A photon has just hit the light harvesting complex of the reaction center of pho

Question

A photon has just hit the light harvesting complex of the reaction center of photosystem II and an electron has been excited from P680. Follow up this electron and give an overview of light reaction in producing the ATP and NADPH required for the next stage of photosynthesis that is Calvin cycle in producing sugar as the final product of photosynthesis. In your writing, you do not want to miss stating how water is used and the oxygen we breathe is produced. The two diagrams below should dictate your writing. Please make your writing brief but complete to earn points.

Explanation / Answer

The light reactions take place in membrane of thylakoids called chloroplasts. There are two photosystems, which are complexes of pigments and proteins, specialized to capture optimum light energy. In the process called non-cyclic phosphorylation, electrons from water pass through various pigments of PSI and PSII to finally produce ATP and NADPH. The steps are as follows:

1. Light absorption by PSII- Light is absorbes and energy is passed from pigment to pigment to finally reach the reaction center P680. This makes an electron to reach high energy level, which is passed to an accepter molecule and is replaced with an electron from water. This breaks the water and dimerizes the oxygen, to produce O2 for our breathing.

2. Synthesis of ATP- High energy electron is passed down the electron transport chain. The energy released in the process drives pumping of H+ ions from the stroma into the thylakoid interior. The H+ ions from water breaking also adds to it, building a gradient. In a process called chemiosmosis, these H+ ions pass through ATP synthase and cause production of ATP from ADP.

3. Light absorption in PSI- The electron arrives at photosystem I and enters P700 in the reaction center. Here, an electron in P700 is excited to a high energy level and transferred to an acceptor molecule. These eceltron is replaced by a new electron in PSII.

4. NADPH formation- The high-energy electron travels down a short second leg of the electron transport chain. The electron is passed to NADP+ (along with a second electron from the same pathway) to make NADPH.

In the next step called cyclic phosphorylation or Calvin Cycle, the products of light reaction are used to produce G3Ps which finally produce sugar in plants. The steps are as follows:

1. Fixation- Ribulose bisphosphate carboxylase (RuBisCO) and three molecules of ribulose bisphosphate (RuBP) in stroma initiates the Calvin cycle. RuBisCO catalyzes a reaction between CO2 and RuBP. For each CO2 molecule that reacts with one RuBP, two molecules of 3-phosphoglyceric acid (3-PGA) is formed, in each turn of the cycle. The number of carbon atoms remains the same during the reaction from inorganic to organic molecules (3 atoms from 3CO2 + 15 atoms from 3RuBP = 18 atoms in 3 atoms of 3-PGA) and hence the process is called carbon fixation.

2. Reduction- Six molecules of ATP and NADPH from light reaction are used to convert the six molecules of 3-PGA into six molecules of glyceraldehyde 3-phosphate (G3P). This is a reduction reaction as it involves gaining of electrons by 3-PGA. For ATP, energy is released with the loss of the terminal phosphate atom, converting it to ADP; for NADPH, both energy and a hydrogen atom are lost, converting it into NADP+. Both of these molecules return to the nearby light-dependent reactions to be reused and reenergized. One of the G3P molecules is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant.

3. Regeneration- G3P exported from the chloroplast has three carbon atoms, so it takes three turns of the Calvin cycle to fix enough net carbon to export one G3P. In three turns, six G3Ps are produced. One is exported while the remaining five G3P molecules are used to regenerate RuBP for further turns. Three more molecules of ATP are used in these regeneration reactions.

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