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One gram of dry green algae fixes 5 moles of CO2 per hour through photosynthesis. If all the fixed carbon is stored as starch (represented as (C6H12O6)n, with molar mass 180 g per mole of glucose unit), find the time in seconds required for the algae to double its own mass.

1. 12 s
2. 24 s
3. 36 s
4. 48 s

Correct answer: 24 s

Solution

Photosynthesis fixes 6 CO2 per glucose unit: 6CO2 + 6H2O -> C6H12O6 + 6O2. Rate of CO2 fixation = 5 mol/hr. Rate of glucose-unit formation = 5/6 mol/hr. Mass of starch formed per hour = (5/6) * 180 = 150 g/hr. The algae must gain 1 g (to double from 1 g to 2 g). Time = 1 g / (150 g/hr) = 1/150 hr = 3600/150 s = 24 s.

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