- Photolysis in living things
- Photosystems I and II
- Molecular hydrogen
- Non-biological photolysis
- Ozone degradation by ultraviolet light
- Other processes
- References
The photolysis is a chemical process by virtue of which the absorption of light (radiation energy) allows the breakdown of a molecule into smaller components. That is, light provides the energy required to break a molecule into its component parts. It is also known by the names of photodecomposition or photodissociation.
The photolysis of water, for example, is essential for the existence of complex life forms on the planet. This is carried out by plants using sunlight. The breakdown of water molecules (H 2 O) results in molecular oxygen (O 2): hydrogen is used for the storage of reducing power.
In general terms, we can say that photolytic reactions involve the absorption of a photon. This comes from a radiant energy of different wavelengths, and therefore, with different amounts of energy.
Once the photon is absorbed, two things can happen. In one of them, the molecule absorbs energy, gets excited and then ends up relaxing. In the other, that energy allows the breaking of a chemical bond. This is photolysis.
This process can be coupled with the formation of other bonds. The difference between an absorption that generates changes to one that is not called quantum yield.
It is particular to each photon because it depends on the source of energy emission. The quantum yield is defined as the number of modified reactant molecules per absorbed photon.
Photolysis in living things
Photolysis of water is not something that happens spontaneously. That is, sunlight does not break hydrogen bonds with oxygen just because. Photolysis of water is not something that just happens, it is done. And living organisms that are capable of carrying out photosynthesis do.
To carry out this process, photosynthetic organisms resort to the so-called light reactions of photosynthesis. And to achieve this, they obviously use biological molecules, the most important of which is chlorophyll P680.
In the so-called Hill Reaction, several electron transport chains allow molecular oxygen, energy in the form of ATP, and reducing power in the form of NADPH to be obtained from the photolysis of water.
The last two products of this light phase will be used in the dark phase of photosynthesis (or Calvin Cycle) to assimilate CO 2 and produce carbohydrates (sugars).
Photosystems I and II
These transport chains are called photosystems (I and II) and their components are located in the chloroplasts. Each of them uses different pigments, and they absorb light of different wavelengths.
The central element of the whole conglomerate, however, is the light collecting center made up of two types of chlorophyll (a and b), different carotenoids and a 26 kDa protein.
The captured photons are then transferred to the reaction centers in which the reactions already mentioned take place.
Molecular hydrogen
Another way that living things have used the photolysis of water involves the generation of molecular hydrogen (H 2). Although living things can produce molecular hydrogen in other ways (for example, by the action of the bacterial enzyme formatohydrogenolyase), production from water is one of the most economical and efficient.
This is a process that appears as an additional step after or independent of the hydrolysis of water. In this case, organisms capable of carrying out the light reactions are capable of doing something additional.
The use of H + (protons) and e- (electrons) derived from the photolysis of water to create H 2 has only been reported in cyanobacteria and green algae. In the indirect form, the production of H 2 is subsequent to the photolysis of water and the generation of carbohydrates.
It is carried out by both types of organisms. The other way, direct photolysis, is even more interesting and is only carried out by microalgae. This involves the channeling of electrons derived from the light breakdown of photosystem II water directly to the enzyme that produces H 2 (hydrogenase).
This enzyme, however, is highly susceptible to the presence of O 2. The biological production of molecular hydrogen by photolysis of water is an area of active research. It aims to provide cheap and clean energy generation alternatives.
Non-biological photolysis
Ozone degradation by ultraviolet light
One of the most studied non-biological and spontaneous photolysis is that of ozone degradation by ultraviolet (UV) light. Ozone, an azotrope of oxygen, is made up of three atoms of the element.
Ozone is present in various areas of the atmosphere, but it accumulates in one that we call the ozonosphere. This zone of high ozone concentration protects all forms of life from the damaging effects of UV light.
Although UV light plays a very important role in both the generation and degradation of ozone, it represents one of the most emblematic cases of molecular breakdown by radiant energy.
On the one hand, it indicates that not only visible light is capable of providing active photons for degradation. Furthermore, in conjunction with biological activities for the generation of the vital molecule, it contributes to the existence and regulation of the oxygen cycle.
Other processes
Photodissociation is also the main source of breakdown of molecules in interstellar space. Other photolysis processes, this time manipulated by humans, have industrial, basic scientific and applied importance.
The photodegradation of anthropogenic compounds in water is receiving increasing attention. Human activity determines that on many occasions antibiotics, drugs, pesticides, and other compounds of synthetic origin end up in the water.
One way to destroy or at least decrease the activity of these compounds is through reactions that involve the use of light energy to break specific bonds in those molecules.
In the biological sciences it is very common to find complex photoreactive compounds. Once present in cells or tissues, some of them are subjected to some type of light radiation to break them down.
This generates the appearance of another compound whose monitoring or detection allows answering a multitude of basic questions.
In other cases, the study of compounds derived from a photodissociation reaction coupled to a detection system makes it possible to carry out global compositional studies of complex samples.
References
- Brodbelt, JS (2014) Photodissociation mass spectrometry: New tools for characterization of biological molecules. Chemical Society Reviews, 43: 2757-2783.
- Cardona, T., Shao, S., Nixon, PJ (2018) Enhancing photosynthesis in plants: the light reactions. Essays in Biochemistry, 13: 85-94.
- Oey, M., Sawyer,. AL, Ross, IL, Hankamer, B. (2016) Challenges and opportunities for hydrogen production from microalgae. Plant Biotechnology Journal, 14: 1487-1499.
- Shimizu, Y., Boehm, H., Yamaguchi, K., Spatz, JP, Nakanishi, J. (2014) A Photoactivatable Nanopatterned Substrate for Analyzing Collective Cell Migration with Precisely Tuned Cell-Extracellular Matrix Ligand Interactions. PLoS ONE, 9: e91875.
- Yan, S., Song, W. (2014) Photo-transformation of pharmaceutically active compounds in the aqueous environment: a review. Environmental science. Processes & impacts, 16: 697-720.