How Does Photolysis Work in Photosynthesis?
Photolysis meaning is given as, it is a chemical reaction in which molecules are broken down into smaller units by light absorption. The experimental method known as flash photolysis, which is used in the study of short-lived chemical intermediates produced in many photochemical reactions, is one of the most well-known examples of a photolytic process.
About Photolysis
This technique was developed by the English chemists in 1949 named R.G.W. Norrish and George Porter that mainly consists of subjecting either a liquid or gas phase photolysis to an intense burst of light, resulting in lasting some microseconds or milliseconds, followed by a second, which is an ordinarily less intense flash. The first flash dissociates the absorbing compound into the short-lived molecular fragments, whereas the second flash gives a means for their identification by the spectrophotometry technique. This method is a valuable tool for transient chemical intermediates identification and thus for the study of fast chemical reaction mechanisms.
Photolysis in Photosynthesis
Photolysis is defined as either a part of the light-dependent reaction or photochemical phase or light phase or Hill reaction of photosynthesis. Photosynthetic photolysis general reaction can be given as follows:
H2A + 2 Photons(Light) → 2e- + 2H+ + A
The chemical nature of "A" is based on the organism type. In the case of purple sulfur bacteria, hydrogen sulfide (H2S) oxidizes to sulfur (S). And, in oxygenic photosynthesis, the water (H2O) molecule serves as a photolysis substrate resulting in the diatomic oxygen (O2) generation. This is the process that returns oxygen to the atmosphere of the Earth. Photolysis of water takes place in the chloroplasts of green algae and plants and thylakoids of cyanobacteria.
Energy Transfer Models
The semi-classical, conventional model defines the photosynthetic energy transfer process as one, where the excitation energy hops from the light-capturing pigment molecules to the reaction center molecules stepwise down the molecular energy ladder.
The effectiveness of the photons of various wavelengths depends upon the absorption spectra of the photosynthetic pigments present in the organism. Also, chlorophylls absorb the light in the red and violet-blue parts of the spectrum, while the accessory pigments capture other wavelengths too. The red algae's phycobilins absorb blue-green light, which penetrates deeper into water than red light, allowing them to photosynthesize in deep waters.
Every absorbed photon causes the exciton formation (which is an electron, excited to a higher energy state) in the pigment molecule. The exciton energy can be transferred to a chlorophyll molecule (which is P680, where P is the pigment and 680 is the absorption, at a maximum range of 680 nm) in the photosystem's reaction center II via resonance energy transfer. Also, the P680 can directly absorb a photon at a suitable wavelength.
Photolysis during photosynthesis takes place in a series of light-driven oxidation events. The energized electron (which is called exciton) of P680 can be captured by a major electron acceptor of the photosynthetic electron transfer chain and exits photosystem II. In order to repeat this reaction, the electrons present in the reaction center need to be replenished. This happens by the oxidation of water in oxygenic photosynthesis cases. The electron-deficient reaction center of the photosystem II (which is the P680*) is given as the strongest biological oxidizing agent yet discovered that allows it to break besides molecules as stable as water.
Quantum Models
A quantum model was proposed in 2007 by Graham Fleming with his co-workers that include the possibility where the photosynthetic energy transfer might involve quantum oscillations, explaining its unusual high efficiency.
According to Fleming, there is a piece of direct evidence that the long-lived wavelike electronic quantum coherence remarkably plays a considerable part in the processes of energy transfer during the photosynthesis that explains the energy transfer's extreme efficiency because it allows the system to sample all of the potential energy pathways with minimal loss and choose the most efficient one. However, this claim has since been proven to be wrong in many publications.
Further, this specific approach has been investigated by Gregory Scholes with his team at the University of Toronto, where in early 2010 published research results that indicate a few marine algae make the most of quantum-coherent Electronic Energy Transfer - EET to enhance their energy harnessing efficiency.
Photoinduced Proton Transfer
Photoacids are the molecules that, upon the light absorption, undergo a proton transfer to form a photobase.
In these particular reactions, dissociation takes place in the electronically excited state. After the proton transfer and the relaxation to the electronic ground state, acid and proton again recombine to form the photoacid.
In ultrafast laser spectroscopy experiments, photoacids are a handy source for causing pH jumps.
Photolysis in the Atmosphere
Photolysis takes in the atmosphere as part of a reaction series, where the primary pollutants such as nitrogen oxides and hydrocarbons react to form secondary pollutants like peroxyacetyl nitrates.
