The difference IMO is nana versus vinnana.
Nana, ie simple knowing occurs in single faculty living organisms such as plants or very simple animals, and is what lies behind their reactions to environmental stimuli.
Vi-nana (literally double knowing) is knowing that one knows. This is quite crucial as Sentience depends on it. Thus only those beings which have Vinnana - they know that they know and can recognize pleasurable, painful or neutral stimuli - can Suffer.
But at what stage do living organisms turn from having simple nana to vinnana? This, IMO is the cause of the scriptural ambivalence towards creatures such as Insects and worms (and in our modern day - bacteria, maybe viruses too??), which of course, deserve the benefit of the doubt for the sake of compassion.