Pathway of Electrons in Photosynthesis

Energy is required for all processes in all organisms, from bacteria to humans. Many organisms obtain this energy by ingesting other organisms, which allows them to access stored energy. But where does the energy stored in food come from? Photosynthesis is responsible for all of this energy.

Photosynthesis is essential to all life on Earth; it is required by both plants and animals. It is known to be the only biological process capable of capturing energy from outer space (sunlight) and converting it into chemical compounds (carbohydrates) that all organisms use to power their metabolism. In a nutshell, we can say that sunlight energy is captured and used to energise electrons, which are then stored in the covalent bonds of sugar molecules. How long do those covalent bonds last and how stable are they? The energy which is extracted from coal and petroleum products today represents sunlight energy captured and stored by photosynthesis around 300 million years ago.

What is Photosynthesis?

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During photosynthesis, sunlight energy is captured and used to power the synthesis of glucose from CO2 and H2O. Photosynthesis is the ultimate source of metabolic energy for all biological systems because it converts the energy of sunlight into a usable form of potential chemical energy. Photosynthesis occurs in two distinct stages. Sunlight energy drives the synthesis of ATP as well as the synthesis NADPH, which is coupled with the formation of O2 from H2O in the light reactions. The ATP and NADPH produced by the light reactions drive glucose synthesis in the dark reactions, which are so named because they do not require sunlight.

Both the light and dark reactions of photosynthesis occur within chloroplasts in eukaryotic cells—the light reactions in the thylakoid membrane and the dark reactions in the stroma. In this article we will discuss photosynthesis's light reactions, which are related to oxidative phosphorylation in mitochondria.

The Pathway of Electrons

Robert Hill and Fay Bendall proposed the general features of a widely accepted mechanism for photoelectron transfer in 1960, in which two light reactions (light reaction I as well as light reaction II) occur during the transfer of electrons from water (Two hydrogen molecules along with one oxygen) to carbon dioxide. This mechanism is based on the relative potential (which is measured in volts) of the various electron-transfer chain cofactors to be oxidised or to be reduced. Molecules with the highest affinity for electrons in their oxidised form (i.e., strong oxidising agents) have a low relative potential. In contrast, molecules that are difficult to reduce in their oxidised form have a high relative potential once electrons have been accepted.

Molecules with a low relative potential are thought to be strong oxidizers, while those with a high relative potential are thought to be strong reducing agents.

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The actual photochemical steps are typically represented by two vertical arrows in diagrams that describe the light reaction stage of photosynthesis. These arrows indicate that the special pigments P680 and P700 receive light energy from light-harvesting chlorophyll-protein molecules and are elevated in energy from their ground state to excited states. These pigments are known to be extremely strong reducing agents in their excited state, quickly transferring electrons to the first acceptor. These first acceptors are also strong reducing agents, allowing electrons to be rapidly transferred to more stable carriers.

The first acceptor in light reaction II could be pheophytin, a chlorophyll-like molecule with a high reducing potential that quickly transfers electrons to the next acceptor.The following quinones in the series are special quinones. These molecules are similar to plastoquinone in that they accept electrons from pheophytin and transfer them to the intermediate electron carriers (electron pathway), which include the plastoquinone pool and cytochromes b as well as f associated in a complex with an iron-sulfur protein.

In light reaction I, electrons are transferred to iron-sulfur proteins in the lamellar membrane, where they are then transferred to ferredoxin, a small water-soluble iron-sulfur protein. When NADP+ as well as a suitable enzyme are present, two ferredoxin molecules, each carrying one electron, transfer two electrons to NADP+, which picks up a proton (that is., a hydrogen ion) and transforms into NADPH.

What are Cytochromes B and F?

We can describe cytochrome c as a functional component of the mitochondrial electron transport chain. This electron transport is part of the ATP synthesis pathway. The function of cytochrome c is basically to transport electrons from one complex of integral membrane proteins of the inner mitochondrial membrane to another. Cytochrome f is a subunit of the cytochrome b6f complex that participates in photosynthesis in plants, green algae, and cyanobacteria by transferring electrons between photosystems II and I.

Importance of Photosynthesis

Photosynthesis is important for more than just capturing sunlight's energy. On a cold day, a lizard sunning itself can use the sun's energy to warm up. Photosynthesis is an essential process because it evolved as a method of storing the energy in solar radiation (the “photo” part) as high-energy electrons in the carbon-carbon bonds of carbohydrate molecules (the “synthesis” part). Carbohydrates are the energy source used by heterotrophs to power ATP synthesis via respiration. As a result, photosynthesis fuels 99 percent of the Earth's ecosystems. When a top predator, such as a wolf, hunts a deer, the wolf is at the end of an energy path that began with nuclear reactions on the surface of the planet.