Mada za sehemu hiiNutritionMada 6
Light reaction stage of photosynthesis occurs in the thylakoids of the chloroplasts. Normally, chloroplasts are aligned in such a way that their thylakoids are held at right angle to the light source in order to maximize light absorption. Within the thylakoids, chlorophyll pigments are arranged in groups called photosystems, which are specialised form of chlorophyll that commonly exists in two forms namely; Photosystem I and Photosystem II. Chlorophyll photosystems are named in chronological order of their discovery, not in a way they are involved in light reaction Thus, photosystem I was discovered before photosysytem II. Photosystem I has a specialised form of chlorophyll called pigment 700 abbreviated as (P700) while Photosystem II has specialised form of chlorophyll pigment, 680 abbreviated as (P680). The numbers 700 and 680 stand for peak of wavelength of light which excite electrons in these pigments. This implies that the respective absorption peak for P700 and P680 is at the wavelength of 700 nm and 680 nm. Structurally, photosystem is a complex of chlorophyll, accessory pigments, proteins, and other molecules. They are important in the absorption and transfer of light energy through a series of reduction oxidation (redox) reactions.
- cyclic electron pathways (cyclic phosphorylation)
- Non cyclic election pathway (Non cyclic Phosphorylation) directly.
The mechanism of non-cyclic photophosphorylation involves both photosystems I and II in a non-cyclic movement of electrons to produce ATP.
When the chlorophyll in Photosystem II absorbs light, releases an excited pair of electrons which are transferred to a series of electron acceptor and carrier systems including cytochromes. As the electrons pass through different carrier systems, they release energy, but these electrons are not returned to their original position in the photosystem II rather they are handed over to the photosystem I. The energy released in a series of electron flow system is used by an enzyme called ATP synthase located in the thylakoid membrane to synthesize ATP by binding inorganic phosphate to ADP.
This process is called phosphorylation, which literally means phosphate addition. However, the energy used in phosphorylation emanates from the sunlight. Hence, this ATP synthesis is precisely described as photophosphorylation. It should be noted that as the two excited electrons leave the photosystem II in the first stage after being activated by the light of wavelength 680 nm, they are replaced immediately by electrons released from the splitting of water molecules within the thylakoid.
Water molecule splits into hydrogen ion or proton (H+), oxygen and two electrons (H2O →2H+ + ½O2 + 2e-). This process is called photolysis, because it only occurs in the presence of light. Thus, oxygen does not come from carbondioxide, but from the split of water molecules.
Cyclic photophosphorylation involves production of ATP which uses photosystem I but not photosystem II. This begins with photo activation of photosystem I through which the electrons are excited and pass on to a chain of electron carriers.
These electrons travel back along a chain of carriers to their original position in photosystem I (Figure 5.3). As electrons travel back, they release energy which is utilised to bind inorganic phosphate to ADP forming ATP with the aid of ATP synthase. In this cycle, only ATP is produced, but NADPH and oxygen are not produced.
The amount of ATP required in Calvin cycle is much higher than that produced in the non-cyclic photophosphorylation. This makes the cyclic photophosphorylation important to balance the ATP deficit without increasing NADPH
Differences between cyclic and non-cyclic photophosphorylation
| Feature | Cyclic Photophosphorylation | Non-Cyclic Photophosphorylation |
|---|---|---|
| Electron Pathway | Excited electrons always return to the original position or molecule. | Excited electrons do not return to the original position or molecule. |
| Photosystems Involved | Involves only one photosystem (P700). | Involves two photosystems (P700 and P680). |
| Photolysis of Water | Does not occur. | Occurs. |
| Products | Produces only ATP. | Produces ATP and reduced compounds like NADPH. |
| Final Destination of Electrons | The final destination of electrons is Photosystem I. | The final destination of electrons is NADP. |
| First Source of Electrons | Photosystem I. | Water. |
| Oxygen Production | Oxygen is not produced. | Oxygen is produced. |
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