The sentence in which the infinitive phrase functions as the subject is the second one - To convert sunlight and energy into food is the end result of photosynthesis.
To convert sunlight and energy into food is the end result of photosynthesis.
Explanation:
An infinitive phrase uses the "to" form of a verb, used as a noun (subject, object), a subject complement, an adjective, or an adverb. The infinitive phrase carries the object or alterants of the infinitive. Examples. To simply relax is often tough for him. (to relax is the subject, to simply relax is the infinitive phrase) . He seldom tries to win the lottery. (to win is the object of tries, to win the lottery is the infinitive phrase.
