Metabolism is
1. a measure of the hydrogen ion concentration in a solution
2. the energy required to start a chemical reaction
3. all of the chemical reactions in an organism required to sustain life
4. a chemical reaction in which the substrates trade constituent parts

3 : All of the chemical reactions in an organism required to sustain life.

Explanation: Metabolism is the process of converting food into energy form ( ATP). Metabolism is set of biochemical reactions which is completed in presence of necessary enzymes and is essential to sustain life.

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