A fuel cell including primarily (a) a membrane electrode assembly, which comprises (i) a proton exchange membrane having a front face and a rear face, (ii) an anode being coupled to the front face, and (iii) a cathode being coupled to the rear face; (b) a fuel permeation-controlling member positioned in front of the anode; the member being substantially impermeable to an organic fuel or water at an ambient temperature or below, but being permeable at a temperature higher than an activation temperature; (c) heating means in control relation to the fuel permeation-controlling member to activate fuel permeation through the member on demand. The invented fuel cell is compact and lightweight, with significantly reduced fuel crossover and improved fuel utilization efficiency. The fuel cell is particularly useful for powering small vehicles and portable devices such as a notebook computer, a personal digital assistant, a mobile phone, and a digital camera.