FOSSILS ET AL.
The Evolution of Photosynthesis
A look at the past and present of this special biological process and the things that have changed
Significance of photosynthesis
Imagine a world without photosynthesis. There wouldn’t be any to begin with. It is not just the lifeline of plants but also all big and complex beings on the planet. Oxygen, the byproduct of this process casually discarded by plants, is the sole reason for Earth to be full of complex beings like us. It is required to release energy from food that is used to produce ATP that powers the cells; it is required by organic matter to decompose and return to the soil; it is involved in many metabolic processes that are essential to detoxify harmful substances; it prevents the planet from losing water, and after all of this, we need it in our atmosphere as ozone to absorb harmful ultraviolet radiation.
Mars is the closest to what we can imagine a world without oxygen. Barren, lifeless, and hopeless. There would be no long food chains, and life, though possible, would only be visible under a microscope.
The recipe
We are all familiar with the chemical reaction of photosynthesis from school days, where 6 molecules of carbon dioxide combine with 6 molecules of water in the presence of sunlight to create one molecule of glucose and 6 molecules of oxygen gas. But this was not always the case. In the distant past, bacteria used things like iron or hydrogen sulfide instead of water. But what is the significance of these ingredients? It has everything to do with the chloroplasts.
Chloroplasts are organelles in plants that contain two kinds of photosystems, PS I and PS II, and each photosystem has its role in a process called Z scheme. The function of these photosystems is to trap sunlight and produce sugar by utilizing quantum physics and chemistry. In PS II, when a photon hits the electron extracted from water by chloroplasts, a transfer of energy occurs, which vigor the electron to jump to a higher energy state and immediately return to its original energy orbital, releasing energy along the way used for producing ATP. In PS I, when a second photon hits the electron, it jumps again to a higher energy state and is transferred to carbon dioxide, starting the process of making sugar.
Although the process seems normal, two very strange things are happening here. Firstly, carbon dioxide is a very stable gas with a negative electron affinity, which means it does not readily accept electrons. This is only possible through the ATP produced during PS II and NADPH. Secondly and more importantly, water is a very stable compound that does not want to lose an electron. In the Z scheme, it is split into molecular oxygen, protons and electrons in the Oxygen Evolving Complex (OEC) of the chloroplasts, which we will discuss later.
Why shift to water?
Before cyanobacteria evolved to perform oxygenic photosynthesis (photosynthesis that releases oxygen), other types of bacteria used hydrogen sulfide to obtain free electrons. Some even oxidized iron in seawater, as both hydrogen sulfide and iron are better electron donors than water. So why did cyanobacteria switch from these conventional sources to water? Studies show that oxygenic photosynthesis evolved when hydrogen sulfide and iron were still abundant, so the change wasn’t due to a scarcity of these materials. It may have happened because water splitting has higher redox potential that oxidation of hydrogen sulphide, which means that more energy is released when electrons are extracted from water than from hydrogen sulphide.
The role of manganese
The evolution of chloroplasts to split water was a fascinating one. It involved an already existing biochemical process and made use of it. Manganese is a compound that bacteria welcome as they use it as an anti-oxidant. It catches ultraviolet light, throws an electron, and becomes oxidized, thus preventing the bacteria cells from harm. This method was incorporated by chloroplasts by developing an oxygen-evolving complex, as previously mentioned, which is a cluster of four manganese atoms and one calcium atom held together by a lattice of oxygen atoms.
Once the manganese atoms are oxidized, they combine their electron-accepting power to split water, thus providing a way to extract electrons. This process is further accelerated by the presence of chlorophyll, the pigment that is responsible for the green color of the leaves. And this is how the first bacteria evolved to use water and carbon dioxide to get energy.
These evolutions indicate that life always finds a way to thrive in any environment. The biological add-ons that cyanobacteria developed to use the most abundant thing around them is a reminder of life’s efficient approach to sustainability. It also highlights the relationship between the planet and the creatures that live on it. This innovation allowed the Earth to inject oxygen into its atmosphere so that further complex plants and animals could appear. And it would not be a hyperbole to say that the evolution of photosynthesis to use a precious liquid in place of toxic gas is the most important evolution in the history of Earth.
Published in Fossils et al. Follow to learn more about Paleontology.
