Explain why autotrophs must undergo cellular
respiration.

Autotrophs by definition are organisms which can produce their own nutrition, namely glucose.

This glucose is produced by photosynthesis, which requires CO2 and H2O as reactants.

Cellular respiration produces these required reactants, CO2 and H2O.

Once produced, they can be directly used in photosynthesis to give glucose.

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Could you elaborate?