Every living creature on this planet Earth needs to extract energy from the dietary food, a process termed as Catabolism ( breaking down of complex molecules into simpler molecules to yield maximum energy, eg: Glucose gets converted by a process involving Glycolysis, Krebs cycle and electron transport chain – to yield ATP). The opposite cellular process, where smaller molecules are combined to construct functional macro molecules, which are useful for building cell structures by using energy produced during catabolism. This chemical process of creating bigger molecules using smaller components, which are useful for building different cell structures is termed as Anabolism.
Catabolism and anabolism, forms two important divisions of metabolic pathways. So, metabolism can be loosely defined as chain of chemical reactions occurring in our body, where a starting molecule, gets converted into a different molecule or molecules in a regulated manner.
Cells will first collect all the necessary elements such as C, O, H, P, K, N, S and many other essential elements (sugars, amino acids, fats etc.) required for the synthesis of cell structures. These elements mostly part of our regular diet, needs to cross the plasma membrane, in order to gain entry into the cytoplasm of the cell and this requires energy.
Once these elements enter inside the cell, they are assembled into monomers that the cell needs. Sometimes the cells cam also get monomers like sugars, amino acids, vitamins and fatty acids directly from the food. These monomers gets organized into important biological macromolecules; proteins, nucleic acids, and lipids. Once the macromolecules are formed, they can be combine and account for vital structures of the cell: ribosomes, chromosomes, membranes etc.