The two most essential photodissociation reactions in the troposphere are firstly given as follows:
O3 + hv → O2 + O(1D) λ < 320nm
which generates the excited oxygen atom that can react with a water molecule to give the hydroxyl radical, which is represented as follows:
O(1D) + H2O → 2 *OH
FAQs on Photolysis: Meaning, Mechanism & Biological Importance
1. What is photolysis in the context of biology?
Photolysis is a chemical process where a molecule is broken down (lysis) by absorbing energy from photons of light (photo). In biology, this term specifically refers to the light-dependent splitting of water molecules (H₂O) within the chloroplasts during the first stage of photosynthesis.
2. Where exactly does photolysis occur inside a plant cell?
Photolysis in plants takes place on the inner side of the thylakoid membrane, which are flattened sacs located inside the chloroplasts. The enzyme complex responsible for this reaction is associated with Photosystem II (PS II).
3. What is the chemical equation for the photolysis of water?
The overall balanced chemical equation for the photolysis of two water molecules is:
2H₂O → 4H⁺ + 4e⁻ + O₂
This shows that for every two molecules of water split, four protons (hydrogen ions), four electrons, and one molecule of diatomic oxygen are produced.
4. What are the main products of photolysis, and what is their role in photosynthesis?
The three main products of photolysis and their functions are:
Electrons (e⁻): These replace the electrons lost by the reaction centre of Photosystem II, allowing the electron transport chain to continue.
Protons (H⁺): These accumulate inside the thylakoid lumen, creating a proton gradient that drives the synthesis of ATP (adenosine triphosphate).
Oxygen (O₂): This is released as a byproduct of photosynthesis and is the primary source of the oxygen in Earth's atmosphere.
5. Why is the photolysis of water so important for life on Earth?
The photolysis of water is fundamentally important for two main reasons. Firstly, it releases oxygen as a byproduct, which has shaped Earth's atmosphere and is essential for aerobic respiration in most living organisms. Secondly, it provides the electrons and protons (H⁺) that are crucial for producing ATP and NADPH, the energy-rich molecules that power the synthesis of glucose in the second stage of photosynthesis. Without it, the entire process of photosynthesis would halt.
6. How does a plant get the energy to split a stable molecule like water during photolysis?
A plant uses the energy from sunlight, which is captured by chlorophyll and other pigment molecules within Photosystem II (PS II). This light energy excites electrons in the chlorophyll to a higher energy state. This energy, channelled through a special enzyme cluster called the Oxygen-Evolving Complex (OEC), provides the power to overcome the chemical bonds holding the water molecule together, leading to its splitting. It is a direct conversion of light energy into chemical energy.
7. What is the role of the Oxygen-Evolving Complex (OEC) in photolysis?
The Oxygen-Evolving Complex (OEC), also known as the water-splitting complex, is a metalloenzyme cluster containing manganese and calcium ions located at Photosystem II. Its primary role is to catalyse the oxidation of water. It sequentially removes four electrons and four protons from two water molecules, and once this is complete, it releases a molecule of O₂.
8. Can photolysis occur without chlorophyll?
No, in the context of photosynthesis, photolysis is entirely dependent on chlorophyll. Chlorophyll is the primary pigment molecule that absorbs the light energy required to initiate the process. While strong light can break down some compounds directly (photodegradation), the specific and efficient splitting of water in plants to fuel photosynthesis requires the energy-capturing and channelling mechanism provided by chlorophyll and its associated photosystems.
9. What is the difference between photolysis and photosynthesis?
Photolysis and photosynthesis are related but distinct concepts. Photosynthesis is the overall process by which plants convert light energy into chemical energy (glucose), consisting of two main stages. Photolysis is a specific sub-process that occurs only during the first stage (light-dependent reactions). It is the critical first step where water is split to provide the necessary electrons and protons for the rest of the light reactions to proceed.
10. Besides photosynthesis in plants, are there other examples of photolysis in nature?
Yes, photolysis is a fundamental chemical process that occurs in other natural contexts. For example:
Ozone Layer Formation: In the stratosphere, high-energy ultraviolet (UV) light splits oxygen molecules (O₂) into individual oxygen atoms (O). These atoms then combine with other O₂ molecules to form ozone (O₃).
Atmospheric Chemistry: Photolysis drives many reactions in the atmosphere, such as the breakdown of pollutants like nitrogen dioxide (NO₂) by sunlight, which contributes to the formation of urban smog.
