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photolysis definition and example

Definition: splitting of a chemical compound via light energy

Photolysis Definition

We define photolysis as a chemical process in which chemical compounds or molecules are split into small units by the action of photons or absorbing light. It is considered as the interface of one or many photons, having a targeted molecule. Photolysis is also named photodecomposition or photodissociation.

What is Photolysis? 

Photolysis is a type of chemical reaction in which molecules are split into smaller by absorbing light. The process of photolysis is not restricted to visible light only. Any chemical compound or bond that gets adequate energy from the photon can cause photolysis or photodissociation. The energy of the photon is inversely proportional to its wavelength. To carry out the process of photolysis different rays like gamma rays, X-rays, and ultraviolet rays are involved.

photolysis process
Figure 1: Image shows the process of photolysis. Image Source: Maria Victoria Gonzaga of Biology Online.
Biology definition:
Photolysis is the splitting or decomposition of a chemical compound by means of light energy or photons. For example, the photolysis of the water molecule in photosynthesis occurred under the influence of light. When photons are absorbed, it causes the hydrogen to bind to an acceptor, subsequently releasing the oxygen. Etymology: from Ancient Greek “φωτ”- (“phōt”-), from “φῶς” (“phôs”), meaning “light” and “λύσις” (l”úsis”), meaning “decomposition”. Synonym: photodissociation; photodecomposition. See also:  photosynthesis

Photolysis in Photosynthesis

Photolysis is a part of which cycle?

Photolysis is considered a part of light-dependent reactions. It is also considered as a photochemical phase, light phase, or Hill reaction of photosynthesis.

READ: Photosynthesis – Photolysis and Carbon Fixation Tutorial

Hill Reaction

In the incidence of sunlight, electrons are transferred from water to non-physiological oxidants (Hill Reagents). This reaction is known as Hill Reaction. The transfer is in the direction which is against the chemical potential gradient.

The following photolysis equation shows the reaction:

H2A + 2 photons (light) → 2 e + 2 H+ + A

In the above equation, the chemical nature of “A” is based on the form and kind of the individual. The water molecule (H2O) acts as the substrate for photolysis in oxygenic photosynthesis. This will lead to the formation of diatomic oxygen molecule O2.
In purple sulfur bacteria, sulfur (S) is oxidized from hydrogen sulfide (H2S). This is the procedure by which oxygen comes back into the earth’s atmosphere.

Where does photolysis occur?

In the cyanobacteria, photolysis of water takes place in the thylakoids. In plants and green algae, it takes place in the chloroplast.

Energy transfer models

The energy transfer model is conventional and semi-classical. In this transfer model, the excitation energy travels into the reaction center molecules from the light-capturing pigment molecule. The movement of energy is not random or sudden. It moves stepwise in an organized way, following the molecular energy ladder.

There are small energy packets which are known as photons having various wavelengths. The effectiveness and efficiency of the photon are based on the tendency of the absorption ranges of the photosynthetic pigments of each individual. The chlorophyll absorbs a wide range of light energy from the red and violet-blue range of the spectrum. The remaining wavelengths are absorbed by other accessory pigments.

For Example:

Red algae Physcobilins absorb the blue-green light. The penetration of blue-green light is more than the red light in water. The absorption of blue-green light helps them to carry out the process of photosynthesis deep in water.

In the pigment molecule, every absorbed photon of light leads to the excitation formation of the electron into a further higher energy state. The excited energy is transferred to the chlorophyll molecule at the wavelength of P680. Here, P is the pigment, and 680 is the absorption at the maximum range of 680 nm. This process takes place at the photosystem reaction center II with the help of resonance energy transfer. The P680 can absorb the photon directly at the appropriate and suitable wavelength.
In photosynthesis, several steps of light-driven oxidation procedures proceed which carry out the process of photolysis. P680 is also known as an exciton. This exciton (P680) is taken by the major electron acceptor, which is a part of the photosynthetic electron transfer chain. The exciton came out from photosystem II.

For the reaction to continue, the electrons which are present at the reaction center must be restocked or replenished again and again. This refilling takes place by the oxidation of water. The strongest biological oxidizing agent is the electron-deficient reaction center at photosystem II. It allows the photolysis of water and splits it into its components.

photolysis of water molecule
Figure 2: Image shows photolysis of water molecule. Image Source: Maria Victoria Gonzaga of Biology Online.

Quantum Models

In 2007, Graham Fleming along with his coworkers proposed a model which is named the quantum model. In the model, he presents that there is the possibility of the involvement of quantum oscillations in the process of photosynthetic energy transfer. He tries to explain the unusual and infrequent high productivity and efficiency.

According to Fleming, there is an indication that during the process of photosynthesis, long wavelike electronic quantum coherence plays the main part during the processes of energy transfer. This explains the efficiency of energy transfer. This allows the overall system to become more efficient by the minimal loss in energy. However, in many further publications, this logic proves wrong.

Gregory Scholes along with his team investigated this approach at the University of Toronto and published his findings in 2010. The results say that increasing the energy harnessing efficiency of a few marine algae makes most of quantum-coherent Electronic Energy Transfer.

