactions include:

• Breaking down the food you eat into usable energy
• Using that energy to power your organs and tissues
• Converting food into building blocks for your body's cells (such as proteins and amino acids)
• Getting rid of waste products

Think of your metabolism as a giant factory that keeps your body running smoothly. The factory takes in raw materials (food), breaks them down into usable parts (energy and building blocks), uses those parts to build things (cells, tissues, organs), and gets rid of the waste products.

Classification based on enthalpy

We can classify metabolisms into two based on the change in their enthalpy (ΔH). When the enthalpy of the system decreases (ΔH < 0), that means the system exerts heat on the outside. Such reactions are called “exothermic reactions.” When the enthalpy of the system increases (ΔH > 0), that means the system absorbs heat from the outside. Such reactions are called “endothermic reactions.”

• Exothermic reactions (ΔH < 0) exert heat
• Endothermic reactions (ΔH > 0) absorb heat

Classification based on Gibbs free energy

Gibbs free energy can be converted into any other form of energy without any help. If a metabolism decreases a system’s Gibbs free energy (ΔG < 0), reactions occur without any help or spontaneously. We call such reactions “exergonic reactions.”

In contrast to that, the system cannot restore its Gibbs free energy by itself. For this reason, if a metabolism increases a system's Gibbs free energy (ΔG > 0), reactions have to get energy from another system to occur. In other words, such reactions occur non-spontaneously. These reactions are so-called “endergonic reactions.”

• Exergonic reactions (ΔG < 0) happen spontaneously
• Endergonic reactions (ΔG > 0) happen non-spontaneously