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UNIT 3: ENERGY IN LIVING SYSTEMS THE BIG PICTURE All life requires energy. That energy ultimately comes from the sun, gets captured by plants through photosynthesis, and is then released and used by cells through cellular respiration. Understanding how energy moves through living systems starts with understanding how molecules move across membranes, because membranes are central to almost everything in this unit. 1. TRANSPORT — How Things Cross Membranes Before energy can be used, the right mo