Nociceptors.
Of course it's different. Plants have evolved completely different response mechanisms to stress. There is no known mechanism by which plants could be said to experience pain.
Uh oh.... I hope I don't have to haul out my old post detailing the various scientific tests on plant's responses to stimuli and pain?
People think that because plants don't have a brain, that they're not worth considering, yet jellyfish have no known brain, but have eyes, can co-ordinate themselves through complicated terrain and hunt/kill/eat fish and other small creatures. Not only that, but when they detect they're being attacked or killed, they automatically release their sperm/eggs to make sure their species/dna survive.
Some plants eat small animals and insects as well.
Plus the fact that plants use thorns, shells, poison, and other forms of protection to keep themselves alive from predators and other dangers, in some level they are aware that they can be at risk, can be damaged/injured and can be killed.
Case in point:
When tomatoes detect an insect is attacking/eating them, they send out a chemical response that makes it's leaves and other parts harder to digest for the insect and turn them off, among other things.
One could sum these things up as basic electrical stimuli/response and nothing more, yet that's all that occurs in our own brains, so what's the difference besides our own level of self centeredness?
We think that the only way someone can sense pain is if they have a central nervous system and a brain to register the pain.... but as we all know, pain is subjective, as some people may find pleasure in other things people find painful.... All pain is is our own subjective identification of the electrical messages sent to our brain. We either like a sensation or we don't, and we respond accordingly to how our species is designed, much like any other living thing on this planet.
Regardless of all of this, no matter if you only eat animals, reptiles, plants or insects, in order for one thing to live, another must die and what really matters is the amount of lives one takes to sustain their own.
Since you usually have to kill and eat more plants to survive then you do from one animal that can feed multiple people, One has to re-think exactly who are the worst "Murderers" of the bunch ;-)