The correct version of the sentence is the following:
Some plants' nutrients are gained by catching insects.
Possessive nouns are nouns that demonstrate possession. In this case, it means that the nutrients belong to the plants.
Possessive nouns are built by adding 's (apostrophe plus -s) if the noun is singular. Since in the example sentence the noun plants is already a regular plural that takes -s, only the apostrophe is added to indicate possession.
