The photosynthetic revolution

So far in this course, you have learned what energy is and how humans get and use the free energy stored in food chemicals. We now turn to where that energy came from in the first place, the sun. Photosynthesis evolved early in life’s history, probably more than three billion years ago. The first photosynthesizing organisms weren’t plants. They didn’t appear until half a billion years ago. But bacteria. Their descendant, the Cyanobacteria, are still alive today pretty much unchanged. Like plants, they use the energy of sunlight to manufacture carbohydrates from carbon dioxide in the air, making oxygen as they do so. All of the oxygen we breathe and all of the food we eat comes from this process.

Aerobic respiration couldn’t happen without it. The physical and chemical signatures of ancient Cyanobacteria have been found in rocks called stromatolites. More striking, though, is the geological evidence for what happened next. After Cyanobacteria had been photosynthesizing for hundreds of millions of years, oxygen started to build up in the Earth’s atmosphere. At this point, still over two billion years ago, dissolved iron salts in the world’s oceans began to react with atmospheric oxygen. Billions of tonnes of greenish water-soluble iron 2 compounds were oxidised to rusty coloured iron 3. And as they were oxidised, they came out of solution. They precipitated on the ocean floor in banded iron formations that we can still see today.

Plant and Cyanobacteria a green, because they contain the pigment chlorophyll, which absorbs the red and blue wavelength of sunlight and reflects the rest. The energy of the absorbed light is passed by an intricate mechanism to an enzyme called photosystem 2, where the solar energy is concentrated and used to tear electrons away from water. This water-splitting step is the beginning of the light reactions of photosynthesis. They continue rather confusingly in photo system 1. Here, more solar energy is added to water’s electrons to make NADPH, essentially an end NADH molecule with an added phosphate group. All of this machinery evolved in the photosynthetic membranes of Cyanobacteria.

About two billion years ago, not long after the first mitochondria arose, a Cyanobacterium was engulfed by a primitive eukaryotic cell and survived. This second symbiotic event was just as momentous as the mitochondrial one. That photosynthesizing Cyanobacterium became the ancestor of all chloroplasts, an organelle that harvests energy from light. It is the solar power plant of green plants. To us, the most familiar photosynthetic organisms and the basis of our ecosystem. Like mitochondria, chloroplasts have a folded internal membrane system, the thylakoid membrane, inside which electrons are transported and across which a proton gradient is established to store energy. Unlike mitochondria, the thylakoid membrane forms a separate interconnected system and forms stacks called grana.

The high-energy electrons of an NADPH made by Cyanobacteria and chloroplasts eventually find their way to carbon dioxide to produce carbohydrate in the so-called dark reactions. We will take a look at those in our second activity.