The covalent bonds in sugar molecules can be broken to yield energy for living things. What is the ORIGINAL source of this energy?* adenosine triphosphate (ATP) heat sunlight carbon dioxide

Question

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Answer 1

Sunlight 'zaps' electrons held by chlorophyll, energizing them

Two different pathways occur in sequence. In the first (confusingly named Photosystem II), this electron energy is 'harvested' in a transport process similar to that used in mitochondria, and having the same effect--pumping protons, which are in turn harvested to drive a machine that produces ATP

The second pathway, Photosystem I, directly captures the energized electrons and holds them in a molecule called NADPH

In the Calvin cycle, the energized electrons and ATP from above are used to create sugar from CO2, water and electrons (the energized electrons are 'locked up' into glucose)

Remember, covalent bonds ARE shared pairs of electrons--and much of their 'energy' (in the case of glucose) is derived from the 'zapping' that occurred in Photosystem I.

Answer 2

Photosynthesis—> carbon dioxide + water + sunlight—> oxygen + glucose

cellular respiration —> oxygen + glucose —> carbon dioxide + water + ATP