This issue is about the generic use of articles and the differences between definite and indefinite, singular and plural subjects. Thank you in advance Monica. It's certainly true that the three kinds of generic noun phrase are often different, in use and occasionally in sense, besides being different in form. Let's give them some names, so we can talk about them. The tiger is in danger of becoming extinct.
Tigers are in danger of becoming extinct.
A tiger is in danger of becoming extinct. These are constructions , which means that the phrase itself, and its usage, have special grammar and special meanings. As to use and meaning, while there are many, many special cases and idioms, one can roughly equate the three generic noun phrase constructions with three different functions. The tiger , as a prototype, has all the properties of anything we would call a tiger, except that it doesn't exist in an individual physical sense, like all real tigers do. This is a very abstract concept, and its use signals that the speaker is theorizing.