Photosynthesis: A prospect for Green Electricity

Of all the living things created by God, the only one He gave autonomy in terms of feeding is plants. Plants use CO2 and water to generate their own food and also for humans. Generation of foods for our consumption is not just the end of it all. Plants purify our air (atmosphere) by mopping up some harmful gases (such as carbon (IV) oxide and carbon (II) oxide) from our environment, making us live a safe and healthy life.

[credit: pixabay CC0 creative commons license. Author: BrianJClark]

Plants have also been known to generate oxygen which is the major gas needed for cellular respiration in animals. Plants have been implicated in protein generation for animals through nitrogen cycle, where some microbes are involved in the process. After noting all these, I have come to understand that plants are highly recognized by me as a gold mine for the sustenance of humanity and other living things.

In biology, plants are called “autotrophs” (auto = Self; trophie = feeding). The two words are summed up to mean “self-feeders”. Plants trap energy from the sunlight and transform it to a chemical energy, ATP and (glucose) which is then stored as starch through a process called polymerization. This process is called photosynthesis. Plants trap energy from the sunlight and use the trapped energy to split water molecules (photolysis) to release electrons which in turn are used to generate glucose. This reaction is not as simple as it sounds. It involves cascades of reactions.

Processes of photosynthesis

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Photosynthesis takes place in green leaves. Inside the leaves, there is an organelle called stomata. The stomata allows passage of gases such as carbon (IV) oxide and oxygen. A green pigment called chlorophyll is responsible for trapping sun energy which is used to carryout the processes of photosynthesis.

This is why I call leaves a photocell. There are two types chlorophyll; a and b.

Chlorophyll a is responsible for the major energy trapping from sunlight while chlorophyll b called an accessory pigment improves the efficiency of energy trapping by absorbing the energy that chlorophyll a couldn’t absorb.

The chloroplast is the sit of photosynthesis. Chloroplasts contain the stacked pancake-like organelles thylakoids. Thylakoids are collectively called grana. The chlorophylls are embedded in the grana.

Photosynthesis occurs in two stages:

Light dependent reactions

Light-independent reactions

Light dependent reactions:

These reactions take place in the grana and require sunlight as a source of energy and hence the name, light reactions. It is at this stage that electrons are generated. The electron energy is used to power the rest of the reactions. In this stage, the chlorophylls a and b use the absorbed energy to split water molecules into hydrogen ions, oxygen and electrons.

The generated electrons are then used to reduce the reducing equivalent, NADP+ to NAPH which then carries out the rest of the reactions.

Light-independent reactions:

Light-independent reactions occur in the stroma. The summary of these reactions is that the NAPH and ATP produced from the light dependent reactions are used to produce carbohydrates from carbon (iv) oxide through Calvin cycle.

Photosystems

Photosystems are also called electron transfer system. The photosystems are responsible for the absorption of light energy and also electron transport during photosynthesis.
There are two types of photosystems

Photosystem II (PS 680)

Photosystem I (PS 700)

^{[credit: wikimedia. creative commons license (visit link for more information on the license). Author: Jiří Janoušek]}

The numbers given shows the spectrum of light absorption. PSII absorbs light at 680nm while PS 700 absorbs at 700nm. Being in a lower energy level, PSII occurs first while PSI occurs after. PSII was the second to be discovered and that’s why the number designation.

Having looked at the processes of photosynthesis, one has to reason that plants can be used for other things. What other importance could plants be to us?

Plants can play a vital role as an electrochemical cell, serving as an electron supplier which can be converted to electrical energy. Electrochemical cell is cell that converts chemical energy to electrical energy. The electrons generated from photosynthesis can be captured and stored for energy generation.

Though this has not been tried but I have a strong belief that this will definitely work due to the fact that plants have more efficient way of absorbing enough photon energy from the sunlight. This can replace the use of solar panels for photoelectron generation. The resource are cheaper and has no negative effect. It is also sustainable as plants will always carry out the processes of photosynthesis.

