2.4 billion years ago, photosynthetic cyanobacteria, or blue green algae as they are sometimes erroneously called, terraformed our planet into the habitable oasis we have come to call Earth. They absorbed sunlight and carbon dioxide and released oxygen transforming Earth's reducing atmosphere into an oxidative one, shifting the balance of life in favor of more complex oxygen loving organisms like ourselves. Today, another organism is poised to shift that balance in the opposite direction. Humans and their penchant for burning fossil fuels have already unloaded an unprecedented amount of CO2 into the Earth's atmosphere. Since it was photosynthesis that instigated the climate change that gave our ancient aerobic ancestors a chance in the past, it is probably fitting that a little biomimicry could solve the climate and energy woes of today.
Cyanobacteria and plants use photosynthesis to make their own fuel from nothing more than CO2, water, and the sun's rays. Artificial photosynthesis is therefore any process that attempts to mimic nature by chemically storing solar energy inside of a fuel. Artificial photosynthesis also includes any technologies that attempts to replicate any part of the photosynthetic process. The term is often used to describe a variety of technologies, including the photocatalytic splitting of water into hydrogen fuel and oxygen gas, the use of photosynthetic microorganisms for biofuels, or the complete biomimicry of the photosynthetic reaction process to create chemical fuels.
Conventional solar power, , which involves the direct conversion of photons into DC current via the photovoltaic effect, is mired by costly materials, intermittency issues, cloudy weather, and the need to store electricity in a battery. Photosynthesis in nature may just hold the answers to all of these problems. The advantage to creating a fuel is the ability to use existing piping infrastructure, and easily store energy for use when sunlight is unavailable. The protein complexes from photosynthesis could also be used to replace costly rare earth metals used in conventional solar cells, driving costs down.
The first step in biomimetics is to understand the phenomenon we are trying to replicate. Nature has devised a number nuanced ways to draw power from the sun, the most well-known being chloroplasts. Within the chloroplast are small disk-like structures called thylakoids surrounded by a fluid filled space called the stroma. Photosynthesis can be divided into two stages, light dependent reactions in the thylakoid and the light independent reactions which take place in the stroma. In most photoautotrophs, organisms that make their own food using sunlight and CO2, thylakoids capture energy from the sun using two types of photosynthetic reaction centers Photosystem I and Photosystem II.
These photosystems work in tandem to produce the energy that will later be used in the light independent reactions in the stroma to create fuel. The photosystems consist of a complex of specialized proteins, pigments, and other cofactors. The main cofactor in most photoautotrophs is chlorophyll, the pigment that makes leaves green and is responsible for absorbing red and blue light. When a photon strikes a chlorophyll molecule, it excites the electrons to a higher energy state allowing them to be scooped up by electron carrier proteins in the cell's electron transport chain. Let's take a closer look at the molecular machinery behind photosystems I and II. The names photosystem I and photosystem II are paradoxical, and are only due to the order in which they were discovered. Since the electron pathway starts at photosystem II, we will be starting there first.
When a photon hits photosystem II it excites an electron to a higher energy state causing it to leave the reaction center of photosystem II and enter the electron transport chain. The deficit in electrons is regenerated by an oxidative process called photolysis which strips electrons from water to create hydrogen and oxygen gas.
The high energy electrons that left photosystem II power the pumping of hydrogen ions from the stroma into the thylakoid to create a concentration gradient. This gradient powers a protein called ATP synthase which phosphorylates ADP to form ATP, an energy carrier that will be used in the light independent reactions. The energy depleted electrons leave the electron transport chain and enter photosystem I.
When a photon strikes photosystem I, the electrons are re-energized and passed through the electron transport chain to a specialized protein called NADP reductase, which reduces NADP+ to NADPH, the second energy carrier molecule critical to the light independent step of photosynthesis.
The ATP and NADPH produced from the light dependent reactions in the thylakoid can now be subjected to a process called the Calvin Cycle which uses ATP and NADPH to reduce carbon dioxide and produce the carbohydrate glyceraldehyde-3-phosphate. The cycle consists of three steps, the first of which is carbon fixation. In this step CO2 is fixed to ribulose 1,5-bisphosphate.
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