Phosphorylation is considered as an important process during photosynthesis.
Phosphorylation is considered as an important process during photosynthesis. During photosynthesis, the conversion or phosphorylation of adenosine diphosphate (ADP- contains two phosphate group) to adenosine triphosphate (ATP- contains three phosphate groups) using the photon of sunlight is called phosphorylation. The process of photophosphorylation in which an electron expelled by the excited photo-center is returned to it after passing through a series of electron carriers is known as cyclic photophosphorylation is. It takes place under the condition of low light intensity and light of wavelength lower than 680 nm and when CO 2 fixation is inhibited. Plants are capable of producing energy by utilizing photons from sunlight through photophosphorylation. Plants spread their leaves to the sky in order to carry out light reactions of photosynthesis and by doing this, they absorb some energy. This energy is then converted into chemical energy to provide energy for the normal physiological functioning of plants. In this reaction, light provides the energy and water provides the electron-molecule. This is a major difference between photosynthesis and cellular respiration. During this reaction, photosynthetic pigments of plants absorb light that activates series of cellular process that ultimately converts light energy into chemical energy and stored in the bonds of the energy molecule ATP. The process of utilizing light energy and electron transport chain to make ATP is known as photophosphorylation. This reaction’s name itself suggests the process of gaining a phosphate molecule. ADP molecule gains this phosphate molecule and produces a molecule of ATP. Hence, photophosphorylation is also called as light reactions of photosynthesis.
The electron transport chains for photosynthesis is carried out in the thylakoid membranes of chloroplasts. This is mainly due to the availability of chlorophyll molecules and accessory pigments to absorb light energy. These both are must required ingredients in order to produce ATP molecule while utilizing energy from sunlight. Chlorophyll molecule acts as a reaction centers and the remaining molecules such as pigments within the membrane form an antenna complex.
Antenna complex, as the name suggests, it is responsible for the absorption of light energy (also known as photon molecule) and then, it transfers energy into the reaction centers. These reaction centers are key locations where the photon energy is transferred into the electron transport system.
The electrons enter into an excited state i.e. higher energy state when the reaction center chlorophyll receives light energy. This step is causing them to the outer electron orbitals and then to attach a protein in the electron transport chain. This is the step when the plant cell transfer light energy to chemical energy.
The energy from the movement of electrons is used to transport hydrogen ions (H+) across the thylakoid membrane. Every single movement of electron transport is coupled with the movement of hydrogen ions. The energy associated with the movement of hydrogen ions is used to make ATP from ADP and inorganic phosphate. For this reaction to take place, enzyme ATP synthase is required.
NADP + + H + → NADPH The above-mentioned sixth step takes place only during noncyclic photophosphorylation. In cyclic photophosphorylation, the electron, after passing through the electron transport chain, instead of reacting with NADP+, reenter into reaction center to repeat this cycle.
2H 2 O → 4 H + + O 2 + 4 e – Note- Oxygen molecule (O2) released as a part of photosynthesis does not come from carbon dioxide (CO2). As mentioned in the above step, it is produced when the water molecule is split to provide electron. In the above-mentioned seventh step, two molecules of water break down such that it produces oxygen molecule, not an oxygen atom. The electron from the water molecule does not enter into the ATP molecule during the light reaction. The overall electron transport flow can be mentioned as follow:
The pathway of energy flow is different from that of electron transport flow. The energy transfer pathway is mentioned as follow:
Electron transport chain is collectively made up of membrane-embedded proteins and organic molecules. The electronic transport chain components are found in the plasma membrane of prokaryotes, whereas in eukaryotes, many copies of these molecules are found in the inner mitochondrial membrane. The electron transport chain contains proteins such as Fd (ferredoxin), PQ (plastoquinone), Cyt C (cytochrome C), Q (ubiquinone), and PC (plastocyanin). The enzyme NADP reductase is also present. While traveling of an electron through the chain, it enters into a lower energy level from a higher energy level. It means it moves from less electron-hungry molecules to more electron-hungry molecules. Hence, this type of transfer of electron is an example of downhill electron transfer. The above-mentioned different protein complexes use the released energy (released during electron transfer) and that turn out into pumping of a proton from the mitochondrial matrix to the intermembrane space.