Working Principles of solar cells

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A solar cell is a solid p-n junction electrical device that traps energy directly from the sunlight and converts it into electricity by photovoltaic effect.

Photovoltaic effect is the process of creating electrical current when energy from light hits a material. The stroke materials have the tendency of emitting electrons and these emitted electrons are rich in energy. The striking of the electrons creates an energy gradient on the electrons. The energy gained by the electrons is used to create electrical energy which is stored in an electrochemical cell or voltaic cell.

Photosystems work on the same principle and a better or more efficient way. Photosystems trap energy from the sunlight and use it to hydrolyze water molecules. The chemical reaction is called photolysis or photo-oxidation of water. Electrons are generated when this happens and also it transports electrons from a lower to higher energy level which can also be stored as electrical energy.

Working principles of photosystems

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PSII traps light energy from the sun at a lower spectra line, the energy is then used to excite or activate electrons in the chlorophyll. The electron will leave the chlorophyll, making it to be an oxidized chlorophyll. In order to replenish the lost electrons, the light energy is further used in splitting the water molecules. The released electrons from water now occupy the chlorophyll a, replacing the knocked off electrons from the chlorophyll. The excited electrons begins their movement as follows:

[credit: wikimedia. CC4.0 license. Author: Somepics]

The movement is carried out in the electron shuttle or transport chain in the thylakoids. The PSII generates and transfers the electron to the oxidized plastoquinone which accepts electrons and then get reduced to plastoquinol. The electron is then transferred from the plastoquinol to cytochrome b6f with the delivery of H+ into the thylakoid lumen from the stroma. This process leads to an electron gradient between the stroma and the lumen, making the lumen more positively charged and the stroma, more negatively charged.

This will also result to a potential difference, leading to electron volts. The process of transferring proton from the stroma to the thylakoid lumen is called chemiosmosis.
The plastocyanin, another electron transporter then accepts the electron and transfers it to the PSI which traps energy from sunlight at even higher spectra region (700nm). The trapping of energy at this particular wavelength further energizes the electron. The highly excited electron is then transferred to by ferredoxin to be used in the reduction of NADP+ to NADPH and H+ with the aid of ferredoxin-NADP reductase.

What happened to the transferred H+ from the stroma to the lumen? In other to strike a balance in the proton concentration between stroma and the thylakoid lumen, the proton will travel through the ATP synthase back to the stroma. The transportation of protons from the lumen back to the stroma is accompanied with a reductive phosphorylation of ADP molecules, converting ADP to ATP.

ATP is regarded as the physiological energy currency. While the NADPH is used then used in the carbon fixation process on the Calvin cycle to generate carbohydrates. The produced ATP is used in powering the cellular activities of the plant.
One may ask why can’t the protons in the lumen go back directly to the stroma?

The reason is that, the lumen or the thylakoid membrane is highly selective in terms of transportation. Once the proton enters the lumen, the movement is irreversible through the same channel. So the only means of its transportation is through the ATP synthase channel.
How can photosynthesis be used in generating electricity?

Relating the working principles of solar cell and photosystems, we will understand that both of them work in the same way and for the same purpose which is electron excitation. This electron excitation is carried out by solar energy and is called photoexcitation. In the case of solar cells, material that can emit electrons is used as a source of the electron. In photosystems, electron is sourced from the chlorophyll. The chlorophyll contains four Manganese atoms which are the electron carriers, this is the source of electrons in the chlorophyll.

When the PSII traps energy from the sunlight, the energy is used to excite the electrons in the four manganese atoms, this means that at each cycle of photosynthesis, four electrons are generated. The manganese atoms get their electrons back by resorting to water splitting.
The challenge now is how to harness these electrons and store them in the voltaic cells for electricity generation. It has been established that this can replace the solar cell and also have a more efficient performance if made possible.
In the next write up, we will also discuss how microbes can be used in place of plants to generate electricity.

REFERENCES

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1. photosynthesis -nhptv
2. Chloroplast -wikipedia
3. Overview of photosynthesis-lumenlearning
4. photosystems -wikipedia
5. similarities between solar cells and photosynthesis -sciencing

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