Metabolism is the sum total of all chemical and physical changes that take place in any organism that enables it to function and grow. It is nothing different from reactions taking place inside a chemistry laboratory. Just that it tends to happen in a more controlled way inside the body. The metabolic process is dependent on a number of factors such as enzymes and other chemical compounds for its completion. It can be represented as

Enzyme Y
↑
Substrate A→Product B→
Product C↓(Final end product)
Enzyme X

In a few cases, there is a block in any one of the steps of a reaction, either due to absence or deficiency of a particular enzyme. A disease arising out of such a defect is called a metabolic disorder.