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The photosynthetic revolution

The evolution of oxygen-producing (“oxygenic”) photosynthesis was a turning point in Earth’s history. The atmosphere started to fill with oxygen, and then the first chloroplasts arose.


Aerobic respiration continues in specialized organelles of the eukaryotic cell, the mitochondria. These are the descendants of ancient bacteria who evolved to use oxygen to completely oxidize their fuel compounds.

The energy in chemicals

Chemical energy is potential energy. It is related to the energies of the bonding electrons in covalent compounds.

Energy and work in living things

We need energy to do work, and we get that energy from chemicals. In the same way that a hydrogen-powered car uses a fuel cell to get energy from hydrogen, …