Because those chemical reactions help the cell to dispose useless substance after they are used and synthetize substance they need or the body needs through them.
Explanation:
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In this case, since the overall processes inside the cell are constantly producing and getting rid of several substances such as nutrients, proteins, lipids and other, we can say that decomposition reactions serve as a mechanism that the cell uses in order to obtain energy or dispose a substance that is no longer important for it. Moreover, since the cell is the fundamental part wherein the most of the useful substances the body needs are synthetized, we can infer that synthesis reactions must occur in order to allow the body to perform lots of functions, for instance, proteins associated to the hair or nails growing are synthetized inside the cell via the protein synthesis mechanism.
