Lemons contain citric acid and essential oils that are toxic to cats.
Ingesting lemon can cause vomiting, diarrhea, and in large quantities, it can damage a cat's nervous system, resulting in weakness and sensitivity to light.
Additionally, the peel, seeds, and leaves of lemons can cause problems even in small quantities.
Suggestions
It is not recommended for cats to eat lemons at all.
If a cat ingests lemon, it is best to contact a veterinarian immediately.