Plants are capable of converting inorganic substances (carbon dioxide and water) into organic sugar.



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The principle of photosynthesis

Structure of a leaf



Light phase

Dark phase

Artificial leaf



During photosynthesis, plants produce organic material, glucose, from inorganic material, carbon dioxide, using the energy of light. Oxygen is also formed in this process.

Photosynthesis takes place in the green parts of plants, that is, in the leaves, and often in the soft stem. The green colour of plants results from the large amount of chloroplasts in the cells of the assimilation tissue. These chloroplasts are where photosynthesis takes place.

Chloroplasts have a double membrane. The internal membrane forms the disc-like thylakoids, which form stacked membranous structures called grana. The thylakoid membrane contains the key enzymes for the light phase of photosynthesis.

The most important of these are the two photosystems and the electron transport chain between them.
The photosystems contain protein-bound light-absorbing pigments, the most important being chlorophyll.
The central chlorophyll-a molecules of Photosystem II are excited by photons and release electrons, which enter the electron transport system.
The oxidised, electron-deficient chlorophyll replaces its missing electrons from water molecules, that is, it splits water. The oxygen atoms in water molecules combine to form molecular oxygen, while protons accumulate inside the membrane.
The first member of the electron transport chain is plastoquinone, which transfers the electrons to the cytochrome complex. Cytochrome is an iron-containing protein, which transfers electrons to the Plastocyanin while pumping more protons into the thylakoid lumen.
The electrons are transferred to Photosystem I from the electron transport chain. The central chlorophyll molecule of Photosystem I is in an electron-deficient state, since it has previously released electrons, being excited by photons. The electrons are then transferred to the ferredoxin NADP reductase by ferredoxin molecules.
In the light phase, protons accumulate on the inside, that is, the proton concentration of the thylakoid lumen increases and thus becomes positively charged. This creates an outward driving force. Protons pass to the outside through the ATPase, while energy is released, since the system enters a lower-energy state from a higher-energy state due to the equalisation of charge and concentration. The energy released is used in the production of ATP. The protons and electrons released are accepted by the NADP, which converts into NADPH.
To sum up, the energy of the photons causes an unequal distribution of protons. This creates a driving force, which is used for the production of ATP.

The reactions of the dark phase are light-independent. In this phase carbon dioxide is incorporated into an organic compound using the energy of the ATP and the hydrogen ions of the NADPH produced in the light phase.
Let's start with 3 five-carbon sugar molecules. They have 15 carbon atoms altogether. An enzyme protein incorporates 1 carbon dioxide molecule into each sugar molecule, while the products split and 6 three-carbon molecules are formed, with a total of 18 carbon atoms. Then, by using 1 NADPH and 1 ATP for each molecule, 6 glyceraldehyde 3-phosphate molecules are formed. One of these quits the cycle, while the others convert back to 3 five-carbon sugar molecules using 3 ATPs, and the cycle starts all over again. That is, by using ATP and NADPH produced in the light phase, 1 three-carbon molecule is produced in this cycle. Two cycles produce 2 three-carbon molecules, which attach and form a six-carbon glucose molecule. The plant uses glucose in its further metabolic processes for starch synthesis or in its digestive processes for the production of ATP.

Experiments have been carried out in order to create artificial systems that mimic photosynthesis. In an artificial leaf the light reactions and the dark reactions take place in two separate vessels. The light reactions take place in a nitride semiconductor, which decomposes water when exposed to light. Oxygen is released as bubbles, while protons and electrons are transferred into the other vessel, with the latter being transferred through a conductive wire. This vessel is the site of the dark reactions. Here a metal catalyst is used to produce formic acid from carbon dioxide and water. This system makes it possible to use the energy of sunlight, and it may also be helpful in reducing the carbon dioxide content of the atmosphere, which would help to reduce the greenhouse effect and thus global warming.

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