Photoinduced Proton Transfer

The molecules which form a photo base when absorbing light and undergo proton transfer are known as photoacids. In a photo inhibited proton transfer reaction, the breakdown happens in the electronically excited state. The acid and proton again combine and form photoacid, after the transfer of proton and the relaxation into the electronic ground state.

In the experiments of ultrafast laser spectroscopy, the most suitable source to induce pH jump is photoacids.

Photolysis in the Atmosphere

In the atmosphere, the process of photolysis reaction takes place in a series of steps. In this, there is the formation of secondary pollutants like peroxy acyl nitrates from the reaction of primary reactants like nitrogen oxide and hydrocarbons.

In the troposphere, vital photodissociation reactions are as follows. This also shows that what happens to atoms during a chemical reaction

O3 + hν → O2 + O(1D) λ < 320 nm

There is the generation of an excited atom of oxygen which reacts with a water molecule and forms hydroxyl radical.

O(1D) + H2O → 2 • OH

In atmospheric chemistry, the radical of the hydroxyl group is very important because it not only starts the hydrocarbon oxidation but also acts as a detergent.

The other essential atmospheric reaction is the formation of tropospheric ozone is:

NO2 + hν → NO + O

The layer of ozone is formed by the process of photolysis or photodissociation. In the stratosphere, ozone is formed by the striking oxygen molecule having two oxygen atoms O2 with ultraviolet light. It splits O2 into an individual oxygen atom which is known as atomic oxygen. Then the atomic oxygen reacts with the unbroken O2 to create ozone O3.

In the upper atmosphere, chlorofluorocarbons (CFCs) are broken down and thus form free radicals of chlorine which leads to the destruction of the ozone layer. This process is also carried out of photolysis.


Photochemical dissociation is a vital process in astrophysics. In this new molecules are formed when previous molecules break down. Free radicals and molecules can stay or exist for a longer duration due to the vacuum of the interstellar medium.
Photodissociation or photolysis is the foremost part by which molecules are broken down. In the study of the structure of interstellar clouds, the rate of photodissociation is very important. Interstellar clouds are involved in the formation of stars.

In the interstellar medium, the examples of photodissociation are ( is the energy of a single photon of frequency ν that results in dissociation).

Atmospheric Gamma-Ray Bursts

Nowadays, the average speed of orbiting satellites in a day is about one gamma-ray burst. Gamma-ray bursts surround most of the universe and they are observable to distance. A volume of the universe comprises numerous billions of galaxies. It proposes that gamma rays bursts remarkably rarely happen in galaxies.

It is very difficult to measure and calculate the particular and specific rate of gamma-ray bursts. Considering a galaxy that is almost the size of the Milky Way, the estimated rate in one burst of long GRBs is about 100,000 to 1,000,000 years. Just a small percent of it can be beamed on the way to the Earth. It is uncertain to define the estimated rate of short GRBs due to the unknowing beaming fraction.

There will be a remarkable effect on the biosphere if the gamma-ray bursts in the Milky Way beamed towards the Earth. In the atmosphere, the absorption of radiations will lead to the photodissociation of nitrogen N2. It would lead to the formation of nitrogen, which acts as a catalyst in the destruction of the ozone layer.

The atmospheric photodissociation

In 2004, there was a study that states that gamma rate bursts at a distance of about kiloparsec (“parsec” is the unit of a length outside of the solar system; for measuring the distance of astronomical objects, a parsec is used) will abolish and destroy about half of the ozone layer of Earth. The combination of direct ultraviolet irradiation with solar ultraviolet radiation, when passing through the ozone layer, will affect the chain of food and initiate mass destruction. According to the author, in a billion-year one such burst is expected. It is thought that the Ordovician-Silurian destruction event was caused by such a burst.

Multiple Photon Dissociation

For the direct photodissociation of molecules, single photons having the spectral range of infrared might not have an adequate amount of energy. A molecule might get internal energy to overcome the dissociation barrier after the absorption of many infrared photons. There are some ways by which the dissociation of several photons can be achieved. By using high power laser-like free-electron laser or carbon dioxide laser or by collision multiple photon dissociation can be achieved. A technique is known as blackbody infrared radiative dissociation (BIRD) in which the collision method is used for the multiple photon dissociation of black-body radiation.


Try to answer the quiz below to check what you have learned so far about photolysis.


Choose the best answer. 

1. What is photolysis?

2. In photosynthesis, which compound undergoes photolysis?

3. Which part of photosynthesis does photolysis occur?

4. In a plant cell, where does photolysis occur?

5. Which of the following does not involve photolysis?

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  • OxfordReference. (2021). Photolysis. Retrieved October 28, 2021, from https://www.oxfordreference.com/view/10.1093/oi/authority.20110803100324611
  • Toppr. (2021a). An essential process connected with photosynthesis. Retrieved October 28, 2021, from https://www.toppr.com/ask/en-hu/question/an-essential-process-connected-with-photosynthesis-is/
  • Toppr. (2021b). Photolysis of water. Retrieved October 28, 2021, from https://www.toppr.com/ask/question/define-the-photolysis-of-water/
  • Vedantu. (2020). What is photolysis. Retrieved October 28, 2021, from https://www.vedantu.com/biology/photolysis

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