Question

how do some biochemicals function as respiratory substrate?

Answer 1

sorry I didn't get your question

Answer 2

In respiration, oxygen is not a substrate but a terminal electron acceptor. The respiratory substrates are electron donors (where the electrons are almost invariable 'held' in C-C, C-H and H-H bonds), such as glucose, fats and amino acids (and petrol in cars). In photosynthesis water may be considered a 'fuel' (a supplier of electrons) where O2 is now a 'waste' product.