A fire in Somerville that destroyed a bakery and a commercial storage unit sent a firefighter to the hospital last night, a fire official said. The one-alarm blaze, at 316 Somerville Ave., started at 7:02 p.m. and burned for almost two hours, said District Fire Chief Frank Lee. Lee declined to describe the extent of the firefighter’s injury, but said he was conscious and alert before he was taken to Massachusetts General Hospital. Fiesta Bakery was closing for the day when the fire started in the one-story building. Investigators are determining a cause. The fire was difficult to douse after it climbed the walls and spread into a loft space, Lee said. The fire closed part of Somerville Avenue for much of Wednesday evening.