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Section 8.3: The Reactions of Photosynthesis

Exercise 1

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Light-dependent reactions, which occur in the thylakoid membranes of a chloroplast, is the first stage of photosynthesis. In this stage, the light energy, which was absorbed by photosystem II, is used for breaking down the water molecules into hydrogen ions (H$^{+}$), oxygen, and energized electrons. Then, these high-energy electrons are transported to photosystem I. In photosystem I, the same electrons from photosystem II are reenergized. Once NADP$^{+}$ picks up these high-energy electrons and H$^{+}$ ions, NADPH is formed. Since the inside of the thylakoid membranes are filled with positively charged hydrogen ions, the outside of the thylakoid membranes becomes negatively charged. The difference in the charges creates an energy that is used to convert ADP into ATP. ATP and NADPH are used for producing high-energy sugars during the next stage of photosynthesis which is the Calvin cycle.

Result

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Light-dependent reactions, which occur in the thylakoid membranes of a chloroplast, is the first stage of photosynthesis. In this stage, the light energy, which was absorbed by photosystem II, is used to convert ADP into ATP, and NADP$^{+}$ to NADPH. ATP and NADPH are used for producing high-energy sugars during the next stage of photosynthesis which is the Calvin cycle.

Exercise 2

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The Calvin cycle uses carbon dioxide, ATP and NADPH to produce high-energy molecules. In the first stage of this cycle, carbon fixation happens. This is when six carbon molecules from the atmosphere combined with a five-carbon molecule called RuBP. Once they are combined, they are converted into an organic molecule with higher-energy forms. In the next stage, the organic molecule is reduced and it becomes a simple sugar. NADPH donates electrons to molecules to make the sugar. The third step is the regeneration which requires ATP. The regeneration happens when sugar molecules proceed to make glucose and the other molecules are recycled to regenerate RuBP compound, which in turn would accept new carbon molecules.

Result

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The Calvin cycle uses carbon molecules, ATP, and NADPH to produce glucose. The reactions which take place during this cycle include carbon fixation, reduction, and regeneration.

Exercise 3

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Result

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Once the light energy is absorbed by the plants, the thylakoid membranes in the chloroplasts intercept this energy. This is when a light-dependent reaction takes place. This process will convert solar energy into chemical energy in the form of ATP and NADPH.

Exercise 4

Step 1

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NADPH is produced during the light-dependent reactions. It is then used in the Calvin cycle as an electron donor to convert carbon dioxide (CO$_2$) into carbohydrates such as glucose.

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To be more specific, NADPH acts as a reducing agent that becomes oxidized in order to reduce CO$_2$ into sugars. Once it is oxidized, it becomes NADP$^+$ that is then used in the light reactions.

Exercise 5

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The light-dependent reactions and the Calvin cycle are processes that are interdependent of each other. The products of the light-dependent reactions are needed by the Calvin cycle.

Step 2

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ATP and NADPH are produced in the light-dependent reactions. These are then used in the Calvin cycle to produce sugars, as well as ADP and NADP$^+$ that are used in the light reactions.

Step 3

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Without the light-dependent reactions, the Calvin cycle would not be able to proceed due to the lack of ATP and NADPH that were supposed to be produced by the light-dependent reactions.

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Section 8.3: The Reactions of Photosynthesis

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