The substrate is the source of energy and carbon processed by the organism. Due to the many different types of metabolism, numerous substrates can be processed, such as sugar, cellulose, wood, etc. The substrate is a source of energy and carbon that is processed by the organism. For example, we eat sausage bread, metabolize it and excrete the digestive products.
Explanation for children:
We eat sausage bread and then have to go to the toilet later. One can also say that the sausage bread was our substrate. We have used it, converted it into energy and finally it is excreted again in another form. Microorganisms can have very different substrates: Some eat wood, sugar, sulphur, etc..