What type of reaction occurs when complex compounds are broken down into simpler substances?

Complex compounds are broken down to simpler substances in catabolic reactions.

These kinds of reactions often occur in biological systems. In living organisms, complex compounds like lipids, proteins and complex sugar like cellulose are broken down into simpler forms. Products of these reactions are simple sugars, amino acids etc. but a certain amount of energy is also produced and stored in energy molecules for future use.

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