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SUN AND CONVERTS IT INTO CHEMICAL ENERGY, AND THEN STORES IT AS FOOD. SARAH: RIGHT. AND WHAT AFFECTS THE EFFICIENCY OF THAT PROCESS? BRYCE: THE SIZE AND THE SHAPE OF THE LEAVES, COLOR OF THE LIGHT — SARAH: WELL, THINK ABOUT IT AT A CELLULAR LEVEL. BRYCE: WELL, PHOTOSYNTHESIS DEPENDS ON CHLOROPLASTS CONVERTING THE LIGHT ENERGY. I SUPPOSE IF RADIANT EN- ERGY PASSES THROUGH A CELL WITHOUT HITTING A CHLO- ROPLAST, IT’S WASTE ENERGY. SARAH: GOOD EXAMPLE. HERE’S ANOTHER. SUPPOSE SUN- LIGHT HITS A LEAF AT RIGHT ANGLES. THE PLANT RECEIVES MORE ENERGY PER UNIT AREA THAN IT WOULD IF THE LIGHT WERE AT LESS THAN 90 DEGREES. COLLEEN: MAKES SENSE. SARAH: SO OVER TIME, PLANTS HAVE EVOLVED. AND THEY CAN TURN THEIR LEAVES SO THEY FACE THE SUN. WE CALL IT “PHOTOTROPISM.” BRYCE: PHOTOTROPISM? SARAH: FROM THE GREEK. “PHOTO” FOR LIGHT, AND “TROPOS” MEANING TURNING. BRYCE: PHOTOTROPISM. COLLEEN: TURNING TO THE LIGHT. GOT IT! DARREN: THE HUMAN ENGINE CONVERTS FOOD TO ELECTRI- CAL, THERMAL, AND KINETIC ENERGY. HOW WELL WE DO IT DEPENDS ON THE TYPE OF FOOD WE EAT, AND THE KIND OF CONDITION WE’RE IN. STAYING IN SHAPE AND STICKING TO A HEALTHY DIET IS A GREAT WAY TO MAKE SURE YOU HAVE THE ENERGY TO ENJOY YOURSELF. SO...IF WE KNOW THE AMOUNT OF ENERGY INPUT AND THE AMOUNT OF ENERGY OUTPUT OF A SYSTEM, WE CAN FIND THE EFFICIENCY OF ENERGY CON- VERSIONS. AND THAT’S GREAT BECAUSE IT MEANS THAT WE CAN EXAMINE A SINGLE SYSTEM, LIKE A CROSS-COUNTRY SKIER, AND TRY TO FIGURE OUT HOW TO IMPROVE THE EFFI- CIENCY OF HER SKI TECHNIQUE. OR WE CAN COMPARE SYS- TEMS THAT DO THE SAME JOB AND DECIDE WHICH IS MORE EFFICIENT. EFFICIENCY OF ENERGY CONVERSION — IT’S SIM-

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