THE LIGHT REACTION
The Light Dependent Reaction is the first part of photosynthesis and occurs within the chloroplast. Chloroplasts contain small units named thylakoids, the inner region of which is called the thylakoid space and it is where the light dependent reaction takes place.
You have already learned the very basic process of photosynthesis which is the process by which plants synthesize energy for themselves by converting the sun's energy into glucose. The glucose produced by plants allows us to consume them from fruits and vegetables and obtain the glucose we need for cellular respiration (Remember back to the first step of cellular respiration in glycolysis!). Therefore, photosynthesis in plants and cellular respiration in animals are very much connected.
However, photosynthesis is much more complicated than you might have thought. In the light reaction, there are 5 essential complexes embedded into the thylakoid membrane, namely; Photosystem II, b6-f complex, Photosystem I, NADP Reductase, and ATP Synthase. Since this structure makes the Light Dependent Reaction very similar to the reactions in the Electron Transport Chain, it can also be thought of as a train traveling across the border of the thylakoid and making stops along each of these essential complexes. The following are the steps of the non-cyclic light dependent reaction:
You have already learned the very basic process of photosynthesis which is the process by which plants synthesize energy for themselves by converting the sun's energy into glucose. The glucose produced by plants allows us to consume them from fruits and vegetables and obtain the glucose we need for cellular respiration (Remember back to the first step of cellular respiration in glycolysis!). Therefore, photosynthesis in plants and cellular respiration in animals are very much connected.
However, photosynthesis is much more complicated than you might have thought. In the light reaction, there are 5 essential complexes embedded into the thylakoid membrane, namely; Photosystem II, b6-f complex, Photosystem I, NADP Reductase, and ATP Synthase. Since this structure makes the Light Dependent Reaction very similar to the reactions in the Electron Transport Chain, it can also be thought of as a train traveling across the border of the thylakoid and making stops along each of these essential complexes. The following are the steps of the non-cyclic light dependent reaction:
PART 1: Electron Transport System
Step 1: A photon is a particle of light energy from the sun's light waves. The "train" makes a stop at Photosystem II and a photon is absorbed by the system. Think of it as the gasoline that will fuel the train for the rest of the ride. Once the photon is absorbed, the energy is passed onto the P680 molecule in the reaction center of photosystem II. The P680 molecule is much like the engine of the train that absorbs the fuel in this case. As it reaches this molecule, or "engine", the high amount of energy excites an electron from the reaction center and causes it to jump to the electron acceptor. To understand this concept, imagine the sugar-high people often get when they eat too many candies. When one of your friends consumes too much sugar, they are momentarily very hyper and children especially begin to literally jump around - much like the way the electron jumps up to the electron acceptor. Think of this as the electron's "sugar-high". As it receives the energy from the photon, it jumps to the electron acceptor named pheophytin and this can be considered a "train car". This sugar-high results in the loss of an electron from photosystem II meaning it is thus oxidized through an oxidation reaction.
Step 2: When the electron leaves photosystem II and jumps onto the electron acceptor (pheophytin train car), it causes the P680 reaction center to be missing an electron. Since it lost the negatively charged electron, P680 is said to be positively charged at this point and thus has a high affinity for electrons to replace the one it has lost. P680+ can now pull electrons from the 2 H2O molecules that are inside the thylakoid space. A Z protein splits the 2 H2O molecules into its 4H+, 4e-, and O2. Through a reduction reaction, P680 absorbs the 4e- one by one and replaces each electron as it is excited and travels through the electron transport system. The H+ ions remain in the thylakoid space and the O2 that is released in the water-splitting serves as one of the most important products of photosynthesis.
Step 3: The electron is transferred from pheophytin to the electron carrier called plastiquinone. It is then again transferred to the b6-f complex. With each transfer, the excited electron releases energy and the b6-f complex uses this very energy to pump H+ ions from the stroma, across the thylakoid membrane, and into the thylakoid space. These H+ ions join the rest of the H+ ions being produced in the thylakoid space by the water-splitting and eventually create a H+ ion gradient; forming a higher concentration of H+ ions in the thylakoid membrane than in the stroma.
Step 4: During steps 1-3, photosystem I also absorbs light energy and passes the energy onto its reaction center in the P700 molecule. Similar to P680, an electron inside P700 becomes excited and gets a "sugar-high". It also moves onto its own primary electron acceptor outside of photosystem I. The electrons are then passed from this acceptor to another carrier called ferredoxin which then transfers the electron again to the next complex, NADP reductase. This complex uses the energy from the electrons it receives to combine an H+ ion in the stroma to an NADP to form an NADPH molecule. This is called a reduction reaction as NADP is reduced when the H+ ion as added to it.
Step 5: In order to replace the missing electron from photosystem I, the electron that originated from the P680 molecule of photosystem II and is now at the b6-f complex is transferred to the carrier called plastocyanine. Then, the e- is transferred again and reaches the P700 molecule in the reaction center of photosystem I; replacing the electron it had lost. In this way, electrons are continuously replaced in both photosystems and they pass along the electron transport system, providing the energy to pump H+ ions into the thylakoid membrane and form NAPH molecules.
Step 5: In order to replace the missing electron from photosystem I, the electron that originated from the P680 molecule of photosystem II and is now at the b6-f complex is transferred to the carrier called plastocyanine. Then, the e- is transferred again and reaches the P700 molecule in the reaction center of photosystem I; replacing the electron it had lost. In this way, electrons are continuously replaced in both photosystems and they pass along the electron transport system, providing the energy to pump H+ ions into the thylakoid membrane and form NAPH molecules.
PART 2: Chemiosmosis and Photophosphorylation
At the end of the electron transport system, the H+ ion concentration in the thylakoid space is very high due the amount of H+ ions being produced from the splitting of the water. Also, the b6-f complex continuously pumps H+ ions into the thylakoid space from the stroma across the thylakoid membrane using the energy it received from the transfer of electrons. Therefore, at this point there is an H+ ion gradient that is formed. The H+ ions have the tendency to move down their concentration gradient from high to low concentration, in this case from the thylakoid space to the stroma, however in this situation the thylakoid membrane is impermeable to them and they may not cross it except by the means of the last complex embedded in the thylakoid membrane, ATP synthase. ATP synthase allows the H+ ions to move down their gradient and as it does, the energy of the gradient is used to generate ATP molecules by combining ADP and Pi molecules in the stroma. This indirect use of photons to drive the phosphorylation of ADP to produce ATP is referred to as photophosphorylation. The overall use of the energy in the H+ ion gradient to initiate the photophosphorylation is called Chemiosmosis.
Think of the ATP synthase as the single hole in a large dam blocking a river. As the water builds up behind the dam, the pressure increases and it wants to keep flowing however is restricted by the dam. One hole in the dam, however, allows the water to flow through. This hole is represented by the ATP synthase and due to the very high amount of H+ ions moving across the membrane into the thylakoid space, the pressure builds up very much like the water and the speed of the H+ ions pumping through the membrane is very high, therefore increasing efficiency of the production of ATP. Per 1 hydrogen atom, that is pumped 1 ATP molecule is produced.