Metabolic basics for Living
Metabolic pathways can lead to a more complex structure from a simpler structure or lead to a simpler structure from a complex structure.
The former cases are called biosynthetic pathways or anabolic pathways. In the phase of metabolism, simple substances are synthesised into complex materials of living tissues. Anabolic pathways, as expected, consume energy. Assembly of protein from amino acids requires energy input.
The latter constitute degradation and hence are called catabolic pathways. The metabolic breakdown of complex molecules into simpler ones, often results in release of energy.
For example, when glucose is degraded to lactic acid in our skeletal muscle, energy is liberated. This metabolic pathway from glucose to lactic acid which occurs in 10 metabolic steps is called glycolysis.
Living organisms have learnt to trap this energy liberated during degradation and store it in the form of chemical bonds. As and when needed, this bond energy is utilised for biosynthetic, osmotic and mechanical work that we perform. The most important form of energy currency in living systems is the bond energy in a chemical called adenosine triphosphate (ATP).