Metabolism refers to the chemical processes that occur within living organisms to maintain and sustain life. It involves a series of complex reactions that convert nutrients into energy or building blocks for biomolecules.

Option 1: Synthesis of biomolecules is a part of metabolism where new biomolecules are created. However, it does not encompass the entire metabolic process.

Option 2: Breaking down of biomolecules also occurs during metabolism. It involves the breakdown of larger molecules into smaller ones to release energy or to be used as building blocks for other molecules. However, this alone does not represent the entirety of metabolism.

Option 4: Recycling of biomolecules can also be a part of metabolism, but it does not fully explain all the processes involved.

Therefore, option 3, which states that metabolism involves both synthesis and breaking down of biomolecules, is the most comprehensive and accurate description.