Plants and other autotrophs are the producers of the biosphere. In a process called photosynthesis, a plant captures the energy of sunlight and uses it to make food molecules. This occurs in two main steps: the light reactions convert solar energy to the chemical energy of ATP and NADPH, and the Calvin cycle uses ATP and NADPH to convert carbon dioxide to sugar.
10.1 Overview of Photosynthesis
In plants, photosynthesis occurs in chloroplasts, mainly in leaf cells. The light reactions occur along the thylakoid membranes within the chloroplasts, where pigments capture light energy. The sugar-making reactions of the Calvin cycle occur in the stroma, the fluid between the inner membrane of the chloroplast and the thylakoids.
In the light reactions, light is absorbed by chlorophyll and other pigment molecules, exciting their electrons. The energy of excited electrons is then used by electron carriers to produce NADPH molecules and generate a concentration gradient that powers the synthesis of ATP. This process splits water and releases oxygen.
In the Calvin cycle, energy from ATP, electrons from NADPH, and carbon from carbon dioxide are combined to produce sugar molecules.
10.2 Light Reactions
Two photosystems, the electron transport chain, and ATP synthase are the key components of the light reactions of photosynthesis. These parts are embedded in the thylakoid membranes of a chloroplast. Select the correct label for each part from each pull-down menu
The photosystems consist of arrays of chlorophyll molecules. Chlorophyll-- the green pigment of leaves-- absorbs light energy. The absorbed energy excites electrons to a higher energy level. Energized electrons from photosystem I are added to NADP+ to form NADPH, while energized electrons from photosystem II are passed through the electron transport chain. Their energy is used to pump hydrogen ions from the stroma into the thylakoid space, creating a concentration gradient. Electrons leaving the electron transport chain enter photosystem I, replenishing its lost electrons. Photosystem II replenishes its electrons by splitting water. Hydrogen ions and oxygen are released into the thylakoid space. This is where the oxygen gas generated by photosynthesis comes from.
The buildup of hydrogen ions inside the thylakoid space stores potential energy. This energy is harvested by an enzyme called ATP synthase. As hydrogen ions diffuse through the membrane, down their concentration gradient, ATP synthase uses the energy of the moving ions to make ATP. ATP and NADPH are used in the sugar-making process of the Calvin cycle.
10.3 Calvin Cycle
So far, energy from light has been stored in the chemical bonds of ATP and NADPH. In the Calvin cycle, this stored energy is used to produce sugar molecules.<BR
The Calvin cycle is a complex series of chemical reactions carried out in the stroma. The Calvin cycle begins with carbon fixation. Three molecules of carbon dioxide are added to three molecules of a 5-carbon sugar abbreviated RuBP. These molecules then split up to form six 3-carbon molecules
In the next two reactions, products from the light reactions are used to boost the energy of these three-carbon molecules.First, high-energy phosphate groups are added. Click on the molecule from the light reactions that provides them.
One of the G3P molecules represents the three carbon dioxide molecules fixed so far. The other five G3Ps are reshuffled to regenerate the original RuBP molecules. To summarize, so far the Calvin cycle has used the energy of the light reactions to reduce three molecules of carbon dioxide and produce one molecule of G3P.
Three more carbon dioxide molecules are fixed to form G3P in this same complicated way. To make each G3P, the Calvin Cycle consumes 9 ATP molecules and 6 NADPH molecules. These are regenerated in the light reactions. G3P is the actual final product of the Calvin cycle. The cell can combine two G3Ps to make glucose, which stores the energy that chlorophyll originally captured from the sun.
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