(Please check out my video about this topic :))
Photosystems:
Photosystems are light harvesting complexes which directly convert energy from the photons of light to excited electrons. They are embedded in the thylakoid membranes and they consist of a reaction center (where chlorophyll a is present) and many antenna pigments that funnel light energy to the reaction center. There are two photosystems: Photosystem II (P680) and Photosystem I (P700). The two photosystems cooperate, and photosystem II is where the whole series light dependent reactions starts.
What happens when the electrons in the reaction center get excited?
One possibility: Noncyclic photophosphorylation
We have to keep in mind that photosystem II and photosystem I cooperate. However, here I'm going to describe the process starting from photosystem II. So first the light energy is captured by photosystem II, and the antenna pigments in photosystem II pass it on to the reaction center containing chlorophyll a. Next, two electrons in the reaction center get excited by the light energy and thus they move to a higher energy level. They are captured by primary electron acceptor. But: two electrons have just been lost from the reaction center, and they need to be replaced somehow. The solution to this is photolysis. Photolysis means essentially splitting of a water molecule to two protons, two electrons and one oxygen atom. Two oxygen atoms combine to form the diatomic oxygen molecule, and the two electrons will are now replaced by the electrons from water.
After the electrons have been accepted by an electron acceptor, they move through an electron transport chain, which consists of plastoquinone (Pq), a cytochrome complex and plastocyanin (Pc). Keep in mind that as the electrons are transferred from one electron carrier to the next (in other words: as one electron carrier is oxidized and the other gets reduced), protons from the chloroplast stroma move through the thylakoid membrane into the thylakoid space called lumen. These protons are going to be essential in ATP synthesis. The ATP synthase is also embedded in the thylakoid membrane, and the protons that have migrated to the lumen are goig to flow through the ATP synthase, providing energy for the phosphorlyation of ADP into ATP ( the reaction is ADP + Pi -> ATP). This is just like in the electron transport chain in animal cells.
Now, at the end of the first electron transport chain, these excited electrons form photosystem II will replace the excited electrons from photosystem I. What happens in photosystem I is very similar to the scenario in photosystem II. The electrons in the reaction center get excited by the photons of light, but this time, they are replaced by electrons from photosystem II, the same ones that have moved down this first electron transport chain.
The excited electrons from photosystem I also move down an electron transport chain. The difference here, though, is that at the end of this electron transport chain, NADPH is made. What happens is that NADP+ is reduced to NADPH because it accepts two protons and these two electrons that have travelled down this electron transport chain.
Another possibility: Cyclic flow
When there is a shortage of ATP, but not NADPH, more ATP can be produced using the cyclic flow mechanism. The essence of the cyclic flow is recycling of electrons. This takes place when the two excited electrons in photosysthem I are are accepted by primary electron acceptor, but then they travel back to the cytochrome complex in the electron transport chain which they have already passed through (the first electron transport chain). This causes an increase in proton gradient in the thylakoid lumen, which increases the rate of the phosphorylation of ADP into ATP.
Photosystems:
Photosystems are light harvesting complexes which directly convert energy from the photons of light to excited electrons. They are embedded in the thylakoid membranes and they consist of a reaction center (where chlorophyll a is present) and many antenna pigments that funnel light energy to the reaction center. There are two photosystems: Photosystem II (P680) and Photosystem I (P700). The two photosystems cooperate, and photosystem II is where the whole series light dependent reactions starts.
What happens when the electrons in the reaction center get excited?
One possibility: Noncyclic photophosphorylation
We have to keep in mind that photosystem II and photosystem I cooperate. However, here I'm going to describe the process starting from photosystem II. So first the light energy is captured by photosystem II, and the antenna pigments in photosystem II pass it on to the reaction center containing chlorophyll a. Next, two electrons in the reaction center get excited by the light energy and thus they move to a higher energy level. They are captured by primary electron acceptor. But: two electrons have just been lost from the reaction center, and they need to be replaced somehow. The solution to this is photolysis. Photolysis means essentially splitting of a water molecule to two protons, two electrons and one oxygen atom. Two oxygen atoms combine to form the diatomic oxygen molecule, and the two electrons will are now replaced by the electrons from water.
After the electrons have been accepted by an electron acceptor, they move through an electron transport chain, which consists of plastoquinone (Pq), a cytochrome complex and plastocyanin (Pc). Keep in mind that as the electrons are transferred from one electron carrier to the next (in other words: as one electron carrier is oxidized and the other gets reduced), protons from the chloroplast stroma move through the thylakoid membrane into the thylakoid space called lumen. These protons are going to be essential in ATP synthesis. The ATP synthase is also embedded in the thylakoid membrane, and the protons that have migrated to the lumen are goig to flow through the ATP synthase, providing energy for the phosphorlyation of ADP into ATP ( the reaction is ADP + Pi -> ATP). This is just like in the electron transport chain in animal cells.
Now, at the end of the first electron transport chain, these excited electrons form photosystem II will replace the excited electrons from photosystem I. What happens in photosystem I is very similar to the scenario in photosystem II. The electrons in the reaction center get excited by the photons of light, but this time, they are replaced by electrons from photosystem II, the same ones that have moved down this first electron transport chain.
The excited electrons from photosystem I also move down an electron transport chain. The difference here, though, is that at the end of this electron transport chain, NADPH is made. What happens is that NADP+ is reduced to NADPH because it accepts two protons and these two electrons that have travelled down this electron transport chain.
Another possibility: Cyclic flow
When there is a shortage of ATP, but not NADPH, more ATP can be produced using the cyclic flow mechanism. The essence of the cyclic flow is recycling of electrons. This takes place when the two excited electrons in photosysthem I are are accepted by primary electron acceptor, but then they travel back to the cytochrome complex in the electron transport chain which they have already passed through (the first electron transport chain). This causes an increase in proton gradient in the thylakoid lumen, which increases the rate of the phosphorylation of ADP into ATP.
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